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.