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000 FXUS63 KLOT 111718 AFDLOT AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL ISSUED BY NATIONAL WEATHER SERVICE LINCOLN IL 1218 PM CDT MON MAY 11 2009 .DISCUSSION... 324 AM CDT...LONG TERM UPDATED AT 1218 PM CDT DRY WEATHER WILL OCCUR TODAY THROUGH TUESDAY. THUNDERSTORMS ARE LIKELY THURSDAY. SHORT TERM... TODAY THROUGH TUESDAY...DRY WEATHER WILL CONTINUE AS A HIGH MOVES ACROSS ILLINOIS AND INDIANA. WE USED THE MOS GUIDANCE FOR THE HIGH AND LOW TEMPERATURE FORECASTS. THE MOS GUIDANCE HAD THE LOWEST AVERAGE BIAS FOR THE LAST 10 DAYS. SINCE MOST OF THE MODELS WERE RUNNING A COOL BIAS WE RAISED THE TEMPERATURE VALUES UP A LITTLE. THE LOCAL WRF AND MOS GUIDANCE WERE USED IN THE DEWPOINT FORECASTS. THE GFS MODEL WAS USED FOR FORECAST MOVEMENT OF UPPER AIR AND SURFACE FEATURES. THE MODEL HAS BEEN CONSISTENT WITH WEATHER PATTERNS SEEN ON IR SATELLITE IMAGERY. THE SYNOPTIC WIND WILL BE LIGHT ACROSS NORTHEAST ILLINOIS TODAY. BOTH THE NAM AND OUR LOCAL WRF MODELS SHOW A LAKE BREEZE THIS AFTERNOON. WE WILL FORECAST A LOWER HIGH TEMPERATURE ALONG LAKE MICHIGAN TODAY. TUESDAY NIGHT THROUGH THURSDAY...THE HIGH MOVES EAST AND GULF MOISTURE RETURNS. A WAVE TROUGH MOVES INTO IOWA NORTH TO WESTERN ONTARIO BY 00 UTC THURSDAY. THIS TROUGH HAS A LARGE HEIGHT GRADIENT AND THERE IS A LOT VORTICITY FORECAST WITH THIS TROUGH. THIS WILL GIVE A STRONG SOUTH LOW LEVEL WIND INTO NORTHEAST ILLINOIS AND NORTHWEST INDIANA. ON THE SURFACE WE EXPECT A STRONG COLD FRONT. THERE WILL BE THUNDERSTORMS WEDNESDAY THROUGH EARLY THURSDAY MORNING. SOME OF THE THUNDERSTORMS COULD BE VERY STRONG. LONG TERM... THURSDAY NIGHT THROUGH MONDAY...MODELS/ENSEMBLES IN REASONABLE AGREEMENT AT THE START OF THE PERIOD...BUT MODEL SPREAD INCREASES FROM THERE ON OUT. THE MODEL DIFFERENCES ARISE AS THEY STRUGGLE WITH A TRANSITION FROM ZONAL FLOW OVER THE LOWER 48...TO A MUCH MORE AMPLIFIED RIDGE WEST/TROF EAST PATTERN ACROSS NORTH AMERICA. AT THIS POINT...ONLY ONE SYSTEM OF CONSEQUENCE IS EXPECTED TO IMPACT THE REGION DURING THE PERIOD...AND IT HAS BEEN TRENDING SLOWER OVERALL THE PAST FEW RUNS. A WARM FRONT...ASSOCIATED WITH DEVELOPING LOW PRESSURE OVER THE PLAINS...WILL LIFT NORTH THROUGH THE REGION FRIDAY/FRIDAY NIGHT WITH AN ASSOCIATED RISK OF THUNDERSTORMS. AS THE LOW LIFTS INTO THE GREAT LAKES REGION ON SATURDAY...THE TRAILING COLD FRONT WILL SWING THROUGH THE FORECAST AREA SATURDAY/SATURDAY EVENING AND BRING A CONTINUED RISK OF SHOWERS/STORMS WITH IT. QUIETER/COOLER WEATHER WILL FOLLOW THE COLD FRONT SATURDAY NIGHT AND LINGER THROUGH THE REST OF THE PERIOD AS HIGH PRESSURE DOMINATES AT THE SURFACE. && .AVIATION... 1800 UTC TAFS... LARGE PATCH OF MID LEVEL CLOUDS AFFECTING THE CHICAGOLAND TAF SITES HAS STARTED BREAKING UP A BIT...JUST IN TIME FOR DIURNAL CUMULUS TO FORM. AM EXPECTING BROKEN CLOUD COVER TO CONTINUE THROUGH THE AFTERNOON...BEFORE FADING EARLY EVENING. LATEST NAM- WRF MODEL CONTINUES TO SHOW THE LAKE BREEZE PUSHING WESTWARD THROUGH THE AFTERNOON...WITH WINDS TRENDING TOWARD THE SOUTHEAST THROUGH THE NIGHT AS HIGH PRESSURE PUSHES OUT OF THE AREA. MODEL CROSS-SECTIONS SHOWING MAINLY CLEAR SKIES TO PREVAIL OVERNIGHT... WITH JUST SOME HIGH CLOUDS BEGINNING TO MOVE IN TOWARD THE END OF THE TAF PERIOD. GEELHART && .MARINE... 343 AM CDT LIGHT VARIABLE WINDS ON LAKE MI THE FIRST PART OF TODAY WITH BROAD HIGH PRESSURE EXTENDING FROM THE PLAINS TO THE CENTRAL GREAT LAKES. SYNOPTIC FLOW FAVORS A SOUTH TO SOUTHEAST DIRECTION LATE TODAY AS THE ANTICYCLONE WILL BE CENTERED OVER THE SOUTHERN GREAT LAKES AND OHIO VALLEY TONIGHT. LOCAL INFLUENCES AND LAKE BREEZES WILL LIKELY OBSCURE THIS WEAK SYNOPTIC FLOW AND PREVAIL BY MIDDAY. WITH THE HIGH MOVING EAST OVER NEW ENGLAND TUESDAY AND TUESDAY NIGHT AND LOW PRESSURE DEEPENS AS IT MOVES EAST SOUTHEAST TO FAR SOUTHERN MANITOBA BY WEDNESDAY MORNING WINDS OVER THE WESTERN GREAT LAKES WILL FRESHEN OUT OF THE SOUTHEAST AND SOUTH THOUGH VERY STRONG WARM AIR ADVECTION TO BE TAKING PLACE WITH 30 TO 40 KT SOUTH WINDS PROGGED AT 1K FT ABOVE THE SURFACE BY 00Z WED...STRENGTHENING FURTHER TO 35 TO 45 KTS BY WED MORNING. HOWEVER...STRONG INVERSION DUE TO THE COLD WATER WILL SEVERELY LIMIT MOMENTUM TRANSFER DOWN TO THE NEAR THE WATER SURFACE. MODELS DEPICT THE LOW TURNING TO THE NORTHEAST DURING WED AND AS IT APPROACHES JAMES BAY LATE IN THE DAY A STRONG COLD FRONT TRAILING SOUTHWESTWARD FROM THIS LOW BACK INTO THE PLAINS WILL PUSH EAST ACROSS THE UPPER MIDWEST...AND INTO THE WESTERN GREAT LAKES WEDNESDAY EVENING. BY THE TIME LOW LEVEL AIR COLD ENOUGH REMOVE THE INVERSION ARRIVES OVER LAKE MICHIGAN DURING THE DAY THURSDAY...THE CENTER OF THE LOW IS FAR REMOVED HAVING MOVED TO NORTHERN QUEBEC BY THEN. THE RESULT IS A MUCH RELAXED PRESSURE GRADIENT OVER THE WESTERN LAKES BY MIDDAY AND THUS ONLY MODERATE WEST WINDS OF 20 KTS AND DECREASING WITH TIME EXPECTED THU. TRS && .LOT WATCHES/WARNINGS/ADVISORIES... IL...NONE. IN...NONE. LM...NONE. && $$
