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000 FXUS63 KLOT 132344 AFDLOT AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL 644 PM CDT WED MAY 13 2009 .DISCUSSION... MESOSCALE UPDATE 542 PM CDT APPEARS AS THOUGH LARGE SCALE ASCENT COMBINED WITH CONTINUED LOW LEVEL THETA-E ADVECTION IS LEADING TO AN INCREASINGLY UNSTABLE ENVIRONMENT. SERIES OF ACARS SOUNDINGS OUT OF ORD FROM 20Z THROUGH 22Z SHOWS CONSIDERABLE COOLING BETWEEN 900-700MB (ON THE ORDER OF 2C+)...LIKELY INDICATIVE OF THE STRONG SYNOPTIC ASCENT LIFTING THROUGH THE INVERSION. LOW LEVEL FLOW HAS INCREASED CONSIDERABLY ON WINCHESTER PROFILER WITH VERY IMPRESSIVE LOW LEVEL SHEAR PROFILE. IN FACT...SPC MESOANALYSIS PAGE SHOWS A MAXIMUM OF 0-1KM SRH GREATER THAN 300 M2/S2 OVER WESTERN PORTIONS OF THE STATE. RUC FORECASTS SUGGEST THAT THIS VERY STRONG LOW LEVEL SHEAR WILL SPREAD EASTWARD OVER NORTHERN ILLINOIS THIS EVENING. GIVEN VERY MOIST LOW LEVEL ENVIRONMENT AND CURRENT DISCRETE NATURE OF THE STORMS...AND THE LIKELIHOOD THAT STORMS ARE BECOMING INCREASINGLY SURFACE BASED THE TORNADO POTENTIAL MAY BE A BIT HIGHER THAN PREVIOUSLY EXPECTED FOR A COUPLE/FEW HOUR WINDOW EARLY THIS EVENING. IZZI PREV DISCUSSION... 330 PM CDT SEVERE WEATHER POTENTIAL THIS EVENING AND RAINFALL AMOUNTS WILL BE THE MAIN FORECAST CONCERNS. SOME RESIDUAL SHOWERS PERSIST ACROSS MAINLY SOUTHEAST PORTIONS OF THE FORECAST AREA. THUNDERSTORMS FROM EARLIER TODAY HAVE ACTED TO SLOW THE NORTHWARD PROGRESS OF THE MAIN SURFACE WARM FRONT. WARM FRONT MAINLY IDENTIFIED BY DEW POINT FIELD APPROACHING FAR SOUTHERN PORTIONS OF THE CWA THIS AFTERNOON...WHILE IT APPEARS ANOTHER OUTFLOW ENHANCED BOUNDARY LINGERS WELL SOUTH OF THE AREA JUST NORTH OF STL. CLOUDS AND PRECIPITATION ASSOCIATED WITH MORNING SHOWERS/STORMS HAVE LIMITED SURFACE BASED INSTABILITY WHICH IS EVIDENT IN REGIONAL 18Z SOUNDINGS. A RATHER IMPRESSIVE UPPER LEVEL SHORT WAVE ACROSS THE DAKOTAS WILL APPROACH THE MID/UPPER MS RVR VALLEY THIS EVENING INCREASING LARGE SCALE ASCENT AND ACTING TO COOL MID LEVELS. EXPECTING THE MAIN COLD FRONT TO BECOME MORE ACTIVE TOWARD 22Z-00Z AS THIS TROUGH APPROACHES. MAX INSTABILITY MAY TEND TO ALIGN IN FAIRLY NARROW CORRIDOR ALONG AND JUST AHEAD OF THE FRONT IN RESPONSE TO STEEPENING LAPSE RATES WITH COOLING MID LEVEL TEMPS AND SECONDARY LOW LEVEL MOISTURE ADVECTION SURGE AHEAD OF THE FRONT. ONCE THUNDERSTORMS INITIATE LATER THIS AFTERNOON EXPECTING SEVERE WEATHER THREAT TO INCREASE AND POSSIBLY EVOLVE INTO A SQUALL LINE FEATURE WITH POSSIBILITY OF SUPERCELLULAR STRUCTURES EMBEDDED IN THE LINE GIVEN FAVORABLE SHEAR. CANNOT COMPLETELY DISCOUNT POSSIBILITY OF MORE DISCRETE DEVELOPMENT WITH THE NORTHWARD MOVING WARM FRONT ACROSS THE AREA LATE THIS AFTERNOON...BUT POTENTIAL OF THIS ACTIVITY BEING SURFACE BASED APPEARS TO BE MINIMAL FROM THE SAMPLING OF THE 18Z RAOBS. EXPECTING THAT CONVECTIVE LINE SHOULD BE MOVING OUT OF FAR SOUTHEAST PORTIONS OF THE CWA SHORTLY AFTER MIDNIGHT WITH SKIES CLEARING OVERNIGHT. SEVERE WEATHER THREATS STILL APPEAR TO RUN THE FULL RANGE FROM HIGH WINDS...HAIL...AND POSSIBILITY OF ISOLATED TORNADOES. FOR MORE DETAILS REGARDING SEVERE POTENTIAL...PLEASE SEE MESOSCALE DISCUSSION BELOW. LOW LEVEL FRONT WILL STALL SOUTH OF THE FORECAST AREA TODAY SETTING UP A QUIET DAY TOMORROW AS A SURFACE HIGH BUILDS IN FROM THE WEST. LOW LEVEL WESTERLIES WILL BE DIMINISHING TOMORROW WHICH MAY ALLOW A LAKE BREEZE TO FORM CLOSE TO THE SHORELINE. LOW LEVEL MODEL 900 HPA THERMAL PROGS MIX DOWN TO THE SURFACE GENERALLY SUPPORT HIGHS FROM THE LOW 70S FAR SOUTH TO MID TO UPPER 60S NORTH FOR THURSDAY. THE NEXT SIGNIFICANT UPPER LEVEL TROUGH IS ALREADY EVIDENT ON WATER VAPOR IMAGERY ENTERING PACIFIC NORTHWEST COAST AT THIS TIME AND IS EXPECTED TO TAKE A SIMILAR TRACK TO THE SHORT WAVE MOVING THROUGH THE REGION THIS EVENING. ENHANCED SOUTHERLIES AHEAD OF THIS SHORT WAVE WILL ALLOW THE STALLED BOUNDARY TO THE SOUTH TO LIFT BACK NORTHWARD AS A WARM FRONT FRIDAY WITH STRONG ISENTROPIC ASCENT AND INCREASING LOW LEVEL MOISTURE CONVERGENCE ESPECIALLY ACROSS SOUTHERN PORTIONS OF THE FORECAST AREA BY FRIDAY MORNING. HAVE INCREASED POPS A BIT FROM PREVIOUS FORECAST FOR FRIDAY. THIS SYSTEM FOR FRIDAY PERHAPS NOT QUITE AS DYNAMIC AS THE SYSTEM THIS EVENING AND CURRENTLY APPROXIMATELY SOUTHERN HALF IS OUTLOOKED IN SLIGHT RISK FOR DAY 3. FORECAST AREA WILL GET GRAZED BY UNSEASONABLY COLD AIR MASS BEHIND MOVING ACROSS ONTARIO SATURDAY BEHIND THIS SYSTEM...AND THE COOLEST DAY OF THE PERIOD IS EXPECTED FOR SATURDAY. BROAD SURFACE HIGH WILL BUILD INTO THE GREAT LAKES REGION FOR SUNDAY WITH DRY CONDITIONS. MEDIUM RANGE MODELS ARE BEGINNING TO COME INTO A BIT BETTER AGREEMENT FOR EXTENDED PERIOD WITH THE 12Z ECMWF TRENDING MORE TOWARD NCEP ENSEMBLE MEANS/DETERMINISTIC GFS IN SLOWER PROGRESSION WITH NEXT WESTERN CONUS TROUGH AND MORE IN THE WAY OF BROAD DOWNSTREAM RIDGING FROM THE NORTHERN PLAINS INTO THE GREAT LAKES REGION. THIS WOULD SUPPORT DRY CONDITIONS AND MODERATING TEMPERATURES THROUGH THE END OF THE PERIOD. MARSILI && .AVIATION... 00Z TAFS...FORECAST FOCUS THIS PERIOD IS CONVECTION THROUGH EARLY THURSDAY MORNING. ISOLATED/SCATTERED SHOWERS/TS DEVELOPED ACROSS NORTHERN IL IN THE LAST FEW HRS AS THE WARM FRONT CONTINUES TO LIFT NORTH. THIS ACTIVITY SHOULD CONTINUE TO LIFT NORTHEAST OF THE TAFS IN THE NEXT 1-2HRS. MEANWHILE...THE MORE WIDESPREAD AND SIGNIFICANT THUNDERSTORM ACTIVITY AS DEVELOPED ALONG/AHEAD OF THE COLD FRONT FROM SOUTHWEST WI...EASTERN IA INTO NORTHERN MO. MODELS HAVE BEEN IN GOOD AGREEMENT SHOWING THIS LINE QUICKLY MOVING EAST ACROSS THE AREA LATER THIS EVENING. THUS...EXPECT A BREAK IN THE CONVECTION ACROSS NORTHERN IL FOR A FEW HRS THIS EVENING. ONCE THE MAIN LINE ARRIVES...EXPECT A PERIOD OF VERY HEAVY RAIN ALONG WITH STRONG GUSTY WINDS AND LIKELY A BRIEF PERIOD OF IFR OR LIFR CIGS/VIS. COLD FRONT SHIFTS EAST BY EARLY THURS MORNING...06Z OR 07Z...SHIFTING WINDS WESTERLY WITH CLEARING SKIES. WESTERLY WINDS CONTINUE THURSDAY...THOUGH SPEEDS DIMINISH BY LATE AFTERNOON WITH A LAKE BREEZE POSSIBLE BY LATE AFTERNOON OR EARLY EVENING. CMS && .MARINE... 345 PM CDT GUSTY SOUTH WINDS WILL CONTINUE THIS EVENING IN ADVANCE OF A COLD FRONT APPROACHING FROM THE WEST. THIS FRONT WILL TRACK ACROSS LAKE MICHIGAN LATE TONIGHT SHIFTING WINDS TO THE WEST. WINDS WILL DIMINISH THROUGH THE DAY ON THURSDAY AS A RIDGE OF HIGH PRESSURE BUILDS ACROSS THE WESTERN GREAT LAKES BY THURSDAY NIGHT INTO FRIDAY. THIS RIDGE OF HIGH PRESSURE WILL MOVE OFF TO THE EAST THURSDAY NIGHT INTO EARLY FRIDAY SETTING UP A RETURN OF SOUTH SOUTHEAST WINDS. ANOTHER TROUGH OF LOW PRESSURE WILL MOVE ACROSS LAKE MICHIGAN LATE FRIDAY NIGHT INTO EARLY SATURDAY...WITH A RETURN TO WEST NORTHWEST WINDS BEHIND THE FRONT. && .LOT WATCHES/WARNINGS/ADVISORIES... IL...NONE. IN...NONE. LM...SMALL CRAFT ADVISORY...NEARSHORE WATERS UNTIL 7 AM THURSDAY. && $$
