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
