Formation and characteristics of derechos

Verhaegen Yoni

Jun 22, 2019
Hi all,

Following the derecho event in the Midwest USA a few weeks ago, I would like to dig myself into the formation and characteristics of derechos. I have done some self-research on the internet, but some questions remain. I hope someone over here can explain the following things to me:

- Why are unidirectional shear environments favored for derechos? (Does unidirectional shear mean that all winds are orientated parallel to each other over the vertical depth of the atmosphere...? For example SW SW SW SW SW SW or NE NE SW SW SW SW?)
- Why are environments where the wind shear is compacted in the lower 3 km even more favored?
- Does a derecho need a linear forcing mechanism to develop or is it in that case called a squall line (what are the differences between both terms?)

Thanks in advance for your time and effort!
A derecho is a QLCS system (what is commonly called a squall line), that exceeds a certain wind threshold over a set period of time (I forget the requisite time period or exact wind threshold), with no more than I believe 2 hours between wind reports that exceed the threshold. So the same mechanisms are generally involved between a general QLCS and a derecho. As far as to the why on the other two questions, I will defer that to someone who knows more than I do on the mechanics of QLCS type systems.

Jeff House

Jun 1, 2008
Chattanooga, TN
Utilizing Established Techniques in Forecasting the Potential for Derecho Development Part 4 has the derecho checklist. Why is that so hard to find on Google now?

Part 2 is the definition of a derecho. Part 3 is more nowcasting. Part 4 checklist is handy for forecasting. Both 3-4 offer some insight.

Actually some turning of winds with height is helpful; however, it's subtle compared to tornadoes. WNW at 500 mb along a CAPE gradient, fed by SW at 700/850 mb will tear up some real estate.
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