000 FXUS63 KDVN 250901 AFDDVN AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE QUAD CITIES IA IL 300 AM CST WED NOV 25 2009 .SYNOPSIS... A TERRIFICALLY CHALLENGING DYNAMIC UPPER AIR SET UP OCCURRING OVER THE UPPER MIDWEST EARLY TODAY. WITH ONE STRONG DEPARTING UPPER LOW FOUND OVER PEORIA...AND ANOTHER STRONG UPPER LOW PHASING INTO THE TROF UPSTREAM. THIS LOW WAS STILL CREATING IMPRESSIVE HEIGHT FALLS AS IT DROPPED INTO THE TROF ON THE 00Z ANALYSIS. THE SHORT WAVE LENGTH BETWEEN THESE LOWS IS ALLOWING MOISTURE FROM THE FIRST UPPER LOW TO BE ENTRAINED INTO THE SECOND UPSTREAM. THERE IS PLENTY OF COLD AIR BEHIND THE UPSTREAM LOW...BUT THE ATMOSPHERE IS RATHER MILD AND MOIST OVER IOWA AND ADJACENT POINTS NORTH AND EAST. && .SHORT TERM...TODAY AND TONIGHT... THE MAIN CHALLENGES TODAY AND TONIGHT WILL BE PCPN TYPES AND ANY SNOW ACCUMULATIONS. ONE THING FOR SURE...WE GOING TO GET ANOTHER MODERATE PCPN EVENT LATER TODAY THROUGH TONIGHT...AS THE 2ND STRONG UPPER LOW SWINGS THOUGH...WITH THE AREA RIGHT IN THE AXIS OF STRONGEST LIFT. SEVERAL WAVES OF LIFT WILL OCCUR OVER THE CWA TODAY...FROM ISENTROPIC LIFT THIS MORNING...TO INTENSE PVA THIS AFTERNOON...FOLLOWED BY DEFORMATION ZONE PCPN LATE THIS AFTERNOON AND TONIGHT. SINCE MOISTURE IS IN PLACE ALREADY...MEASURABLE PCPN SHOULD BE EASILY ATTAINED AND AN UPDATE TO GO CATEGORICAL IS POSSIBLE TODAY...BUT WILL CARRY LIKELY POPS OUT THE DOOR INITIALLY AT ISSUANCE TIME. TONIGHT...WITH DEFORMATION ZONE PCPN WORKING THROUGH...ACTUALLY MAXIMIZING OVER THE AREA...CATEGORICAL POPS ARE WARRANTED. WHILE THE LIFT IS VERY INTENSE...AND COLD AIR IS CERTAINLY RUSHING IN ABOVE 900MB...WE ARE NOT FORECAST TO COOL LOW LEVELS DURING THE PROCESS. THIS IS TROUBLESOME. ALL MODEL DATA AVAILABLE KEEPS THE LAYER BELOW 900MB MOIST AND ABOVE FREEZING THROUGH 06Z...BEFORE SLOWLY TRANSITIONING TO ALL SUB FREEZING THROUGH 12Z. HEAR LIES THE UNCERTAINTY...TYPICALLY...STRONG DYNAMIC STORMS CAN COOL THE LAYER WHERE PRECIP IS GENERATED...AND THIS WILL CERTAINLY BE THE CASE...BUT AS TO WHAT DEGREES CAN THE MOIST ATMOSPHERE BE COOLED BENEATH IS UNCERTAIN...AS IT WILL NOT SEE ANY SIGNIFICANT EVAPORATIVE COOLING. HASHING OUT THE DETAILS COULD TAKE A BOOK AFD TONIGHT...BUT IN THE END...I SEE A PROLONGED PERIOD OF MODERATE RAIN...MIXED AT TIMES WITH SNOW AND SLEET...IN THE DEFORMATION ZONE THIS EVENING...WITH A TRANSITION TO WET SNOW WEST OF THE MISSISSIPPI AROUND 3 OR 4 AM. ALONG AND EAST OF THE MISSISSIPPI RIVER...IT LOOKS TO TRANSITION TO SNOW CLOSER TO 8 AM...OR TOWARD THE END OF THE EVENT. SNOW ACCUMULATION WOULD BE VERY LOW IN THIS FORECAST THOUGHT PROCESS...AS MOST THE QPF WOULD BE LIQUID OR LIQUEFIED ON ARRIVAL. HOWEVER...IF...WE COULD COOL THE LAYER BELOW 900MB THIS EVENING...AND MAINTAIN THAT FOR ANY TIME...SEVERAL INCHES OF WET SNOW WOULD BE LIKELY IN THE DEF ZONE. SO...I WILL ADD AROUND 1 INCH TO SNOW GRIDS TONIGHT...TRYING TO GET THE MOST LIKELY SCENARIO OUT THERE. IN THE END...IT IS THE STRONG SYSTEMS CIRCULATION PULLING IN MILD TEMPS VIA A TROWAL PROCESS THAT COULD PREVENT US FROM SEEING ANY ACCUMULATIONS. NORMALLY THIS WOULD BE A SIGNAL FOR HEAVY SNOW...BUT TONIGHT A TROUBLESOME INDICATION OF A PROLONGED MIXED OR RAIN EVENT. ..ERVIN.. .LONG TERM...THANKSGIVING DAY THROUGH NEXT TUESDAY... WILL WALK OUT WHATS LEFT OF THE SNOW THANKSGIVING MORNING AS THE DEFORMATION ZONE MOVES QUICKLY OFF TO THE EAST. NOT EXPECTING ANY ADDITIONAL ACCUMULATION AND WILL ONLY HAVE CHANCE POPS FOR EXTREME EASTERN IA AND OUR IL COUNTIES IN THE MORNING. HOWEVER MODELS HANG ON TO PLENTY OF LOW LEVEL MOISTURE FOR MUCH OF THE DAY SO A RATHER CLOUDY DAY IN STORE. LATER IN THE DAY THERE MAY BE SOME BREAKS IN THE CLOUDS IN OUR WESTERN CWA. SHOULD BE A CHILLY TURKEY DAY WITH NORTHWEST WINDS GUSTING TO 25 TO 30 MPH. BUT WINDS SHOULD DIMINISH LATER IN THE AFTERNOON AS STORM SYSTEM LIFTS INTO THE EASTERN GREAT LAKES AND OHIO VALLEY...FOLLOWED BY SURFACE RIDGE MOVING TO WESTERN IA. 850 MB TEMPS WILL RANGE FROM -4C IN NW IL TO -2C IN OUR WESTERN CWA. HAVE LOWERED MAX TEMPS INTO THE 30S MOST OF THE AREA WITH ABOUT 40 IN THE SOUTHWEST CWA. BELIEVE THAT ANY SNOW THAT FALLS FROM THIS SYSTEM WILL HAVE MELTED IN THE MORNING. MET/MAV/MEX GUIDANCE SEEMS A BIT WARM WITH THIS SCENARIO DESCRIBED. RAPID WARM-UP IN THE OFFING FRIDAY AND SATURDAY AS UPPER FLOW BECOMES ZONAL AND THEN SOUTHWEST...AS ANOTHER STRONG TROUGH SLIDES DOWN THE WEST COAST AND INTO THE SOUTHERN ROCKIES. DRY SOUTHWEST SURFACE WINDS WILL USHER WARMER TEMPS INTO THE REGION WITH ECMWF INDICATING 850 MB TEMPS OF +12C BY 12Z SATURDAY. HAVE RAISED TEMPS BOTH FRIDAY AND SATURDAY WITH READINGS PUSHING INTO THE 50S ACROSS THE ENTIRE CWA ON SATURDAY. BUFKIT RH SUGGESTS LITTLE IN THE WAY OF CLOUDS. PATTERN RECOGNITION SUGGESTS GUIDANCE TEMPS MAY BE A BIT TOO COOL. AS THE TROUGH THAT MOVES INTO THE SOUTHERN ROCKIES BY THE WEEKEND HEADS TO THE SOUTHERN PLAINS BY MONDAY...ANOTHER NORTHERN STREAM TROUGH DIVES INTO THE UPPER MIDWEST. COLDER AIR WILL BE ARRIVING DURING THE SUNDAY-MONDAY TIME FRAME. MODELS HAVING PHASING ISSUES AND NOW TAKE STORM SYSTEM FROM THE SOUTHERN ROCKIES TO THE OHIO VALLEY EARLY NEXT WEEK. THIS IS MUCH DIFFERENT THAN YESTERDAY/S RUN THAT DEPICTED A POWERFUL STORM MOVING NORTHWARD ACROSS THE MIDWEST. CURRENT ECMWF/GFS/CANADIAN GEM MODELS KEEP BULK OF PRECIPITATION SOUTH OF THE DVN CWA...JUST BARELY GRAZING OUR FAR SOUTHERN CWA. IF THIS FARTHER SOUTH STORM TRACK HOLDS THEN LATER SHIFTS MAY NEED TO REMOVE POPS THAT ARE CURRENTLY IN THE GRIDS FOR SUNDAY THROUGH TUESDAY. ..HAASE.. && .AVIATION... LOW PRESSURE ALOFT WILL SWEEP THROUGH THE AREA AGAIN TODAY AND TONIGHT...BRINGING A CONTINUATION OF THE VARIABLE MVFR TO LIFT CIGS AND MVFR TO IFR VISIBILITIES. HIGHLY CHAOTIC CIGS MAY VARY FROM 300 FT AGL...TO 2000 FT AGL WITHIN THE SAME HOUR THROUGH TODAY. RAIN SHOULD SPREAD OVER ALL SITES DURING THE AFTERNOON ONCE AGAIN...BUT A BRIEF SHOWER OR DRIZZLE IS ALSO POSSIBLE THIS MORNING. BY THIS EVENING...STEADY RAIN MIXING WITH SNOW AND SLEET TOWARDS 06Z AT CID...AND AROUND 09Z AT MLI...BRL...DBQ WILL BE FOUND. A TRANSITION TO A BRIEF PERIOD OF MODERATE SNOW IS POSSIBLE AT ALL SITES AFTER 09Z. WINDS WILL BE FROM THE WEST TO NORTHWEST AROUND 12 KTS MUCH OF THE PERIOD. && .DVN WATCHES/WARNINGS/ADVISORIES... IA...NONE. IL...NONE. MO...NONE. && $$ HAASE/ERVIN