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000 FXUS63 KLOT 110848 AFDLOT AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL 348 AM CDT MON MAY 11 2009 .DISCUSSION... 311 PM CDT MAIN FORECAST FOCUS TODAY IS WITH ESCORTING CURRENT SHORTWAVE OUT OF THE AREA THIS EVENING...FOLLOWED BY NEXT...MUCH MORE SIGNIFICANT...WAVE PROGGED TO ARRIVE MIDWEEK. CONSIDERABLE SUNSHINE THIS AFTERNOON HAS ALLOWED HIGHS TO REACH THE CONVECTIVE TEMPS...WITH RECENT ACARS SOUNDINGS FROM ORD AND SPC`S MESOANALYSIS PAGE BOTH SUGGESTING SBCAPES HAVE CLIMBED TO JUST OVER 250 J/KG WITH LITTLE TO NO INHIBITION REMAINING. THIS ALONG WITH WEAK UPPER SUPPORT AND BROAD LOW LEVEL CONVERGENCE HAS BEEN ENOUGH TO ALLOW SCT SHOWERS/ISOLD THUNDERSTORMS TO DEVELOP THIS AFTERNOON. INSTABILITY IS WEAK...HOWEVER STRONG EFFECTIVE DEEP LAYER SHEAR AND LOW FREEZING LEVELS COULD ALLOW FOR A FEW MORE SMALL HAILERS WITH THE STRONGEST UPDRAFTS. LOSS OF DAYTIME HEATING SHOULD TAKE A TOLL ON THE SHOWERS...SO EXPECT ANY RAIN TO END OVER SE CWA BY ABOUT 02Z AT THE LATEST. NEXT CONCERN FOR TONIGHT WILL BE WITH CLOUD COVER. FAIRLY LARGE AREA OF DIURNALLY ENHANCED STRATOCUMULUS COVERS MUCH OF WI AND SE MN THIS AFTERNOON. THIS CLOUDINESS WILL BEGIN DISSIPATING AFTER SUNSET...THOUGH IF LAST NIGHT IS ANY INDICATION OF WHAT IS TO COME TONIGHT THEN SOME STRATOCU COULD LINGER THROUGH THE NIGHT. THE AMOUNT OF CLOUD COVER TONIGHT WILL BE KEY TO POTENTIAL FOR SOME PATCHY FROST TO DEVELOP. CONFIDENCE IN FROST OCCURRING IS LOW GIVEN EXPECTED MARGINAL TEMPS AND POSSIBILITY OF SOME CLOUDINESS...BUT SINCE FROST IS IN THE GRIDS/FORECAST ALREADY COURSE OF LEAST REGRET IS TO PROBABLY RETAIN IT FOR NORTHERN SUBURBS WEST INTO NC IL. QUIET AND SEASONABLE CONDITIONS ARE EXPECTED MONDAY INTO TUESDAY. WHILE LAKE BREEZES ARE LIKELY BOTH DAYS...MOST SIGNIFICANT INLAND PENETRATION WILL LIKELY TAKE PLACE MONDAY SINCE GRADIENT WILL BE LIGHTEST. BY TUESDAY AFTERNOON A MORE SOUTHERLY GRADIENT WILL FAVOR MOST SIGNIFICANT INLAND PENETRATION ALONG THE NORTH SHORE AREAS. OUTSIDE OF THE LAKE COOLING...TEMPS WILL MODERATE MONDAY AND TUESDAY WITH HIGHS NEARING THE 70 DEGREE MARK INLAND BY TUESDAY AFTERNOON. WEATHER WILL TAKE A TURN FOR THE MORE INTERESTING COME TUESDAY NIGHT INTO WEDNESDAY AS THE POWERFUL UPPER LOW OFF THE BRITISH COLUMBIA COAST OPENS UP INTO AN OPEN...ALBEIT SHARP...UPPER TROUGH WHILE TRACKING QUICKLY EASTWARD ACROSS THE NORTHERN TIER OF STATES. MOIST AIRMASS FESTERING ABOUT ALONG THE GULF STATES SHOULD BE DRAWN RAPIDLY NORTHWARD BY 50KT+ LOW LEVEL JET IN ADVANCE OF THIS SYSTEM. WOULD BE ABSURD TO START GETTING BOGGED DOWN IN TALKING ABOUT DETAILS OF A CONVECTIVE EVENT THIS FAR OUT...HAVING SAID THE OVERALL SYNOPTIC SET-UP IS CERTAINLY LOOKING INTERESTING FOR A POTENTIAL SEVERE WEATHER EVENT EITHER HERE OR NEARBY WEDNESDAY AFTERNOON/EVENING. SHOULD SEE A QUICK SHOT OF COOLER/DRIER CONDITIONS THURSDAY AND POSSIBLY INTO FRIDAY...BEFORE NEXT WAVE ARRIVES BY NEXT WEEKEND. MUCH MORE UNCERTAINTY WITH THE DETAILS OF THIS SYSTEM...SO JUST GOING TO RUN WITH CHANCE POPS THROUGH THE WEEKEND. DETAILS WILL UNDOUBTEDLY BECOME CLEARER WITH TIME... IZZI && .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... 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 LATER TODAY AS THE ANTICYCLONE WILL BE CENTERED OVER THE SOUTHERN GREAT LAKES AND OHIO VALLEY TONIGHT BY LOCAL INFLUENCES AND LAKE BREEZES WILL LIKELY OBSCURE THIS WEAK SYNOPTIC FLOW. 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. && $$
