382
FXUS63 KDVN 252110
AFDDVN
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE QUAD CITIES IA IL
310 PM CST WED NOV 25 2009
SYNOPSIS
ANOTHER STRONG UPPER LOW WAS BEARING DOWN ON THE REGION THIS
AFTERNOON...ON THE HEALS OF YESTERDAY/S RAINY SYSTEM EXITING OVER
THE GREAT LAKES. OVER 140 METER HT FALLS WERE EVIDENT ON 12Z
ANALYSIS AT H5 OVER EASTERN SD WITH THE STRENGTHENING LOW CENTERED
OVER SW MN EARLY THIS AFTERNOON. A STRONG VORT WAS ROTATING AROUND
THE BOTTOM OF THIS LOW ACROSS NRN MO WITH RESULTING FORCING AND
UPPER DIFLUENCE PRODUCING A BROAD BAND OF LIGHT TO MODERATE RAIN
ACROSS FAR EASTERN IA INTO NW MO EARLY THIS AFTERNOON. A DRY SLOT
WAS EVIDENT JUST BEHIND THIS BAND... WITH ANOTHER AREA OF DEVELOPING
PRECIPITATION ON DMX 88D IN THE FORM OF RAIN AND SNOW IN CENTRAL IA
IN THE AREA OF PRIMARY AREA OF FORCING AHEAD OF MAIN SE MOVING UPPER
VORT. AT THE SURFACE...THE PRIMARY LOW HAD SLOWED TO A NEAR
STANDSTILL OVER NRN LAKE MI AT 18Z WITH A DEVELOPING TROUGH
EXTENDING WEST ACROSS WI. AS THE APPROACHING FORCING TAPS INTO THE
HIGHER MOISTURE WRAPPING AROUND THE H8 LOW OVER THE GREAT
LAKES...PRECIPITATION IS EXPECTED TO INTENSIFY AND OVERSPREAD MOST
OF THE FORECAST AREA TONIGHT.
SHORT TERM...TONIGHT AND THURSDAY
PRIMARY CHALLENGE IS PINNING DOWN THE TIMING OF THE TRANSITION FROM
RAIN TO SNOW AND DETERMINING SNOW ACCUMULATIONS TONIGHT. WHILE UPPER
LEVEL FORCING AND MID LEVEL INSTABILITY ARE IMPRESSIVE WITH THIS
SYSTEM...THE LACK OF A STRONG PUSH OF ARCTIC AIR AND COMPLEX THERMAL
AND MOISTURE FIELDS DUE TO THE INTERACTION WITH THE LOW OVER THE
GREAT LAKES CONTINUE TO CREATE A CHALLENGING FORECAST.
OF THE 12Z MODELS...GFS AND ECMWF HAD A BETTER INITIALIZATION OF H8
TEMPERATURES OVER THE NORTHERN PLAINS...WITH WRF/NAM LAGGING A BIT
WITH THE -6 C CONTOUR IN THE COLD ADVECTION ACROSS THE DAKOTAS. THE
HOWEVER...BY 18Z...THEY ALL APPEARED TO BE TOO FAST IN DROPPING
THE UPPER LOW SOUTHWARD INTO NRN IA...WHICH APPEARED TO BE STILL
CENTERED OVER FAR SW MN AT 20Z. THIS MAY DELAY THE ONSET OF THE
PRECIPITATION THIS EVENING AND SLOW ITS EXIT ON THU...OTHERWISE...
ONLY MINOR DIFFERENCES AMONG THE MODELS AND UTILIZED A BLEND.
ISENTROPIC LIFT TIED CLOSELY TO THE CENTRAL IA PRECIP IS SHOWN
RAPIDLY OVERSPREADING THE NW 2/3RDS OF THE CWA THIS EVENING. UPPER
LEVEL FORCING REMAINS STRONGEST ALONG AND NORTH OF THE H5 LOW ACROSS
EAST CENTRAL IA AND NW IL AROUND 06Z. THIS IS SHOWN ACCOMPANIED BY
STEEP H5 TO H7 LAPSE RATES. SHOULD THUS SEE WIDESPREAD RAIN
DEVELOPING THIS EVENING. COMPLICATING THE PHASE TRANSITION...
A WARM TROUGH FROM THE SURFACE TO H8 ROTATING AROUND THE H8 GREAT
LAKES LOW PUSHES SOUTH ACROSS WI INTO NW IL LATE TONIGHT. RESULTING
LOW LEVEL...NEARLY SATURATED...ABOVE FREEZING AIRMASS WITH THIS
FEATURE WILL LIKELY DELAY A COMPLETE CHANGEOVER AND KEEP A MIX GOING
IN THE NORTH THROUGH MUCH OF THE NIGHT. THE INITIAL THRUST OF COLD
AIR AT 925 AND 850 MB WILL BE STRONGEST ACROSS THE SOUTHERN CWA THIS
EVENING...BUT FORCING THERE WILL BE MORE LIMITED.
CONSIDERING THIS...AND INITIAL PAVEMENT/GROUND TEMPERATURES WELL
INTO THE 40S...WILL COUNT ON AN EXTENDED PERIOD OF RAIN/SNOW MIX AND
EVENTUAL CHANGE TO ALL SNOW OVERNIGHT. THIS SHOULD AMOUNT TO LITTLE
MORE THAN A LIGHT ACCUMULATION ON GRASSY SURFACES MAINLY NORTH OF
I-80 WHERE UPPER FORCING WILL BE STRONGEST. HAVE KEPT MENTION OF
ACCUMULATIONS OF NEAR AN INCH ACROSS THE NW...MAINLY NW OF A LINE
FROM WILLIAMSBURG TO DUBUQUE...WHERE CHANGE TO ALL SNOW WILL OCCUR
EARLIEST AND LOW LEVEL THERMAL FIELDS ARE MORE SUPPORTIVE.
THANKSGIVING WILL HAVE LIGHT SNOW IN THE EAST EXITING EARLY. COULD
SEE SOME TRACE TO DUSTING AMOUNTS AFTER SUNRISE EAST OF THE MS
RIVER. OTHERWISE...CYCLONIC FLOW GIVES WAY TO RIDGING BY EVENING
WITH LINGERING CLOUDS AND WET GROUND HOLDING TEMPERATURES IN THE
UPPER 30S TO LOWER 40S.
..SHEETS..
LONG TERM...THURSDAY NIGHT THROUGH NEXT WEDNESDAY
THURSDAY NIGHT
THROUGH FRIDAY NIGHT WILL BE QUIET WITH THE LARGE UPPER TROF WELL
EAST OF THE AREA AND UPPER RIDGE BUILDING OVER THE PLAINS. A NARROW
SURFACE RIDGE WILL MOVE THROUGH THE MID MS VALLEY THURSDAY NIGHT
WHICH WILL AIDE RADIATIONAL COOLING. BUT BOTH THE NAM AND GFS
DEVELOP MID LEVEL WAA RH/CLOUDS OVER THE AREA LATE THURSDAY NIGHT
WHICH SHOULD STOP OR AT LEAST SLOW THE DROP IN TEMPERATURES. SO THE
WARMER MAV GUIDANCE WAS FAVORED FOR MIN TEMPS THURSDAY NIGHT. USED
THE EXPECTED MIXING DEPTH AROUND H925 AS A BASIS FOR MAX TEMPS
MAINLY IN THE 40S FOR FRIDAY. THE SHALLOW MIXING DEPTH LOOKED
REASONABLE WITH WEAK SLY WINDS DEVELOPING BEHIND THE DEPARTING
RIDGE. WINDS WILL STILL BE LIGHT THROUGH FRIDAY EVENING OVER THE
EASTERN CWA TO AIDE RADIATIONAL COOLING...SO FAVORED THE COLDER
GUIDANCE THERE AND WENT CLOSE TO OR JUST ABOVE THE WARMER MAV
NUMBERS IN THE SW WITH STRONGER SLY WINDS LIMITING THE DROP IN TEMPS.
FOR SATURDAY INTO MONDAY THE MODELS ARE IN GOOD AGREEMENT WITH A NRN
STREAM TROF MOVING INTO THE PACIFIC NW AND DEEPENING AS IT CROSSES
THE ROCKIES WHILE ENERGY IN THE SRN STREAM FORMS A CLOSED LOW OVER
SRN CA/NRN MEX BY SUNDAY. THE NRN STREAM TROF WILL DRAG COLD FRONT
THROUGH THE REGION SATURDAY NIGHT WITH MODELS SHOWING SOME LIGHT
POST FRONTAL PCPN SUNDAY INTO MONDAY. TEMPS LOOK COLD ENOUGH TO
SUPPORT RN OR RN/SN MIX CHANGING TO ALL -SN BY SUNDAY NIGHT.
THERE IS STILL A LOT OF UNCERTAINTY FOR TUESDAY AND WEDNESDAY AS THE
SWRN U.S. UPPER LOW LIFTS NE AHEAD OF ANOTHER S/W DIGGING OVER THE
CENTRAL AND NRN ROCKIES. THE GFS SHOWS THE SWRN U.S. ENERGY LIFTING
ALMOST DUE N THROUGH MS VALLEY WEDNESDAY INTO THURSDAY GIVING US
WIDESPREAD PCPN. SINCE THE REGION WOULD BE ON THE COLD SIDE OF THE
SYSTEM THIS WOULD LIKE BE MAINLY SNOW. HOWEVER THE ECMWF CONTINUES
TO TAKE A MORE NELY TRACK WITH THIS ENERGY LEAVING THE MID/UPPER MS
VALLEY DRY THROUGH MID WEEK. HAVE LEANED TOWARD THE ECMWF GUIDANCE
WHICH HAS BEEN CONSISTENT THE LAST COUPLE OF RUNS AND IS IN LINE
WITH HPC GUIDANCE...KEEPING THE REGION DRY TUESDAY THROUGH
THURSDAY. TEMPS THROUGH THE PERIOD LOOK NEAR NORMAL FOR EARLY
DECEMBER. ...DLF...
AVIATION
POTENT UPPER LOW MOVING SE THROUGH ERN IA AND CENTRAL IL WILL SPREAD
A MIX OF PRECIPITATION OVER AREA THE REST OF THIS AFTERNOON AND
TONIGHT. WIDESPREAD MVFR/LOWER VFR CONDITIONS THIS AFTERNOON WILL
DETERIORATE TO MAINLY IFR CIGS/VSBYS THIS EVENING AND OVERNIGHT IN A
MIX OF RN/SN BECOMING ALL -SN. THE TAFS INDICATE A FASTER CHANGE
FORM RN TO SN THAN THE 12Z NAM/GFS SUGGESTS. THIS QUICKER CHANGE
WAS SUPPORTED BY THE NAM/GFS NOT CAPTURING THE SRN EXTENT OF THE
COLDEST AIR OVER THE NRN PLAINS THIS MORNING. ALSO THE GFS/MAV
GUIDANCE WAS SEVERAL HOURS BEHIND WITH THE CHANGE FROM RN TO SN
OVER NW IA LATE THIS MORNING. THERE IS POTENTIAL THAT KCID/KDBQ
COULD SEE A PERIOD OF TWO OF LIFR VSBYS/CIGS IN MOD SN BETWEEN 03Z
AND 09Z BUT CONFIDENCE IS NOT GREAT ENUF TO INCLUDE IN THE TAF AT
THIS TIME. BY THURSDAY MORNING CONDITIONS WILL IMPROVE TO MAINLY
MVFR AS -SN ENDS FROM W TO E. EXPECT NW WINDS TO INCREASE THIS
EVENING WITH GUSTS BTWN 20 AND 25KTS AT ALL TAFS SITES EXPECTED
OVERNIGHT.
..DLF
DVN WATCHES/WARNINGS/ADVISORIES
IA...NONE.
IL...NONE.
MO...NONE.
SHEETS/DLF
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