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000 FXUS63 KLOT 241743 AFDLOT AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL 1143 AM CST TUE NOV 24 2009 .DISCUSSION... 333 AM CST DENSE FOG THIS MORNING...RAIN AND ISOLATED THUNDER CHANCES THIS EVENING...AND EVOLUTION OF PRECIP TYPE WEDNESDAY THROUGH FRIDAY ARE MAIN CHALLENGES FOR THIS FORECAST PACKAGE. FIRST THINGS FIRST...DENSE FOG HAS ONCE AGAIN FORMED ACROSS THE AREA THIS MORNING. MANY OBS OUTSIDE OF URBAN AREAS DIPPED TO OR VERY NEAR 1/4 MILE BY MIDNIGHT...WITH URBAN SITES STAYING IN THE 1-3 MILE RANGE SO FAR. EXPECT VERY LOW VIS TO MAINTAIN ITSELF THROUGH MID MORNING FOR THOSE LOCATIONS THAT HAVE ALREADY DIPPED TO 1 MILE OR LESS...AND WOULD ANTICIPATE AT LEAST INTERMITTENT DROPS BELOW 1 MILE FOR URBAN SITES. LOW STRATUS DECK CONTINUES TO BUILD IN ACROSS THE AREA IN A SIMILAR FASHION TO LAST NIGHT...AND EXPECT EVOLUTION OF FOG TO ALSO BE SIMILAR TO YESTERDAY...WITH A GENERAL LIFT INTO THE LOW STRATUS BY MID MORNING. THIS AFTERNOON...RAIN BAND ASSOCIATED WITH OCCLUDED SURFACE FRONT AND UPPER LOW CURRENTLY OVER FAR SOUTHWESTERN IOWA WILL BEGIN TO MOVE ACROSS THE FORECAST AREA FROM SOUTHWEST TO NORTHEAST. AFTER MUCH DEBATE...DECIDED TO ADD IN ISOLATED THUNDER MENTION AS AXIS OF HEAVIEST RAIN AND UVV MOVES ACROSS THE AREA FROM 21-06Z TONIGHT. THUNDER CURRENTLY OCCURRING OVER NORTH CENTRAL IOWA WITH 7-7.5 DEGREE MID LEVEL LAPSE RATES PER SPC MESOANALYSIS...AND NAM/SREF/RUC IN GENERAL AGREEMENT IN BRINGING SIMILAR LAPSE RATES ACROSS THE AREA. BY NO MEANS IS THUNDER EXPECTED TO BE WIDESPREAD...AND MOST LOCATIONS WOULD LIKELY HEAR JUST A RUMBLE OR TWO AT BEST. AFTER MAIN BAND OF RAIN MOVES ACROSS THE AREA TONIGHT...MAY SEE A SHORT BREAK IN WIDESPREAD PRECIP BY WEDNESDAY MORNING AS SHORTWAVE RIDGING BEHIND FIRST UPPER TROUGH/LOW MOVES THROUGH. SECOND UPPER WAVE WILL BE JUST ON ITS HEELS THOUGH... WITH POPS INCREASING FROM THE NORTHWEST WEDNESDAY NIGHT. WITH COLDER AIR ALOFT SPREADING INTO THE AREA WITH THIS SECOND WAVE...STILL ANTICIPATE A CHANGEOVER TO A MIX OF RAIN AND SNOW LATE WEDNESDAY AND INTO THURSDAY. WRAP AROUND MOISTURE AND ASSOCIATED PRECIP ON THE WESTERN SIDE OF THE SECOND UPPER LOW WILL BEGIN TO MOVE INTO THE AREA THURSDAY. HAVE MAINTAINED THE RAIN/SNOW MIX...AND FORECAST SOUNDINGS CONTINUE TO INDICATE THAT WHILE MUCH OF THE COLUMN WILL BE BELOW FREEZING...IT MAY NOT BE COLD ENOUGH TO SUPPORT WIDESPREAD ICE CRYSTAL GENERATION. IF THIS ENDS UP BEING THE CASE...COLD DRIZZLE MIXED WITH LIGHT SNOW AT TIMES WOULD BE THE LIKELY P-TYPE. UPPER TROUGH FINALLY BEGINS TO LIFT OUT ON FRIDAY...WITH JUST A FEW RESIDUAL SNOW SHOWERS REMAINING UNTIL UPPER FLOW TURNS ANTICYCLONIC. TEMPERATURES WILL ALSO BE TRICKY...WITH MOS GUIDANCE SEEMINGLY TOO WARM WEDNESDAY THROUGH FRIDAY. INSTEAD GENERALLY FOLLOWED DETERMINISTIC GFS OUTPUT...WITH UPPER 40S DURING THE DAY WEDNESDAY DROPPING TO UPPER 30S TO AROUND 40 FOR HIGHS THURSDAY...AND RIGHT AROUND 30 BOTH WEDNESDAY AND THURSDAY NIGHT. NO CHANGES MADE TO PREVIOUS FORECAST SATURDAY THROUGH NEXT MONDAY. BOXELL && .AVIATION... 1143 AM CST 1800 UTC TAFS...IFR CONDITIONS ARE EXPECTED TO PERSIST THROUGHOUT MOST OF THE FORECAST PERIOD AS DECENT LOW LEVEL MOISTURE REMAINS IN PLACE AHEAD OF AN OCCLUDED LOW PRESSURE SYSTEM CURRENTLY IN SOUTH CENTRAL IOWA. WHILE VISIBILITY IS EXPECTED TO GRADUALLY IMPROVE THROUGH THE AFTERNOON AS SFC TEMPS SLOWLY INCREASE...CIGS ARE EXPECTED TO REMAIN IFR. A BRIEF IMPROVEMENT PERIOD UP TO LOW END MVFR CIGS IS EXPECTED LATE THIS AFTERNOON INTO THE EARLY EVENING HOURS AHEAD OF THE MAIN BATCH OF RAIN/THUNDERSTORMS MOVING NORTH ACROSS CENTRAL ILLINOIS. AS THE LIGHTER RAIN TRANSITIONS TO A HEAVIER RAIN THIS EVENING...CONDITIONS SHOULD REVERT BACK TO IFR. CONFIDENCE IN TIMING OF ANY POSSIBLE THUNDER WAS LOW ENOUGH AT THIS POINT TO LEAVE IT OUT. UPWARD MOTIONS APPEAR TO BE THE BEST THIS EVENING ALTHOUGH INSTABILITY IS MEAGER...SO BEST CHANCES FOR ANY THUNDER THAT DOES OCCUR WOULD LIKELY BE THIS EVENING. AS THE OCCLUSION COME THROUGH OVERNIGHT...WINDS WILL RAPIDLY VEER TO THE SOUTH-SOUTHWEST WITH THE MAIN BATCH OF PRECIP ENDING. SOME VERY BRIEF IMPROVEMENTS BACK TO MVFR MAY OCCUR...BUT OVERALL AM EXPECTING IFR CONDITIONS TO PERSIST THROUGH WEDNESDAY WITH SHOWERY TYPE OF RAIN CONTINUING. WINDS ARE EXPECTED TO BECOME GUSTY WEDNESDAY AFTERNOON OUT OF THE WEST AS COOLER AIR STARTS MOVING INTO THE REGION. APPEARS AS THOUGH SFC/LOW LEVEL TEMPS SHOULD REMAIN WARM ENOUGH TO SUPPORT LIQUID PRECIP TYPE THROUGH WEDNESDAY AFTERNOON. EXPECT MAINLY IFR CONDITIONS THROUGHOUT THE PERIOD WITH SOME BRIEF POP UPS TO MVFR POSSIBLE. HALBACH && .MARINE... 158 AM CST MAIN CONCERN FOR MARINE FORECAST IS PERIOD OF STRONG NORTHWEST WINDS LATE THURSDAY INTO EARLY FRIDAY BEHIND LOW PRESSURE SYSTEM AND ASSOCIATED PUSH OF COLD AIR ACROSS THE WESTERN LAKES. LOW PRESSURE CURRENTLY ANALYZED OVER NORTHEASTERN KANSAS TO MOVE NORTHEAST ACROSS WISCONSIN AND NORTHERN/CENTRAL LAKE MICHIGAN THOUGH WEDNESDAY EVENING. WITH LOW MOVING ACROSS CENTRAL/NORTHERN LAKE MICHIGAN...MAY SEE A BRIEF PERIOD OF GUSTY 20-25 KNOT SOUTHWEST/WEST WINDS ON FAR SOUTH END LATE WEDNESDAY/WEDNESDAY NIGHT. MORE IMPORTANTLY...SECOND UPPER DISTURBANCE DIGGING INTO TROUGH ALOFT WILL MAINTAIN SLIGHT DEEPENING OF TRAILING TROUGH/COLD FRONT WEDNESDAY NIGHT INTO THURSDAY. THIS TRAILING TROUGH WILL MOVE ACROSS THE LAKE THURSDAY...WITH TIGHTENING OF PRESSURE GRADIENT AND SURGE OF LOW LEVEL COLD AIR EXPECTED TO PRODUCE NORTHWEST WINDS AROUND 30 KNOTS LATE THURSDAY/THURSDAY NIGHT. MODEL FORECAST SOUNDINGS OVER THE LAKE DURING THIS PERIOD INDICATE GUSTS JUST BELOW GALE FORCE...THOUGH A FEW GALE FORCE GUSTS ARE POSSIBLE. PRESSURE GRADIENT AND ASSOCIATED NORTHWEST WINDS WILL THEN GRADUALLY DECREASE FRIDAY...AS LOW CONTINUES TO PULL AWAY TO THE NORTHEAST AND WEAK HIGH PRESSURE RIDGE SPREADS ACROSS THE MISSISSIPPI VALLEY AND WESTERN LAKES FRIDAY NIGHT. RATZER && .LOT WATCHES/WARNINGS/ADVISORIES... IL...NONE. IN...NONE. LM...NONE. && $$
