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000 FXUS63 KLOT 241938 AFDLOT AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL 138 PM 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... 137 PM CST WINDS WILL BE ON THE INCREASE ACROSS THE LAKE TONIGHT AS A LOW PRESSURE SYSTEM CURRENTLY IN SOUTHERN IOWA LIFTS NORTHEAST TOWARD THE SOUTHERN HALF OF THE LAKE. THIS LOW WILL THEN TAKE A JAUNT NORTH AND HEAD TOWARD FAR EASTERN LAKE SUPERIOR. THE STRONGEST WINDS WILL COME IN ACROSS THE LAKE ON THURSDAY AS A SECONDARY COLD FRONT DROPS DOWN THROUGH THE GREAT LAKES. WINDS WILL BE GUSTING UP TO 30 KT GIVEN THE STRONG COLD AIR ADVECTION AND WARM LAKE WATERS. LOOKS A BIT MARGINAL FOR GALES AT THIS POINT...SO WILL HOLD OFF ON ISSUING ANY GALE HEADLINES...BUT WILL HAVE TO MONITOR ON WEDNESDAY/THURSDAY. SMALL CRAFT CONDITIONS WILL LIKELY BE MET TOMORROW AFTERNOON AND THURSDAY AFTERNOON INTO FRIDAY MORNING. HALBACH && .LOT WATCHES/WARNINGS/ADVISORIES... IL...NONE. IN...NONE. LM...NONE. && $$
