Wednesday, June 22, 2016

That Midwest Severe Weather Outbreak Today Looks Really Bad

Areas in red and orange are most likely to experiemce
tornadoes and damaging thunderstorm winds today.  
A broad area around the southern Great Lakes and Ohio Valley is bracing today for a very dangerous weather day.

An unusually strong weather  disturbance for this time of year is interacting with very hot, very humid air to create the risk for tornadoes, a few of which might be strong, very damaging winds, flash flooding and maybe even a derecho.

Derecho are relative rare, and are often more dangerous than tornadoes or individual severe thunderstorms because they cover such a wide area.

Most areas in the eastern two thirds of the United States experience a derecho once a year at most. Almost all areas get them less often than that, maybe one or two every four years.

According to NOAA's Storm Prediction Center, a derecho is defined as a swath of wind damage that extends for more than 250 miles, which includes quite a few locations within the derecho's path of gettings gusts of at least 58 mph,  and also several well-separated 75 mph or greater gusts.

Sometimes winds in a couple spots along a derecho's path can get ridiculous. One derecho in May, 1998 created a wind gust of 128 mph in southern Wisconsin.

Derechos move forward fast - often between 50 and 70 mph -  so they come on you suddenly. They're particularly dangerous in urban areas, since they toss down lots of trees and power lines that hit houses and cars and cause long-lasting power outages.

Whether or not a derecho gets going with this outbreak or not, things will get dicey in the southern Great Lakes and other parts of the Midwest.

They already are dicey, in fact. Heavy thunderstorms over eastern Iowa with this storm system early this morning have caused some flash floods.

But the real dangerous stuff starts this afternoon. Initially, some supercell thunderstorms, with the potential for a couple strong tornadoes, will get going in eastern Iowa or northwestern Illinois.

The storms will then evolve into a line of bad storms barreling east to southeast across Illinois, Indiana and Ohio. That might be the derecho we're talking about. Even it it ultimately does not meet the definition of a derecho, many communities along the path of the storms will get strong damaging winds, sometimes in excess of 70 mph.

There will be a tornado threat in this area as it comes through later today, too,  especially in eastern Iowa, parts of Illinois including in the southern half of the Chicago metro area, and maybe on toward South Bend and Indianapolis, Indiana and Columbus, Ohio, says the Storm Prediction Center

The complex of storms will plow through the central Appalachians tonight and possible into the mid-Atlantic states, where severe storms and a couple tornadoes caused damage yesterday.

Here's a video taken in 2011 that shows you don't need a tornado to make a storm like today's expected outbreak dangerous. These are straight line winds from a thunderstorm


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