FXUS63 KDVN 212200
AFDDVN

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE QUAD CITIES IA IL
355 PM CST MON DEC 21 2009

.SYNOPSIS...
12Z UA ANALYSIS HAS H85 LOWS ASSOCIATED WITH A CLIPPER SYSTEM IN
NORTHERN MONTANA AND THE WYOMING/SOUTH DAKOTA BORDER. A MODERATELY
STRONG BAROCLINIC ZONE RAN SOUTHEAST FROM THE NORTHERN PLAINS INTO
THE OHIO AND TENNESSEE VALLEYS. 18Z SFC ANALYSIS HAD A LOW IN THE
NEBRASKA PANHANDLE WITH A WARM FRONT RUNNING EAST TO FAR WESTERN
IOWA. SATELLITE TRENDS HAD PLENTY OF LOW LEVEL STRATUS WITH WAA MID
LEVEL CLOUDS STREAMING IN FROM THE PLAINS. AREA RADARS WERE PICKING
UP MAINLY CLOUDS BUT SOME SPORADIC -SN WAS BEING REPORTED ACROSS
NORTHERN IOWA. DEW POINTS WERE IN THE 20S WITH A TONGUE OF 30 DEW
POINTS MOVING NORTH THROUGH KANSAS.

&&

.SHORT TERM...TONIGHT AND TUESDAY...
THE OVERALL QUESTION IS WHEN WILL SATURATION ALOFT OCCUR AS LIFT
INCREASES ACROSS THE AREA TONIGHT. NO MODEL IS HANDLING THE CURRENT
SITUATION WELL BUT THE 285-295K THETA CONDENSATION PRESSURE DEFICITS
FROM THE GEM ARE NOT OVERLY BAD. FORCING INCREASES THIS EVENING AS
CONDENSATION PRESSURE DEFICITS LOWER. SOME -SN/FLURRIES SHOULD
DEVELOP THIS EVENING NORTH OF HWY 30 WITH FLURRIES POSSIBLE DOWN TO
I-80. SNOW WILL QUICKLY BREAK OUT NORTH OF I-80 LATE THIS EVENING
AND CONTINUE OVERNIGHT. AS SATURATION INCREASES -SN WILL BE POSSIBLE
DOWN TO ABOUT HWY 34. DRYING OUT ALOFT IN THE FCST SOUNDINGS LATE
TONIGHT BRING THE PROSPECT OF FREEZING DRIZZLE FOR AREAS ALONG/SOUTH
OF HWY 34. THE BULK OF THE SNOW WILL BE PRIOR TO SUNRISE WITH 1-3
INCHES NORTH OF I-80. THE HIGHEST AMOUNTS WILL BE ALONG THE HWY 20
CORRIDOR.

WAA ON TUESDAY WILL ALLOW ANY FREEZING PRECIP TO CHANGE OVER TO JUST
DRIZZLE OR -RA SOUTH OF HWY 34 DURING THE MORNING WHILE SNOW
CONTINUES OVER ABOUT THE NORTHEAST HALF OF THE CWFA. FREEZING
DRIZZLE WITH SOME -SN WILL BE SEEN IN BETWEEN THESE TWO AREAS BY
LATE MORNING. WAA CONTINUES INTO TUESDAY AFTERNOON WITH
SPRINKLES/-RA EXPECTED SOUTH OF HWY 34 WITH A RA/SN MIX POSSIBLE
SOUTH OF I-80 AND JUST NORTH OF HWY 34. FCST SOUNDINGS STILL SUGGEST
THE POTENTIAL FOR FREEZING DRIZZLE ALONG/NORTH OF I-80 THROUGH THE
AFTERNOON WITH FCST SOUNDINGS DRYING OUT ALOFT.    ..08..

.LONG TERM...TUESDAY NIGHT THROUGH NEXT MONDAY...
MOST OF FOCUS THIS PERIOD ON POTENTIAL HIGH IMPACT STORM DURING THE
X-MAS HOLIDAY WINDOW. ENSEMBLES OF FAVORED SHORT AND MEDIUM RANGE
MODELS HAVE GENERALLY TRENDED TO THE WESTERN TRACK OF THE EARLIER
ECMWF RUNS AND WILL HAMMER OUT A PRELIMINARY FORECAST USING A BLEND
OF THE 12Z RUN GFS AND ECMWF BUT MORE EMPHASIS ON THE GFS. STILL
MUCH UNCERTAINTY AND TIME TO CHANGE IN UP COMING MODEL RUNS WITH
THE SYSTEM JUST NOW GETTING ON COAST OFF OF THE PACIFIC NW AND B.C...
WITH TONIGHT/S 00Z RUNS AND ESPECIALLY THE 12Z TUE RUNS HOPEFULLY
COMING TOGETHER FOR HIGHER CONFIDENCE IN DETAILS OF THE EVENT.

TUE NIGHT...LIGHT PRECIP SPREADING UP WITH THERMAL PROFILES
SUGGESTING MAINLY FREEZING DRIZZLE OR MAYBE FLURRIES ALONG AND
NORTH OF I80 WITH LACK OF MOISTURE IN THE -10 TO -20C LAYER.
POSSIBLY JUST LIGHT RAIN AFTER AN INITIAL ZR MIX TUE EVENING ALONG
AND SOUTH OF HWY 34 IN NE MO...SE IA AND WEST CENTRAL IL.

WED...AS INVERTED TROF COMPLEX OFF DEEP SOUTHERN PLAINS CYCLOGENESIS
TAKES SHAPE...FIRST ROUND OF LIFT AND ELEVATED WARM MOIST CONVEYOR
TO FUEL A POTENTIALLY SIGNIFICANT ICE STORM ALONG AND 25-40 MILES
EITHER SIDE OF THE I80 CORRIDOR...POSSIBLE 1/4 TO 1/23 INCH ICE
ACCUMULATIONS. FROM CID TO MAQUOKETA AND TO FREEPORT IL ON
NORTHWARD THEY MAY GET 1-3 OR 2-4 INCHES OF SNOW DEPENDING ON HOW
MUCH GOES INTO SLEET/IP PRODUCTION. SOUTH OF A FFL IA TO MONMOUTH IL
LINE MAINLY RAIN AND SOME SLEET FOR WED. WED NIGHT...AFTER
LINGERING LIGHT FREEZING RAIN AND SLEET PERSISTS UNTIL AROUND 02Z...
EXPECT REST OF NIGHT TO BE KIND OF A LULL PERIOD WITH OCCASIONAL
LIGHT RAIN AND MAINLY DRIZZLE...POSSIBLE FREEZING DRIZZLE NORTH OF
I80 WHERE SFC TEMPS CAN DIP BELOW FREEZING. OTHERWISE SFC TEMPS TO
HOLD STEADY OVERNIGHT FROM EVENING VALUES WITH THE BULK OF THE CWA AT
32 DEGREES OR MILDER. PRE-STORM BRISK EAST WINDS OF 15 TO 25 MPH MAY
BE MORE OF AN IMPACT THEN ON THE WEST SIDE OF THE SYSTEM FRI
AFTERNOON AND EVENING...ESPECIALLY ON ANY ICE COATED TREE LIMBS AND
POWER LINES WED AND WED EVENING.

THU...THE MAIN EVENT TAKING INTO ACCOUNT THE GFS SFC LOW PROGRESSION
FROM FAR SOUTHWEST MO AT 12Z THU...TO NEAR THE NORTH CENTRAL
MO/IA BORDER BU 00Z FRI AT SUB 990 MB PRESSURE VALUES/988 MB/. THIS
FEATURE WILL THEN LOOK TO SLOW-ROLL NORTHWARD AND FILL SOME TO SOUTH
OF DSM BY 12Z X-MAS MORNING. THIS POSITIONING WOULD PLACE THE BULK
OF THE CWA IN WARM SECTOR AND FULL BRUNT OF WARM MOIST CONVEYOR OFF
THE WESTERN GULF. UNSEASONABLY HIGH PW/S NEARING AND INCH AND DEEP
SATURATION COULD DRIVE LOCALLY HEAVY RAINS ON THU OF AN INCH OR
MORE OVER THE SOUTHEASTERN 3/4S OF THE DVN CWA...WHILE THE
NORTHWESTERN 2 TIERS OF COUNTIES FROM CID TO FREEPORT IL GETS IN ON
HEAVY WET ACCUMULATING SNOW AND SLEET. RAIN OF THIS MAGNITUDE WILL
RUN-OFF ON PARTIALLY FROZEN SOILS AND COMBINE WITH LINGERING MELTING
SNOWPACK TO DRIVE RIVERS AND STREAMS UP...AS WELL AS PRODUCE
POSSIBLE ICE JAMS AS ICE ENCRUSTED RIVERS RISE AND BUST THE ICE
CRUST. FLASH-FLOODING OF LOW-LYING INLAND AREAS ALSO A VERY REAL
THREAT IF THIS WESTERN STORM TRACK VERIFIES. IN THE SNOW AREAS
GENERALLY ALONG AND NORTH OF A CID TO STERLING IL LINE...EARLY
PROGNOSTICS AND FORCING TOOLS SUGGEST AN EASY 4-8 INCHES OF SNOW BY
THU EVENING....AND TAKING INTO ACCOUNT THE WED SNOWS COULD ADD UP
TO 5 TO 11+ BY THE TIME THE SNOW WINDS DOWN LATE THU NIGHT. IN-
WRAPPING LLVL COLD CONVEYOR TO SWITCH BULK OF PRECIP OVER TO SNOW
AFTER MIDNIGHT THU NIGHT IN ALMOST THE ENTIRE CWA...BUT HEAVY DEF
ZONE PRECIP BANDS WILL LOOK TO MIGRATE NORTH OF THE LOCAL AREA BY
THEN AND JUST BE LIGHTER SNOWS WITH MINIMAL ADDITIONAL ACCUMULATION.
A WINTER STORM WATCH HAS BEEN POSTED FOR THE AREAS WITH THE
POTENTIAL HVY SNOW ACCUMULATION FOR WED THROUGH FRI. BUT AGAIN BULK
OF THIS TO BE ON THU AND THU EVENING WITH CURRENT TIMING. A REAL
POSSIBILITY THIS WILL HAVE TO BE EXPANDED SOUTHEASTWARD TO COVER
FREEZING RAIN AND WINTERY MIX THREAT. ALSO IF NEW PROGS SUGGEST A
FURTHER SOUTHEASTWARD TRACK...THE HEAVY SNOW THREAT WILL HAVE TO BE
EXPANDED SOUTHEASTWARD AS WELL. A BUNCH OF VARIETIES AND AMOUNTS OF
PRECIP EACH WITH THEIR OWN IMPACTS SURE LOOKS IN LINE TO MAKE FOR A
HIGH IMPACT DURING THE HOLIDAY PERIOD.

FRI INTO SAT...SFC LOW TO FILL AND PICK UP PACE NORTHEASTWARD
ACRS EASTERN WI WITH LINGERING SNOW SHOWERS FRI INTO SAT. MAY BE JUST
FLURRIES BY FRI NIGHT. SFC WINDS NOT TO PICK UP AGAIN WITH SLOW LOW
DEPARTING TIME UNTIL LATER FRI NIGHT...BUT SNOW PACK MAY BE WET
ENOUGH TO ALLEVIATE ANY BLOWING SNOW AND JUST FOSTER LOW BASED
DRIFTING.      ..12..

&&

.AVIATION...
IFR/LIFR TO VLIFR CIGS WILL BE SEEN THROUGH AT LEAST 00Z/23. ALL
MODELS SHOW A STRONG INVERSION TRAPPING LOW LVL MOISTURE. VSBYS
SHOULD REMAIN AT 1-2SM AT KDBQ/KCID THROUGH 03Z/22. KMLI/KBRL SHOULD
REMAIN AT 3-5SM. CONDENSATION PRESSURE DEFICITS IN THE 285-295K
THETA LAYER DROP AFT 03Z/22 WITH INCREASED FORCING. EXPECT -SN TO
DVLP 03Z-06Z AT KCID/KDBQ WITH VSBYS DROPPING BLO 1SM. KMLI TO SEE
-SN AFT 06Z WITH VSBYS DROPPING TO 1SM. KBRL TO REMAIN PRECIP FREE
THROUGH 12Z. FZDZ WL DVLP FM SOUTH TO NORTH AT ALL SITES AFT 12Z.
..08..

&&

.DVN WATCHES/WARNINGS/ADVISORIES...
IA...WINTER STORM WATCH FROM WEDNESDAY MORNING THROUGH FRIDAY
     AFTERNOON FOR BENTON-BUCHANAN-DELAWARE-DUBUQUE-JACKSON-
     JONES-LINN.

IL...WINTER STORM WATCH FROM WEDNESDAY MORNING THROUGH FRIDAY
     AFTERNOON FOR JO DAVIESS.

MO...NONE.

&&

$$

08/12