Issued by NWS Chicago, IL
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000 FXUS63 KLOT 100851 AFDLOT AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL 351 AM CDT SUN MAY 10 2009 .DISCUSSION... 344 AM CDT SCATTERED LIGHT SHOWERS MAY OCCUR TODAY AND THUNDERSTORM ARE LIKELY WEDNESDAY. THE HIGH TEMPERATURE WILL CLIMB INTO THE 70S WEDNESDAY AND SATURDAY. TODAY...THE 500 MB ANALYSIS AT 00 UTC SHOWS VERY COLD AIR MOVING INTO NORTHERN ILLINOIS AND NORTHERN INDIANA. THE COLD AIR ALOFT WILL CREATE SOME VERY STEEP LAPSE RATES THIS MORNING WHEN THE SUN IS HEATING THE GROUND. THE NAM MODEL AND LOCAL WRF MODEL SHOW THIS INSTABILITY IN THEIR BUFKIT FORECAST SOUNDINGS. A 3 KM WRF MODEL AND OUR LOCAL WRF SHOWS SCATTERED SMALL AMOUNTS OF PRECIPITATION ACCUMULATING THIS AFTERNOON. THE AMOUNTS ARE VERY LOW. THE LOW DEWPOINTS WILL PREVENT A LOT OF SHOWERS OR HEAVY RAIN. WE ARE FORECASTING SCATTERED VERY LIGHT SHOWERS THIS MORNING AND ISOLATED SHOWERS THIS AFTERNOON. THE HIGH WILL SETTLE OVER MISSOURI AND ILLINOIS TONIGHT. THE COLDEST AIR WILL BE OVER NORTHERN ILLINOIS TONIGHT AND MONDAY MORNING. SOME LOW TEMPERATURES MAY REACH A LITTLE BELOW 38 DEGREES FAHRENHEIT. WILL ADD SOME PATCHY FROST FOR THE NORTHERN COUNTIES...AROUND THE AURORA VALLEY AND IN EASTERN PORTER COUNTY FOR TONIGHT. MONDAY THROUGH TUESDAY...A LARGE HIGH MOVES ACROSS ILLINOIS AND INDIANA MONDAY AND MONDAY NIGHT. THE SURFACE WIND WILL BECOME SOUTH BY TUESDAY MORNING. THE 850 MB FORECAST FROM THE GFS SHOWS AN INCREASING SOUTH WIND AND WARM AIR ADVECTION. THE HIGH TEMPERATURES WILL BE IN THE UPPER 60S TO LOWER 70S TUESDAY. THE 850 MB WIND ARE FORECAST TO INCREASE TO 40 KNOTS LATE TUESDAY NIGHT. THERE IS FORECAST A LARGE INCREASE IN MOISTURE TRANSPORT FROM THE SOUTH. THE SURFACE TO 850 MB SHEAR IS FORECAST TO INCREASE AS WELL. WE WILL ADD A CHANCE OF THUNDERSTORMS TUESDAY NIGHT. THIS IS AHEAD OF A STRONG COLD FRONT THAT WILL APPROACH ILLINOIS TUESDAY NIGHT. WEDNESDAY THROUGH SATURDAY...A STRONG COLD FRONT WILL MOVE THROUGH OUR FORECAST AREA WEDNESDAY. THE FORECAST HODOGRAPHS SHOW SOME SIGNIFICANT SHEAR OVER NORTHEAST ILLINOIS WEDNESDAY AFTERNOON. CAPE VALUES REACH 1350 J/KG AROUND FORD COUNTY. THERE WILL BE THUNDERSTORMS AND SOME OF THE THUNDERSTORMS MAY BE VERY STRONG SOUTH OF A LINE FROM HEBRON INDIANA TO STREATOR ILLINOIS. THIS IS ONLY AN OUTLOOK. A HIGH MOVES OVER ILLINOIS AND INDIANA THURSDAY AND FRIDAY. A WARM FRONT RETURNS FRIDAY. THE RETURNING WARM AIR AND INSTABILITY WILL GIVE A CHANCE OF THUNDERSTORMS FOR SATURDAY. THE HIGH TEMPERATURE WILL BE IN TH 60S THURSDAY AND FRIDAY AND IN THE 70S SATURDAY. && .AVIATION... 139 AM CDT 0600 UTC TAFS...MODELS SHOW MID LEVEL LOW CURRENTLY OVER CENTRAL MN OPENING AND WEAKENING AS IT CONTINUES MOVING SOUTHEASTWARD ACROSS WI AND LAKE MI TODAY...WITH AREA OF ISENTROPIC LIFT IN 050-100 LAYER AHEAD OF THE SHORT WAVE MOVING ACROSS THE LOCAL TERMINALS THIS MORNING AND EARLY AFTERNOON. EXPECT PATCHY CLOUDS IN THIS LAYER CURRENTLY JUST UPSTREAM ACROSS IA...WESTERN WI AND SOUTHERN MN TO CONTINUE TO ADVANCE SOUTHEASTWARD DURING THE EARLY MORNING. WEAK SURFACE LOW REFLECTION OF MID LEVEL LOW LOCATED IN WEST CENTRAL WI PROGGED TO BECOME EVEN MORE ILL-DEFINED WITH TIME TODAY AS SHORT WAVE TROUGH WEAKENS AS IT APPROACHES SOUTHEAST WI. MODELS DO INDICATE A WEAK SURFACE TROUGH/COLD FRONT DROPPING SOUTH THROUGH NORTHERN IL DURING THE AFTERNOON AS THE SHORT WAVE MOVES OVER SOUTHERN LAKE MI. THIS WILL SHIFT SYNOPTIC LOW LEVEL FLOW TO NORTHWESTERLY AT THE TERMINALS...WITH DIRECTION VEERING AROUND TO NORTHEASTERLY DUE TO WEAK GRADIENT AND LAND/LAKE DIFFERENTIAL HEATING. HEATING WILL WARM SURFACE TO CONVECTIVE TEMPERATURE OF 15C WITH HIGHER BASED CU DEVELOPING MIDDAY THEN DISSIPATING WITH SUNDOWN. WEAK PRESSURE GRADIENT OVER AREA WILL RESULT IN LIGHT OR NIL WIND AFTER SUNSET. TRS && .MARINE... 243 AM CDT LIGHT WINDS TODAY AND TONIGHT AS WEAK LOW PRESSURE FILLS AS IT MOVES SOUTHEASTWARD ACROSS WI TODAY...AND AS HIGH PRESSURE CENTERED OVER SOUTH CENTRAL CANADA SETTLES SOUTHEASTWARD ACROSS THE DAKOTAS AND UPPER MIDWEST TODAY AND THE MS VALLEY AND WESTERN GREAT LAKES TONIGHT. THE AREA OF HIGH PRESSURE SHIFTS EASTWARD ACROSS THE GREAT LAKES AND OHIO VALLEY MONDAY NIGHT. WINDS BECOME SOUTHEASTERLY AND INCREASE MONDAY NIGHT AND TUESDAY AS LOW PRESSURE MOVES EAST ACROSS THE CANADIAN PRAIRIE. AS THE LOW CONTINUES ON INTO ONTARIO DURING WEDNESDAY A TRAILING COLD FRONT WILL PUSH EAST THROUGH THE WESTERN GREAT LAKES VEERING WINDS ON LAKE MI FROM SOUTHEAST TO SOUTH THEN TO SOUTHWEST WITH THE PASSAGE OF THE COLD FRONT LATE IN THE DAY. TRS && .LOT WATCHES/WARNINGS/ADVISORIES... IL...NONE. IN...NONE. LM...NONE. && $$
