000
FXUS63 KLOT 032052
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
252 PM CST WED DEC 3 2008
.DISCUSSION...
252 PM CST
NEAR TERM FORECAST FOR THIS EVENING AND OVERNIGHT...MAIN CONCERN
WILL BE THE ONGOING SNOWFALL EVENT. THE BAND OF LIGHT TO MODERATE
SNOW IS MOVING THROUGH NRN ILLINOIS...GENERALLY NORTH AND WEST OF A
LINE FROM WAUKEGAN TO CARPENTERSVILLE TO PAW PAW AT ISSUANCE TIME.
THE LEADING EDGE OF THE SNOW IS JUST BEHIND THE COLD FRONT...AND IS
SLOWLY ADVANCING SOUTH AND EAST. EXPECT THE SNOW TO REACH CHICAGO
TO JOLIET TO STREATOR IN TIME FOR THE EVENING RUSH HOUR. THE
HEAVIEST SNOWFALL IS STILL EXPECTED TO BE OVER THE NRN PORTIONS OF
THE CWA...WITH 3 TO 4 INCHES ALONG THE WISCONSIN BORDER AND 2 TO 3
INCHES SOUTH OF A LINE FROM CHICAGO TO OTTAWA. SEE CHIWSWLOT FOR
COMPLETE DETAILS.
SHORT TERM FORECAST BEYOND 12Z THURSDAY...FOLLOWING THE PASSAGE OF
THE COLD FRONT AND THE ASSOCIATED SNOWFALL...THE MAIN CONCERN FOR
THE REMAINDER OF THE SHORT TERM FORECAST WILL BE TEMPERATURES.
EXPECT TEMPERATURES TO DROP OFF RAPIDLY OVERNIGHT TONIGHT AS SKIES
CLEAR AND LOW DEWPOINT/DRY AIR OVERSPREADS THE REGION. WITH
UPSTREAM SFC DEWPOINTS ONLY IN THE LOW SINGLE DIGITS...HAVE MADE
SIGNIFICANT ADJUSTMENTS TO MIN TEMPS FOR THURSDAY AND FRIDAY
MORNING. BASED ON THE UPSTREAM DEWPOINT TRENDS AND FRESH
SNOWCOVER...HAVE LOWERED MIN TEMPS OVER NWRN PORTIONS OF THE CWA
TO THE MIDDLE SINGLE DIGITS FOR THURSDAY MORNING AND LOWER SINGLE
DIGITS FOR FRIDAY MORNING. THE SERN PORTIONS OF THE CWA SHOULD BE
SLIGHTLY WARMER...WITH MIN TEMPS IN THE LOW TO MIDDLE TEENS...AND
DOWNTOWN CHICAGO IN THE UPPER TEENS. WITH WINDS REMAINING RATHER
BREEZY OVERNIGHT TONIGHT...IN THE 10 TO 15 MPH RANGE...WIND CHILL
READINGS SHOULD BE NEAR OR BELOW 0F. FOR FRIDAY MORNING...WIND
CHILL READINGS SHOULD BE BELOW 0F AS WELL. FOR THE DAYTIME HOURS
THURSDAY AND FRIDAY...EXPECT DRY CONDITIONS WITH ONLY A PASSING
FLURRY THURSDAY AND DECREASING CLOUD COVER AND NO PCPN FOR FRIDAY
AS SFC HIGH PRESSURE BUILDS ACROSS THE MIDDLE MISSISSIPPI VALLEY.
THE MODELS ARE IN RELATIVELY GOOD AGREEMENT ON MAINTAINING A SPLIT
FLOW PATTERN WITH A STRONG SUBTROPICAL JET OVER THE GULF COAST AND
A STRONG POLAR JET DIVING OUT OF CENTRAL CANADA AND INTO THE
NORTHERN PLAINS. THOUGH THE MODELS SHOW SOME MINOR TIMING
DIFFERENCES WITH THE SHORT WAVE ENERGY MOVING THROUGH THE UPPER
GREAT LAKES THURSDAY AND FRIDAY...THERE SHOULD BE LITTLE
IMPACT...IF ANY...ON THE LOCAL SENSIBLE WEATHER FEATURES.
THE NEXT CHANCE FOR SNOW WILL COME LATE FRIDAY NIGHT AND SATURDAY.
THE MODELS ARE HAVING A MORE DIFFICULT TIME IN COMING TO A CONSENSUS
ON A STRONG CLIPPER DROPPING OUT OF THE CANADIAN ROCKIES FRIDAY
NIGHT AND SWINGING ACROSS THE LOWER GREAT LAKES SATURDAY. THE GFS
AND ECMWF ARE IN GOOD AGREEMENT ON THIS FEATURE...TRENDING
FASTER...STRONGER AND MORE AMPLIFIED THAN THE NAM. THE NAM ALSO
MAINTAINS A MORE NORTHERLY TRACK TO THE SFC LOW CENTER. AT THIS
POINT...PREFER THE FASTER...MORE AMPLIFIED GFS/ECMWF
SOLUTION...GIVEN THE INITIAL STRENGTH OF THE ASSOCIATED SHORT WAVE
AND THE INITIALLY HIGH AMPLITUDE PATTERN. EXPECT THAT THERE WILL BE
SOME LIGHT SNOW WITH THE PASSAGE OF THE CLIPPER...BUT AMOUNTS SHOULD
BE RATHER LIGHT GIVEN THAT THE LOCAL AREA WILL BE CUT OFF FROM ANY
SIGNIFICANT LOW LEVEL MOISTURE INFLUX. ALSO...THE STRONGEST UPPER
LEVEL FORCING ASSOCIATED WITH THE SHORT WAVE WILL BE WELL TO THE
NORTH...ACROSS THE UPPER GREAT LAKES.
LOOKING INTO THE LONGER TERM...A PERIOD OF COOLER THAN NORMAL
TEMPERATURES AS A LARGE SCALE TROUGH CARVES OUT OVER THE ERN CONUS
AND PERSISTENT NWLY FLOW DEVELOPS OVER THE CENTRAL CONUS. THE GFS
AND ECMWF BRING A LARGE AREA OF HIGH PRESSURE ACROSS THE NORTHERN
PLAINS AND INTO THE UPPER MIDWEST SUNDAY AND MONDAY. BEST CHANCE
FOR MORE PCPN LOOKS TO BE MONDAY NIGHT AND TUESDAY AS THE NEXT
CLIPPER IMPACTS THE AREA. AT THIS TIME...THE MAIN SURFACE LOW TRACK
LOOKS TO BE VERY SIMILAR TO THE SCENARIO DEVELOPING FRIDAY AND
SATURDAY...AND IF THE CURRENT GFS/ECMWF RUNS VERIFY...THE SFC LOW
MIGHT EVEN BE A LITTLE FARTHER NORTH AND SOME OF THE PCPN COULD BE
RAIN OR AN RAIN/SNOW MIX BEFORE TURNING OVER TO ALL SNOW BY THE END
OF THE EVENT.
IT IS STILL A BIT EARLY TO TALK ABOUT AMOUNTS FOR BOTH OF THE
CLIPPER SYSTEMS...BUT INITIAL INDICATIONS ARE THAT BOTH SHOULD BRING
ONLY LIGHT PCPN AMOUNTS.
KREIN
&&
.AVIATION...
1800 UTC TAFS...RELATIVELY NARROW...THOUGH FAIRLY INTENSE BAND OF
SNOW ASSOCIATED WITH STRENGTHENING DEEP FRONTOGENETIC CIRCULATION
ASSOCIATED WITH THE AMPLIFYING TROUGH. MODELS INDICATE THAT THIS
DEEP FRONTAL CIRCULATION WILL PERSIST THROUGH THE AFTERNOON AND
POSSIBLY EVEN STRENGTHEN A BIT...SO NOWCASTING TOOLS REALLY AREN`T
GIVING INDICATION OF THIS BAND OF SNOW WEAKENING BEFORE ARRIVING.
BEST GUESS AT TIMING OF ONSET IS MID AFTERNOON FOR CHGO METRO
TERMINALS AND LATE AFTERNOON INTO GYY. ONSET OF SNOW SHOULD BE
SUDDEN WITH VSBY RAPIDLY DROPPING FROM VFR TO IFR AND LIKELY
REMAINING IFR FOR SEVERAL HOURS WITH PERIODIC BURSTS OF HEAVIER
SNOW RESULTING IN LIFR/VLIFR CONDITIONS.
RAPID IMPROVEMENT CONDITIONS EXPECTED MID-EVENING FOLLOWING THE
PASSAGE OF THIS BAND WITH VFR CONDITIONS ANTICIPATED OVERNIGHT
AND THROUGH THE THURSDAY. INITIALLY SW WINDS WILL VEER TO NW AS SFC
FRONT MOVES THROUGH THIS AFTERNOON...WITH WINDS GRADUALLY
INCREASING IN VELOCITY SHORTLY AFTER SUNSET AS LOW LEVEL COLD AIR
ADVECTION INTENSIFIES. ONCE SKIES CLEAR LATE TONIGHT SOME SLIGHT
DECOUPLING COULD RESULT IN WINDS SLACKENING A BIT...PARTICULARLY
AS THE GRADIENT WEAKENS VERY LATE TONIGHT AND MORES0 THURSDAY.
IZZI
&&
.MARINE...
155 PM CST
COLD FRONT MAKING ITS WAY ACROSS THE LAKE THIS AFTERNOON SHOULD
CLEAR THE SE SHORES BY EARLY EVENING. NORTHWEST WINDS BEHIND THE
FRONT WILL GUST TO BETWEEN 25-30KT FOR A TIME TONIGHT BEFORE
DIMINISHING LATE TONIGHT AND MORE SO THURSDAY. WINDS SHOULD BACK
AND BEGIN TO INCREASE FRIDAY NIGHT INTO SATURDAY IN ADVANCE OF
THE NEXT CLIPPER SYSTEM RACING SE TOWARD THE REGION. FOR NOW HAVE
JUST BUMPED WINDS UP TO 30KT...THOUGH SOME OF THE GUIDANCE
SUGGESTS THAT A PERIOD OF SOUTH TO SOUTHWEST GALES COULD OCCUR
AHEAD OF THE LOW SATURDAY. IN THE WAKE OF THE CLIPPER STILL
APPEARS PROBABLE THAT WE WILL SEE A PERIOD OF NORTHWESTERLY GALES
SATURDAY NIGHT INTO SUNDAY AS THE NEXT CANADIAN HIGH BUILDS INTO
THE AREA IN THE WAKE OF THE CLIPPER.
IZZI
&&
.LOT WATCHES/WARNINGS/ADVISORIES...
IL...WINTER WEATHER ADVISORY...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-
ILZ010-ILZ011-ILZ012-ILZ013-ILZ014 UNTIL MIDNIGHT THURSDAY.
IN...NONE.
LM...SMALL CRAFT ADVISORY...LMZ740-LMZ741-LMZ742-LMZ743...6 PM
WEDNESDAY TO 4 PM THURSDAY.
SMALL CRAFT ADVISORY...LMZ744-LMZ745...6 PM WEDNESDAY TO 10 PM
THURSDAY.
&&
$$
