FXUS63 KLOT 142351
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
551 PM CST MON DEC 14 2009
.DISCUSSION...
229 PM CST
RADAR AND SATLT TRENDS CONFIRM THAT VORT MAX NOW MOVING THROUGH
NERN IL IS CONTINUING TO SHEAR OUT AS IT ACCELERATES NEWD.
COVERAGE OF SNOW BAND ASSD WITH SOUTHERN PORTION OF ENHANCED
CLOUDINESS TO N OF THIS FEATURE HAS BEEN DECREASING STEADILY AND
THIS TREND IS EXPECTED TO CONTINUE. CLOUD TOPS ASSD WITH COMMA
HEAD HAVE ALSO BEEN WARMING. AS A RESULT...WILL KEEP FCST DRY
DESPITE FAIRLY STRONG COLD ADVECTION EXPECTED TO DVLP OVERNIGHT.
THIS LEAD SYSTEM IS BEING EJECTED BY A MORE ENERGETIC FEATURE IN
THE NRN STREAM...NOW DIGGING ESEWD ALONG NORTH DAKOTA/SASKATCHEWAN
BORDER. THIS SHORT WAVE WILL AMPLIFY AS IT SHIFTS EWD ACROSS UPR
GRTLKS BY TUESDAY MORNING...BEFORE FORMING A CLOSED LOW OVER ERN
ONTARIO. THIS SYSTEM'S IMPACT ON THE MIDWEST WILL BE IN THE FORM
OF WIND AND COLD ADVECTION AS SFC HIGH PRESSURE BUILDS UPSTREAM
OVER THE NRN PLAINS AND MO VLY. AS THIS SCENARIO EVOLVES...AREA OF
MODEST PRESSURE RISES TO W OF MS RVR WILL COMBINE WITH COLD
ADVECTION LATER TONIGHT AND TUESDAY MORNING AS H85 THERMAL RIBBON
PASSES ACROSS AREA. THIS SHOULD KICK UP NWLY WINDS INTO 20 MPH
NEIGHBORHOOD FOR A WHILE LATE TONIGHT AND TUESDAY MORNING...BEFORE
GRADUALLY SUBSIDING DURING THE AFTERNOON AS MAIN PUSH OF COLD AIR
SHIFTS OFF TO E.
HIGH CENTER TO MOVE OVERHEAD TUESDAY NIGHT SETTING UP GOOD
RADIATION AL COOLING SITUATION...ESPECIALLY OVER SNOW COVER. HAVE
MASSAGED LOW TEMP GRIDS ACCORDINGLY WITH LOWS TUES NIGHT SUBZERO
IN RFD AREA...TO MID TEENS E CONTROL IL-NW RN INDIANA.
HIGH SHIFTS E OF AREA ON WED...BUT AFC/BOUNDARY LYR FLOW REMAINS
QUITE LIGHT SO DESPITE WARMING ALOFT...SFC TEMPS LIKELY TO REMAIN
CHILLY...AGAIN MAINLY OVER SNOW COVER. MOS GUIDANCE TEMPS REFLECT
THIS WELL WITH TEMPS WED AFTERNOON FCST TO REMAIN WELL BELOW
FREEZING.
RATHER BENIGN WX PERIOD TO PERSIST THROUGH END OF
WEEK...HOWEVER...AFTER A PERIOD OF MORE ZONAL FLOW WEDNESDAY NIGHT
AND THURSDAY...A SERIES OF DIGGING SHORT WAVES ARE FCST TO AMPLIFY
LARGER SCALE TROFFING OVER CNTRL AND ERN CANADA AND ERN U.S. RIGHT
NOW SNOW CHANCES LOOK MEAGER AND UNSATISFACTORY...BUT ONE OF THESE
CLIPPER TYPE SYSTEMS COULD BRING A DUSTING HERE OR THERE FRIDAY
NIGHT INTO SATURDAY. OF GREATER RELIABILITY WILL BE A TREND TOWARD
BELOW AVERAGE TEMPERATURES HEADING INTO WEEKEND. THE MAGNITUDE OF
THIS COOLING VARIES QUITE A BIT DEPENDING ON GUIDANCE WITH SOME
FAIRLY SUBSTANTIAL DIFFERENCES IN UPPER FLOW EVOLVING AMONG
MODELS. AS AN EXAMPLE...12Z EUROPEAN ROTATED A LOBE OR WRN
EXTENSION OF UPR LOW OVER CANADIAN MARITIMES DOWN ACROSS
MIDWEST...THEN PROGRESSES THIS EWD THROUGH OH VLY BY DAY 7.
MEANWHILE...GFS RETROGRADES THE ENTIRE CLOSED LOW BACK ACROSS THE
WRN GRTLKS BEFORE THIS CIRCULATION SHIFTS EWD ACROSS THE LOWER
LAKES BY DAY 7. FOR NOW HAVE JUST STAYED WITH HPC GUIDANCE THAT
REFLECTS A MODEST COOL PERIOD THROUGH EXTENDED TIME FRAME.
MERZLOCK
&&
.AVIATION...
0000 UTC TAFS...MVFR CEILINGS AND GUSTY NORTHWEST SFC WINDS MAIN
AVIATION FORECAST CONCERNS THIS EVENING.
EARLY EVENING SFC ANALYSIS SHOWS LOW PRESSURE NOW LOCATED NEAR PORT
HURON MICHIGAN...HEADING OFF TO THE EAST INTO SOUTHERN ONTARIO. HAVE
GENERALLY DISCOUNTED NEAR TERM MODEL FORECASTS OF IFR/LIFR
CONDITIONS TONIGHT...AS REGIONAL OBS AND SATELLITE IMAGERY CURRENTLY
INDICATE THAT AN EXTENSIVE AREA OF MVFR CLOUDINESS LINGERS ON BACK
SIDE OF DEPARTING LOW ACROSS WI/IL AND EASTERN IA AND DRIER AIR IS
HELPING TO GRADUALLY RAISE CEILINGS FROM THE WEST. WHILE CHICAGO
AREA TERMINALS HAVE CURRENTLY SCATTERED OUT LOW LAYERS AND
TEMPORARILY IMPROVED TO VFR...UPSTREAM OBS AND SHORT TERM MODEL
GUIDANCE WOULD SUGGEST A RETURN TO MVFR CIGS WITHIN A COUPLE OF
HOURS. WOULD THEN EXPECT GRADUAL IMPROVEMENT THROUGH MVFR RANGE
THROUGH TUESDAY MORNING BEFORE SKIES CLEAR LATE MORNING/MIDDAY.
WHILE A STRAY SNOW FLAKE OR TWO NOT COMPLETELY OUT OF THE QUESTION
THIS EVENING ACROSS NORTHERN IL TERMINALS...SHALLOW MOIST LAYER
SUGGESTS SHOULD NOT BE ENOUGH TO RESTRICT VISIBILITY OR ACCUMULATE.
AS FOR WINDS...AREA OBS DEPICT A SHIFT TO A 300-320 DEG DIRECTION
AND THIS IS EXPECTED TO CONTINUE THROUGH THE NIGHT BEFORE BACKING
SLIGHTLY MORE WEST-NORTHWEST TUESDAY. SFC PRESSURE GRADIENT/PRESSURE
RISES AND STEEPENING OF LOW LEVEL LAPSE RATES DUE TO COLD ADVECTION
PRODUCING GUSTS TO AROUND 20 KTS OVERNIGHT/EARLY TUES...THEN WILL
DIMINISH TUES AFTN AS SFC HIGH PRESSURE BUILDS INTO THE REGION.
RATZER
&&
.MARINE...
227 PM CST
A PERIOD OF NORTHWEST FLOW WILL PERSIST ACROSS LAKE MICHIGAN THE
NEXT 24 HOURS STARTING THIS EVENING AS LOW PRESSURE NEAR PORT
HURON CONTINUES TO SLIDE OFF TO THE EAST AND HIGH PRESSURE OVER
THE HIGH PLAINS BUILDS SLOWLY EASTWARD INTO THE MISSISSIPPI
VALLEY. SPRAWLING HIGH PRESSURE AT 1037 MB (30.60 INCHES) WILL
THEN MOVE TO CENTRAL ILLINOIS BY WEDNESDAY NIGHT AS A WEAK
TROUGH OF LOW PRESSURE DEVELOPS OVER LAKE SUPERIOR THROUGH THE
STRAITS OF MACKINAC.
THIS WEAK TROUGH DRIFTS SOUTH INTO THE CENTRAL PART OF THE LAKE BY
THURSDAY MORNING BUT THEN WASHES OUT WITHIN THE LARGER AREA OF
HIGH PRESSURE PERSISTING BOTH ACROSS THE OHIO VALLEY AND PASSING
NORTH OF LAKE SUPERIOR. LOW PRESSURE THEN ORGANIZING OVER THE
CENTRAL PLAINS MOVES TO SOUTHERN ILLINOIS BY FRIDAY MORNING WITH
AN INVERTED TROUGH EXTENDING NORTHWARD FROM THE LOW. THIS FEATURE
SLIDES OFF TO THE EAST OF LAKE MICHIGAN FRIDAY NIGHT ALLOWING FOR
STRONGER NORTHWEST FLOW TO DEVELOP BY SATURDAY.
ED F
&&
.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...NEARSHORE WINTHROP HARBOR TO GARY IN UNTIL
MIDNIGHT WEDNESDAY.
SMALL CRAFT ADVISORY...LMZ744-LMZ745...9 PM MONDAY TO 6 AM
WEDNESDAY.
&&
$$
