First HIGH RISK of the Year Today

The Storm Prediction Center will upgrade an area of northern Illinois and northern Indiana to a High Risk of severe weather, citing 15% tornado probabilities and 60% straight line wind probabilities in the 1630z (12:30 PM EDT/11:30AM CDT) Day 1 convective outlook. Major severe weather, including some large, violent tornadoes, and later on the possibility of a major derecho. A derecho is when a line of storms causes significant wind damage over a large area of at least 240 miles. The most recent example of one was the event of June 29-30 of last year, where storms formed in a line west of Chicago, eventually converging into one large line, and causing major damage around the DC metro area, among others, during a massive heat wave. See below for the SPC discussion:

 MESOSCALE DISCUSSION 1040
   NWS STORM PREDICTION CENTER NORMAN OK
   1026 AM CDT WED JUN 12 2013

   AREAS AFFECTED...EXTREME ERN IA...NRN IL...NRN IND...NWRN OH

   CONCERNING...OUTLOOK UPGRADE 

   VALID 121526Z - 121630Z

   SUMMARY...THE 1630Z CONVECTIVE OUTLOOK WILL INCLUDE A CATEGORICAL
   UPGRADE TO HIGH RISK FOR DAMAGING WIND POTENTIAL FROM EXTREME
   EASTERN IOWA INTO NORTHERN PORTIONS OF ILLINOIS AND INDIANA...AND
   NORTHWEST PORTIONS OF OHIO. IN ADDITION...TORNADO PROBABILITIES WILL
   BE INCREASED TO 15 PERCENT OVER PORTIONS OF EXTREME EASTERN IOWA
   INTO NORTHWEST ILLINOIS.

   DISCUSSION...SVR TSTMS ARE EXPECTED TO DEVELOP THIS AFTERNOON IN THE
   VICINITY OF EASTERN IOWA/NORTHWEST ILLINOIS AND QUICKLY BECOME
   SEVERE SUPERCELLS WITH A RISK FOR TORNADOES...POSSIBLY
   SIGNIFICANT...WITH UPSCALE GROWTH INTO A FAST-MOVING AND
   FORWARD-PROPAGATING MESOSCALE CONVECTIVE SYSTEM. WIDESPREAD DAMAGING
   WINDS ARE LIKELY ACROSS THE HIGH RISK AREA...WITH SOME SIGNIFICANT
   SEVERE GUSTS POSSIBLE.

   DETAILS WILL BE FORTHCOMING IN THE 1630Z CONVECTIVE OUTLOOK.