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000 FXUS63 KLOT 220012 AFDLOT AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL 612 PM CST SAT NOV 21 2009 .DISCUSSION... 230 PM CST NOT MUCH CHANGE IN CONCERNS FROM THIS MRNG. FOG POTENTIAL STILL LOOMS TONIGHT DESPITE PASSING AC ADVECTION NWD THRU WRN IL. DEWPTS STILL NR 40 TNGT AND WITH CALM WINDS AND NEARLY CLEAR SKIES...VSBYS SHUD START LOWERING AFTER MDNGT AND PROGRESS TOWARD ZERO NR SUNRISE. AC PASSAGE NWD THRU WESTERN IL WILL LIKELY KEEP VSBY UP ABV ADVISORY CRITERIA FOR SECTIONS WEST OF I-39. THIS PASSING AC TO CONTINUE THRU MONDAY AND PROMPT PARTLY CLOUDY CONDITIONS UNTIL MONDAY NIGHT. WHILE MAX TEMPS TRY TO WARM UP EARLY IN THE WEEK UNDER INCREASING SW FLOW ALOFT...INCREASED CLOUD COVER AND LOWER SUN ANGLE WILL INHIBIT THE USUAL TEMP JUMPS ASSOCIATED WITH EARLY AUTUMN WARM AIR ADVECTION...BUT RESPOND READILY TO ANY UPCOMING COLD AIR ADVECTION. THUS HIGHS EARLY THIS WEEK WILL HOVER IN THE 50S BUT SETTLE INTO THE 40S AFTER TUESDAY. TUESDAY THEN IS THE FOCUS OF NEXT CONCERN. THE UPR LVL PATTERN ASSOCIATED WITH THIS UPCOMING CHANGE HASNT EVEN MADE IT ONTO THE WEST COAST AS YET...BUT SHUD BE MAKING LANDFALL LATE TONIGHT AND CROSSING THE NRN ROCKIES SUNDAY NIGHT. DEEPENING MONDAY INTO A STRONG CLOSED LOW IN THE PLAINS...EXPECT TO SEE AC AND CI ADVECTING INTO THE MS VLY AND PCPN CHANCES INCREASE THU ILLINOIS LATE MON NGT INTO TUESDAY. ALL MODELS ARE TRENDING TOWARD OCCLUDING THIS SYSTEM FROM THE MID AND UPR MS VLY TO THE GREAT LAKES BY MIDWEEK. TIMING AND STORM CENTER LOCATION DIFFERENCES ARE STILL APPARENT AMONG THEM. THE ECMWF IS CONSIDERABLY SLOWER THAN THE GFS AND NAM WITH THE NAM BEARING A MORE SLY TRACK THAN EITHER OF THE TWO. AT THIS POINT...A BLEND IS PROBABLY PRUDENT. THE GENERAL TREND THO IS FOR RAIN TO BEGIN TUESDAY WITH COLD AIR WRAPAROUND FILTERING IN WEDNESDAY AND MIXING WITH SNOW WED NIGHT BEFORE DIMINISHING TO SCATTERED FLURRIES THURSDAY. GFS AND ECMWF ACTUALLY RESOLVE TWO DISTINCT CLOSED UPR TROFS SWINGING THRU LATE IN THE WEEK. THE FIRST TROF ASSOCIATED WITH THE RAIN ONSET ON TUESDAY SHUD SWING THRU ON WEDNESDAY. WEAK UPR RIDGING MAY PUT AN END TO THE RAIN WED AFTN BEFORE A SECONDARY CLIPPER-LIKE SYSTEM SWINGS DOWN FROM MANITOBA TO NRN IL WED NGT. THE SECOND SHOT OF CAA FOLLOWING THIS SECOND TROF PASSAGE WILL BE THE IMPULSE THAT PULLS DOWN AIR COLD ENUF TO CHANGEOVER TO A SNOW MIXTURE. UPR RIDGING WORKING IN FRIDAY AND SATURDAY WILL END ANY LINGERING FLURRIE CHANCES WHILE RETURNING WARM AIR ADVECTION FOR THE WEEKEND. RLB && .AVIATION... 0000 UTC TAFS...THE WIND WILL BE NEAR CALM TONIGHT. A LARGE HIGH OVER OHIO WILL SPREAD EAST INTO SOUTHERN QUEBEC BY SUNDAY MORNING. BY SUNDAY EVENING THE HIGH CENTER WILL BE OVER NEW BRUNSWICK AND THE HIGH WILL SPREAD WEST TO IOWA. THIS WILL GIVE A SOUTHEAST WIND 5 TO 10 KNOTS. WE EXPECT MORE MOISTURE IN THE ATMOSPHERE NEAR THE GROUND OVERNIGHT. THE AIR ABOVE 3000 FEET WILL BE RATHER DRY BY SUNDAY MORNING. WE WILL FORECAST FOG AND THE VISIBILITY WILL FALL TO 3 TO 4 MILES BY 04 UTC AT MOST OF THE AIRPORTS. BECAUSE OF THE SURROUNDING LAND USE...WE WOULD EXPECT MORE FOG AND THE VISIBILITY AS LOW AS 1 TO 3 MILES AT ROCKFORD AND DUPAGE. THOSE AIRPORTS ARE IN MORE RURAL LOCATIONS. THE FOG WILL BURN OFF BY 16 UTC SUNDAY. WE EXPECT CLOUDS AT 25000 FEET AS AN UPPER AIR TROUGH MOVES INTO UPPER MISSISSIPPI VALLEY. && .MARINE... 145 PM CST NO SIGNIFICANT WINDS/WAVE EXPECTED THROUGH THE NEXT SEVERAL DAYS WITH HIGH PRESSURE IN CONTROL OF THE WEATHER. LIGHT/VARIABLE WINDS WILL TEND TO TURN MORE SOUTHERLY TONIGHT...ALBEIT NOT VERY STRONGLY...AND GENERALLY CONTINUE IN A GENERAL SOUTHEASTERLY DIRECTION THROUGH MIDWEEK. IZZI && .LOT WATCHES/WARNINGS/ADVISORIES... IL...NONE. IN...NONE. LM...NONE. && $$
