Issued by NWS Chicago, IL
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000 FXUS63 KLOT 221240 AFDLOT AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL 640 AM CST SUN NOV 22 2009 .DISCUSSION... 329 AM CST SHORT TERM FOG CONCERNS AND EVOLUTION OF MID WEEK STORM SYSTEM CONTINUE TO BE THE FOCI FOR THIS FORECAST PACKAGE. THIS MORNING...SURFACE RIDGE THAT GAVE THE AREA A BEAUTIFUL SATURDAY CONTINUES TO SHIFT OFF TO THE EAST. CALM WINDS AND GENERALLY CLEAR SKIES ARE STILL IN PLACE OVER THE REGION...AND AREAS OF FOG HAVE ONCE AGAIN FORMED THIS MORNING. DEWPOINT DEPRESSIONS ARE A FEW DEGREES LARGER THIS MORNING AS OPPOSED TO LAST...AND AS A RESULT VISIBILITIES HAVE GENERALLY STAYED IN THE 3-5 SM RANGE...AND REALLY DON`T EXPECT VIS TO DROP TOO MUCH MORE THIS MORNING. AFTER ANY RESIDUAL FOG BURNS OFF BY MID MORNING...TODAY LOOKS TO BE A NEAR REPEAT OF SATURDAY...SAVE FOR PERHAPS A BIT MORE MID AND HIGH LEVEL CLOUD COVER...ESPECIALLY ACROSS THE WESTERN COUNTIES...AS WARM AIR ADVECTION CONTINUES TO ORGANIZE OFF TO THE SOUTH AND WEST OF THE FORECAST AREA. CLOUD COVER WILL STEADILY INCREASE TONIGHT AND MONDAY MORNING FROM THE WEST AS PREVIOUSLY ADVERTISED STORM SYSTEM TAKES SHAPE OVER WESTERN HIGH PLAINS. NUMERICAL GUIDANCE HAS COME INTO CONSIDERABLY BETTER AGREEMENT WITHIN THE LAST 24 HOURS REGARDING THE EVOLUTION OF THIS SYSTEM. APPEARS THAT A LEAD SHORTWAVE WILL BEGIN TO MOVE NORTHEAST TOWARD THE AREA DURING THE DAY MONDAY. AT THIS TIME...LOOKS AS IF DRY EASTERLY FLOW WILL KEEP RAIN CHANCES WITH THIS FIRST WAVE TO BELOW SLIGHT CHANCE MENTION. ON TUESDAY...MAIN AREA OF RAIN IN ASSOCIATION WITH AN OCCLUDED FRONT IS EXPECTED TO START MOVING ACROSS THE AREA FROM SOUTHWEST TO NORTHEAST. RAIN SHOULD EXPAND ACROSS THE ENTIRE FORECAST AREA BY TUESDAY NIGHT AND EARLY WEDNESDAY MORNING AS PARENT UPPER TROUGH TRAVERSES THE AREA. THERMAL PROFILES TUESDAY AND DURING THE EARLY MORNING WEDNESDAY APPEAR TO BE TOO WARM TO SUPPORT ANYTHING OTHER THAN JUST RAIN. SECONDARY SHOT OF COLD AIR ASSOCIATED WITH YET ANOTHER SHORTWAVE/UPPER LOW WILL BEGIN TO MOVE THROUGH THE AREA WEDNESDAY AFTERNOON AND EVENING. AS COLDER AIR SPILLS SOUTH ACROSS THE AREA...EXPECT PRECIPITATION TO CHANGE TO A RAIN/SNOW MIX BEFORE TRANSITIONING TO ALL SNOW BY EARLY THURSDAY MORNING. SCATTERED SNOW SHOWERS APPEAR TO BE POSSIBLE THROUGH THANKSGIVING DAY AS UPPER LOW CONTINUES TO SPIN ACROSS UPPER GREAT LAKES. AT THIS TIME...WHILE THERMAL PROFILE LOOKS TO BE COLD ENOUGH TO SUPPORT SNOW...SURFACE TEMPERATURES ARE EXPECTED TO BE WARM ENOUGH TO PRECLUDE ANYTHING IN THE WAY OF SIGNIFICANT ACCUMULATIONS. LITTLE CHANGE MADE TO FORECAST FOR FRIDAY THROUGH SUNDAY. A FEW SNOW SHOWERS LOOK TO REMAIN POSSIBLE ACROSS PORTIONS OF THE AREA FRIDAY WITH THE AREA SITUATED UNDER COLD CYCLONIC FLOW ALOFT. DEPENDING ON EXACT TRACK OF TROUGH AND SURFACE FEATURES...ALSO COULDN`T RULE OUT A FEW LIGHT LAKE EFFECT SNOW SHOWERS ACROSS PORTIONS OF PORTER COUNTY INDIANA ON FRIDAY. BOXELL && .AVIATION... 630 AM CST 1200 UTC TAFS...SURFACE HIGH EXTENDING FROM SOUTHEAST CANADA INTO THE GREAT LAKES REGION WILL CONTINUE TO SLOWLY DRIFT SOUTHEAST TODAY. UPPER LEVEL RIDGE WILL BE SLOW TO EXIT THE GREAT LAKES REGION TODAY WHICH WILL ALLOW WEAK SFC RIDGE AXIS TO LINGER ACROSS THE AREA. THUS...SFC PRESSURE GRADIENT WILL REMAIN FAIRLY WEAK TODAY AND HAVE NOT MADE MANY CHANGES WITH SOUTHEAST WINDS THIS MORNING OF 5 TO 10 KNOTS. WINDS SHOULD BACK MORE EAST SOUTHEAST LATE THIS AFTERNOON AS THE SFC ANTICYCLONE EAST OF THE AREA SLOWLY TRACKS NORTHEAST. CLOUD DECK ASSOCIATED WITH LOW LEVEL WARM/MOISTURE ADVECTION WILL LIKELY REMAIN WEST OF NORTHEAST ILLINOIS TERMINALS...AND ANY CIGS AT RFD THIS MORNING WILL REMAIN VFR. PATCHY FOG AFFECTING TERMINALS AGAIN THIS MORNING AND LIKELY VERY SHALLOW IN NATURE. DID INCLUDE TEMPO IFR VSBYS AT DPA THROUGH 14Z BUT AT RFD WILL KEEP VSBYS MVFR DUE TO INFLUENCE OF AFOREMENTIONED CLOUDS AFFECTING NORTH CENTRAL ILLINOIS THIS MORNING. FOR TONIGHT...LIGHT EAST WINDS TO CONTINUE WITH JUST SOME INCREASING HIGH CLOUDINESS AS UPPER LEVEL DISTURBANCE ACROSS THE CENTRAL PLAINS APPROACHES. DEPENDING ON EXTENT OF HIGH CLOUDINESS...MAY SEE A REPEAT OF SOME SHALLOW GROUND FOG AND HAVE INCLUDED MVFR VSBYS FOR GYY/DPA/RFD AFTER 09Z. MARSILI && .MARINE... 638 AM CST CONTINUED EASTWARD PROGRESSION OF BROAD SURFACE ANTICYCLONE ACROSS SOUTHEAST CANADA INTO NORTHEAST UNITED STATES WILL RESULT IN SLIGHTLY STRENGTHENING SURFACE PRESSURE GRADIENT ACROSS WESTERN GREAT LAKES REGION TODAY...MOST NOTABLY ACROSS NORTHERN HALF OF THE LAKE WITH A WEAK SFC RIDGE AXIS EXPECTED TO LINGER ACROSS SOUTHERN PORTIONS OF THE LAKE. EXPECTING LIGHT SOUTH WINDS TO GENERALLY INCREASE INTO 10 TO 15 KT RANGE THIS MORNING...WITH SOME GUSTS TO AROUND 20 KT THIS AFTERNOON ACROSS NORTHERN PART OF THE LAKE. ON MONDAY...AN AREA OF SURFACE LOW PRESSURE WILL BEGIN TO TAKE SHAPE ACROSS THE CENTRAL PLAINS AND WILL EVENTUALLY APPROACH SOUTHERN LAKE MICHIGAN ON WEDNESDAY. THIS SYSTEM WILL BEGIN TO OCCLUDE WEDNESDAY/THURSDAY WHICH WILL RESULT IN SFC LOW DEVELOPING FURTHER BACK INTO COLD AIR WEST OF THE LAKE. AS UPPER LEVEL SYSTEM BEGINS TO PULL OUT LATE THURSDAY...SFC LOW WILL TRACK INTO EASTERN GREAT LAKES...WITH WINDS SHIFTING TO THE NORTHWEST AND MORE GUSTY IN NATURE IN COLD ADVECTION AND STRONGER GRADIENT BEHIND THIS SYSTEM. MARSILI && .LOT WATCHES/WARNINGS/ADVISORIES... IL...NONE. IN...NONE. LM...NONE. && $$
