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000
FXUS63 KLOT 030016
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
616 PM CST MON DEC 2 2013

.DISCUSSION...
609 PM CST

I HAVE UPDATED THE FORECAST FOR THIS EVENING TO INCREASE P0PS AND
ALSO ADD A SLIGHT CHANCE OF SOME LIGHT SNOW AND SLEET. AREA AMDAR
SOUNDINGS IN CONJUNCTION WITH SURFACE OBS AND MODEL BUFFER
SOUNDINGS INDICATE THAT THE PRECIP MAY MIX WITH SOME LIGHT SNOW OR
SLEET AT THE ONSET THIS EVENING ACROSS NORTHEASTERN ILLINOIS. AREA
DEW POINTS ACROSS NORTHEASTERN ILLINOIS ARE GENERALLY IN THE UPPER
20S WITH SURFACE TEMPS IN THE UPPER 30S TO NEAR 40. THEREFORE...
EVAPORATION COOLING WILL LIKELY ALLOW THE FREEZING HEIGHTS TO
LOWER JUST ENOUGH TO GET SOME WINTER PRECIP TO THE SURFACE. THERE
HAVE EVEN BEEN A FEW REPORTS OF LIGHT SNOW MIXING IN WITH THE RAIN
OUT WEST. ANY WINTRY PRECIP SHOULD BE SHORT LIVED...LIKELY ENDING
BY LATE EVENING. NO ACCUMULATION IS EXPECTED.

KJB

&&

.PREV DISCUSSION...
341 PM CST

A LOOK AT THE 09Z SREF TEMPERATURE PLUMES THIS MORNING FOR ORD TOLD
THE QUICK STORY FOR THE WEEK...NAMELY GRADUALLY WARMER IN THE FIRST
HALF...A COLD FRONT PASSAGE SOMETIME WEDNESDAY...AND MUCH COOLER
AGAIN FOR THE LATTER HALF AND INTO THE WEEKEND.  WHAT WAS
INTERESTING ABOUT THE 09Z SREF RUN WAS THE SPREAD WEDNESDAY
AFTERNOON FROM 59 DEGREES IN THE WARMEST MEMBER TO THE UPPER 20S IN
THE COOLEST MEMBERS.  THE 15Z RUN NARROWED THIS SLIGHTLY BY WARMING
THE COLDEST MEMBERS INTO THE LOWER 30S.  SO WE HAVE IT DOWN TO A 30
DEGREE RANGE.  TIMING OF THE FRONT OBVIOUSLY WILL BE CRITICAL.
BEFORE AND AFTER THE FRONT THE SPREAD IS MUCH SMALLER.

THE WEEK BEGINS WITH A BROAD LONGWAVE UPPER TROUGH CENTERED OVER THE
GREAT LAKES WITH NORTHWEST FLOW ACROSS THE WESTERN CONUS.  A CLOSED
UPPER LOW MOVING ONSHORE IN THE PACIFIC NORTHWEST WAS ASSISTING
SURFACE CYCLOGENESIS DOWNSTREAM OVER THE NORTHERN PLAINS IN THE LEE
OF THE ROCKIES.  BY TOMORROW THE PATTERN TRANSITIONS TO SOMETHING
MORE ZONAL BEFORE A DEEPER AND BROADER LONGWAVE TROUGH DIGS ACROSS
THE WESTERN CONUS FOR LATE IN THE WEEK INTO THE WEEKEND.

THE HANDLING OF THE UPPER AND SURFACE LOWS TOMORROW AND WEDNESDAY
DURING THE BRIEF TRANSITION TO ZONAL FLOW IS WHERE THE GEM/ECM
MODELS ARE HAVING A HARD TIME AGREEING WITH THE GFS/NAM.  ALL FOUR
SHOW THE LEE CYLONE STRENGTHENING AND DROPPING INTO THE CENTRAL
PLAINS WHILE A WARM FRONT LIFTS THROUGH THE LOCAL AREA EARLY
TOMORROW...BRINGING A CHANCE OF LIGHT RAIN STARTING TONIGHT AND
MILDER TEMPERATURES TOMORROW.

DIFFERENCES THEN BECOME FAIRLY PRONOUNCED AS EARLY AS TOMORROW
EVENING.  THE GFS EJECTS A MUCH STRONGER AND NEGATIVELY TILTED
SHORTWAVE ACROSS THE NORTHERN CONUS WHILE THE GEM/ECM MODELS ARE
SHOWING EITHER A WEAKER OR A SLOWER MANIFESTATION OF THIS ENERGY.
THE NAM SPLITS THE DIFFERENCE AND THE SREF LEANS TOWARD THE
GFS...THOUGH WITH CONSIDERABLE SPREAD AS MENTIONED ABOVE.  HAVE
COORDINATED WITH NEIGBORS IN LEANING TOWARD THE SLOWER NON NCEP
SOLUTIONS...AS ALSO SUPPORTED BY WPC...BUT WITH A NUDGE TOWARD THE
COOLER AND FASTER SREF ENSEMBLE.  SO THIS KEEPS WEDNESDAY MILD FOR
MOST OF THE AREA...AT LEAST UNTIL OPINIONS CHANGE AND THE GFS/NAM
BECOME MORE FAVORED.

THE FORECAST SIMPLIFIES SOMEWHAT AFTER THE FRONT DOES CLEAR THE
AREA...WHENEVER THAT HAPPENS TO BE.  REGARDLESS OF HOW WARM THINGS
GET BEFORE THE FROPA WEDNESDAY...THURSDAY BRINGS A RETURN TO WINTER
WITH HIGHS ONLY IN THE UPPER 20S TO MIDDLE 30S...THEN FRIDAY AND
SATURDAY COOL US EVEN FURTHER UNDER THE INFLUENCE OF NORTH TO
NORTHEAST WINDS AHEAD OF A BROAD ARCTIC HIGH PASSING TO OUR NORTH.
PERIODS OF LIGHT AND NON ACCUMULATING SNOW ARE POSSIBLE IN THE ARTIC
AIR.

TEMPERATURES LOOK TO MODERATE SOMEWHAT BY SUNDAY AND MONDAY AS THE
ARCTIC HIGH DEPARTS TO THE EAST...BUT ONLY TO THE MIDDLE OR UPPER
20S FOR HIGHS.  WITH COLD AIR STILL IN PLACE...SHORTWAVE ENERGY
ALOFT AND A SYSTEM LIFTING OUT OF THE GULF MAY BRING A CHANCE OF
SNOW TO THE AREA SUNDAY/SUNDAY NIGHT.

LENNING

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 00Z...

* EAST WINDS ARND 10KT THRU DAYBREAK TUE.

* VFR CIGS STEADILY LOWER TO MVFR BY 03Z...THEN CONTINUE TO LOWER
  TO ARND IFR OR 700FT AGL BY 06Z AND REMAIN IFR THRU TUE MORNING.

* LIGHT RAIN/DRIZZLE AND POSSIBLY A PERIOD OF LIGHT WET SNOW THIS
  EVENING AT THE ONSET OF PRECIP.

* BULK OF PRECIP ENDS ARND 06-08Z WITH SOME PATCHY DZ AND REDUCED
  VSBYS TO 4-5SM OVERNIGHT.

BEACHLER

//DISCUSSION...UPDATED 00Z...

REGIONAL RADAR SHOWS A CHANNEL OF PRECIPITATION EXTENDING
SOUTHEAST FROM SOUTHWEST WISCONSIN THROUGH EAST-CENTRAL ILLINOIS.
THIS CHANNEL OF PRECIP WILL CONITNUE TO SLIDE EAST THIS
EVENING...REACHING ORD/MDW ARND 3Z. TEMPERATURES ARE HOVERING IN
THE MID/UPR 30S...HOWEVER WITH GUIDANCE INDICATING SOME COOLING TO
TAKE PLACE THIS EVENING...SOME OF THE PRECIP MAY BEGIN TO FALL AS
LIGHT SNOW/SLEET OR A MIX OF RAIN/SNOW. IT IS EXPECTED TO REMAIN
VERY LIGHT AND NOT POSE ANY PROBLEMS...THEN THE BULK OF THE PRECIP
SHUD COME TO AN END ARND 06-08Z ACROSS THE TAF SITES. ALTHOUGH
DESPITE THE PRECIP COMING TO AN END...LINGERING LLVL MOISTURE WILL
KEEP SOME PATCHY DZ OR MIST IN THE AREA. GUIDANCE CONTINUES TO
INDICATE CIGS STEADILY LOWERING FROM VFR TO MVFR BY 03Z...THEN
DROP TO IFR CONDS OR 700FT AGL ARND 06Z OR CLOSER TO WHEN PRECIP
ENDS. THEN WITH THE LACK OF MIXING AND DRY AIR...IFR CLOUDS LOOK
TO LINGER THRU DAYBREAK TUE AND LIKELY PERSIST THRU MIDDAY. SOME
IMPROVEMENT TO CIGS ARND MVFR BY 00Z WED. WINDS THROUGHOUT THE
NEXT 12 HOURS WILL REMAIN 110-130 DEG WITH SPEEDS ARND 10KT. THEN
WINDS MAY BEGIN TO TURN 140-160 DEG BY DAYBREAK TUE.

BEACHLER

//ORD AND MDW CONFIDENCE...UPDATED 00Z...

* HIGH CONFIDENCE IN PRECIP TIMING/MVFR TIMING.
* MEDIUM/LOW CONFIDENCE IN ANY PRECIP FALLING AT ORD/MDW AS LIGHT
  SNOW/SLEET.
* HIGH CONFIDENCE IN WIND DIR/SPEED FORECAST.
* MEDIUM/HIGH CONFIDENCE IN PRECIP ENDING TIME.
* MEDIUM/HIGH CONFIDENCE IN CIGS FORECAST OF IFR OVERNIGHT THRU
  TUE MORNING.

BEACHLER

//OUTLOOK FOR ORD/MDW FOR 12Z WEDNESDAY-12Z MONDAY...UPDATED 00Z...

WEDNESDAY...MVFR CIGS POSSIBLE. CHC OF RAIN.
THURSDAY...VFR. POSSIBLE PERIOD OF LIGHT SNOW/FLURRIES.
FRIDAY THROUGH SUNDAY...VFR.

BEACHLER

&&

.MARINE...
234 PM CST

AS HIGH PRESSURE SCOOTS AWAY FROM THE LAKE...LOW PRESSURE OVER THE
CENTRAL U.S. WILL HAVE LONG-LIVED INFLUENCE ON WINDS AND WAVES OVER
THE LAKE THROUGH THE MIDDLE PART OF THIS WEEK. THE PRESSURE GRADIENT
REALLY BEGINS TO TIGHTEN OVER THE LAKE BEGINNING LATE TONIGHT AND
FURTHER INTO TUESDAY. THE WARM AIR ADVECTION OVER THE LAKE WILL
PROVIDE AN INVERSION AND THE STRENGTH OF THIS VARIES ON MODEL
GUIDANCE. HOWEVER...THERE IS A CONSENSUS THAT DESPITE THAT VARYING
DEPTH OF THE INVERSION...THE MAXIMUM WINDS IN THE SOUTHERLY FLOW
WILL PRIMARILY IF NOT ENTIRELY STAY BELOW GALE FORCE. THE LOW CENTER
ITSELF IS EXPECTED TO MOVE NORTHEASTWARD NEAR OR OVER THE NORTHERN
THIRD OF THE LAKE. THE TIME IN WHICH THIS OCCURS IS HIGHLY
UNCERTAIN...BUT AFTER IT DOES THERE SHOULD BE A DECENT BOUT OF COLD
AIR ADVECTION WHERE GALES WOULD BE MOST FAVORED THIS WEEK. RIGHT NOW
THE FAVORED TIME USING A MEAN OF MODEL GUIDANCE IS WEDNESDAY NIGHT
AND POSSIBLY INTO THURSDAY. A SECONDARY COLD FRONTAL PASSAGE MAY
OCCUR LATE IN THE WEEK AND PROVIDE WINDS POSSIBLY NEAR GALE FORCE
AGAIN. OVERALL A PROLONGED NORTHWESTERLY PATTERN IS EXPECTED FROM
THURSDAY ONWARD.

MTF

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...NONE.
&&

$$

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