Highs Blow, Lows Suck

A while ago, Steph came across this saying written on a meteorological t-shirt.  She asked me what it meant so I decided that I would answer it here..... it just took longer than normal for me to get around to it!  Anyways, on to the topic!

The creation of high and low pressure in the atmosphere is all about the convergence and divergence of air high in the atmosphere.  When air "converges" into a given air column, the mass of air in that air column increases over time.  When you add mass to a column, the amount of force, or pressure, that it places on the surface below increases as well, creating an area of high pressure.  Similarly, when air "diverges", the mass of air in that column decreases over time and reduces the amount of pressure on the surface below, creating an area of low pressure.

An example of how convergence and divergence in the
upper atmosphere creates high and low pressure systems at the surface.

In addition, there are vertical motions that take place with convergence/divergence.  When air converges (diverges) in the upper atmosphere, it has no where to go besides down (up).  Therefore, there is sinking (rising) motion associated with high (low) pressure systems.  At the surface, this results in divergence (convergence) under high (low) pressure systems - hence the saying "highs blow, lows suck."  In high pressure systems, air is forced down from upper levels to the surface and spread out (the blowing).  Meanwhile, in low pressure systems, air is pulled up do to the upper level divergence, pulling the air into the column at the surface (the sucking).  However, there is much much more to pressure systems that just this since this concept does not take into account one factor.... rotation and the jet stream!

To understand how air flows around areas of high and low pressure, you need to under stand the Coriolis force.  As air flows around the globe, the Coriolis force turns the flow to the right in the northern hemisphere and to the left in the southern hemisphere, creating a curved flow.  (to understand how the Coriolis force does this, I suggest you watch this video:

 

As a result, the divergent flow under a high pressure system is turned to the right, creating a clockwise rotation.  This kind of rotation is called anti-cyclonic.  Meanwhile the converging winds under a low pressure system are also turned to the right, causing a counter-clockwise or cyclonic rotation.  The opposite is true in the southern hemisphere where winds are turned to the left.  High pressure systems there are cyclonic and low pressure systems are anti-cyclonic.

Now in the upper atmosphere, the jet stream flows around areas of high and low pressure creating the ribbon-like appearance we are use to seeing.  As the jet stream flows curves around high pressure, the winds increase in speed, becoming what we call "supergeostrophic" (faster than the normal geostrophic value) while at the base of the trough, the winds decrease in speed or "subgeostrophic" (slower than the normal geostrophic value).  Therefore, the air must accelerate from the bast of the trough to the crest of the ridge and decelerate from the crest of the ridge to the base of the next trough.

Now, let's imagine that this flow is actually like bumper-to-bumper traffic with cars crossing the "ridge" faster than cars in the trough.  Travelling from the first trough to the next ridge, the distance separating the cars increases as their speed increase, or they diverge.  But between the ridge and the trough, the cars begin to pile up as the faster cars in the back slam into the slower moving cars in front.  As a result, they converge.  The divergence between the trough and next ridge decreases the pressure at the surface thereby strengthening a surface low pressure system or weakening a surface high pressure system.  Conversely, the convergence between the ridge and the trough increases the surface pressure, thereby weakening a surface low or strengthening a surface high underneath.

Obviously, there are some other factors that influence high and low pressure systems (including jet streaks!), but that would make for a very lengthy article that could be a bit boring to read if you're not that interested.  So with that I will cut this article short, and maybe dive back into it at a later date if you are at all interested!

Tornadoes... Mother Nature's best kept secret!

If you ask meteorologists what got them interested in the studying the weather in the first place, you'll find that a majority of them will mention something about a tornado.  And why not?  Tornadoes are often found to be the most fascinating of weather phenomenon amongst both professional meteorologists and amateur weather enthusiasts.  At the same time, they are actually one of the least understood of all weather phenomenon... and that's because they are so dangerous and destructive!

So what exactly causes tornadoes?
Tornadoes most commonly form in supercell thunderstorms (although they can form in thunderstorms along squall lines, near the ends of thunderstorm bow echos, and within land falling hurricanes).  These type of thunderstorms form in areas of high vertical wind shear, where winds increase in speed as you increase in altitude.  This results in rotation about a horizontal axis inside the thunderstorm.  When the updraft within the storm hits this rotation, it tilts it into the vertical (as shown to the right), creating a rotating updraft located in the area encircled by the red, dashed line in the image below.  This is a part of a storm circulation known as a mesocyclone.

Diagram of the structure of a supercell thunderstorm.

Now the mesocyclone really gets spinning.  In a process called stretching, air is stretched in a narrower and narrower column, causing it to rotate faster and faster (imagine how a skater spins faster by drawing in their arms and legs towards the axis of rotation).  Once the forward flank downdraft and the rear flank downdraft meet under the mesocyclone , the rear flank downdraft surrounds and isolates the rising updraft at the mesocyclone's center.  The low-level part of the updraft circulation now rises more slowly than the updraft aloft causing the entire air column to stretch.  Just as in the skater scenario, as the low-level rotation is stretched, it's rotational speed is increased dramatically (this is known as vortex stretching). 

Now it gets complicated.  Since the exact process of tornadogenesis (the process of making a tornado) is still uncertain, there are three different theories.  The first is known as the "dynamic pipe effect" in which the tornado descends from the mid-levels of the storm and then emerges from the base of the wall cloud.  To understand this concept, think of a narrowly constricted flow in the middle atmosphere that might develop when the mid-level mesocyclone is stretched.  Imagine this as a pipe.  Air entering this narrowly constricted pipe region from below must itself constrict as it approaches the entry point.  That constriction actually extends/lowers the pipe downward.  This process can then continue to the ground as long as the air below the pipe is rotating.  When it reaches the ground, a tornado is born.

The dynamic pipe effect.

The second concept is more of a bottom-up approach.  This process is believed to occur due to the tilting of the horizontal circulation along the forward flank gust front as it moves under the mesocyclone's updraft.  The air behind the gust front is cooler and sinking, while the air ahead of the gust front is warm and rising, which leads to a sense of rotation along the boundary.    If the rotation advances under the strong updraft of the mesocyclone, it can be tilted into the vertical, leading to rapid rotation very close to the ground.  With further vortex stretching, this rotation can become a "bottom-up" tornado.

The final concept actually came from the original VORTEX (Verification of the Origins of Rotation in Tornadoes EXperiment) study from the 1994-1995 storm chasing season.  This concept describes a process called "vortex breakdown" in which a tornado expands to a very large size.  As it gets extremely large, a downdraft develops at it's core.  As this downdraft progresses down to the vortex to the surface, the tornado expands and can become up to a half mile wide.  Then, smaller vortices known as suction vortices develop within the circulation of the tornado when the central downdraft merges with the outer rising air.  The most violent part of the tornado are the suction vortices where the strongest winds occur.  During the VORTEX study, a similar process was observed in the mesocyclone during tornadogenesis.  In this case, a central downdraft occurred within the mesocyclone circulation.  The resulting tornado occurred when the central downdraft merged with the rotating air in the outer part of the surface mesocyclone.  In a similar scenario as suction vortices, one of the resulting vortices in the mesocyclone spun up to form the tornado.

So what about tornadoes?  Any interesting facts?

Well, tornadoes are generally on the ground for a few minutes, but can last as long as an hour or even more!  The lifespan of a tornado generally depends on how quickly the rear flanking downdraft can wrap completely around the tornado's circulation, cutting off it's updraft.  However, supercell thunderstorms that do produce a tornado can go through the tornadogenesis process several times during its lifetime.  Each cycle is associated with a new updraft that develops just outside of the precious cell that produced the earlier tornado.  The resulting tornadoes form a group known as a tornado family, with more than one member of the family being possible on the ground at the same time.

How are tornadoes rated?
Recently, the National Weather Surface updated their criteria for tornado intensity, upgrading the classic Fujita scale to the new Enhanced Fujita scale (or EF scale).  Below you can see how the two scales compare in wind speeds... and then you can see the Enhanced Fujita scale in detail.

Okay... so your head is probably ready to explode after all of that.  This post got a bit lengthy and technical to boot.  If you have any questions, post a comment and I'll do my best to answer them there!  Until next time, keep your eye on the sky!

Lightning.... it's not caused by some dude with a hammer!

Lightning.  It can often be one of the scariest parts of thunderstorms, especially to a little kid (if you've even been woken at 2 a.m. during a thunderstorm by your scared son or daughter... or have done it to your parents, you know what I'm talking about).  Lightning is the severe weather element most associated with thunderstorms and is easily identified world-wide.  It can cause awe and wonder while at the same time fear and worry.  So how and why does lightning happen?

To understand how lightning forms, you need a little background on the electric field of the Earth-atmosphere system.  The Earth has what is known as a fair weather electric field that exists in the absence of clouds.  In the clear air, the atmosphere will carry a net positive charge while the Earth below carries a net negative charge.  This results from the action of past thunderstorms, which act to deposit electrons on the Earth's surface and remove them from the atmosphere.  The fair weather electric field is about 100 volts per meter which, compared to the field just before a lightning strike (1,000,000 volts per meter), is quite small.  Luckily, air is an excellent insulator and so its conductivity is close to zero.

So now let's examine what happens when a rain cloud is in the area.  It all begins in the cloud where ice crystals and hail collide with each other, transferring electrical charges with each other as they collide.  The smaller ice crystals gain a positive charge while the larger particles, like hail, gain a negative charge.  The smaller, positively charged ice crystals are then swept higher into the cloud by the storm's updraft, while the larger, negatively charged particles fall toward the cloud base.  In so doing, the top of the cloud becomes positively charges, while the cloud base near the surface becomes negatively charged.    The negatively charged cloud base then induces a positive charge at the ground level (it's like a magnet... opposites attract!).

The negative charges in the cloud base then begin searching for a path of least resistance to the ground, creating the stepped leader which ionizes the air around it, forming a narrow conductive path.  As one of the branches of the stepped leader approaches the surface, the electric field between the surface and the stepped leader becomes so great that positive charges jump upward off the object to meet the descending stepped leader.  When this traveling spark connects with the stepped leader, the channel for electrons flow opens up and the bright and powerful return stroke occurs.  This process continues until the negative charge in the cloud base has been drained and deposited on the ground. 

What about thunder?
When lightning occurs, the air surrounding the stroke is heated to 54,000°F, causing the air to expand explosively.  This creates a shock wave that evolves rapidly into crashing sound waves and results in the noise we know as thunder.  The sound waves travel at approximately 330 meters per second, which means that it takes the sound about 5 seconds to travel a mile while the light from the flash travels so fast that it essentially arrives at our eyes in an instant.  Therefore, you can estimate the approximate distance between you and a lightning strike by counting the seconds between the flash of light and the sound of the thunder.

So now when you see lightning and hear thunder occur during a thunderstorm, you will know the cause behind it.  It's just a discharge of negatively charged particles from the cloud base to the ground followed by sound waves caused by exploding, superheated air.  That's right... it's not Thor with his mighty hammer.  Sorry big guy.

I think I just made Thor mad.

Look for the Bow!

Strong winds associated with thunderstorms are usually the culprits that get blamed for knocking over large trees, buildings, and other structures.  And rightly so.  When a thunderstorm is classified as severe, it may be capable of producing winds in excess of 60 mph!  That's not far away fro the winds of a a very weak category 1 hurricane (tropical cyclones must have winds of at least 74 mph to be considered a hurricane)!  So what causes these strong winds?

First, you need to know a little about how air flows in a thunderstorm.  When a thunderstorm firsts

Evolution of a gust front.

starts, it has a central updraft that brings warm, moist air from the surface up to build the thunderstorm.  These updrafts are relatively upright or slightly tilted.  As the rain develops and falls into the lower atmosphere, it evaporates and cools the air through which it falls.  This leads to the development of a cold pool (of air... no swimming here folks) at the base of the storm.  As the cold pool develops, this cooler air spreads outward in all directions, including towards the warm moist air feeding the thunderstorm.  As the cold pool grows deep enough and cold enough that the air begins to rush outward toward the warm air feeding the thunderstorm.  The leading edge of this out rushing air is called the gust front.  It is at this time that severe straight-line winds can occur at the surface.  As the cold pool continues to strengthen and the out rush of air begins, air in the evaporation region in the mid-altitudes of the storm flows forward toward the line of storms, creating a feature known as the rear inflow jet which can evolve and strengthen the gust front.

Winds behind the gust front can be severe, sometimes reaching up to 115 mph, but in most cases the winds range from 25 to 60 mph.  Often, a shelf cloud (also known as a roll cloud) will develop over the gust front and can make it easy to see as it approaches you.

So how do meteorologists detect these strong winds?

The first indication of a gust front/strong winds is a fine line of lower radar reflectivity that appears just ahead of the higher reflectivity of the storm.  Another, tell tale sign of strong winds is one that any radar watcher can easily identify.  A bow echo appears on radar as an arched thunderstorm echo and provides a distinct signature of strong straight-line winds.  It is at the center of the bow where the strongest winds will be located, since it is closer to the rear inflow jet, and the out rushing air in the section will be moving in generally the same direction as the rear inflow jet.

A classic example of a bow echo indicating strong straight line winds along the storms leading edge.

Another tool meteorologists use to identify areas of strong winds is the radar velocity feature which measures wind speeds moving toward and away from the radar site (greens indicate winds moving toward the radar while reds indicate winds moving away from the radar).  If this product "lights" (the colors get brighter and brighter... almost glowing like a light saber... okay, that went a bit too far) up with returns of higher wind speeds, this would indicate strong winds in a thunderstorm in the area.

 

Strong straight line winds indicated by the glowing reds/pinks on the radar velocity image on the left.  Normal radar reflectivity is on the right.  Glowing reds???.... these winds must be from the dark side....

And of course we also rely on observations reported by trained weather spotters and local law enforcement!  Those reports help a lot to verify what we are seeing on our computer monitors!

 

So if you just keep your eye on the radar when a storm is moving in, you may be able to spot one of the tell tale signs of strong winds.  If you're outside, look for the roll/shelf cloud... and then get to safety.  You don't want to be outside when that hits!

Mother Nature's Yo-yo

Hail.  That icy ball of wonder that occasionally falls from thunderstorms, covers the ground, and even causes damage.  It can be the size of peas, or as large as bowling balls.  So how does it form... and how can meteorologists detect it in time to warn the public?

How Hail Forms

Hail: Mother Nature's Yo-yo.

A hailstone begins as a frozen raindrop or ice crystal way up high near the top of a thunderstorm.  As it falls, it begins to melt some and collects more rain droplets as it collides with them.  This wet, icy mass then encounters a strong updraft of warm air, forcing it back up into the top of the thunderstorm where it refreezes.  It then continues this process until it becomes too heavy for the updraft to lift it back up into the storm and it falls to the surface and creates a strong, cold down draft.  This repeated up and down motion (which is why I called it a yo-yo!) and thawing and refreezing results in the onion-like layers that people often see when they cut a hail stone open.

How do meteorologists detect hail?

The most useful tool meteorologists use to detect
hail is radar.  Before dual polarized (dual pol) radars, the best indication on the radar of hail falling over an area was very high radar reflectivity values.  On the radar, these would appear as purple or white areas on a thunderstorm echo (you normally see greens, yellows, oranges, and reds).  The reason why the reflectivity value is so large is because the hail is bigger than the raindrops in the cloud!  Size matters!

The white and purple on this radar
image indicates the presence of hail.

With the dawn of dual pol technology, meteorologists now have more tools at their disposal.  With the Differential Reflectivity (which basically measures how spherical things are), hail shafts can be more easily identified because hail often takes a more spherical shape (compared to rain which is more elongated in one direction).  Values near zero from this product appearing in a return from a thunderstorm would likely suggest the presence of hail in the thunderstorm.






Another tool, known as the Correlation Coefficient, allows meteorologists to see the sizes of different types of radar targets (rain, hail, etc.).  This tool can be very useful in the detection of very large hail in a thunderstorm, which can be very dangerous and deadly.

With these new tools, meteorologists are now able to better detect the presence of hail in a thunderstorm so that the public can be warned in advance.  Hail is a common sight around the nation during the spring and summer, and if you do see some, collect a few pieces AFTER the storm has passed.  If you cut it open, you can satisfy your inner weather nerd by looking at the neat layers, like this:


A cross section of a hail stone.  Look at the layers!  How cool is that?

Water, water everywhere!

Welcome to severe weather awareness week in Wisconsin and Minnesota.  Each day this week, I will be covering a severe weather threat and tell you how it typically happens/forms and how meteorologist discover and track them.  Today's topic is Flash Flooding.  So let's jump in!


Want to go for a walk on that bridge???
Yeah, me neither.

Flash flooding occurs when the water level in creeks, streams, and rivers rise rapidly and and rush over their banks affecting areas in their flood plain.  Flash flooding can occur within minutes and is usually the result of heavy rain falling over an area for an extended period of time, although they do occur from broken dams and levees.  The threat of flash flooding is even greater in early spring when heavy rain can combine with a melting snow pack on grounds that not absorb much moisture due to the presence of frost.
So how do meteorologists determine when a flash flood is imminent?  First, we look at weather reports from the past few hours and days.  If an area's ground is already saturated or contains frost, it is at a higher risk for flash flooding.  Then, we determine how much rain an area has seen from the event.  It only takes a few inches of rain over a short time on saturated ground to cause a flash flood, especially if that ground is steep terrain! 


An example of training.

We also watch the radar for an event known as "training."  Training occurs when a band of rain moves over an area constantly for an extended period of time.  Think of the area as a train track and the storm as the train.... if the "track" doesn't get to see anything but the "train," it is very likely that flash flooding is imminent. 

Another tool that meteorologists can now use (thanks to the new dual polarized radars!) is known as the specific differential phase (KDP) product of dual polarized radars.  KDP allows meteorologists to look inside the storm and examine how concentrated the rain is, letting them know rainfall rates in storms.  This lets meteorologists to know for sure of a heavy rain event so they can better issue flash flooding watches, warnings, and statements based on what is actually happening.  An example of KDP is below, compared to normal radar reflectivity.  In the image, an area of high reflectivity (right) is being examined for heavy rainfall.  Higher values of KDP (the blues and greens in the picture)indicate areas of heavy rain fall, which could lead to flash flooding in those areas.  While the northern/upper circled area does indicate heavy rainfall, the southern/lower area does not.  Before dual polarized radar, meteorologists would not have had advanced knowledge like this... they would have had to wait for the actual storm reports to come in!  Now with the KDP, they can predict a flash flood event in advance, issue a statement/watch, and then once actual reports come in, they can issue a warning if need be.

Radar reflectivity (left) and Specific Differential Phase (KDP, right) of a storm near Norman, OK.



Armed with the radar reflectivity, specific differential phase, and storm reports from people like you, meteorologists can promptly warn the public of flash flooding dangers in the area.  Flash flooding events are serious and can cause a lot of damage to property, infrastructure, the economy, and not to mention is a danger to public safety.  Take precautions.  If a flash flood warning is issued in your area, get out safely.  Plan ahead and have an emergency action plan & kit in place, and you will be prepared for a flash flooding event.  For more flood safety tips, see my post from last week on flood safety.



I don't want to end up in Oz!

Hang on, Toto!!!!

Anyone who has seen the classic movie "The Wizard of Oz" knows how Dorothy and Toto got to the
land of Oz from the farm stead in Kansas.  The iconic scene of a tornado lifting the farm house up into the air with Dorothy and Toto inside still frightens many people.  The thought of being picked up high into the sky by a devastating tornado and then flung far away is a very scary thought.  What is even scarier is the possibility of losing your life due to a tornado.

Luckily, there are steps you can take to protect yourself.  First, before a tornado even gets a chance to come your way, it is suggested that you have a plan in place based on the kind of dwelling in which you live.  Know where you can take shelter in a matter of seconds and practice a tornado drill at least once a year.  Also, have a pre-determined place to meet after a disaster in case you become separated before, during, or after the storm.  Since flying debris is the greatest danger during a tornado, be sure to store protective coverings (mattress, sleeping bags, thick blankets, etc.) in or near your shelter place so you can use them on a few seconds' notice.  I also suggest buying a NOAA Weather Radio to alert you in the event a tornado is possible or is imminent.

When a tornado watch is issued for your area, this means that conditions are favorable for the development of tornadoes.  Stay alert for warnings from local television and radio stations and your NOAA Weather Radio.  Also, keep an eye on the sky!  Some tornadoes do occur without a tornado warning (weather forecasting science is not perfect - who knew?)!  Therefore, there is no substitute for staying alert to the sky.  Here are some things to look and listen for even if a tornado is not visibly evident:

1. A strong, persistent rotation in the base of the cloud.
2. Whirling dust or debris on the ground under a cloud base - sometimes tornadoes have no visible funnel!
3. Hail or heavy rain followed by either dead calm or a fast, intense wind shift.  Many tornadoes are wrapped in heavy precipitation and can't be seen!
4. A loud, continuous roar or rumble which doesn't fade in a few seconds like thunder.
5. At night, look for small, bright, blue-green to white flashes at ground level near a thunderstorm (as opposed to lightning up in the clouds).  These mean power lines are being snapped by a very strong wind or perhaps a tornado.
6. At night, us the storm's lightning to help you look for a tornado.  Look for a persistent lowering from the base of the clouds, illuminated or silhouetted by the storm's lightning, especially if it is on the ground or there are the blue-green to white flashes described in #5.

Do not panic.... follow your emergency plan!

When a tornado warning is issued for your area (which means a tornado is imminent or has been
spotted) or you identify a tornado from the previous tips, take action immediately.  You may have only moments to seek shelter!

• In a home with a basement:  Avoid ALL windows.  Get in the basement and under some kind of sturdy protection (like a heavy table or work bench) and/or cover yourself with a mattress or sleeping bag.  Know where the very heavy objects are on the floor above (like refrigerators, freezers, stoves, pianos, etc.) and do NOT go under them.  If the floor becomes weakened, they could fall through and crush you.  Also, if you have a helmet, you can put that on too to offer some protection for your head.
• In a home with NO basement, a dorm, or an apartment:  Again, avoid ALL windows.  Get to the lowest floor, a small center room (like a bathroom or closet), under a stairwell, or in an interior hallway with no windows.  Crouch as low as possible to the floor, facing down and cover your head with your hands (just like you practiced in school tornado drills).  If you can climb in a bathtub, it may also offer a shell of partial protection.  Even in an interior room, cover yourself with some sort of thick padding to protect your self from falling debris in case the roof and ceiling fail.
• In an office building, hospital, nursing home, or skyscraper:  Go directly to an enclosed, windowless area in the center of the building, away from glass and on the lowest floor possible.  Then crouch down and cover your head.  Interior stairwells are usually good places to take shelter, and if not crowded, allow you to get to a lower level quickly.  Stay off the elevators as you could be trapped inside them if the power is lost.
• In a mobile home:  Get out... Get out... GET OUT!!!!!  Even if your home is tied down, it is not as safe as an underground shelter or permanent, sturdy building.  Go to one of those shelters or a nearby permanent structure.  Most tornadoes can destroy even tied-down mobile homes, and it is best not to play the low odds that yours will make it.
• At school:  Follow the drill!  Go to an interior hallway or room in an orderly way as you are told.  Crouch low, head down, and protect the back of your head with your arms.  Stay away from windows and large open rooms like gyms and auditoriums.
• In a car or truck:  Vehicles are extremely risky in a tornado since they are easily tossed!  There is no safe option when caught in a tornado in a car, just slightly less-dangerous ones.  If the tornado is visible, far away, and the traffic is light, you may be able to drive out of its path by moving at right angles to the tornado.  Seek shelter in a sturdy building or underground if possible.  If you are caught by extreme winds or flying debris, park the car as quickly and safely as possible and out of the traffic lanes.  Stay in the car with the seat belt on.  Put your head down below the windows, cover your head with your hands and a blanket, coat, or cushion if possible.  If you can safely get noticeably lower than the level of the roadway, leave your car and lie in that area, covering your head with your hands.  Avoid seeking shelter under bridges and overpasses, which can create deadly traffic hazards while offering little protection against flying debris.
• In the open outdoors:  If possible, seek shelter in a sturdy building.  If not, lie flat and face down on low ground, protecting the back of your head with your arms.  Get as far away from trees and cars as you can since they may be blown onto you in a tornado.
• In a shopping mall or large store:  Do not panic.  Watch for others and move as quickly as possible to an interior bathroom, storage room, or other small enclosed area away from windows.
• In a church or theater:  Again, so not panic.  If possible, move quickly and orderly to an interior bathroom or hallway, away from windows.  Crouch face down and protect your head with your arms.  If there is no time to do that, get under the seats or pews, protecting your head with your arms or hands.

After the tornado, keep your family together and wait for emergency personnel to arrive.  Carefully render aid to those who are injured.  Stay away from power lines and puddles with wires in them since they may still be carrying electricity.  Watch your step to avoid broken glass, nails, and other sharp objects.  Stay out of any heavily damaged houses or buildings since they could collapse at any time.  Do NOT use matches or lighters in case of leaking natural gas pipes or fuel tanks nearby.  Remain calm and alert, and listen for information and instructions from emergency crews or local officials.

It is wise to have an emergency survival kit on hand
for any emergency, including tornadoes.

If you follow these tips, you may be able to survive a direct hit from a tornado.  Be smart and practice tornado safety at least once a year, maybe even during a state-wide tornado drill.  Wisconsin's state-wide tornado drill is this coming Thursday, April 18th at 1:00 p.m..  In the event that there is severe weather that day, it will be postponed until Friday, April 19th.  Also, it is also wise to have a disaster preparedness kit ready, to help you with and after any emergency, including a tornado.  To learn more about creating a disaster preparedness kit, go to http://www.ready.gov/build-a-kit.  You can also buy them online at many, many locations.  Just be prepared.  You never know what Mother Nature has in store.  Just ask Dorothy and Toto!

Lightning - Shocking.... isn't it?

So I seem to be on a roll with the bad puns.  Anyways, today's topic is lightning safety.  Lightning is a serious issue during thunderstorms, and you don't have to be directly under a thunderstorm cloud to get struck (did you know that lightning can travel as far as 10 miles away from the cloud?)!  Lightning is often in the presence of heavy rain or hail, strong winds, and even tornadoes, so they are in "good" company. 



Good advice!

Luckily, lightning has a friend.  A friend that gives you advanced warning that it is coming.  Thunder.  A saying presented by the National Weather Service for safety purposes is "When thunder roars, go indoors!"  The point here is, that when you hear thunder, that means that there is a lightning-producing thunderstorm in the vicinity and that you should move indoors for your own safety (not only from lightning, but from other severe weather that may be present as well).  Simply put - if you don't want to get struck by lightning (and who does?), get inside.  And stay there for at least 30 minutes after the storm has ended, since lightning could still strike you as the storm moves away.

But what if I'm inside????  Lightning can't hit me then, right?
Well, I hate to pop your bubble, but it can actually affect you in your home.  Stay off of electrical devices connected to the power grid (telephones, computers, etc.) as lightning can cause a power surge and destroy these devices.  The best idea is to unplug these when a storm moves in.  Also, stay out of the shower/bath.  If lightning strikes nearby your home, it's electrical current can actually follow your home's plumbing!  Not to mention, if your home is struck and manages to catch fire, you'd have just moments to get out due to the fire... and if you come out of the shower/bath, you'd be naked and would likely waste precious time putting clothes on!  So just stay away from water and electrical devices, just to be safe.

Also, the water point brings up another issue.  If you are outside fishing or swimming in a body of water and hear thunder or see lightning, leave immediately!  Water is a perfect conductor for electricity!!!!!

If you are traveling in your vehicle, you can relax if your vehicle has a hard-metal topped roof.  If lightning strikes your vehicle, it will safely pass to the ground beneath you... but it will also likely kill your car's on-board computer, which is a costly repair.  You'll be safe, but you car will be dead.  The best bet is to not travel through a thunderstorm unless absolutely necessary!  Stay home or pull off the road and wait for the storm to pass.

No one wants to get struck by lightning.  So just get inside and play it safe, staying away from water sources and electrical devices.

Wind just blows me away!

Okay.... so that was a bad pun.  But hey, at least it is accurate!  Wind is a crucial part of the atmospheric system... but when it becomes strong, it can make things dangerous!  Strong winds associated with severe thunderstorms are often, I feel, the most neglected factor of the storm by the public.  People think they can handle a little wind.  But what happens when that wind gets too strong?  Things happen.

Straight-line wind damage in Burnett County, WI on July 1st, 2011.
Courtesy of WQOW-Eau Claire, WI.

Strong winds associated with severe thunderstorms can be very destructive, especially when they are
straight-line winds.  Straight-line winds are very powerful because of their non-divergence and can easily topple entire forests (a past event like this is still evident along a few bluffs south of La Crosse, WI) and damage homes. 

The National Weather Service will issue severe thunderstorm warnings for storms that produce winds in excess of 60 mph to alert the public of the danger.  When such a warning is issued, it is advised that you get into a basement or interior hallway and stay away from windows since strong thunderstorm winds can blow in your windows and send glass flying around the room - injuring anyone in the path of the glass.  Strong winds can even topple old or poorly constructed buildings, so choose a study structure to seek shelter in.



Image of a microburst in Australia.

In addition to strong winds produced by thunderstorms, there is another type of strong wind that can cause significant damage to property and the public.  A microburst is a brief, localized, but powerful gust of wind that originates from quickly sinking air.  They can produce damaging straight-line winds at the surface that are similar to tornadoes.  And there are two types of microburst, wet and dry... and you can probably figure out the difference there!  Just like thunderstorm winds, microbursts are also destructive, so you should seek shelter immediately if you are warned about them (although there the time between an issued warning and being able to take cover is still very short!).

So all in all, be careful with the wind.  Sure it can help you rake the yard in the fall (by blowing all your leaves into your neighbor's yard), but it can also be very destructive.  Take the warnings seriously and stay clear of those windows!

Hail hurts!

Most people east of the Rocky Mountains have seen this type of precipitation during a severe
thunderstorm.  Hail, which is a frozen ball of ice, can cause significant damage to property and even cause people their life!  From crop damage to automotive damage and hospital stays, hail can have a dramatic impact on a person's life.

So what do I do?

When large hail (greater than 1" in diameter - this is the stuff that causes the most damage) is indicated by radar in a near by thunderstorm, the local National Weather Service office will issue a severe thunderstorm warning to alert the public.  If this warning is issued for your area, stay inside!  If you have some time before the storm hits, you may be able to get your vehicle inside a structure, but your safety should come first and foremost.  Hail is often accompanied by strong winds, heavy rains, and even tornadoes, so you should take shelter immediately.

The record holder 8" hailstone that fell near
Vivian, South Dakota on July 23, 2010. 
It had a circumference of 18.625" and weighed just shy of 2 lbs!

In addition, if you are outside, seek shelter immediately.  Hail does hurt... especially large hail.  While most hail is often small, it can and have gotten as large as a bowling ball!  Imagine that hitting you on the head over and over!

So just play it safe.  If hail is expected or IS falling in your area.  Stay inside.  You can go outside after the storm has passed to look at and collect some of the hail if you wish.  Just keep in mind..... hail hurts!

Turn Around.... Don't Drown!

If you are one of the many people out there who watch television and radio weather reports or get information directly from the National Weather Service (NWS), you have heard this saying before.  If this saying is new to you, commit it to memory - it could save your life!

"Turn Around, Don't Drown" is the common safety tip for dealing with floods while traveling in a car or on foot.  If the roadway in front of you has water over it (even a small amount), turn around and find a different route.  The water could actually be concealing a huge gap in the roadway created by the moving flood water.  Also, you may have no clue as to how fast the water is moving, and it only takes a few inches or feet (depending the vehicle) of water to float a vehicle.  Which means you could very easily be swept downstream by the rapidly moving flood water.

Okay, so what if I'm not in a vehicle.  What do I do then?
First, I would recommend getting a NOAA Weather Radio that can alert you to the watches, warnings, and advisories that the NWS issues.  They come in portable and non-portable varieties.  These devices issue alerts for floods and flash floods along with information to keep safe.  Secondly, if a watch or advisory is issued for your area, be on the look out for rapidly rising water and take necessary precautions!  Get ready to evacuate if necessary and make arrangements in advance.  However, if a warning is issued and you live in a flood prone area, abandon your home immediately and get to higher ground.  Flash floods can develop in minutes or hours dues to excessive rainfall, a dam or levee failure, or the release of water behind an ice jam - so you may not have much time to evacuate before your escape route is cut off by flood water.

I also want to mention that you should never, ever walk, run, or swim through flood water.  It only takes 6" of fast-moving water to knock you off your feet and the current is often extremely fast.  Again, turn around, don't drown.  And please don't think that you can outrun a flash flood!  Many have sadly tried and have lost their lives because of it!  Just stop, turn around, and find another escape route.

Remember... Turn Around, Don't Drown!

Oh where, oh where did my twister go?

Even though storm chasers are missing tornadoes....
I'm quite positive the public is not!

If you have noticed a lack of tornadoes across the nation so far this spring, you’re not alone.  Many meteorologists and storm chasers have indeed noticed the shortage of twisters across the U.S. so far this spring.  Tornado season usually begins to kick into high gear in March, but this year has been relatively quiet thus far.

If you may recall, last year was quite active.  By early April 2012, there had already been 290 tornadoes (which resulted in 60 tornado-related deaths).  Now fast forward to early April 2013, and the nation has only seen 137 reported tornadoes and, thankfully, only 2 fatalities.  So what is going on?  Why is the amount of tornadoes less than half of what we saw last year?  What changed?

The main reason that is has been so quiet (tornado-wise) thus far is also one of the biggest topics you hear or have talked about among your friends – the colder than normal spring and late winter! The cold air over the central and eastern U.S. can largely be blamed on a stationary high pressure system over Greenland and the North Atlantic which has blocked the normal flow of storm systems across the nation by altering the jet stream.  Normally, the jet stream begins to bring warm, moist air (a critical component for tornadogenesis) from the Gulf of Mexico into the central U.S. this time of year.  But this year, the flow has been the opposite, bringing cool dry air in from the north.

So when will this pattern change?  The process is well underway.  The blocking high pressure system over the North Atlantic has been weakening and warmer temperatures and moist air have moved back in from the Gulf of Mexico in recent days.  As a result, the first significant outbreak of severe weather across the nation could occur early next week as several storm systems slide across the central part of the nation.  The first of these systems will come through the Midwest on Sunday into Monday with the potential for gusty winds and some large hail.  This first system could then set the stage for a much larger severe weather outbreak by Tuesday and Wednesday when a second storm system slides through.

The categorical (left) and probabilistic (right) outlooks for severe weather for Monday, April 8th, 2013.
Courtesy of the Storm Prediction Center.

The severe weather outlook areas for Tuesday, April 9th (in red)
and Wednesday, April 10th, 2013 (in purple).
Courtesy of the Storm Prediction Center.

So what about folks up here in Wisconsin?  Although we can see severe weather and tornadoes now, our tornado season begins to ramp up in late April and continue into August (but they CAN occur anytime of the year, as evidenced by the tornado that hit near Kenosha on January 7th, 2008).  But at least for next week, we should remain safe in the severe weather department (unlike 2011 when the earliest outbreak in WI was on April 10th!).

Just out of curiosity, looking back on the past few years, 2012 was actually a very quiet year for tornadoes in Wisconsin.  According to the National Weather Service, there were only 4 reported tornadoes in Wisconsin in all of 2012 compared to 38 in 2011 and 46 in 2010.  So let's hope for another quiet year in 2013! 

Also, please note that Severe Weather & Tornado Awareness Week in Wisconsin in April 15th through the 19th, with a statewide tornado drill on Thursday, April 18th around 1 p.m. (should severe weather happen on that day, the drill will be postponed until Friday, April 19th).  Next week, I plan on posting regularly informing you how to stay safe in various weather situations, and then during Severe Weather & Tornado Awareness Week, I plan on posting on the various types of severe weather we see!

Hope to see you back here soon!  :)

Cold + Snow in March = Crabby People. Why is it so cold?

If you are like the many people I've heard from in the past week or so, you are very sick of the cold and snow that the area has seen over the past few weeks.  Many people are longing for the warmer and snow-less days of spring that typically begin around this time of year.  However, we have been stuck in a cold and snowy pattern that doesn't seem to let up.  This has led to many crabby people around the region!  And it has left many wondering why.

Negative and positive phases
of the Arctic Oscillation

It all has to do with the Arctic Oscillation (AO).  The AO is an index of sea-level pressure variations north of 20°N latitude and is either given a positive or negative value.  This index is directly related to the degree to which Arctic air can penetrate into the mid-latitudes, where we live here in the Midwest.  When the AO is positive, this symbolizes low pressure in the Arctic.  This allows the mid-latitude jet stream to blow strongly and consistently from west to east in what is known as a zonal or near-zonal flow.  This keeps the cold Arctic air locked into the polar region.  However, when the AO becomes negative, this means that there is high pressure in the polar region, resulting in larger swings of the jet stream (much less zonal) and allows for greater movement of very cold polar air into the mid-latitudes.  These large swings in the jet stream also result in strong storms sliding along the stream which can produce several inches of snow at a time.

Well, right now, the AO is negative.... very negative.  As the chart below shows, the AO is currently at its lowest level of the winter with an index value near -5.5!  This means that the high pressure over the arctic is strong, and is sending plenty of cold air southward into the eastern half of the nation (as you are probably well aware of)!

The Arctic Oscillation Index during the 2012-2013 winter months. The black line is the observed number, while the red lines are the predicted values for the near future.

Luckily, the AO index is predicted to rise dramatically through April 1st, which, if correct, means that the cold air will be retreating back into the polar regions and more spring-like air will move into the region.  Rejoice spring lovers!  Granted, it does not mean a complete reversal of the pattern, but it will mean that the cold conditions we are seeing across the eastern half of the country will ease up.

The April temperature outlook (left) and precipitation outlook (right) released by the Climate Prediction Center on Thursday, March 21st, 2013

And the April outlook released today from the Climate Prediction Center tends to agree.  They are anticipating slightly above average temperatures for the eastern and southern portions of the United States during the month of April, with below normal temperatures along the west coast.  And if you are wondering about precipitation, it appears the Great Lakes is expected to see above normal amounts of precipitation while the coastal areas may see below normal precipitation.

So it appears that the month of April should be near normal temperatures but wet for the Midwest.  So, yes spring lovers.  Rejoice.  Spring is on the way!

 

Time for a bigger rain gauge!

Wait…. What?  We’re in the middle of a drought and you’re talking about getting a bigger rain gauge?  What’s going on here?


The map shows location of selected rain gauges,
with blue (red) triangles depicting sites with
significant increasing (decreasing) trends,
and white circles showing sites
 with little or no change.
(Credit: Adapted from Villarini et al. (2013))

Even with the current drought situation plaguing much of the Midwest, some of you may actually want to consider getting a bigger rain gauge if you enjoy weather-watching.  Why?  Well, heavy rain events are becoming more common!  A study by the University of Iowa recently published in the Journal of Climate suggests that heavy rains have become more frequent in the Upper Midwest over the past 60 years, holding true even with the current drought situation.  The study also found that temperatures over the center of the nation are rising too, which helps to explain the heavy rain issue.  Warmer air can hold more water vapor, and when more water is available in the atmosphere for precipitation, a greater chance for heavy rains exists.

Lead author Gabriele Villarini stated “We have found that there is a tendency toward increasing trends in heavy rainfall in the northern part of the study region, roughly the Upper Mississippi River basin.  We tried to explain these results in light of changes in temperatures.  We found that the northern part of the study region – including Minnesota, Wisconsin, Iowa, and Illinois – is also the area experiencing large increasing trends in temperature, resulting in an increase in atmospheric water vapor.”

However, the current drought that has affected the Midwest is not part of the study.  It has occurred too recently (over the past year or two) while the study focused on rainfall and temperature data from about 1950 to 2010.  Also, it should be noted that the study was not looking at average annual rainfall, but rather just heavy rainfall events.  So while the area may currently be experiencing a deficit in overall rainfall, the area may be in the normal range when it comes to the number of heavy rainfall events/days.

In addition to the rising temperatures adding to the water vapor in the atmosphere, other studies point to changes in irrigation over the Ogallala Aquifer (which runs from Nebraska to northern Texas) as a source of increased water vapor in the atmosphere.  This suggests that human changes in land use, land cover, and agricultural practice also contribute to the increased frequency of heavy rains.

So in our warming atmosphere, you can expect to see more heavy rainfall events across not only the Upper Midwest but possibly the globe as a whole.  This will also increase the frequency and risk of flash flooding in the area as heavy rains quickly flow into area streams, creeks, and rivers before the ground can absorb the moisture (for those of you in the Coulee Region, think back to August of 2007 when parts of the area received over a foot of rain in about 2 days).  Keep this in mind if you live in or plan to move to a flood prone area in the future so you are aware of the risk.  It also raises the importance of early warnings from the National Weather Service and local media.  I will be going over the meanings of the warnings, watches, advisories, etc. that the National Weather Service issues coming up in April during Wisconsin’s Severe Weather Awareness Week (April 15^th – 19^th), so watch for that.  It never gets old going over these important messages since they save lives!

Does Snow = Drought Relief?

The counties in red on this map all
declared drought disasters in 2012!

In the past several years, states in the southern portion of the Great Plains have suffered from extreme drought conditions.  Last year, these conditions spread to include a large portion of the nation and had a major impact on crops nationwide.  However, little gets said about the drought during the winter months.  Many people wonder why that is.  The answer can be summed up in one word: snow.

During the winter months, much of the nation sees precipitation fall in form of snow or other frozen precipitation.  This frozen precipitation falling on a cold ground does not get absorbed into the soil until it begins melting AND any frost has left the ground.  In other words, even though it is snowing like crazy outside, it will not improve the drought conditions until it melts.  And how quickly it melts is important as well.  If we get a fast melt of the snow pack, there will be plenty of runoff with the water flowing right into area river ways (even more so if the frost has not left the ground yet)!  Thus, not much of the water is absorbed by the drought stricken ground.

Another reason why snow has little impact on drought conditions is the fact that there is actually very little water in most snow.  Most snow that falls in the Upper Midwest has a snow to rain ratio of 12: 1, meaning that 12 inches of snow is equivalent to just one inch of rainfall!  And sometimes this ration can be as high as 30:1!!!  Talk about dry snow.  The truth is that there typically is just not enough water in snow to offset our drought conditions.

U.S. Seasonal Drought Outlook valid Feb. 21 - May 31, 2013.
Released Feb. 21, 2013.
Courtesy of the Climate Prediction Center.

So what does that mean for our drought conditions now?  Well, the drought has improved very little over the winter months.  And the drought is expected to persist for a good chunk of the nation (especially over the Great Plains) into the next several months.

On another note, some may be wondering what caused the drought in the first place.  Well, scientists of the Potsdam Institute for Climate Impact Research (PIK) have proposed that a common physical cause is to blame.  They suggest due to man-made climate change, the atmospheric flow pattern (known as the jet stream) is repeatedly disturbed through a subtle resonance mechanism.



Meridional windfield over four different timespans.
(Credit: Image courtesy of Potsdam
Institute for Climate Impact Research (PIK))

The jet stream, that fast current of air circulating the globe high up in the atmosphere, oscillates between the tropical and arctic regions of the globe, bringing cold, dry air from the arctic toward the tropics and warm, moist air in the other direction.  However, lead author Vladimir Petoukhov stated “What we found is that during several recent extreme weather events these planetary waves almost freeze in their tracks for weeks. So instead of bringing in cool air after having brought warm air in before, the heat just stays. In fact, we observe a strong amplification of the usually weak, slowly moving component of these waves.”  And when the heat stays, the heat keeps building and building which leads to a heat wave like what the United States and Russia have seen in the past few years.

Example of a heat dome on a weather map.

In other words, the droughts are caused by heat waves?  In a way.  Like stated above, the heat is brought in from the south repeated, and forms what is called a heat dome under a stationary ridge of high pressure.  As the heat keeps building, the ridge remains quite strong and prevents storm systems from passing through.  And if one or two do happen to slide through the ridge, they are so weak that they produce little in the way of precipitation!

Example of the jet streams moving air around the Earth.

But how does climate change cause the jet stream to freeze in its track?  Man-made climate change does not mean uniform global warming.  The relative increase in temperatures in the Arctic is higher than on average because it is amplified by the loss of snow and ice.  This in turn reduces the temperature difference between the Arctic and say Europe (Temperature difference is the main driver of air flow around the globe!).  In addition, continents generally warm and cool more easily and quickly than the oceans.  These two factors result in an unnatural pattern of the mid-latitude jet stream so that for extended periods slow synoptic waves (ridges and troughs, i.e. highs and lows on the weather map) get trapped in place.

So will we see this pattern over the United States again this year?  Only time will tell.  But for now, let’s hope, and pray that our drought stricken areas get much needed rainfall this spring to help relieve the drought!