The official definition of a derecho from the AMS Gloassary:

derecho—A widespread convectively induced straight-line windstorm.

Specifically, the term is defined as any family of downburst clusters produced by an extratropical mesoscale convective system. Derechos may or may not be accompanied by tornadoes. Such events were first recognized in the Corn Belt region of the United States, but have since been observed in many other areas of the midlatitudes. 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:

Scott Bachmeier on the CIMMS Satellite blog (truly superb wx geekdom) has a fab loop of the mcc/derecho that crossed Chicago Monday.

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