000 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. && $$