FXUS63 KIWX 050542
AFDIWX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE NORTHERN INDIANA
1242 AM EST SAT DEC 5 2009

.AVIATION.../06Z TAFS/
MVFR CIGS WITH SCT FLURRY ACTIVITY WILL CONTINUE TO SHIFT NE
THROUGH THE PERIOD WITH A TREND TO VFR CONDITIONS OVER MUCH OF
THE FA BY MID TO LATE MORNING. SCT MVFR CIGS WILL PERSIST OVER
PORTIONS OF BERRIEN AND CASS COUNTIES PER A CONTINUED LAKE
RESPONSE...WHICH WILL BEGIN TO WANE BY LATE MORNING...WITH A TREND
TO VFR FLIGHT CONDITIONS.

&&

.PREV DISCUSSION... /ISSUED 407 PM EST FRI DEC 4 2009/

SHORT TERM...
FORECAST CONCERNS LIE WITH LINGERING LAKE EFFECT IN EXTREME 
NORTHWEST AREA AND CLOUD COVER AND TEMPS REMAINDER OF AREA. SHORT 
WAVE TROUGH UPSTREAM FROM WI TO EASTERN MO THIS AFTERNOON WILL 
GRADUALLY MOVE EAST TONIGHT. AREA OF PERSISTENT CLOUDS AHEAD OF THIS 
TROUGH IN CENTRAL IL HAS BEEN EXPANDING EAST AND NORTHEAST TODAY. 
MODELS NOT HANDLING THIS AREA WELL WITH RUC HAVING BEST HANDLE IN 
ITS LOW LEVEL RH PROGS. MODELS SHOW DEVELOPING INVERSION OVERNIGHT 
AND THIS GIVES ME CONCERN AS LOW LEVEL MOISTURE WOULD BECOME TRAPPED 
AND WOULD HAVE TO RELY ON SUBSIDENCE FOR DISSIPATION. HAVE INCREASED 
SKY COVER GRIDS LATE THIS AFTERNOON AND EVENING BASED ON SAT TRENDS 
WITH A SLOW DISSIPATION OVERNIGHT. HAVE KEPT PARTLY CLOUDY SKY COVER 
PERCENTAGES AS A PRUDENT HEDGE. IF SKIES CAN CLEAR AS MODELS 
SUGGEST...LOWS WILL BOTTOM OUT TOWARD DAYBREAK IN MID TO UPPER TEENS 
FOR COLDEST LOWS OF THE SEASON. 

SOUTHWEST FLOW WILL CONTINUE INTO BERRIEN AND EXTREME NORTHWEST CASS 
COUNTY TONIGHT. AS WE HAVE BEEN MENTIONING THE SOUTHWEST FLOW IS 
LEAST FAVORABLE FOR LAKE EFFECT SNOW ACCUMULATIONS WITH SHORT FETCH. 
HOWEVER...DELTA T VALUES INTO THE LOWER 20S WITH WINDS ONLY IN 10 TO 
20 KNOT RANGE SHOULD STILL YIELD SOME GOOD SNOW SHOWERS. QUESTION 
REMAINS WHETHER TRAJECTORIES WILL KEEP IT JUST NORTH OF BERRIEN OR 
INTO THE NORTHERN TIP. KBEH HAS SEEN VIS DOWN TO 1/2SM MILE THIS 
AFTERNOON WITH THIS SHORTER FETCH. EXPECT UP TO A COUPLE INCHES 
STILL POSSIBLE ACROSS NORTHERN TIP OF BERRIEN COUNTY OVERNIGHT WITH 
SOME LOCALLY HEAVIER AMOUNTS POSSIBLE THOUGH AREA IS QUITE SMALL 
WITHIN OUR CWA. STILL A CHANCE FOR LINGERING SNOW SHOWERS SATURDAY 
BUT MOISTURE BEGINS TO WANE AND FOCUS SHIFTS NORTH LATER IN THE DAY 
AND ESPECIALLY SAT NIGHT.

WEAK HIGH PRESSURE RIDGE TO SLIDE ACROSS THE AREA SATURDAY AND 
SATURDAY NIGHT. TEMPS MIXED FROM 925MB ON SATURDAY ONLY YIELDS UPPER 
20S TO LOWER 30S. WILL NUDGE UP JUST A BIT AS NAM MAY BE A BIT TOO 
COOL BUT WILL STAY BELOW MOS THROUGH THE PERIOD.  

LONG TERM...
SUNDAY THROUGH FRIDAY...THE MAIN CHALLENGES THIS PERIOD INCLUDE THE 
CONCERNS FOR WINTRY WEATHER DEVELOPING TUESDAY AND PERSISTING INTO 
THURSDAY AS A POTENT UPPER LEVEL SYSTEM AND SFC LOW MOVE INTO THE 
AREA. FOR THIS PACKAGE...PREFER A SFC LOW TRACK NORTHEAST THROUGH 
ABOUT FORT WAYNE WEDNESDAY MORNING. THIS TRACK HAS SUPPORT FROM THE 
CANADIAN OPERATIONAL RUN...AND THE GFS ENSEMBLES. IT IS ALSO A BLEND 
OF THE ECMWF AND THE GFS OPERATIONAL RUNS AND IS REASONABLY 
CONSISTENT WITH THESE PREVIOUS RUNS. KEPT THE CHANCE FOR FREEZING 
RAIN TUESDAY INTO EARLY WEDNESDAY AS WARM AND MOIST AIR RIDES OVER A 
WEDGE OF COLD AIR. STILL HARD TO TELL HOW MUCH ICE ACCUMULATION 
POTENTIAL GIVEN TEMPS CLOSE TO FREEZING AND THE POTENTIAL ALSO FOR 
SLEET AND SNOW BESIDES RAIN. IT APPEARS THE BEST CHANCES FOR SNOW 
ACCUMULATION IS OVER NW AREAS INCLUDING SOUTH BEND AND SW LOWER 
MICH. WINDS WILL INCREASE RAPIDLY AFTER THE SFC LOW RACES NORTHEAST 
WEDNESDAY AFTERNOON WITH RAIN CHANGING TO ALL SNOW. LAKE ENHANCED 
PRECIPITATION WILL CHANGE TO ALL SNOW BY LATE WEDNESDAY WITH AT 
LEAST SOME ACCUMULATIONS LIKELY DOWNWIND OF LAKE MICHIGAN. WINDY 
CONDITIONS ARE ALSO EXPECTED...SO WITH THE POTENTIAL FOR 
ACCUMULATING SNOW AND STRONG WINDS...WINTER WEATHER HEADLINES WILL 
LIKELY BE NEEDED AT SOME POINT. LAKE EFFECT SNOW SHOULD PERSIST INTO 
THURSDAY AS DELTA T VALUES APPROACH -20C. HAVE CONTINUED TO TREND 
HIGH TEMPS THURSDAY DOWN GIVEN VERY COLD AIR AND PRECIPITATION.

&&

.IWX WATCHES/WARNINGS/ADVISORIES...
IN...NONE.
MI...NONE.
OH...NONE.
LM...NONE.
&&

$$

SHORT TERM...LASHLEY
LONG TERM...SKIPPER
AVIATION...JC