SPC MESOSCALE DISCUSSION 1839 FOR NRN IA...SERN MN...SWRN WI AND NWRN IL
CONCERNING...SEVERE THUNDERSTORM POTENTIAL... VALID 212258Z - 220200Z

STORMS ARE EXPECTED TO DEVELOP AND INTENSIFY WITHIN THE NEXT 2-3 HOURS FROM
NRN IA THROUGH SERN MN AND SWRN WI THEN SPREAD EWD INTO NWRN IL. CURRENT
THINKING IS THAT STORMS WILL DEVELOP FAR ENOUGH NORTH THAT THEY WILL REMAIN
SOMEWHAT ELEVATED WITH LARGE HAIL THE INITIAL PRIMARY THREAT. HOWEVER...THREAT
FOR DAMAGING WIND MAY INCREASE LATER THIS EVENING AS STORMS EVOLVE INTO AN MCS.
IF STORMS DEVELOP A LITTLE FARTHER S TOWARD CNTRL OR N CNTRL IA...THEY MAY BECOME
SURFACE BASED WHICH WOULD INCREASE THREAT FOR ISOLATED TORNADOES. AREA WILL
CONTINUE TO BE MONITORED FOR A POSSIBLE WW. WARM FRONT EXTENDS FROM NRN NEB
EWD THROUGH CNTRL IA INTO CNTRL IL. SURFACE HEATING...STEEP LAPSE RATES AND RICH
LOW LEVEL MOISTURE HAS CONTRIBUTED TO STRONG INSTABILITY ALONG AND S OF THE
BOUNDARY WITH MLCAPES FROM 3000 TO 3500 J/KG. HOWEVER...ATMOSPHERE REMAINS
CAPPED TO SURFACE BASED CONVECTION...SUGGESTING MOST LIKELY AREA OF STORM
INITIATION AND INTENSIFICATION WILL BE FARTHER N ACROSS NRN IA/SRN MN AND SWRN WI
WHERE WARM ADVECTION AND ISENTROPIC LIFT ASSOCIATED WITH INTENSIFYING SWLY LOW
LEVEL JET SHOULD BE SUFFICIENT TO INITIATE STORMS LATER THIS EVENING. VERY STRONG
VERTICAL SHEAR EXISTS ALONG AND JUST N OF BOUNDARY WITH LOW LEVEL HELICITY FROM
300 TO 400 M2/S2 AND DEEP LAYER SHEAR FROM 40 TO 50 KT. SUFFICIENT SHEAR WILL EXIST
THROUGH THE CLOUD LAYER TO SUPPORT STORM ORGANIZATION INCLUDING SUPERCELLS.
THERE IS ALSO CONCERN THAT IF STORMS CAN INITIATE CLOSER TO BOUNDARY...THREAT FOR
ISOLATED TORNADOES WILL INCREASE.

..DIAL.. 08/21/01