Issued by NWS Chicago, IL
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000 FXUS63 KLOT 292327 AFDLOT AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL 627 PM CDT SAT AUG 29 2009 .DISCUSSION... 211 PM CDT AUTUMNLIKE TEMPERATURES CONTINUE TO ADVECT INTO REGION THIS AFTERNOON AND EVENING. AT 00Z...TIGHT 1000-500 MB THKNS RIBBON IS STILL FCST TO BE PASSING SWD THROUGH MIDWEST...ACCOMPANIED BY SOMEWHAT BRISK NWLY WINDS. THIS HAS ALLOWED COLD ADVECTION SRATOCU FIELD TO EXPAND WITH THIS AFTERNOON/S ATTEMPT AT DIURNAL HEATING. EXPECT TO SEE SOME DCRS IN CLOUD COVG LATER THIS AFTN...BUT WITH HANG BACK UPR TROF NOW EXTENDING WWD FROM NRN LWR MI-SRN MN CONTINUING TO ROTATE SWD/SSEWD...DO NOT EXPECT MUCH IN THE WAY OF CLRG TONIGHT. IN FACT...CURRENT VIS IMAGERY SHOWS LEADING EDGE OF RATHER SOLID CLOUD DECK ALONG ROUGHLY A MSP-LNR-MKE LINE...AND ROTATING SWD TOWARD NRN IL. AS A RESULT...WILL MAINTAIN MOSTLY CLOUDY SKIES THROUGH TONIGHT. ALSO SUSPECT SOME SPRINKLES WILL OCCUR WITH APCH OF MID LVL THERMAL TROF. THEREFORE SEE NO NEED TO MAKE ANY MAJOR CHGS TO PREVIOUS FCST AND WILL KEEP POP/WX GRIDS PRETTY MUCH UNCHANGED. GIVEN ELEVATED WIND AND CLOUD COVER TONIGHT...LOW TEMPS SHOULD HOLD UP IN LWR 50S MOST AREAS...EXCEPT PERHAPS FAR WRN AND SRN PORTION OF FA WHERE CLOUD COVER MAY NOT BE AS EXTENSIVE. WILL FAVOR RATHER CONSISTENT MOS NUMBERS FOR LOWS TONIGHT. UPR TROF AXIS TO SHIFT E OF REGION BY SUNDAY AFTN. DECREASE IN COLD ADVECTION AND SUBSIDENCE IN WAKE OF TROF SUGGEST SKIES WILL BECOME GENLY CLR AS THE DAY PROGRESSES. EXCEPTION MAY BE IN NWRN INDIANA WHERE LAKE EFFECT CLOUDINESS AND SOME SHOWERS MAY PERSIST FROM SAT NGT INTO SUNDAY MORNING. WITH MORE SUNSHINE AND DECREASING WIND...TEMPS SHOULD GET A DEG OR TWO WARMER SUNDAY AFTN. TRAJECTORY FCSTS...MOS...AND FCST SOUNDINGS SUGGEST UPR 60S FOR HIGHS MOST AREAS...WHICH...THOUGH QUITE COOL...IS ONLY 10-12 DEGREES BLO SEASONAL NORMS. ALSO CONCUR WITH PREVIOUS FCST THAT SUN NGT WILL BE WHEN WE BOTTOM OUT ON TEMPS. AS SFC HIGH SETTLES OVR AREA...GOOD RADIATIONAL COOLING SET UP SHOULD ALLOW TEMPS TO DROP TO NR RECORD LOWS. REMAINDER OF FCST PERIOD PROMISES AN UNUSUALLY PROLONGED PERIOD OF MAINLY CLR SKIES AND SLOW DAY-TO-DAY AIR MASS MODIFICATION. MODELS ARE FAIRLY CONSISTENT IN FORMING SOMEWHAT OF AN OMEGA PATTERN OVR NRN PLAINS BY MONDAY...THEN SHIFTING THIS EWD ACROSS UPR GRTLKS BY MIDWEEK. THIS IS IN RESPONSE TO SMALL...BUT CONCENTRATED CUTOFF LOW NOW PARKED OVR PAC NW. AS THIS CUTOFF TRIES TO PUNCH THROUGH LARGER SCALE RIDGE OVR WRN CONUS...IT IS EXPECTED TO CONTINUE TO INDUCE RIDGING TO ITS N AND E. THIS IS SIGNIFICANT FOR US AS IT WILL CAUSE SFC HIGH PRESSURE TO SIT OVR SRN GRTLKS REGION FOR SVRL DAYS THIS WEEK...RESULTING IN GENLY CLR SKIES...LOW DEW POINTS AND A SLOW DAY-TO-DAY INCR IN HIGH TEMPS GIVEN SET UP FOR LARGER THAN NORMAL DIURNAL TEMPERATURE SWINGS. SOME DIVERGENCE IN MODEL SOLNS IN EXTENDED TIME FRAME HEADING INTO LABOR DAY WEEKEND SO WILL KEEP FCST DRY ATTM. HOWEVER...OVERALL TREND BASED ON CONSENSUS IS FOR LARGER SCALE TROFFING TO EVOLVE OVR WRN U.S. AS UPR RIDGE SHIFTS EWD TO EXTEND FROM SRN PLAINS TO GRTLKS. THIS WOULD SUGGEST A CONTINUED WARMING TREND TO ABOVE NORMAL TEMPERATURES. MERZLOCK && .AVIATION... 627 PM CDT 0000 UTC TAFS...CEILING TREND TONIGHT INTO SUNDAY MORNING IS THE PRIMARY AVIATION FORECAST ISSUE TO RESOLVE. SECONDARY SURFACE TROUGH CURRENTLY MOVING THROUGH SOUTHEAST WISCONSIN WILL TRACK ACROSS NORTHERN ILLINOIS THIS EVENING BRINGING REINFORCING COLD ADVECTION AND A LESS DIURNALLY DRIVEN LOW CLOUD DECK WITH IT. SURFACE OBSERVATIONS INDICATE FAIRLY WIDESPREAD MVFR CIGS BEING REPORTED BEHIND THIS TROUGH ACROSS CENTRAL WISCONSIN AND SEE LITTLE REASON TO DEVIATE MUCH FROM THE CURRENT TAFS WHICH CAPTURE TREND TO MVFR CIGS AFTER 02Z THIS EVENING. SOME LOWER END MVFR CIGS BETWEEN 1000-1500 FT ALSO ARE POSSIBLE OVERNIGHT BUT ESPECIALLY FOR GYY WHERE FLOW OFF LAKE MICHIGAN MAY PROVIDE SOME SLIGHTLY LOWER CIGS. SHOULD BE ENOUGH LOW LEVEL MIXING WITH THE COLD ADVECTION TO KEEP SOME GUSTS TO AROUND 15 KNOTS GOING THROUGH THE OVERNIGHT HOURS. WILL ALSO HAVE TO WATCH EVOLUTION OF RAIN ACROSS WISCONSIN/LAKE MICHIGAN AND THE POSSIBILITY OF SOME LIGHT RAIN SHOWERS/SPRINKLES POSSIBLY SURVIVING AS FAR SOUTH AS NORTHERN ILLINOIS LATE IN THE EVENING INTO THE OVERNIGHT HOURS. STRONGER UPPER LEVEL VORT MAX SHOULD STAY NORTH OF THE AREA SO CONFIDENCE NOT HIGH HOW FAR SOUTH SHOWERS WILL MAKE IT AND WILL KEEP TAFS DRY FOR NOW. CORE OF COLDEST LOW LEVEL AIR WILL SHIFT INTO THE OHIO RIVER VALLEY EARLY SUNDAY AND EXPECTING STEADY IMPROVEMENT IN CIGS SUNDAY MORNING...EVENTUALLY TO VFR BY MID MORNING. STEEP LOW LEVEL LAPSE RATES SHOULD ALLOW FOR SCT-BKN VFR CU TO DEVELOP AGAIN TOMORROW AFTERNOON MAINLY AWAY FROM LAKE MICHIGAN. CLOSER TO THE LAKE...MAY HAVE A MINIMUM IN CU DUE TO STABILIZING INFLUENCE OF NORTHERLY FLOW OFF THE LAKE. STRONGER SYNOPTIC SCALE SUBSIDENCE WILL ALSO BE A CONTRIBUTING FACTOR TO LESS CU FORMATION SUNDAY. SFC GRADIENT WILL BEGIN TO SLACKEN SUNDAY AFTERNOON AS RIDGE AXIS TO THE WEST BEGINS TO BUILD IN SO HAVE ENDED GUSTS AFTER 18Z. MARSILI && .MARINE... 114 PM CDT AN UNSEASONABLY DEEP AREA OF LOW PRESSURE WAS OVER SOUTHEAST ONTARIO THIS AFTERNOON WITH A TROF EXTENDING WEST FROM THE LOW ACROSS NORTHERN LAKE MI AND NORTHERN WI. THE TROF WILL MOVE SOUTH ACROSS LAKE MI BY THIS EVENING AS THE LOW MOVES INTO SOUTHWEST QUEBEC. THIS WILL SHIFT WINDS FROM NORTHWEST TO NORTH. WINDS WILL BE GENERALLY 15-25 KNOTS WITH A FEW GUSTS TO AROUND 30 KNOTS TONIGHT AND SUNDAY MORNING. THE LOW WILL CONTINUE MOVING NORTHEAST AND WEAKENING SUNDAY. THIS WILL ALLOW WINDS TO GRADUALLY DIMINISH SUNDAY AND SUNDAY NIGHT. MEANWHILE LARGE HIGH PRESSURE...30.3 INCHES OVER SOUTH CENTRAL CANADA THIS AFTERNOON WILL MOVE SOUTHEAST ACROSS THE UPPER MIDWEST TONIGHT AND SUNDAY...AND OVER THE GREAT LAKES SUNDAY NIGHT THROUGH THE MIDDLE OF NEXT WEEK. THIS WILL BRING AN EXTENDED PERIOD OF TRANQUIL WEATHER TO LAKE MICHIGAN. ALLSOPP && .LOT WATCHES/WARNINGS/ADVISORIES... IL...NONE. IN...NONE. LM...SMALL CRAFT ADVISORY...NEARSHORE GARY TO MICHIGAN CITY IN UNTIL 1 AM MONDAY. SMALL CRAFT ADVISORY...NEARSHORE WINTHROP HARBOR TO GARY IN UNTIL 7 PM SUNDAY. && $$
