FXUS63 KLOT 251524
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
924 AM CST WED NOV 25 2009

.DISCUSSION...
924 AM CST

MADE A FEW ADJUSTMENTS TO THE WX/POP FORECAST FOR THE REST OF THIS
MORNING AND AFTERNOON. WITH UPPER LEVEL RIDGING IN PLACE ACROSS
THE REGION...AM EXPECTING JUST SOME PATCHY DRIZZLE...IF EVEN
THAT...THROUGH THE REST OF THE MORNING. MID LEVEL DIFLUENT AXIS
THAT IS CAUSING SOME LIGHT TO MODERATE RAIN ACROSS EASTERN IOWA
WILL SHIFT EASTWARD INTO NORTHERN ILLINOIS AND INTO THE I-39
CORRIDOR RIGHT AROUND LUNCHTIME. THIS AXIS WILL THEN SHIFT EAST-
NORTHEAST THROUGH THE AFTERNOON BRINGING BETTER CHANCES INTO THE
CHICAGO AREA TOWARD MID TO LATE AFTERNOON. ANY PRECIP AMOUNTS ARE
EXPECTED TO BE LIGHT AT A TENTH OF AN INCH OR LESS THROUGH THIS
AFTERNOON. TEMPERATURES WILL ALSO REMAIN WARM ENOUGH THAT ANY
PRECIP THAT DOES FALL WILL BE IN LIQUID FORM. UPDATED ZFP
FORTHCOMING...

HALBACH

&&

.PREV DISCUSSION...
321 AM CST

COMPLICATED AND GENERALLY MESSY FORECAST WITH RAIN AND DRIZZLE
LIFTING THROUGH THE AREA TODAY AND ADDITIONAL HEAVIER PRECIP MOVING INTO
THE AREA LATER TONIGHT WITH SECOND UPPER LOW. PRECIP TYPE
EVOLUTION TONIGHT THROUGH FRIDAY CONTINUES TO BE A
PARTICULAR CHALLENGE. 

EARLY MORNING SURFACE ANALYSIS INDICATES SURFACE LOW AND OCCLUDED
FRONT MOVING ACROSS FAR SOUTHERN LAKE MICHIGAN...WITH MAIN BAND OF
RAIN JUST AHEAD OF IT. AS THIS LIFTS NORTHEAST TODAY...EXPECT THE
ASSOCIATED BAND OF RAIN TO MOVE NORTH WITH IT. ADDITIONAL LIGHT
SHOWERS AND DRIZZLE HAVE DEVELOPED EARLY THIS MORNING IN
ASSOCIATION WITH UPPER LOW THAT IS TRACKING JUST BEHIND THE
SURFACE LOW. ADDITIONAL RAIN WILL BE POSSIBLE DURING THE FIRST
HALF OF THE MORNING AS SOME WRAP AROUND MOISTURE/DEFORMATION RAIN
MOVES ACROSS THE AREA.

STARTING LATE THIS MORNING AND EARLY AFTERNOON...EXPECT THAT WE'LL
LIKELY SEE A REPRIEVE IN THE WIDESPREAD RAIN SHOWER ACTIVITY AS
SHORTWAVE RIDGING MOVES THROUGH BEHIND THE FIRST UPPER LOW. LOOKS
LIKE BEST CHANCE OF A BREAK FROM THE RAIN WILL BE ACROSS OUR
SOUTHERN COUNTIES...FURTHEST AWAY FROM THE INFLUENCE OF THE
SURFACE AND UPPER LOWS.  

SECOND UPPER WAVE CURRENTLY OVER THE EASTERN DAKOTAS WILL BE RIGHT ON
THE HEELS OF THE FIRST...WITH RAIN EXPECTED TO DEVELOP JUST TO THE
NORTH OF THE UPPER CIRCULATION ALONG AN AXIS OF INCREASED
CONVERGENCE COINCIDING WITH SOME WEAK FRONTOGENESIS AND A LOW TO
MID LEVEL WARM CONVEYOR BELT-LIKE STRUCTURE. EXPECT THIS BAND OF
PRECIPITATION TO ROTATE FROM A NW TO SE ORIENTATION TO A SW TO NE
ORIENTATION AS UPPER CIRCULATION MOVES ACROSS THE AREA. ALSO
STARTING LATE THIS EVENING AND OVERNIGHT...COLDER AIR SPILLING IN
FROM THE NORTHWEST SHOULD RESULT IN A GRADUAL CHANGE FROM RAIN TO
A MIX OF RAIN AND SNOW FOR THE WESTERN COUNTIES BY MIDNIGHT...
AND THEN TRANSITIONING TO A MIX FOR MOST OF THE REST OF THE
FORECAST AREA FOR THANKSGIVING MORNING. ONLY EXCEPTION MAY BE
ACROSS THE FAR SOUTHWESTERN AREA OF THE CWA...WHERE COLUMN MAY BE
JUST COLD ENOUGH TO SUPPORT ALL LIGHT SNOW. P-TYPE FORECAST IS
STILL VERY LOW CONFIDENCE WITH MODEL GUIDANCE GENERALLY ON THE
FENCE BETWEEN RAIN VS. SNOW THROUGH THURSDAY MID-DAY. AT ANY
RATE..SURFACE TEMPERATURES SHOULD REMAIN WARM ENOUGH TO AVOID ANY
SIGNIFICANT ACCUMULATION.

BY THURSDAY AFTERNOON AND EARLY EVENING...EXPECT THAT LINGERING SNOW
SHOWERS WILL MOVE OUT AS UPPER LOW MOVES ACROSS THE EASTERN GREAT
LAKES. ONLY REMAINING SNOW CHANCES OVERNIGHT THURSDAY INTO EARLY
MORNING FRIDAY WILL BE FOR FAR NORTHWESTERN PORTIONS OF PORTER COUNTY
WHERE JUST A FEW LAKE EFFECT SNOW SHOWERS COULD CLIP PORTIONS OF
THE COUNTY AS COLDEST 850 HPA AIR MOVES OVER. WITH SOUTHERN LAKE
MICHIGAN WATER TEMPS AROUND 10 C AND 850 T AROUND -6 OR SO...DELTA
T MAY BE JUST HIGH ENOUGH TO GET A FEW LIGHT SNOW SHOWERS OFF THE
LAKE. CERTAINLY DON'T EXPECT THEM TO AMOUNT TO TOO MUCH THOUGH.
 
SATURDAY STILL LOOKS TO BE DRY BEFORE YET ANOTHER SYSTEM BEGINS TO
MOVE INTO THE AREA ON SUNDAY. ATTM...NO CHANGES MADE BEYOND
FRIDAY.  

BOXELL

&&

.AVIATION...

1200 UTC TAFS...AVIATION FORECAST CONCERNS CONTINUE TO BE
PRECIPITATION TIMING THIS MORNING AND AGAIN TONIGHT...AND
POTENTIAL FOR IFR/LIFR CEILINGS AND VISIBILITY THROUGH THE
PERIOD.

UPPER LOW LOCATION CLEARLY DELINEATED BY REFLECTIVITY SWIRL IN 
REGIONAL RADAR MOSAIC THIS MORNING. UPPER LOW TO CONTINUE TO MOVE 
SLOWLY EAST/NORTHEAST OUT OF NORTHEAST IL THIS MORNING...WITH AREA 
OF RAIN DIMINISHING FROM WEST TO EAST ACROSS TERMINALS THROUGH 15Z. 
CEILINGS CURRENTLY VARYING BETWEEN ABOUT 600-1600 FT IN STEADIER 
RAIN...THOUGH OBS AND 11-3.9 MICRON GOES IMAGERY INDICATES 
WIDESPREAD AREA OF LOWER STRATUS BACK INTO EASTERN IA/SOUTHEAST MN. 
THUS WOULD EXPECT DOWNWARD TREND INTO IFR CONDITIONS THIS MORNING AS 
UPPER LOW AND ASSOCIATED RAIN PULL AWAY. APPEARS THERE WILL BE A 
BREAK IN PRECIPITATION MIDDAY...AS SHORT WAVE RIDGE DEVELOPS ALOFT 
BEHIND DEPARTING UPPER LOW...AND AHEAD OF NEXT STRONG WAVE DIGGING 
ACROSS THE UPPER MIDWEST. FORECAST SOUNDINGS FROM WRF SUGGEST 
DIURNAL IMPROVEMENT TO MVFR CEILINGS BY EARLY AFTERNOON...BEFORE 
NEXT BATCH OF RAIN WITH SECOND UPPER SYSTEM MOVES INTO THE AREA LATE 
THIS AFTERNOON/EVENING. FORECAST SOUNDINGS/TIME HEIGHT SECTIONS AND 
STATISTICAL MOS OUTPUT ALL INDICATE CONDITIONS DETERIORATING BACK 
INTO IFR OR EVEN LIFR RANGE DURING THE NIGHT...EXTENDING INTO 
THURSDAY MORNING AS DEFORMATION PRECIPITATION BAND SHIFTS EAST 
ACROSS NORTHERN ILLINOIS. WRF SOUNDINGS INDICATE COLUMN COOLS ENOUGH 
FOR PRECIP TO MIX/CHANGE TO WET SNOW EARLY THURSDAY MORNING...THOUGH 
GFS A LITTLE FARTHER SOUTH WITH TRACK OF UPPER LOW AND WRAP-AROUND 
COLD AIR SUGGESTING MIX OR COLD RAIN MORE LIKELY. THUS SOMEWHAT LOW 
CONFIDENCE ON PRECIP TYPE THURSDAY MORNING.

AS FOR WINDS...SFC LOW ANALYZED OVER CENTRAL LAKE MICHIGAN THIS 
MORNING WITH TROUGH TRAILING THROUGH NORTH CENTRAL IL. SFC LOW TO 
PROPAGATE TO THE NORTHEAST LEAVING TROUGH AXIS ACROSS FORECAST 
AREA...WHICH SHOULD SUPPORT SOUTHWEST WINDS SHIFTING MORE WESTERLY 
DURING THE DAY. WIND SPEEDS GENERALLY IN 10-15 KT RANGE EXPECTED AS 
SFC PRESSURE RISES AND TIGHTER GRADIENT REMAIN MAINLY ACROSS 
WESTERN/SOUTHERN PARTS OF THE FORECAST AREA. WINDS MAY BACK A LITTLE 
MORE SOUTHWESTERLY TONIGHT AS SECOND UPPER WAVE SHARPENS SFC TROUGH 
A BIT. TROUGH/COLD FRONT THEN MOVE ACROSS AREA THURSDAY MORNING 
SHIFTING WINDS TO THE NORTHWEST AND BECOMING GUSTY BY MIDDAY.

RATZER

&&

.MARINE...
146 AM CST

MAIN MARINE CONCERNS REMAIN GUSTY NORTH/NORTHWEST WINDS DEVELOPING 
THURSDAY AS COLD AIR SURGES ACROSS THE WESTERN LAKES BEHIND 
DEPARTING AREA LOW PRESSURE.

OCCLUDED LOW ANALYZED ALONG WI/IL BORDER EARLY THIS MORNING. THIS 
LOW PROGGED TO LIFT NORTHEAST ACROSS CENTRAL LAKE MICHIGAN THIS 
MORNING...AND ACROSS THE NORTHERN LOWER PENINSULA AND INTO ONTARIO 
BY THIS EVENING. TROUGH EXPECTED TO BECOME MORE ELONGATED FROM 
SOUTHWEST TO NORTHEAST ALONG LOW TRACK HOWEVER...AS SECOND UPPER 
DISTURBANCE DIGS INTO REGION THROUGH TONIGHT. THIS EXPECTED TO
KEEP PRESSURE GRADIENT A LITTLE BAGGY OVER THE LAKE TODAY...THOUGH
WITH WEST/SOUTHWEST WINDS GUSTING INTO THE 20-25 KNOT RANGE ON THE
FAR SOUTH END OF THE LAKE. GIVEN MARGINAL GUSTS AND OFF SHORE FLOW
PRODUCING WAVES GENERALLY 3 FEET OR LESS ALONG IL/IN SHORES...HAVE
DECIDED AGAINST ISSUING SMALL CRAFT ADVISORY FOR TODAY AT THIS
TIME.

STRONGER WINDS EXPECTED TO DEVELOP THURSDAY MORNING...AS TROUGH/COLD 
FRONT SLIPS OFF TO THE EAST OF THE LAKE AND COLD AIR PUSHES 
SOUTHEAST ACROSS THE REGION. COUPLED WITH TIGHT PRESSURE GRADIENT ON 
BACK SIDE OF DEPARTING SYSTEM...LOW LEVEL COLD AIR ADVECTION WILL 
RESULT IN DEEPER MIXED BOUNDARY LAYER WITH GUIDANCE CONSISTENT IN 
DEPICTING NORTHWEST WINDS INCREASING TO 30 KNOTS OR SO THURSDAY AND 
THURSDAY EVENING. COULD BE SOME GUSTS TO MARGINAL GALE 
FORCE...THOUGH SUSTAINED WINDS EXPECTED TO REMAIN AROUND 30 KNOTS. 
WILL NEED SMALL CRAFT ADVISORY THURSDAY MORNING...THOUGH IN 
AGREEMENT WITH MKX AND IWX WFOS WILL HOLD OFF ISSUANCE FOR NOW. 
WINDS WILL THEN GRADUALLY DIMINISH LATE THURSDAY NIGHT THROUGH 
FRIDAY...AS HIGH PRESSURE RIDGE SPREADS ACROSS UPPER MIDWEST AND 
WESTERN LAKES BY FRIDAY EVENING. 

RATZER

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...NONE.
&&

$$