FXUS63 KLOT 260503
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1103 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...
1045 PM CST

0600 UTC TAFS...MAIN FORECAST FOR THE OVERNIGHT HOURS WILL FOCUS
ON SNOWFALL COVERAGE AND IMPACT ON VISIBILITY. LATEST RADAR
IMAGERY SHOWS THAT THE SNOW HAS BECOME RATHER SPOTTY...BUT FEEL
THAT RADAR MAY BE UNDER REPRESENTING SNOWFALL AS MOST OF THE
METARS INDICATE SOME FORM OF SNOW IN THE PRESENT WEATHER
INDICATOR. THUS FEEL THAT THE RADAR RETURNS REPRESENT THE HEAVIER
SNOW AND LIGHT SNOW MAY BE ESCAPING DETECTION. SO...TIMING OF
INDIVIDUAL BURSTS OF SNOW REDUCING VISIBILITY WILL BE QUITE A
CHALLENGE OVERNIGHT. GIVEN THE SPOTTY NATURE OF THE HEAVIER
SNOW...HAVE GONE WITH PREVAILING 6SM -SN TO START OUT THE 06Z
UPDATE...WITH A TEMPO 2SM. CONFIDENCE IS LOW AT THIS TIME FOR
TIMING AND DURATION OF IFR VIS RESTRICTIONS...BUT FEEL THAT THERE
WILL BE PERIODS OF IFR VIS DURG THE REMAINDER OF THE NIGHT.
CEILINGS SHOULD BE LESS PROBLEMATIC. WHILE CIGS HAVE GENERALLY
IMPROVED TO VFR ACROSS NRN IL...SHORT TERM GUIDANCE AND UPSTREAM
OBS SHOW SOME TENDENCY FOR MVFR CIGS...SO HAVE GONE WITH MVFR CIGS
IN THE TEMPO AND THEN RETURNED TO PREVAILING MVFR DURG THE MORNING
HOURS. 

GUIDANCE STILL INDICATES A MODEST MID LEVEL IMPULSE ROTATING
AROUND THE MAIN UPPER LOW...SO WILL KEEP A PERIOD OF 2SM VIS
REDUCTION FROM LATE MORNING TO LATE EVENING. EVENTUALLY...THE
UPPER LOW WILL BEGIN TO EJECT NEWD TOMORROW AFTERNOON OR
EVENING...AND WINDS WILL EVENTUALLY SHIFT FROM PREVAILING SLY TO
MORE WLY. DECREASING MOISTURE SHOULD ALLOW FOR PCPN INTENSITY TO
TAPER OFF...BUT CYCLONIC CURVATURE SHOULD BE SUFFICIENT TO ALLOW
SOME LIGHT SNOW TO CONTINUE INTO THE NIGHT.

KREIN

&&

.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...SMALL CRAFT ADVISORY...NEARSHORE WATERS 
     UNTIL 4 AM SATURDAY.

&&

$$