From the NWS Quad Cities’ Area Forecast Discussion

…POTENTIALLY NEAR HISTORIC SNOW EVENT OVER SOUTHEAST 1/2 TO 1/3 OF THE FORECAST AREA ON TAP…

OVERVIEW…KEY FEATURES ARE SHOWING UP WHICH INCREASES CONFIDENCE ON EVOLUTION OF THIS WINTER STORM SYSTEM. SPECIFICALLY…ONSHORE DATA SUPPORTS NOWCAST TOOLS OF RAPID OCCLUSION TO OCCUR WITH PARENT SFC LOW TO SLIDE NE…THEN N…THEN BRIEFLY NW TO NEAR UIN ALLOWING FOR HEAVIER PRECIPITATION TO OCCUR. INITIALIZATION AND VERIFICATION SUPPORTS PRIMARY USE OF HI-RES ECMWF WITH FORCING INPUTS FROM GFS AND 06Z 80KM NAM-WRF INTO DAY 3. WENT WITH BLEND OF ECMWF AND GFS BEYOND DAY 4.

KEY COMMENT…THIS SYSTEM APPEARS WILL PHASE AND NEARLY PERFECTLY WITH CANADIAN ALBERTA UPPER LOW…ARCTIC AIR…GULF MOISTURE AND SHEAR PROFILES FOR MAXIMUM INTENSITY OF SNOW AND THUNDER-SNOW OVER AT LEAST SE 1/3 OF AREA. THIS IS ROUGHLY LIKE SEVERAL SMALL WAVES COMING TOGETHER TO FORM A BIG WAVE…ANOTHER DESCRIPTION IS CONSTRUCTIVE PHASING OR INTERFERENCE. IF SFC LOW OCCLUDES RAPIDLY ENOUGH…THIS WOULD SUPPORT NEED TO UPGRADE TO BLIZZARD WATCH DUE TO ISALLOBARIC PUSH TO HIGHER WIND VALUES OF 5+ MPH. OCCLUDING LOW WILL ALSO BE VITAL TO HOW MUCH THUNDER-SNOW WILL OCCUR IN FORECAST AREA.

TUESDAY AND WEDNESDAY…RAPIDLY DEEPENING AND OCCLUDING SFC LOW WILL SLIDE TO NEAR STL AT TRIPLE POINT WITH PARENT OCCLUDED LOW TO NEAR UIN. THIS WILL RESULT IN SNOW…HEAVY AT TIMES WITH ACCUMULATION RATES OF 1 TO 2 INCHES AN HOUR OVER SE 1/2 OF AREA. THUNDER-SNOW WILL BE POSSIBLE FAR SOUTH BY 21Z AND SLIDE TO AT LEAST HIGHWAY 34 CORRIDOR THROUGH 09Z. IT MAY APPROACH I-80 IN ILLINOIS. NW WINDS WILL INCREASE TO 25 TO 35 MPH WITH SOME GUSTS TO NEAR 45 MPH RESULTING IN NEARLY BLIZZARD CONDITIONS. WENT THROUGH SEVERAL LOCAL CALCULATIONS AND CAME UP JUST SHORT OF UPGRADING WATCH TO A BLIZZARD WATCH. PASS THIS CONCERN TO DAY SHIFT AS A PRIMARY ISSUE. AGAIN OCCLUDING SFC LOW WILL BE KEY.

SNOW AMOUNT GRADIENTS WILL BE SIGNIFICANT AND WENT WITH MIDDLE OF THE ROAD VALUES WITH MOST SOLUTIONS QPF VALUES SUGGESTING POOR ENTROPY OR ENERGY DISSIPATION ISSUES. HEAVIEST SNOWS SHOULD OCCUR +/- 30 MILES EITHER SIDE OF A QUINCY ILLINOIS PEORIA ILLINOIS TO NEAR KANKAKEE LINE. SNOW TOTALS TO RANGE FROM 15 TO 20 INCHES SOUTH OF A STERLING TO QUAD CITY TO FAIRFIELD LINE. NORTH OF THIS LINE TO A GALENA TO CEDAR RAPIDS TO MARENGO LINE 10 TO 15 INCHES ARE EXPECTED. NORTH OF THIS LINE SECOND LINE THE GRADIENT OF SNOW WILL RAPIDLY DECREASE TO 5 TO 10 INCHES WITH 5 TO 7 INCHES WITH THIS STORM SUGGESTED NORTH OF HIGHWAY 20 NEAR MANCHESTER AND INDEPENDENCE.
FINAL POINT…SNOW VALUES OF OVER 20 INCHES ARE POSSIBLE IF SFC OCCLUSION IS AGAIN SIGNIFICANT ENOUGH AND WILL PASS THIS TO DAY SHIFT ALONG FAVORED AFOREMENTIONED HEAVIEST SNOW TRACK.

WILL EMPHASIZE NEAR BLIZZARD CONDITIONS WITH POTENTIAL THUNDER-SNOW IN CHIWSWDVN…OR NEXT WINTER STORM WATCH PRODUCT.