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!