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000 FXUS63 KLOT 091924 AFDLOT AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL 224 PM 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 DEW POINT 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... 18Z TAFS...MAIN CONCERN THIS PERIOD ARE WINDS INTO THIS EVENING AND CIGS. LOW PRESSURE RESPONSIBLE FOR STRONG WINDS EARLIER THIS MORNING HAS RAPIDLY MOVED EAST AND IS NOW LOCATED NEAR LAKE ONTARIO. PRESSURE GRADIENT ACROSS THE AREA HAS WEAKENED QUITE A BIT...SO MUCH SO THAT A LAKE BREEZE FORMED A FEW HRS AGO AND HAS SINCE PUSHED WEST THRU ORD/MDW AS WELL AS GYY. WIND DIRECTION HAS TURNED NORTHEAST WITH SPEEDS GENERALLY 10KTS OR LESS. THE NAM MODEL SEEMS TO BE HANDLING THIS LAKE BREEZE AND ASSOCIATED DIRECTIONS THE BEST BUT INDICATES THE LAKE BREEZE MAY SLOW OR EVEN STALL. STILL SEEMS TO HAVE A STEADY SPEED AS IT MOVES INTO DUPAGE AND WILL COUNTIES SO HAVE GONE WITH A PREVAILING NORTHEASTERLY WIND AT ORD/MDW/GYY BUT HELD SHORT OF DPA AND WILL NEED TO MONITOR ITS MOVEMENT OVER THE NEXT FEW HRS. AS SUNSET APPROACHES THE LAKE BREEZE WILL WASH OUT...WHERE EVER IT ENDS UP...AND AT SOME POINT DURING THE EARLY/MID EVENING...EXPECT WINDS TO TURN BACK WEST OR NORTHWESTERLY BUT UNDER 10KTS. AS HIGH PRESSURE BUILDS ACROSS THE REGION TONIGHT...WINDS WILL LIKELY GO LIGHT WESTERLY OR LIGHT AND VRB BY MORNING. WEST/NORTHWEST WINDS INCREASE TO 10 TO 15 KTS SUNDAY MORNING BUT EXPECT ANOTHER LAKE BREEZE TO MOVE THRU THE TERMINALS EARLY/MID AFTERNOON. CIGS HAVE GENERALLY GONE VFR AT MOST OF THE TAFS. LOWER CLOUDS IN THE 1-2KFT RANGE ACROSS SOUTHERN WI ARE SLOWLY LIFTING... ESPECIALLY AS THEY MOVE SOUTH INTO NORTHERN IL WHERE SFC TEMPS ARE FEW DEGREES WARMER. OVERALL...EXPECT PREVAILING 3-4KFT CIGS BUT CAN/T COMPLETELY RULE OUT SOME HIGH MVFR...MAINLY THRU MID AFTERNOON. 12Z MODELS ALSO HINT AT SOME SHOWERS DEVELOPING OVER NORTHEAST IL LATER THIS AFTERNOON...20Z-23Z...THEN QUICKLY SHIFTING SOUTHEAST. INSTABILITY RATHER LIMITED ALONG WITH AN INVERSION IN THE 700-800 MB LEVEL OFF MODEL SOUNDINGS. LAKE BREEZE MIGHT PROVIDE ENOUGH SFC CONVERGENCE FOR SOME ISOLATED ACTIVITY. THOUGH CONFIDENCE TOO LOW TO INCLUDE IN THIS FORECAST. CMS && .MARINE... 224 PM...LOW PRESSURE OVER NEW YORK WILL CONTINUE TO QUICKLY LIFT ACROSS NEW ENGLAND TONIGHT. BROAD BUT WEAK HIGH PRESSURE OVER THE NORTHERN AND CENTRAL PLAINS WILL MOVE EAST ACROSS THE LAKES AND THE OHIO VALLEY SUNDAY INTO MONDAY AS IT STRENGTHENS. BY TUESDAY... THIS HIGH WILL EXTEND FROM NEW ENGLAND TO THE SOUTHEAST U.S. AS IT CONTINUES TO SLOWLY MOVE EAST. THIS HIGH WILL ALLOW A LAKE BREEZE TO FORM ACROSS THE IL/IN NEARSHORE SUNDAY AND MONDAY AFTERNOONS. A TROUGH OF LOW PRESSURE WILL DEVELOP OVER THE PLAINS TUESDAY NIGHT INTO WEDNESDAY WITH DEEP LOW PRESSURE MOVING NORTHEAST ACROSS ONTARIO WEDNESDAY INTO WEDNESDAY NIGHT. CMS && .LOT WATCHES/WARNINGS/ADVISORIES... IL...NONE. IN...NONE. LM...SMALL CRAFT ADVISORY...NEARSHORE WATERS UNTIL 1 AM SUNDAY. && $$
