FXUS63 KLOT 252103
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
303 PM CST FRI DEC 25 2009

.DISCUSSION...
300 PM CST

SHORT WAVE TROF NOW ROTATING NORTHWARD FROM CENTRAL IL AND
SOUTHERN IND EARLY THIS AFTERNOON. AREA OF PRECIP BECAME MORE
DEVELOPED ACROSS CENTRAL IL DURING THE MORNING AS THE TROF TURNED 
NORTHEASTWARD ACROSS THE MID MS RIVER TO SOUTHERN IL IN RESPONSE
TO THE APPROACHING TROF. 

PRECIP HAS SPREAD ACROSS NW IND AND NORTHERN IL INTO SOUTHERN WI
DURING THE LATE MORNING AND EARLY AFTERNOON. RA QUICKLY CHANGING
OVER TO SN AND WILL DO SO ACROSS FAR NE IL AND NW IND IN NEXT HR
OR TWO AS COLD LOW LEVEL AIR THAT HAS NOW WRAPPED ALL THE WAY
AROUND TO THE SW AND S SIDE OF THE SYSTEM RAPIDLY ADVECTS NNE ON
25-35KT LOW LEVEL FLOW.

WILL BE SOME MINOR ACCUMULATING SN MID AFTERNOON INTO EARLY
EVENING TIL UPPER TROF CROSSES OVER FAR NORTHERN IL. THIS LIGHT SNOW
ACCUMULATION...ALONG WITH THE RAPID DROP INTO THE 20S AS THE
OCCLUDED FRONT PUSHES THRU THE REMAINDER OF THE FA WILL LIKELY
RESULT IN SLICK SPOTS AS RAINFALL OF LAST NIGHT AND THIS MORNING
LIKELY WASHED ALL SALT AND OTHER MELTING AGENTS FROM PAVEMENT SO
ANY WET SPOTS TO QUICKLY FREEZE...ESPECIALLY USUAL CULPRITS:
BRIDGES...ELEVATED RAMPS...ETC AS THEY EXPERIENCE FREEZING TEMPS
FROM BOTH ABOVE AND BELOW. 

THIS PD OF SNOW TO TAPER OFF/END DURING THE FIRST PART OF THE
EVENING BUT THEN THE NEARLY CONSOLIDATED UPPER LOW...CURRENTLY
OVER THE MO-IA BORDER...DRIFTS ENE THEN NE TO SE WI BY 12Z SAT.
EXPECT AT FIRST JUST FLURRIES AND SCATTERED A -SHSN NEAR/UNDER THE
UPPER LOW.

OVERNIGHT AND SAT H7 AND H8 LOWS PROGGED TO TRACK NEWRD FROM SE IA
TO OVR/NEAR S CENTRAL WI. MODELS SHOW AREA OF FAIRLY STRONG UVV
FROM ISENTROPIC UPGLIDE AS WAA TO BE OCCURRING IN THE LOWER-MID 
LEVELS. MODELS GIVING 0.15 TO 0.2 QPF DURING THE DAY SAT AS A
RESULT SO FORECASTING A SNOWFALL OF 1-3 INCHES ACROSS MUCH OF FA.

UPPER LOW TO DO AN COUNTER-CLOCKWISE LOOP OVER SRN WI AND NORTHERN IL
DURING SAT AND SUN. THIS KEEPS CHANCES OF LIGHT SNOW AND SNOW
SHOWERS AND ACCOMPANYING MINOR ACCUMULATIONS GOING FOR THE AREA
TIL LATE SUN NIGHT WHEN UPPER LOW FINALLY MOVES FAR ENOUGH OFF TO
THE E OVR THE CENTRAL GRTLKS.

RIDGING BUILDS IN FROM THE PLAINS LATE MON INTO TUE AS ANOTHER UPPER
TROF MOVES FM THE ROCKIES OUT OVR THE HI PLAINS. AS THIS TROF
CROSSES THE CENTRAL MO VALLEY AND APPROACHES THE MID MS VALLEY
OVERNIGHT TUE CHC OF -SN WILL ALSO SPREAD E ACROSS THE REGION FOR
WED.  

REGARDING HYDROLOGIC ISSUES...WATER LEVELS ON STREAMS AND RIVERS
WILL CONTINUE TO RISE TODAY AND TONIGHT THOUGH LATEST FORECASTS
ARE FOR CRESTS TO BE BELOW FLOOD STAGE...EXCEPT MINOR FLOODING
FOR VERMILION AND ILLINOIS RIVERS.

TRS

&&

.AVIATION...
1142 AM CST

1800 UTC TAFS...RATHER CHALLENGING AVIATION FORECAST TODAY AS
STRONG STORM SYSTEM OVER IOWA CONTINUES TO SLOWLY OCCLUDE. GUSTY
SOUTHEAST WINDS SHOULD CONTINUE IN ADVANCE OF APPROACHING OCCLUDED
FRONT. FEEDER BANDS OF SHOWERS CONTINUE TO PINWHEEL IN TOWARD THE
CENTER OF THE MASSIVE CYCLONE AND SHOULD PROVIDE FOR AT LEAST OCNL
SHRA THROUGH THE AFTERNOON. OUTSIDE OF SHRA ACTIVITY EXPECT VFR
VSBYS. CIGS SHOULD GENERALLY HOLD IN THE MVFR RANGE THROUGH THE
AFTERNOON THOUGH BRIEF SCATTERING OUT TO VFR ISNT OUT OF THE
QUESTION. 

AS FRONT PASSES LATE THIS AFTERNOON LOOK FOR COLD AIR TO COME
FLOODING INTO THE AREA AND RESULTANT BANDS OF SHOWERS TO SWITCH
OVER TO SNOW. RADAR REFLECTIVITY UPSTREAM REALLY NOT ALL THAT
IMPRESSIVE...HOWEVER SURFACE OBS INDICATE A LARGE AREA OF LIGHT
SNOW WITH WDSPR MVFR AND SCT IFR VSBYS FROM WESTERN IL WEST ACROSS
MO INTO NE/KS. PROLONGED PERIOD OF LIGHT SNOW WITH PERIODIC BURSTS
OF HEAVIER SNOW SHOWERS IS EXPECTED THIS EVENING AND CONTINUING
THROUGH THE DAY SATURDAY. VSBYS WILL FREQUENTLY BE REDUCED TO MVFR
WITH OCNL IFR VSBYS. IMPOSSIBLE TO REALLY PINPOINT MOST LIKELY
TIMING FOR IFR VSBYS AT THIS DISTANCE...SO WILL JUST STICK WITH
PREVAILING MVFR AND ANTICIPATE LATER ISSUANCES WILL NEED TO
INTRODUCE AT LEAST TEMPOS FOR IFR VSBY TONIGHT INTO SATURDAY. 

WOULD ANTICIPATE THAT THE SOUTHEAST WINDS THAT ARE CAUSING
PROBLEMS FOR ORD/MDW TRAFFIC FLOW SHOULD VEER TO MORE S OR SSE BY
MID AFTERNOON BEFORE SETTLING TO BETWEEN 180-200 TONIGHT AND
SATURDAY. GUSTINESS WILL VERY GRADUALLY WANE TONIGHT INTO SATURDAY
AS DEEP SURFACE LOW OVER IOWA SLOWLY WEAKENS. 

IZZI

&&

.MARINE...
123 PM CST

WINDS ARE GRADUALLY SUBSIDING OVER THE LAKE TODAY AS A LOW OVER
IOWA BEGINS TO OCCLUDE AND GRADUALLY FILL. ANTICIPATE BEING ABLE
TO CANCEL GALE WARNING EARLY FOR SOUTHERN LAKE MICHIGAN AND
NORTHERN PORTIONS SHOULD BE ABLE TO EXPIRE ON SCHEDULE THIS
EVENING. SFC LOW SHOULD WANDER AROUND IOWA THROUGH TOMORROW BEFORE
A MORE DECISIVE NORTHEASTERLY MOTION COMMENCES SATURDAY NIGHT. THE
LOW IS EXPECTED TO MOVE ACROSS NORTHERN PORTIONS OF THE LAKE EARLY
SUNDAY WITH MODEST WNW WINDS LOCKING IN BEHIND THE LOW. COLD FRONT
EXPECTED TO DROP DOWN THE LAKE MONDAY WITH RENEWED COLD AIR
ADVECTION AND TIGHTENING PRESSURE GRADIENT LEADING TO A FRESHENING
UP OF THE WINDS TO AROUND 30 KNOTS. FAIRLY QUICK DROP OFF IN WINDS
EXPECTED MONDAY NIGHT AS SURFACE RIDGE BUILDS EASTWARD ALLOWING
GRADIENT TO BEGIN RELAXING.

IZZI

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...GALE WARNING...LMZ080-LMZ669-LMZ671-LMZ673-LMZ675-LMZ740-LMZ741-
     LMZ742-LMZ743-LMZ744-LMZ745-LMZ777-LMZ779-LMZ870-LMZ872-
     LMZ874-LMZ876-LMZ878 UNTIL 3 PM FRIDAY.

     GALE WARNING...LMZ261-LMZ362-LMZ364-LMZ366-LMZ563-LMZ565-LMZ567-
     LMZ868 UNTIL 9 PM FRIDAY.

&&

$$