Animated Gif of a thunderstormAugust 09, 2000 Appalachian-crossing DerechosAnimated Gif of a thunderstorm

Last updated 5/18/01 by Steve Keighton

 

Preprint article submitted to the Wx Analysis and Fcstg Conf 2001


On August 9, 2000, two separate large scale severe bow echoes moved across the RNK CWA from the northwest, and both made it through the mountains producing severe weather (mainly wind damage) in their wake. The first moved more from west to east and affected the northeast half of the CWA during the afternoon through early evening hours. The second, and even larger scale bow echo moved in from the north-northwest after dark and brought severe weather to practically all but the far southwest corner of the CWA (even though farther west in TN also experienced severe wx from part of this southward moving line). It exited the southeast portion of the CWA after 1 am. 

GOES IR Loop of event

Radar Composite Loop

 

The wind damage and severe wind reports (including a few tornadoes) from each of the MCS events easily fits the definition of a derecho, and was relatively rare in that both systems (but especially the second one) managed to cross the Appalachians with only slight weakening. 

Plot of counties with severe weather reports for derecho #1

Plot of counties with severe weather reports for derecho #2

Soundings showed the atmosphere was unusually unstable with dew points in the low to mid 70s across the mountians and CAPEs in the 2000-3000 J/kg range in the center of the Appalachians (RNK) during the afternoon and evening. The convective parcels in the soundings below were all lifted using the mean temp/dew pt in the lowest 150mb; if the surface parcel was lifted in the evening, CAPEs were well over 5000!  Also, dry air at mid level resulted in steep theta-e lapse rates and available downdraft energy to support bow echoes. Hodographs show generally NW flow, with moderate wind shear, but certainly not strong; again, favorable for bow echoes.

RNK 12Z Aug 9, 2000         Skew T             Hodo

RNK 18Z Aug 9, 2000        Skew T            Hodo

RNK 00Z Aug 10, 2000        Skew T             Hodo

GSO 00Z Aug 10, 2000        Skew T             Hodo

 

See the preprint article above for a 12Z surface frontal analysis and a 21Z surface theta-E analysis, showing the surface moisture ridge the convection formed in and then followed later in the afternoon. It appeared the first derecho-producing MCS left an outflow boundary and associated tight theta-E and instability gradient due to moisture pooling which the second one was able to survive along as it corssed the Appalachians after dark.

Finally, check out the BLKV GPS PW plot for the day. The PW peaked at the time the second MCS/bow echo passed through around 11pm (03 UTC). The first bow echo passed by just to the north of RNK about 21 UTC (the first little rise before a sharp, but short-lived dip).

BLKV GPS PW plot