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000 FXUS63 KLOT 250923 AFDLOT AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL 323 AM CST WED NOV 25 2009 .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... 1030 PM CST 0600 UTC TAFS...A VERY MESSY PERIOD EXPECTED FOR AVIATION WEATHER CONCERNS. A VERTICALLY STACKED LOW PRESSURE SYSTEM IS NOW APPROACHING THE NORTHWEST CORNER OF ILLINOIS AND WILL CONTINUE TO TRACK TO THE NORTHEAST CNTRL LAKE MICHIGAN BY 12Z. THE MAIN PCPN SHIELD IS PUSHING NORTH OF THE TERMINALS...BUT AS THE UPPER LOW CENTER MOVES OVER THE REGION...PCPN WILL BECOME MORE DRIZZLE. THE LATEST SATELLITE IMAGERY SHOWS A MID LEVEL DRY PUNCH MOVING OVER THE AREA WHICH CONFIRMS THIS IDEA. STILL EXPECT THAT CIGS SHOULD DROP TO IFR...WITH AT LEAST PERIODS OF LIFR OVERNIGHT...BUT FOR THE PAST FEW HOURS...CIGS HAVE BOUNCED AROUND QUITE A BIT. HOWEVER...AS THE UPPER LOW MOVES OVERHEAD...FEEL FAIRLY CONFIDENT THAT THE CORE OF THE COLD AIR ALOFT WILL BE RIGHT OVERHEAD AND THERE SHOULD BE PERIODS OF CIGS DROPPING TO LIFR. AT THIS POINT...THE MOST LIKELY TIME FOR CIGS TO DROP TO LIFR WILL BE DURG THE PRE-DAWN HOURS. FOR PCPN...HAVE GONE WITH DRIZZLE FOR THE PERIOD AFTER MIDNIGHT AS THE SATELLITE IMAGERY INDICATES THE MID LEVEL DRY SLOT MOVING ACROSS MUCH OF ILLINOIS. THE SHORT RANGE MODEL GUIDANCE IS WELL INITIALIZED WITH THIS FEATURE...WITH RH TIME SECTIONS SHOWING THE LAYER BELOW 700MB SATURATED AND WITH SIGNIFICANTLY DRIER AIR ABOVE. HOWEVER...WILL KEEP THE IDEA OF MORE SHOWERY RAIN WITH THE FRONTAL PASSAGE AS THERE WILL BE BETTER UPPER DYNAMICS AND SFC FORCING. THE GENERAL TREND FOR THE PERIOD WILL BE FOR CIGS TO DROP TO IFR OVERNIGHT...AND REMAINING IFR UNTIL THE FRONTAL PASSAGE. HOWEVER...WITH THE LOW PRESSURE SYSTEM MOVING RIGHT OVER THE REGION...THERE WILL BE PERIODS OF LIFR CIGS DURG THE EARLY MORNING HOURS. WINDS WILL REMAIN SELY UNTIL THE FRONTAL PASSAGE AND THEN ABRUPTLY SHIFT WSWLY AS THE FRONT SWEEPS ACROSS THE AREA. VISIBILITY WILL BE A CONCERN AS WELL...AND IS THE BIGGEST QUESTION MARK AT THIS TIME. VISIBILITY WILL LIKELY REMAIN IN THE 3-5SM RANGE...THOUGH PERIODS OF IFR VIS ARE QUITE POSSIBLE...PARTICULARLY WHEN THE PCPN TRANSITIONS OVER TO DRIZZLE. KREIN && .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. && $$
