Formation and characteristics of derechos

[jones123]

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!

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[Uncle Bobby]

If you had just posted this I wouldn't answer because I'm no expert, but since nobody else has answered in 2½ days I'll offer you what I have read.

Unidirectional shear, also often referred to as a straight-line hodograph, means parallel orientation with height as you describe.  I suppose you don't get such violent straight-line surface winds if there is directional shear (veering or backing with height) because then you get rotation in the convection, if only a little.

I don't know what wind shear compaction means.

I have read that a strong jet at ~500mb is required for a derecho to occur, so I believe that would be your linear forcing mechanism.  A squall line develops when you have unidirectional winds parallel to the squall line, e.g. winds out of the SW at (pretty much) all heights along a cold front and the squall develops from, say, Wichita to KC, or Louisville to Columbus OH.  A derecho will have a strong 500mb jet perpendicular to the derecho's orientation.  I have no idea, I never looked at any of the data, but I therefore assume that there was a 500mb jet coming out of the west in Iowa, and that the derecho convection was oriented in a N-S line, or bow, and moved rapidly eastward.

I was actually myself caught up in two of the most famous derechos in history; July 4, 1970 in Cleveland OH and May 27, 2001 in SW KS.