Issued by NWS Chicago, IL
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000 FXUS63 KLOT 250437 AFDLOT AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL 1037 PM CST TUE NOV 24 2009 .DISCUSSION... 752 PM CST MAIN CONCERN FOR THE NEAR TERM THROUGH THE OVERNIGHT HOURS IS BROAD AREA OF PRECIP THAT IS CURRENTLY MOVING THROUGH THE CWA. THIS AREA OF PRECIP HAS BEEN RIDING OUT AHEAD OF UPPER LEVEL CLOSED LOW...AND ATTENDANT SURFACE LOW. AS THIS LOW HAS SLOWLY PROGRESSED TO THE EAST...IT HAS DRAWN A FAIR AMOUNT OF MOISTURE NORTH INTO THE REGION WHICH HAS BEEN EVIDENT ON SATELLITE IMAGERY ALONG WITH THE UPPER 40S/LOW 50S SURFACE DEW POINTS REPORTED THROUGHOUT THE CWA TODAY. THIS HAS HELP PROVIDE DECENT RAINFALL TOTALS OF UP TO HALF AN INCH...INCLUDED IN AREAS WHERE LIGHT TO MODERATE RAIN IS STILL BEING REPORTED. EXPECT AN ADDITIONAL QUARTER TO HALF AN INCH OF RAINFALL ACROSS PORTIONS OF NORTHERN ILLINOIS AND NORTHWEST INDIANA OVER THE NEXT COUPLE OF HOURS BEFORE THIS MAIN PRECIP SHIELD LIFTS TO THE NORTHEAST. A LACK OF ANY LIGHTNING STRIKES OVER THE LAST COUPLE OF HOURS ASSOCIATED WITH THIS PRECIP SHIELD GIVES PRUDENCE TO CURRENT RUC ANALYSIS WHICH HAS SHOWN A DOWNWARD TREND FOR ANY ELEVATED INSTABILITY. MODEL GUIDANCE CONTINUES WITH THIS TREND WITH VERY WEAK INSTABILITY THROUGH TONIGHT. WITH THESE TRENDS IN MIND...HAVE DECIDED TO PULL ANY MENTION OF THUNDER IN THE FORECAST. RODRIGUEZ && .AVIATION... 1030 PM CST 0000 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... 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. && $$
