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000 FXUS63 KLOT 121138 AFDLOT AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL 638 AM CDT TUE MAY 12 2009 .DISCUSSION... 403 AM CDT THUNDERSTORM POTENTIAL OVERNIGHT TONIGHT AND THREAT OF STRONG TO SEVERE STORMS WEDNESDAY AFTERNOON AND EARLY EVENING ARE MAIN FORECAST CONCERNS. LITTLE CHANGE IN FORECAST ELEMENTS FOR THE NEXT FEW DAYS NECESSARY AS LATEST MODEL OUTPUT HAVE SIMILAR SOLUTIONS TO PREVIOUS RUNS THOUGH LATEST RUNS HAVE TRENDED SOMEWHAT DEEPER/SHARPER WITH THE UPPER TROUGH AS IT MOVES EAST ACROSS THE SOUTHERN CANADIAN PRAIRIE AND THE NORTHERN PLAINS DURING WEDNESDAY. STILL...THE STRONG COLD FRONT IS STILL PROGGED TO BE POSITIONED FROM SOUTH CENTRAL WI ACROSS NORTHWEST AND FAR WEST CENTRAL IL AT 00Z THU. THREAT OF THUNDERSTORMS TO INCREASE FOR OVERNIGHT AS STRONG SOUTH TO SOUTHWEST FLOW IN PLACE ACROSS THE PLAINS EARLY THIS MORNING ADVANCES EASTWARD ACROSS THE MS VALLEY DURING THE EVENING AS THE WESTERN UPPER TROUGH PROGRESSES TO OVER THE NORTHERN AND CENTRAL ROCKIES. THIS RESULTS IN STRONG MID AND EVENTUALLY LOWER LEVEL THETA E ADVECTION AND ISENTROPIC LIFT ACROSS THE REGION. MODELS IN GOOD AGREEMENT IN LIFTING A WARM FRONT NORTHEASTWARD THROUGH THE LOCAL FORECAST DURING WEDNESDAY MORNING TEMPORARILY ENDING THE THUNDERSTORM CHANCES BEFORE STRONG LOW LEVEL CONVERGENCE AHEAD OF THE COLD FRONT APPROACHING FROM THE NORTHWEST AND STRONG LIFT IN DIVERGENT MID-UPPER LEVEL FLOW SPREADS EAST ACROSS THE MID MS VALLEY AHEAD OF THE UPPER TROUGH AXIS MOVING FROM THE PLAINS OVER THE MO RIVER VALLEY LATER IN THE DAY. BASED ON STABILITY AND ENERGY PARAMETERS AND WHERE LOW LEVEL AND UPPER LEVEL FORCING INTERSECT AGREE WITH SPC ASSESSMENT OF AREA OF GREATEST RISK OF STRONG TO SEVERE BEING ACROSS CENTRAL IL. SEE DAY 2 CONVECTIVE OUTLOOK /SWODY2/ FOR FURTHER DETAILS. COLD FRONT TO HAVE PASSED THROUGH ALL BUT FAR EASTERN AND SOUTHEASTERN PORTION OF THE FORECAST AREA BY 06Z THU WITH POP VALUES DECREASING WEST TO EAST DURING THE NIGHT AS THE MID-UPPER TROUGH PASSES BY DURING THE OVERNIGHT. LINGERING ANOTHER SHORT WAVE TO BE MOVING ACROSS SOUTHERN CANADA AND THE NORTHERN AND CENTRAL PLAINS FRIDAY DEVELOPING A STRONG WARM AIR ADVECTION PATTERN WITH SHIFTING EAST FROM THE PLAINS ACROSS THE MID MS VALLEY DURING FRIDAY MORNING BRINGING THE THREAT OF MORE SHOWERS AND THUNDERSTORMS BACK TO THE AREA. TRS && .AVIATION... 1200 UTC TAFS...THE HIGH OVER SOUTHERN MICHIGAN WILL MOVE EAST TO THE EASTERN PART OF LAKE ERIE BY 00 UTC. THE WIND WILL BECOME SOUTHEAST AND THE SPEED WILL INCREASE TO 15 KNOTS. THE CLOUDS WILL INCREASE WITH A CEILING OF 20000 F00T BROKEN CIRROSTRATUS THIS AFTERNOON. THEN 8000 FOOT BROKEN ALTOCUMULUS CLOUDS WILL MOVE IN AFTER 04 UTC TONIGHT. ONCE THE INVERSION DISSIPATES LATER THIS MORNING...A SOUTHEAST WIND WILL INCREASE TO 10 TO 15 KNOTS. THE INTENSE LOW... THAT WILL MOVE FROM MANITOBA TO ONTARIO TONIGHT...WILL CREATE A LARGE PRESSURE GRADIENT. THIS WILL KEEP THE WIND SPEED IN THE 10 TO 15 KNOT RANGE WITH GUSTS OVER NIGHT. THE FORECAST MODELS SUCH AS THE GFS AND OUR LOCALLY RUN WRF MODELS SHOW PRECIPITATION WEDNESDAY MORNING. THE MORNING THUNDERSTORMS WILL BE ALONG A WARM FRONT THAT WILL MOVE NORTH THROUGH NORTHERN ILLINOIS AND NORTHWEST INDIANA. WE WILL FORECAST TEMPO THUNDERSTORMS FROM 11 UTC TO 12 UTC. THUNDERSTORMS WILL END AT OHARE AIRPORT AROUND 15 UTC WEDNESDAY. && .MARINE... 224 AM CDT A LARGE HIGH WAS OVER SOUTHWEST MICHIGAN THIS EARLY MORNING. THIS HIGH WILL MOVE EAST TO NEW YORK STATE BY WEDNESDAY MORNING. A LOW WILL MOVE INTO SOUTHERN MANITOBA AND NORTHERN MINNESOTA WEDNESDAY MORNING. THE PRESSURE GRADIENT BETWEEN THE HIGH AND THE LOW WILL INCREASE OVER LAKE MICHIGAN. GALES COULD OCCUR WEDNESDAY. HOWEVER THERE IS AN INVERSION OVER LAKE MICHIGAN AND THIS WILL BLOCK ANY TRANSFER OF ENERGY FROM THE FASTER WIND ALOFT. WE WILL FORECAST WIND SPEED JUST BELOW GALE. WE WILL NOT ISSUE ANY GALE WARNING AT THIS TIME. THE LOW WILL MOVE TO NORTHERN ONTARIO BY THURSDAY MORNING. A COLD FRONT TRAILING SOUTH FROM THE LOW WILL WILL MOVE ACROSS LAKE MICHIGAN. THE WIND THEN...BE NORTHWEST AND THE SPEED WILL DIMINISH && .LOT WATCHES/WARNINGS/ADVISORIES... IL...NONE. IN...NONE. LM...SMALL CRAFT ADVISORY...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744- LMZ745...7 AM WEDNESDAY TO 7 AM THURSDAY. && $$