MESOSCALE DISCUSSION 0074 NWS STORM PREDICTION CENTER NORMAN OK 0339 PM CST FRI JAN 20 2006

AREAS AFFECTED...NRN/ERN IA...SRN/ERN WI...NRN IL...FAR NWRN IND...SWRN/CENTRAL AND NRN LOWER MI

CONCERNING...PRECIPITATION TRENDS...WINTER PRECIPITATION

VALID 202139Z - 210345Z

HEAVY SNOW BANDS WITH HRLY RATES UP TO 1 INCH WILL CONTINUE TO DEVELOP ENEWD FROM NRN/CENTRAL IA INTO SRN WI AND EVENTUALLY WCENTRAL/NRN LOWER MI BY 03Z. SECONDARY AREA OF HEAVY SNOW WILL CONTINUE TO DEVELOP OVER NRN IL THROUGH 00Z...WHILE MOVING INTO SWRN AND CENTRAL LOWER MI FROM 01Z-03Z. HRLY SNOWFALL RATES AT TIMES WILL EXCEED 1 INCH WITH ISOLATED 2 INCH/HR RATES POSSIBLE OVER THE CHI METRO AREA NEWD INTO SWRN LOWER MI. A MIX OF SLEET WILL BE POSSIBLE AS WELL...MAINLY AT THE ONSET OF THE PRECIP AND OVER PORTIONS OF NRN IL/NWRN IND INTO SWRN LOWER MI.

IR IMAGERY SHOWS A WELL DEFINED MID LVL DRY SLOT THAT CONTINUES TO WORK SLOWLY ENEWD ACROSS NRN MO AND FAR SRN IA. NORTH OF THIS DRY SLOT...STRONG MID LEVEL FRONTOGENETIC FORCING IN A VERY FAVORABLE THERMODYNAMIC PROFILE WAS AIDING IN BANDS OF HEAVY SNOW FROM CENTRAL INTO NERN IA. THIS FORCING FOR HEAVY SNOW BANDS IS FORECAST TO CONTINUE LIFTING ENEWD WITH THE UPPER SYSTEM...AND MOVE INTO SRN/ERN WI THROUGH 00Z...AND EVENTUALLY INTO WCENTRAL AND PORTIONS OF NRN LOWER MI BETWEEN 00Z-03Z. SOUTH OF THIS ZONE...STRONG WAA EVIDENT BY REGIONAL VWP DATA...CONVECTIVE INSTABILITY EVIDENT BY LIGHTNING STRIKES/VISIBLE SAT IMAGERY AND SFC PRESSURE FALLS SUGGESTS THAT MODERATE-HVY SNOW WILL LIKELY DEVELOP....ONCE SUFFICIENT MID LEVEL SATURATION OCCURS IN THE NEXT 1-3 HOURS OVER SCENTRAL/SERN WI AND NRN IL. FORECAST SOUNDINGS OVER NRN IL SUGGEST SUFFICIENTLY DEEP ELEVATED WARM LAYER CENTERED AROUND 800 MB THAT PRECIP SHOULD START AS SLEET AND MAY AGAIN CHANGE OVER TO SLEET BRIEFLY DURING THE HEAVIEST PRECIP FROM 00-03Z. GIVEN THE DEGREE OF LOW-MID LEVEL FORCING AND CONVECTIVE INSTABILITY SNOWFALL RATES MAY REACH 1-2 INCHES/HR FOR BRIEF PERIODS ACROSS NRN IL/SERN WI FROM 23-02Z. STRONGEST VERTICAL MOTION AND CONSEQUENT HEAVY SNOW POTENTIAL WILL BEGIN TO MOVE INTO SWRN/CENTRAL MI FROM 01-03Z. IN ADDITION LOW LEVEL COLD ADVECTION SHOULD BEGIN TO INCREASE OVER NRN IL AND SWRN LOWER MI/NWRN IND AS THE SURFACE LOW PRESSURE CENTER APPROACHES SRN IL. THIS SHOULD AID IN A SWD MOVEMENT OF THE SURFACE FRONT AND SUB-FREEZING TEMPERATURES INTO THE EVENING HOURS. BEST ESTIMATE ON THREAT FOR WINTER PRECIP SHOULD BE ALONG AND NORTH OF I-80/I-90 IN NRN IL AND NORTH OF I-94 IN SWRN LOWER MI.

..CROSBIE.. 01/20/2006

ATTN...WFO...DTX...APX...IWX...GRR...GRB...LOT...ILX...MKX...DVN...ARX...DMX...

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