Issued by NWS Chicago, IL
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000 FXUS63 KLOT 110856 AFDLOT AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL 356 AM CDT MON MAY 11 2009 .DISCUSSION... 324 AM CDT DRY WEATHER WILL OCCUR TODAY THROUGH TUESDAY. THUNDERSTORMS ARE LIKELY THURSDAY. 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. TUESDAY NIGHT THROUGH THURSDAY MORNING... 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. THURSDAY MORNING THROUGH MONDAY...HIGH PRESSURE WILL MOVE INTO ILLINOIS AND INDIANA. THE TEMPERATURES WILL BE A LITTLE LOWER WITH THE HIGH TEMPERATURE IN MIDDLE TO UPPER 60S ON THURSDAY AND IN THE LOWER 70S FRIDAY. ANOTHER LOW WILL MOVE THROUGH WISCONSIN FRIDAY NIGHT THROUGH SATURDAY. THIS WILL BRING A CHANCE OF SHOWERS AND THUNDERSTORMS FOR THE WEEKEND. && .AVIATION... 0600 UTC TAFS...GLOB OF SC OVER SRN WI HAS BEEN ERODING SLOWLY ALTHO SOME SWD PROGRESS HAS BEEN EVIDENT VIA SATL PIX. THIS SWD ADVANCE BEING OFFSET BY DISSIPATING NEAR THE WI BORDER. WILL LEAVE MENTION OF 5K SC AT RFD BUT NOT TAKE IT INTO A CEILING. THIS STILL A POSSIBILITY HOWEVER. OTHERWISE...WEST TO EAST STREAMING AC/CI IN UPPER LVL CONVERGENCE ZONE FROM THE CENTRAL PLAINS THROUGH ILLINOIS WILL SLOWLY DROP SOUTH OVERNIGHT AND LEAVE CLEAR SKIES AT TERMINALS BY MORNING. LOOKS LIKE ANTICYCLONIC FLOW PREVAILS UNDER SFC HIGH PRESSURE ON MONDAY. MAY BE A FEW CU NEAR 5K CONSIDERING A CONVECTIVE TEMP OF 60 WHILE FORECASTING HIGHS IN THE LOW TO MID 60S. WITH LIGHT NORTHERLY WIND FIELD ON MONDAY...CAN EXPECT ANOTHER EARLY TO MID AFTN LAKE BREEZE TO KICK IN AND SHIFT WINDS TO THE EAST. AGAIN THESE WILL DROP OFF NEAR SUNSET TO LIGHT AND VARIABLE. HAS BEEN DRYING LAST COUPLE OF DAYS AND SEE NO PROSPECT FOR DENSE FOG FORMATION TONIGHT OR MONDAY NIGHT. BUT SINCE IT WILL BE RADIATING VERY WELL OVERNIGHT...WOULD NOT BE SURPRISED TO SEE SOME MORNING GROUND FOG IN FIELDS THAT LIFT BRIEFLY TO LOWER MORNING VSBY DOWN TO 4 OR 5 MILES. RLB && .MARINE... 210 PM...A WEAK AREA OF LOW PRESSURE OVER SOUTHERN LAKE MICHIGAN THIS AFTERNOON WILL MOVE SOUTHEAST ACROSS NORTHERN INDIANA AND OHIO TONIGHT. A LARGE AREA OF HIGH PRESSURE OVER SOUTHERN CANADA AND THE PLAINS WILL MOVE EAST ACROSS THE LAKES REGION AND THE OHIO VALLEY TONIGHT THROUGH MONDAY NIGHT...REACHING THE ATLANTIC COAST BY WEDNESDAY MORNING. LOW PRESSURE WILL MOVE EAST ACROSS SOUTHERN CANADA TUESDAY NIGHT REACHING JAMES BAY BY THURSDAY MORNING. A TRAILING FRONT/TROUGH OF LOW PRESSURE FROM THIS LOW WILL MOVE EAST ACROSS LAKE MICHIGAN WEDNESDAY EVENING. AHEAD OF THIS FRONT...THE GRADIENT WILL TIGHTEN WITH SOUTHERLY WINDS INCREASING TO 15-25KTS WITH SOME HIGHER GUSTS POSSIBLE. WINDS TURN WESTERLY AND DIMINISH SOME BEHIND THE FRONT. CMS && .LOT WATCHES/WARNINGS/ADVISORIES... IL...NONE. IN...NONE. LM...NONE. && $$
