STEVE KAHN

Strong to severe thunderstorms are spreading northeast from west central Illinpis. Severe thunderstorm warnings valid until 8:15 pm have just been issued for portions of central Illinois including the Peoria area. Wind gusts of 60 mph or higher and hail as large as ping-pong balls can be expected. These storms are on a northeast track and headed towards the Chicago Metro area later this evening.

The latest timescale discussion #736 from the Storm Prediction Center indicates a tornado or severe thunderstorm watch might be issued later this evening.

SPC Mesoscale discussion #736

MESOSCALE DISCUSSION 0736
   NWS STORM PREDICTION CENTER NORMAN OK
   0717 PM CDT MON MAY 20 2013

   AREAS AFFECTED...CNTRL INTO NRN IL

   CONCERNING...SEVERE POTENTIAL...WATCH POSSIBLE 

   VALID 210017Z - 210115Z

   PROBABILITY OF WATCH ISSUANCE...60 PERCENT

   SUMMARY...THE THREAT FOR SEVERE STORMS CAPABLE OF DAMAGING
   WINDS...LARGE HAIL...AND PERHAPS A TORNADO IS EXPECTED TO DEVELOP
   NEWD ACROSS THE AREA THIS EVENING INTO TONIGHT.  CONVECTIVE TRENDS
   ARE BEING MONITORED FOR A POSSIBLE WW.

   DISCUSSION...RECENT TRENDS IN REGIONAL RADAR DATA INDICATE TSTMS
   DEVELOPING E OF THE MS RIVER /NW OF SPI/ IN ADVANCE OF AN EVOLVING
   QLCS OVER MO.  VISIBLE SATELLITE IMAGERY AND SURFACE OBSERVATIONS
   SUGGEST THAT THE ERN-MOST STORMS ARE FORMING NEAR AN OUTFLOW
   BOUNDARY LIFTING NWD THROUGH CNTRL IL.  THE 00Z ILX APPEARS TO BE A
   REASONABLE PROXY FOR THE ENVIRONMENT S OF THE OUTFLOW BOUNDARY WHERE
   STEEP LAPSE RATES AND A MOIST PBL /LOWEST 100-MB MEAN-MIXING RATIOS
   OF AROUND 13 G PER KG/ WERE YIELDING A MODERATELY UNSTABLE
   ENVIRONMENT.  WHEN COUPLED WITH A VERTICALLY VEERING WIND PROFILE
   WITH 40-50 KT SWLY MIDLEVEL WINDS...THE ENVIRONMENT APPEARS
   SUPPORTIVE OF ORGANIZED STORM MODES...INCLUDING SUPERCELLS AND
   BOWING STRUCTURES WITH A RISK FOR DAMAGING WINDS...LARGE HAIL...AND
   PERHAPS A TORNADO.

Strong to severe thunderstorms continue to build northeast out of Missouri into west-central Illinois. The weather remains quiet across the Chicago Metro area, but our latest in-house "RPM" model continues to bring a round of potent thunderstorms into the area by midnight.

Here is the very-latest "RPM" forecast.

Latest in-house"RPM" forecast valid 11PM CDT this Monday evening.

Downstate areas into Chicago south and Indiana suburbs being monitored for possible severe weather watch

MESOSCALE DISCUSSION 0729
   NWS STORM PREDICTION CENTER NORMAN OK
   0327 PM CDT MON MAY 20 2013

   AREAS AFFECTED...NERN MO...CNTRL IL...W-CNTRL IND

   CONCERNING...SEVERE POTENTIAL...WATCH POSSIBLE 

   VALID 202027Z - 202130Z

   PROBABILITY OF WATCH ISSUANCE...60 PERCENT

   SUMMARY...PROSPECTS FOR TSTM DEVELOPMENT ARE SOMEWHAT UNCERTAIN
   ACROSS W-CNTRL IND WWD INTO NERN MO...BUT THE ENVIRONMENT IS
   FAVORABLE FOR DMGG WINDS AND HAIL GIVEN STRONG
   HEATING/DESTABILIZATION.  CONVECTIVE TRENDS ARE BEING MONITORED FOR
   A POSSIBLE WW...PARTICULARLY ACTIVITY DEVELOPING NEWD FROM SWRN MO
   TOWARDS THE MID-MS VALLEY.

   DISCUSSION...RADAR/SATELLITE TRENDS SHOW A DECAYING TSTM CLUSTER/MCV
   MOVING ACROSS N-CNTRL IL...WITH AN INCREASE IN SHALLOW BOUNDARY
   LAYER CU NOTED ACROSS E-CNTRL IL. THIS APPEARS TO BE DEVELOPING
   WITHIN A WEAK CONFLUENCE AXIS WHICH EXTENDS FROM 30 SE MMO TO 20 SSW
   SPI. BOTH MODIFIED 12Z RAOBS AND 12Z NAM FORECAST SOUNDINGS SUGGEST
   MLCAPE VALUES HAVE INCREASED TO 1000-1500 J/KG. WHILE THERE IS STILL
   SOME UNCERTAINTY TO WHAT DEGREE TSTMS WILL INTENSIFY/ORGANIZE OVER
   IL...A THREAT FOR DMGG WINDS AND LARGE HAIL COULD DEVELOP GIVEN
   STEEP LOW-LEVEL LAPSE RATES AND MODERATELY STRONG DEEP-LAYER SHEAR.
   ADDITIONALLY...TSTMS CURRENTLY DEVELOPING OVER WRN MO SHOULD
   EXPAND/DEVELOP NEWD TOWARDS E-CNTRL MO THROUGH LATE AFTERNOON. IT IS
   UNCERTAIN WHETHER A WW WILL BE REQUIRED WITHIN THE NEXT 1-2
   HRS...BUT CONVECTIVE TRENDS CONTINUE TO BE MONITORED.

   ..ROGERS/KERR.. 05/20/2013