FXUS63 KLOT 100259
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
859 PM CST WED DEC 9 2009
.DISCUSSION...
859 PM CST
HAVE EXTENDED EXISTING WINTER WEATHER ADVISORY THROUGH MIDNIGHT FOR
CONTINUED SNOW/BLOWING SNOW.
EVENING UPPER AIR ANALYSIS INDICATES UPPER LOW HAS PULLED AWAY TO
THE NORTHEAST...THOUGH WEAKER TROUGH NOTED IN MID LEVELS MOVING
THROUGH THE UPPER MISSISSIPPI VALLEY. WEAK THERMAL RIDGE/RESIDUAL
TROWAL-LIKE FEATURE NOTED ABOVE BOUNDARY LAYER IN 800-850 HPA LAYER
WHICH MAY BE HELPING MAINTAIN SOME WEAK WARM ADVECTION FORCING WITH
THIS TROUGH ACROSS WESTERN IL...WHERE REGIONAL RADAR MOSAICS
CONTINUE TO INDICATE SLIGHTLY ENHANCED REFLECTIVITY IN LIGHT SNOW.
SHORT TERM GUIDANCE AND RADAR TRENDS INDICATE THIS SHOULD WEAKEN
OVER THE NEXT FEW HOURS AS THIS MOVES EAST ACROSS ADVISORY
AREA...THOUGH COMBINATION OF LIGHT FALLING SNOW AND BLOWING SNOW
WILL CONTINUE TO AFFECT ESPECIALLY NORTH/SOUTH ROADS WITH SOME
DRIFTING AND SLIPPERY TRAVEL CONDITIONS FOR THE NEXT FEW HOURS. WITH
THE END OF THE LIGHT SNOWFALL AND GRADUAL DECREASE IN WINDS...SHOULD
BE ABLE TO ALLOW WINTER WX HEADLINES EXPIRE...JUST IN TIME FOR THE
WIND CHILL ADVISORY TO KICK IN. HAVE MADE MINOR CHANGES TO LATE
EVENING WEATHER/POP AND WINTER WX ADVISORY GRIDS.
WINDS STILL GUSTING ABOVE 30 MPH AT TIMES WILL GRADUALLY DIMINISH
OVERNIGHT THOUGH SUSTAINED WINDS IN 15-20 MPH RANGE COMBINED WITH
TEMPS FALLING INTO THE SINGLE DIGITS BELOW ZERO ACROSS NORTH CENTRAL
IL STILL EXPECTED TO PRODUCE WIND CHILLS TO OR SLIGHTLY BELOW -20 F.
THUS WIND CHILL HEADLINES APPROPRIATE AND WILL MAKE NO CHANGES
THERE. BLUSTERY AND COLD CONDITIONS CONTINUE INTO THURSDAY AND HAVE
MADE NO CHANGES TO FORECAST BEYOND THIS EVENING.
RATZER
&&
.PREV DISCUSSION...
336 PM CST
TONIGHT THROUGH FRIDAY...
OVERALL NO MAJOR CHANGES TO FORECAST MADE FROM PREVIOUS THINKING
EARLIER TODAY...WITH MAIN CHANGES BEING TO HEADLINES FOR TONIGHT.
WILL BE LETTING THE WINTER STORM WARNINGS EXPIRE WITH FORECAST
ISSUANCE THIS AFTERNOON AND WILL REPLACE WARNINGS WITH A WINTER
WEATHER ADVISORY FOR THIS EVENING WITH LINGERING GUSTY WINDS AND
AREAS OF BLOWING SNOW REDUCING VISIBILITIES...ESPECIALLY ACROSS OPEN
AREAS. A WIND CHILL ADVISORY WILL ALSO BE ISSUED FOR MUCH OF NORTH
CENTRAL ILLINOIS.
STRONGEST ISALLOBARIC COUPLET IS NOW SHIFTING INTO WEST CENTRAL
LOWER MICHIGAN ALTHOUGH BY PURE MIXING ALONE SEEING CONTINUED GUSTS
TO 40 TO 45 MPH ACROSS PORTIONS OF THE AREA...AND SHOULD GRADUALLY
SEE THESE GUSTS DIMINISH THIS EVENING. BAND OF SNOW WHICH SET UP
ALONG INTERSTATE 80 CORRIDOR PUT OUT SOME LIGHT AMOUNTS OF SNOW BUT
REPORTS FROM THIS AREA INDICATING OPEN AREAS BEING AFFECTED BY QUITE
LOW VISIBILITIES IN BLOWING SNOW. HAVE OPTED TO CANCEL THE WINTER
STORM WARNING WHICH WAS IN EFFECT THROUGH 00Z SINCE MOST OF THE
ACCUMULATING SNOWFALL HAS ENDED...BUT DUE TO STRONG WIND GUSTS
THROUGH EARLY EVENING WILL REPLACE THE WARNING WITH A WINTER
WEATHER ADVISORY FOR BLOWING SNOW. THE AREA OF THE WINTER WEATHER
ADVISORY HAS ALSO BEEN EXTENDED SOUTHWARD TO INCLUDE WILL COUNTY
DUE TO PRESENCE OF LIGHT SNOW BAND THAT PUT DOWN LIGHT DUSTING OF
SNOW. COULD SEE AS MUCH AS AN INCH OF ADDITIONAL SNOW EARLY THIS
EVENING WITH LAST IN SERIES OF MID LEVEL SHORT WAVES MOVING
THROUGH...BUT MOST LOCATIONS WILL LIKELY REMAIN UNDER AN INCH FOR
NEW SNOW ACCUMULATION.
OTHER CONCERN TONIGHT WILL BE ADVECTION OF BRUTALLY COLD AIR MASS
INTO THE REGION WITH 850 HPA TEMPS LOWERING TO -20 DEG C ACROSS FAR
NORTH CENTRAL ILLINOIS BY MORNING. GRADIENT WILL REMAIN SOMEWHAT
STRONG THROUGH THE NIGHT AND WOULD SUSPECT STRONG COLD AIR ADVECTION
AND LOW LEVEL INVERSION WILL ALLOW LOW CLOUDS TO STAY IN DURING THE
OVERNIGHT HOURS. DESPITE THESE FACTORS...PRESENCE OF FRESH SNOW PACK
AND UPSTREAM TREND SUGGESTING TRENDING TOWARD COLDER MET
GUIDANCE...OR EVEN A BIT COLDER THAN THIS. DID INCLUDE OVERNIGHT
LOWS IN THE ZERO TO 4 BELOW RANGE FOR FAR NORTHWEST COUNTIES AND
COMBINATION OF THESE TEMPERATURES WITH GUSTY WINDS TO 20 TO 25 MPH
THROUGH THE OVERNIGHT HOURS WILL PRODUCE WIND CHILLS APPROACHING
20 BELOW OR SLIGHTLY COLDER LATE TONIGHT INTO EARLY THURSDAY
MORNING. DID GO AHEAD AND ISSUE A WIND CHILL ADVISORY FOR MUCH OF
NORTH CENTRAL ILLINOIS COUNTIES THAT RUNS FROM 09Z TONIGHT THROUGH
THURSDAY MORNING.
SKIES WILL PARTIALLY CLEAR TOMORROW WITH CONTINUED DRY AIR ADVECTION
IN LOW LEVELS. DESPITE POTENTIAL FOR SOME SUN TOMORROW...CORE OF
BITTERLY COLD AIR MASS WILL BE IN PLACE AND WOULD SUSPECT AREAS WITH
SUBSTANTIAL NEW SNOW COVER WILL HAVE TROUBLE REACHING DOUBLE
DIGITS ABOVE ZERO TOMORROW. WILL START TO SEE LOW LEVEL WARM
ADVECTION FRIDAY AS SFC HIGH SLIDING ACROSS LOWER/MID MS RIVER
VALLEY TRACKS EASTWARD...ALTHOUGH TEMPS WILL BE SLOW TO MODERATE
ESPECIALLY FOR SNOW COVERED AREAS.
MARSILI
FRIDAY NIGHT THROUGH WEDNESDAY...
FOR THE PERIOD FROM FRIDAY NIGHT ONWARD...FEW CHANGES TO PREVIOUS
FORECAST EXCEPT TO INTRODUCE LOW CHANCE POPS SAT NIGHT INTO SUNDAY
FOR WEAK SHORTWAVE ENERGY PASSING THROUGH AS INDICATED BY GFS AND
ECMWF AS WELL AS ENSEMBLES. TEMPERATURES SHOULD MODERATE THROUGH
THE WEEKEND INTO EARLY NEXT WEEK AS WINDS TURN TO THE SOUTH BEHIND A
DEPARTING RIDGE AND THE NEXT APPROACHING LOW PRESSURE SYSTEM...
THOUGH EXPECT VALUES TO TREND TOWARD THE COOLER GUIDANCE NUMBERS
ESPECIALLY IN AREAS WITH EXTENSIVE SNOW COVER. HAVE OPTED TO KEEP
LOW CHANCE POPS FOR EARLY NEXT WEEK AS A LONGWAVE TROUGH SETS UP
OVER THE WESTERN HALF OF THE US AND EJECTS PULSES OF SHORTWAVE
ENERGY ACROSS THE AREA. GFS AND ECMWF DIFFER ON HOW EFFECTIVE
THIS ENERGY WILL BE AT DEVELOPING SURFACE CYCLONES...WITH THE GFS
ACTIVE ACROSS THE OHIO AND TENNESSEE VALLEYS AND THE ECMWF TAKING
ITS SURFACE SYSTEMS THROUGH THE GREAT LAKES. WILL WAIT TO PIN
THINGS DOWN BETTER AFTER THE MODELS COME INTO CLOSER AGREEMENT.
LENNING
&&
.AVIATION...
00Z TAFS...FORECAST CONCERNS THIS PERIOD FOCUS ON SNOW...BLOWING
SNOW AND ASSOCIATED VIS/CIGS THRU EVENING...AS WELL AS WIND SPEEDS.
LAST SHORT WAVE ROTATING ACROSS THE REGION LATE THIS AFTERNOON
ALONG WITH A SECONDARY FRONT HELPING TO KEEP LIGHT...TO AT TIMES
MODERATE SNOW GOING ACROSS THE TERMINALS. AS THIS FEATURE
CONTINUES TO SHIFT EAST THIS EVENING...EXPECT LIGHT SNOW TO TAPER
OFF TO FLURRIES. BUT THAT TRANSITION WILL LIKELY BE MASKED AS
GUSTY WEST WINDS PRODUCE BLOWING SNOW. CIGS APPEAR TO BE LIFTING
AND WHILE IFR VIS AROUND 1SM STILL POSSIBLE FOR A FEW HRS...EXPECT
CIGS TO REMAIN MVFR OR BETTER AND SLOWLY BECOME VFR OVERNIGHT.
VISIBILITIES ONCE FALLING SNOW ENDS DIFFICULT TO PREDICT FOR
BLOWING SNOW BUT WILL MAINTAIN AT LEAST MVFR THRU MIDNIGHT. CIGS
CONTINUE TO LIFT/SCATTER OUT THURSDAY MORNING AND LIKELY WILL GO
SKC BY SUNSET THURSDAY.
STRONGEST OF THE WINDS SEEMS TO HAVE PASSED WITH MOST LOCATIONS
ACROSS NORTHERN IL GUSTING BELOW 30 KTS. EXPECT WINDS TO SLOWLY
DIMINISH WITH GUSTS 20-25KTS STILL LIKELY BY MORNING. WINDS NEAR
LAKE MICHIGAN INCLUDING GYY WILL BE STRONGER AND SLOWER TO
DIMINISH. WINDS MAY INCREASE AGAIN THURSDAY MORNING THOUGH GUSTS
LIKELY TO REMAIN BELOW 30KTS THEN GUSTS SHOULD DIMINISH COMPLETELY
WITH SUNSET THURSDAY. CMS
&&
.MARINE...
1230 PM CST
DEEP SURFACE LOW OVER LAKE HURON THIS EVENING SHOULD KICK OUT FOR
SRN QUEBEC BY MRNG BUT STILL MAINTAIN GALE FORCE WINDS OVER THE
LAKE. EVEN WITH LOW TRACKING NE TO NRN QUEBEC FRIDAY...PRESSURE
GRADIENT ACROSS THE LAKES STILL QUITE STRONG AS HEFTY HIGH
PRESSURE PATTERN FROM THE NRN ROCKIES AND PLAINS BUILDS SE ACROSS
THE MS VLY TO THE MID ATLANTIC COAST THRU THAT PERIOD. FINALLY GET
SOME RELAXATION OF WINDS LATE FRIDAY NIGHT INTO SATURDAY AS SFC
LOW PULLS OFF THE EAST COAST OF CANADA...AND THE WESTERN PORTION
OF RIDGING ACROSS THE OHIO VLY BUILDS NORTHWEST OVER THE LAKES.
WINDS OVER LAKE GENERALLY WEST TO NW AT GALES FORCE THRU FRIDAY
BUT FINALLY BACKING TO SW LATE FRI NGT AND SAT.
RLB
&&
.LOT WATCHES/WARNINGS/ADVISORIES...
IL...WINTER WEATHER ADVISORY...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-
ILZ010-ILZ011-ILZ012-ILZ013-ILZ014-ILZ019-ILZ020-ILZ021-
ILZ022 UNTIL MIDNIGHT THURSDAY.
WIND CHILL ADVISORY...ILZ003-ILZ004-ILZ005-ILZ008-ILZ010-ILZ011-
ILZ012-ILZ019...3 AM THURSDAY TO NOON THURSDAY.
IN...NONE.
LM...GALE WARNING...LMZ080-LMZ261-LMZ362-LMZ364-LMZ366-LMZ563-LMZ565-
LMZ567-LMZ669-LMZ671-LMZ673-LMZ675-LMZ777-LMZ779-LMZ868-
LMZ870-LMZ872-LMZ874-LMZ876-LMZ878 UNTIL 1 AM SATURDAY.
GALE WARNING...NEARSHORE WATERS UNTIL 6
PM THURSDAY.
&&
$$
