Issued by NWS Chicago, IL
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000 FXUS63 KLOT 090927 CCA AFDLOT AREA FORECAST DISCUSSION...CORRECTED NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL 424 AM CDT SAT MAY 9 2009 .DISCUSSION... 350 AM CDT THE NEXT SEVERAL DAYS WILL HAVE DRY WEATHER. TODAY THROUGH SUNDAY...THE 500 MB TROUGH WILL BE OVER MICHIGAN BY THIS AFTERNOON. THE SCATTERED LINGERING SHOWERS WILL MOVE EAST OF NORTHERN ILLINOIS AND NORTHWEST INDIANA BY THIS AFTERNOON. THIS IS BASED ON MOVEMENT FROM THE SATELLITE IR IMAGERY LOOPS AND USING THE LOCAL WRF MODEL AND GFS MODEL. A SURFACE LOW HAS INTENSIFIED...WITH THE PRESSURE FALLING 4.1 MB FROM 01 UTC TO 08 UTC. THIS IS 1.0 MB MORE DECREASE IN PRESSURE THAN THE LOCAL WRF OR GFS MODELS SHOW. THE WIND WILL BE FROM THE NORTHWEST TODAY WITH SOME GUST TO 26 MPH. THIS IS BECAUSE THE LOW DECREASES IN PRESSURE AND RESULTS IN AN INCREASE IN THE PRESSURE GRADIENT WEST OF THE LOW. WE WILL USE THE MOS GUIDANCE FROM THE NAM MODELS FOR TEMPERATURE FORECAST. WE NOTICE A 2 DEGREE MEAN ABSOLUTE ERROR OR THE SIZE OF THE ERRORS...FOR THE LAST TEN DAYS IN THE LOCAL WRF AND NAM MODEL TEMPERATURE FORECASTS. THE NAM HAD A LOWER BIAS AND THIS BIAS WAS ON THE COOL SIDE OF THE ACTUAL HIGH TEMPERATURES. WE WILL USE THIS BIAS TO ADJUST THE TEMPERATURE FORECAST FOR TODAY THROUGH MONDAY. THE LOCAL WRF PERFORMED BETTER WITH THE HOURLY DEWPOINT FORECASTS. DRY AND COOLER WEATHER WILL OCCUR ON SUNDAY...AS A LARGE HIGH MOVES INTO NORTHERN ILLINOIS. MONDAY THROUGH TUESDAY...THERE WILL BE ZONAL FLOW OVER THE GREAT LAKES MONDAY NIGHT AT 500 MB. A 500 MB RIDGE BUILDS OVER THE GREAT LAKES DURING TUESDAY. THERE WILL BE A FRONT AND TWO LOWS IN THE GREAT PLAINS BY TUESDAY NIGHT. WE USED THE GFS MODEL BECAUSE OF ITS TIMING WITH THE SUNDAYS SURFACE LOW. THE WEATHER WILL BECOME WARMER ON TUESDAY. A 500 MB TROUGH AND FRONT WILL MOVE TOWARDS ILLINOIS WEDNESDAY. WE EXPECT THUNDERSTORMS ON WEDNESDAY AND SHOWERS ON THURSDAY. BY THURSDAY EVENING ANOTHER HIGH WILL MOVE INTO ILLINOIS. WE EXPECT DRY WEATHER FOR FRIDAY. && .AVIATION... 155 AM CDT 0600 UTC TAFS...AT 05Z UPPER LOW MOVING FROM SOUTHEAST MN INTO WEST CENTRAL WI WITH DEEPENING SURFACE LOW OVER FAR SOUTHWEST WI WITH ALL OF RAIN AND THUNDERSTORMS IN THE WARM AIR ADVECTION/ISENTROPIC LIFT ZONE LIFTING NORTHEASTWARD ACROSS SOUTHEASTERN WI AND SOUTHERN LAKE MI WHILE A COLD FRONT MOVES QUICKLY ACROSS FAR SOUTHWEST WI AND EASTERN IA. MVFR STRATOCU ADVANCING INTO NORTHERN IL WITH THE LOW LEVEL CAA AND WILL CONTINUE TO SPREAD ACROSS THE LOCAL TERMINALS EARLY THIS MORNING. DEEPENING SURFACE LOW RESULTING IN INCREASING SOUTHWEST WINDS AHEAD OF THE COLD FRONT AND MODERATELY STRONG WEST TO WEST-NORTHWEST WINDS GUSTING TO AROUND 30 KNOTS BEHIND THE COLD FRONT. STRONGEST SURFACE PRESSURE RISES AND LOW LEVEL COLD ADVECTION PROGGED ACROSS SOUTHERN WI...NORTHERN IL AND SOUTHERN LAKE MI INTO SOUTHWEST LOWER MI AND NORTHWEST IN THIS MORNING AND EXPECT THE GUSTY WEST TO WEST-NORTHWEST TO SPREAD ACROSS THESE AREAS THOUGH TO DROP OFF PRETTY QUICKLY DURING THE AFTERNOON AS THE SURFACE LOW MOVES RAPIDLY TO THE CHAMPLAIN VALLEY/UPPER ST. LAWRENCE VALLEY BY 00Z. RAIN WRAPPED BACK AROUND THE NORTH AND NORTHWEST SIDE OF THE MID/UPPER LOW TO OVER SOUTHEAST MN AND NORTHEAST IA TO REMAIN NORTH OF THE STATE LINE AS THE SYSTEM LIFTS EAST-NORTHEAST ACROSS THE WESTERN GREAT LAKES TODAY. WITH LOW LEVEL DRYING UNDER WAY ACROSS THE LOCAL AREA TODAY THE MVFR STRATOCU CEILINGS TO LIFT TO VFR AS DEW POINT DEPRESSIONS INCREASE AS A RESULT OF ADVECTION OF MID AND LOWER 40S SURFACE DEW POINTS ACROSS INTO THE AREA AS WELL AS DIURNAL TEMPERATURE RISE. TRS && .MARINE... 424 AM CDT DEEPENING SURFACE LOW MOVING ACROSS FAR SOUTHERN LAKE MI RESULTING IN INCREASING SOUTHWEST WINDS FAR SOUTH AND INCREASING NORTH TO NORTHEAST WINDS CENTRAL AND NORTH PORTIONS EARLY THIS MORNING. AS THE LOW MOVES QUICKLY EASTWARD TO THE EASTERN GREAT LAKES TODAY WINDS WILL DROP OFF PRETTY QUICKLY DURING THE AFTERNOON AS THE PRESSURE GRADIENT ACROSS THE LAKE DROPS OFF SIGNIFICANTLY BY MID TO LATE AFTERNOON WITH THE LOW WELL REMOVED TO THE EAST AND THE CENTER OF A HIGH PRESSURE RIDGE WELL WEST OVER THE CANADIAN PRAIRIE AND THE NORTHERN AND CENTRAL PLAINS. THIS RIDGE TO GRADUALLY DRIFT EAST AND SOUTHEASTWARD OVER THE MS VALLEY...UPPER GREAT LAKES AND OH VALLEY BY SUNDAY NIGHT AND MONDAY RESULTING IN LIGHT WIN REGIME OVER THE UPPER LAKES UNTIL LATE TUESDAY-TUESDAY NIGHT WHEN THE HIGH MOVES OF TO THE EAST AND A TROUGH WILL BE MOVING EAST ACROSS CENTRAL CANADA AND THE PLAINS WITH INCREASING SOUTHEAST TO SOUTH WINDS TUESDAY NIGHT AND WEDNESDAY. TRS && .LOT WATCHES/WARNINGS/ADVISORIES... IL...NONE. IN...NONE. LM...GALE WARNING...LMZ080-LMZ567-LMZ669-LMZ671-LMZ673-LMZ675-LMZ740- LMZ741-LMZ742-LMZ743-LMZ744-LMZ745-LMZ777-LMZ779-LMZ868- LMZ870-LMZ872-LMZ874-LMZ876-LMZ878 UNTIL 4 PM SATURDAY. && $$