12 July 2011
The official definition of a derecho from the AMS Gloassary:
derecho—A widespread convectively induced straight-line windstorm.
Johns, R. H., and W. O. Hirt, 1987: Derechos: Widespread convectively induced windstorms. Wea. Forecasting, 2, 32–49.
One of these beasties hit Chicago this morning and caused the most widespread power outage in history, along with winds gusting to over 70 mph. The winds were so strong it caused a seiche to develop on Lake Michigan!
These events are usually seen when there is extreme heat and humidity that causes very high instability. The wind shear in the atmosphere is usually significant but all from one direction so the tornado threat is very low. Derechoes can sometimes travel hundreds of miles in a few hours and produce a swatch of wind damage across several states.
This one made it to southern Ohio by 5 PM and it may leave a boundary that triggers more convection on Tuesday. This was a classic set up for a derecho to develop, and more may be on the way with the intense upper level high over the plains forecast to strengthen.
Update 13 July:
Storm reports from Chicago:
0705 AM TSTM WND GST ROCKFORD 42.27N 89.06W 07/11/2011 M50.00 MPH WINNEBAGO IL AMATEUR RADIO BYPASS 20 AND 11TH STREET. 0705 AM TSTM WND GST ROCKFORD 42.27N 89.06W 07/11/2011 M55.00 MPH WINNEBAGO IL AMATEUR RADIO MEASURED AT 6TH AND BROADWAY. 0709 AM TSTM WND GST FRANKLIN GROVE 41.84N 89.30W 07/11/2011 E70.00 MPH LEE IL TRAINED SPOTTER 12 INCH DIAMETER TREES DOWN. 0715 AM TSTM WND GST ROCKFORD 42.27N 89.06W 07/11/2011 E80.00 MPH WINNEBAGO IL TRAINED SPOTTER ESTIMATED 70 TO 80 MPH WINDS ON EAST SIDE OF ROCKFORD. 0715 AM TSTM WND DMG 2 SW COMPTON 41.67N 89.11W 07/11/2011 LEE IL EMERGENCY MNGR RT 251 BLOCKED BY DOWN POWER LINES NEAR SPURR RD. 0715 AM TSTM WND DMG 2 SW COMPTON 41.67N 89.11W 07/11/2011 LEE IL EMERGENCY MNGR ROUNT 251 BLOCKED BY DOWNED POWER LINES NEAR SPURR RD. 0717 AM TSTM WND DMG POPLAR GROVE 42.37N 88.82W 07/11/2011 BOONE IL BROADCAST MEDIA MULTIPLE REPORTS OF TREES DOWN IN CANDLEWICK LAKE. RELAYED BY WREX. 0717 AM TSTM WND DMG POPLAR GROVE 42.37N 88.82W 07/11/2011 BOONE IL BROADCAST MEDIA OVER 30 LARGE TREES DOWN. RELAYED BY WREX. 0720 AM TSTM WND DMG DE KALB 41.93N 88.75W 07/11/2011 DE KALB IL EMERGENCY MNGR TREES SNAPPED OFF AT THE BASE AT ANNIE GLIDDEN RD SOUTH OF RT 38. CORN ALSO FLATTENED NORTH AND SOUTH OF DE KALB. 0726 AM TSTM WND GST 10 W ROCKFORD 42.27N 89.26W 07/11/2011 E55.00 MPH WINNEBAGO IL TRAINED SPOTTER SUSTAINED WINDS AT 40 TO 45 MPH. FREQUENT CG LIGHTNING. DITCHES STARTING TO FILL WITH WATER. 0728 AM TSTM WND GST NW DE KALB 41.93N 88.75W 07/11/2011 E70.00 MPH DE KALB IL TRAINED SPOTTER TREES SNAPPING. 0728 AM TSTM WND GST 6 NNE ROCKFORD 42.35N 89.02W 07/11/2011 E60.00 MPH WINNEBAGO IL TRAINED SPOTTER SUSTAINED WIND AT 40 TO 45 MPH. POWER OUTAGES REPORTED. 0730 AM TSTM WND GST SYCAMORE 41.98N 88.70W 07/11/2011 E60.00 MPH DEKALB IL TRAINED SPOTTER