High Risk Issued for November 17

The Storm Prediction Center has issued a High Risk outlook for today, as the threat for severe weather has been deemed to be very high.

The High Risk has been issued for eastern Illinois and much of Indiana, likely pointing to the Storm Prediction Center's concern over potentially tornadic cells that may develop as they coagulate with one another to form a powerful squall line. Categorical outlooks issued below tell the story; tornadoes and wind are the top two concerns for today.

Hail Outlook

Tornado Outlook

Damaging Wind Outlook

High resolution models have been continually projecting discrete cells in Illinois developing into an extreme squall line over Indiana and Ohio. The cells that develop in northern and central Illinois will have great potential to be tornadic, especially if they become supercells right off the bat. This environment is very dynamic with a 120kt+ jet stream and 60kt+ lower level winds combining with strong winds from the south to produce incredible shear and a very potent place for thunderstorms.

Initiation of these storms is expected around the noon hour Central Time in Illinois, and further intensification will occur explosively until the 3:00 PM hour, before coagulation occurs and the storms race eastward towards the Ohio Valley at over 50 knots, per recent sounding analysis.

This is a dangerous situation. If you are in the Moderate Risk (red) or High Risk (pink) areas on the top image, you are advised to prepare for potentially extreme weather, which may include tornadoes (some strong), potentially extreme damaging winds, and hail. The scope of this now-likely outbreak is displayed on excerpts of the Storm Prediction Center write-up below.

...TORNADO OUTBREAK WITH MULTIPLE SIGNIFICANT TORNADOES AND
   WIDESPREAD DAMAGING WINDS EXPECTED ACROSS THE OH VALLEY AND SRN
   GREAT LAKES REGION TODAY...
...THE DEVELOPMENT OF A SQUALL-LINE APPEARS LIKELY DURING
   THE LATE AFTERNOON ACROSS THE OH VALLEY WITH THE LINE DEVELOPING SWD
   INTO THE TN VALLEY DURING THE EARLY EVENING.
0-3 KM STORM RELATIVE HELICITIES OF 400 TO 450 M2/S2 WILL BE
   FAVORABLE FOR STRONG TORNADOES WITH SEVERAL LONG-TRACK DAMAGING
   TORNADOES EXPECTED TO OCCUR FROM ERN IL ENEWD ACROSS MUCH OF WRN AND
   CNTRL IND FROM LATE MORNING INTO EARLY AFTERNOON. AS CELL COVERAGE
   INCREASES AHEAD OF THE FAST MOVING FRONT...A SQUALL-LINE SHOULD
   ORGANIZE LATE THIS AFTERNOON. WIDESPREAD WIND DAMAGE WILL BE
   ASSOCIATED WITH THE LINE ALONG WITH TORNADOES WITH ROTATING CELLS
   EMBEDDED IN THE LINE. THE SEVERE THREAT SHOULD RAMP UP SOUTH OF THE
   OH RIVER DURING THE LATE AFTERNOON AS CELLS INITIATE SWD ALONG THE
   FRONT. THE POTENTIAL FOR WIND DAMAGE AND TORNADOES SHOULD DEVELOP AS
   A FAR SOUTH AS THE TN VALLEY DURING THE EVENING AS AN EXTENSIVE LINE
   OF SEVERE STORMS MOVES EWD INTO THE UPPER OH VALLEY AND CNTRL
   APPALACHIAN MTNS.
 SEVERAL FACTORS APPEAR TO BE COMING TOGETHER FOR A TORNADO OUTBREAK
   ACROSS THE OH VALLEY. THE FIRST FACTOR IS THAT THE SYSTEM IS
   NEGATIVELY-TITLED AND VERY WELL-ORGANIZED. THE SECOND IS THAT THE
   LOW AND MID-LEVEL JETS WILL BECOME FAVORABLY COUPLED IN THE OH
   VALLEY TODAY COINCIDING WITH AN OUTBREAK OF STORMS ACROSS THE WARM
   SECTOR. THE THIRD IS THAT SUPERCELLS ARE EXPECTED TO REMAIN DISCRETE
   INTO THE MID-AFTERNOON. THESE FACTORS COMBINED WITH EXTREMELY
   FAVORABLE LOW-LEVEL SHEAR PROFILES JUSTIFIES THE ISSUANCE OF A HIGH
   RISK ACROSS ERN IL...IND AND SW LOWER MI.

Andrew