THE DAY OF THE DERECHO...
What a day of weather Monday was for many of you back in the great Midwest. A vicious Derecho raked a large portion of the central Midwest (including much of my area) producing extensive damage with many reports of wind gusts 70 to 112 mph! Power in some areas will be out for 3-4 days and damage will be in the millions, if not more!
SO WHAT IS A DERECHO?
A derecho (pronounced similar to "deh-REY-cho" in English, is a widespread, long-lived wind storm. Derechos are associated with bands of rapidly moving showers or thunderstorms variously known as bow echoes, squall lines, or quasi-linear convective systems. Although a derecho can produce destruction similar to that of a tornado, the damage typically occurs in one direction along a relatively straight path. As a result, the term "straight-line wind damage" sometimes is used to describe derecho damage. By definition, if the swath of wind damage extends for more than 250 miles, includes wind gusts of at least 58 mph along most of its length, and also includes several, well-separated 75 mph or greater gusts, then the event may be classified as a derecho. Recently, an updated definition of "derecho" was proposed. The primary aim of this work is to make the definition more physically-based, i.e. focused on the meteorological processes believed responsible for the production of organized, damaging surface winds. The proposal requires the presence of certain radar-observed storm structural features such as bow echoes and rear-inflow jets.
EARLY SIGNS OF TROUBLE
It was apparent early Monday trouble was brewing as a line of storms entered NW Iowa. As they progressed east, They entered a very unstable atmosphere in central and eastern Iowa that included high CAPE and vertical shear known as BULK SHEAR.