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000
FXUS63 KLOT 060916
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
316 AM CST FRI DEC 6 2013

.DISCUSSION...
316 AM CST

PRIMARY FORECAST CHALLENGE REVOLVES AROUND TEMPS AND SUNDAY/SUNDAY
EVENING`S SNOW.

DEEP LONG WAVE TROUGH HAS ESTABLISHED ITSELF OVER THE WESTERN HALF
OF THE CONUS EAST INTO THE GREAT LAKES REGION. TIGHT GEOPOTENTIAL
HEIGHT GRADIENT AND STRONG POLAR JET SEPARATES THE ARCTIC AIR THAT
SPILLED SOUTH INTO THE WESTERN AND NORTHERN CONUS AND THE
UNSEASONABLY WARM AIR MASS TO ITS SOUTH. A 100-200MI WIDE SWATH OF
PRECIPITATION CONTINUES TO TRAIN NORTHEASTWARD TO THE NORTH OF THE
POLAR FRONT WELL TO THE SOUTH OF OUR CWA. WATER VAPOR IMAGERY SHOWS
A SHORTWAVE TROUGH ROUNDING THE BASE OF THE WESTERN TROUGH OVER NEW
MEXICO WITH MODELS LIFTING THAT LOW AMPLITUDE WAVE NORTHEASTWARD AS
IT SHEARS OUT INTO THE MID MISSISSIPPI VALLEY THIS AFTERNOON. EXPECT
THIS TO RESULT IN A NORTHWARD INFLECTION OF THE PRECIP BAND IN
ADVANCE OF THE WAVE...WITH NORTHERN MOST EXTENT OF THE PRECIP
POTENTIALLY GETTING INTO OUR FAR SOUTHEAST CWA THIS AFTERNOON.

IN THE WAKE OF THIS WAVE ANTICIPATE SKIES TO CLEAR TONIGHT AND ALLOW
FOR A VERY COLD NIGHT ACROSS THE AREA. GRADIENT IS PROGGED TO REMAIN
STRONG ENOUGH TO ALLOW PREVENT IDEAL RADIATIONAL COOLING SET-UP AND
LACK OF SNOW COVER WILL ALSO HELP SPARE US FROM WHAT WOULD OTHERWISE
BE A BITTERLY COLD NIGHT. EVEN WITH A LITTLE WIND AND NO SNOW EXPECT
LOWS TO BOTTOM OUT JUST BARELY ABOVE ZERO IN THE TYPICALLY COLD
SPOTS TO 10-15 IN THE URBAN "HEAT" ISLAND OF CHICAGO. WINDS AROUND
10 MPH WILL PRODUCE SUB ZERO WIND CHILLS...POSSIBLY BOTTOMING OUT
AROUND 10 TO 15 BELOW. DESPITE SUNNY SKIES SATURDAY WILL BE VERY
COLD WITH HIGHS STRUGGLING TO REACH 20.

WESTERN EXTENT OF THE LONG WAVE TROUGH WILL RE-LOAD THIS WEEKEND AS
STRONG SHORTWAVE OFF THE PAC NW COAST DIGS SOUTH INTO THE FOUR
CORNERS REGION SATURDAY...BEFORE HEADING EAST INTO THE PLAINS
SUNDAY. MID-HIGH LEVEL CLOUDINESS WILL BE ON THE INCREASE IN ADVANCE
OF THIS SYSTEM SATURDAY NIGHT WITH SKIES EVENTUALLY CLOUDING OVER.
LOWS SATURDAY NIGHT LIKELY TO BE IN THE EVENING...ESPECIALLY OUT
WEST BEFORE TURNING STEADY OR EVEN RECOVERING A FEW DEGREES BY
MORNING.

SURFACE RIDGE OVER THE AREA SATURDAY EVENING WILL MOVE EAST
OVERNIGHT WITH LOW LEVEL FLOW TURNING EASTERLY AFTER MIDNIGHT.
THERMODYNAMICALLY CONDITIONS WILL REMAIN MARGINALLY FAVORABLE FOR
LAKE EFFECT CLOUDS AND PROBABLY SOME SNOW SHOWERS AS WELL AS DELTA T
VALUES REMAIN AROUND 15C AND LAKE INDUCED EQUILIBRIUM LEVELS REMAIN
AROUND 4-5K FT. GIVEN THE LOW EL`S WOULDNT ANTICIPATE ANY
SIGNIFICANT ACCUMS BUT LAKE EFFECT SNOW SHOWERS COULD CONTINUE
SUNDAY MORNING OVER MAINLY LAKE IL AND NORTHERN COOK COUNTIES.

SYSTEM SNOWS WILL BEGIN TO SPREAD EAST INTO ILLINOIS SUNDAY
AFTERNOON THOUGH TOP DOWN SATURATION WILL NEED TO TAKE PLACE BEFORE
SNOW REACHES THE GROUND. COULD SEE SYSTEM SNOWS SEED THE LAKE EFFECT
CLOUDINESS AND RESULT IN EITHER AN INCREASE IN LES SHOWERS SUNDAY
AFTERNOON OR AN EARLY START TO THE SYNOPTIC SNOWS THANKS TO THE LAKE
MOISTURE. MODELS GENERALLY IN PRETTY GOOD AGREEMENT ON ABOUT A 6
HOUR PERIOD OF SOME LIGHT TO MODERATE SNOW SUNDAY
EVENING...ESPECIALLY NORTHERN CWA WHERE STRONGER FORCING AND DEEPER
MOISTURE WILL RESIDE. FORECAST SOUNDINGS OVER SOUTHERN PORTIONS OF
THE AREA SHOW MID LEVEL DRY AIR IN PLACE MEANING THAT IF PRECIP DOES
FALL IT COULD BE MORE OF A FREEZING SNIZZLE OR FREEZING DRIZZLE WITH
LITTLE/NO ACCUMULATION. FARTHER NORTH TOWARD THE WI BORDER MODEL QPF
OF AROUND A TENTH INCH COULD FLUFF UP TO A COUPLE INCHES OF SNOW
GIVEN THE EXPECTED FAIRLY HIGH SNOW:LIQUID RATIOS. AGAIN...SOME LOW
LEVEL COMPONENT TO THE WIND OFF THE LAKE COULD RESULT IN SOME MINOR
LAKE ENHANCEMENTS OVER LAKE COUNTY ILLINOIS EARLY SUNDAY EVENING
BEFORE WINDS BECOME OFFSHORE.

RE-ENFORCING SHOT OF ARCTIC AIR WILL BEGIN TO FILTER INTO THE AREA
SUNDAY NIGHT INTO MONDAY...BUT GFS/ECMWF ARE NOW BRINGING A FAIRLY
STRONG CLIPPER INTO THE UPPER GREAT LAKES BY TUES MORNING WHICH
WOULD RESULT IN WARM-ISH AIR ADVECTION AS WINDS BACK SOUTHERLY
MONDAY NIGHT INTO TUESDAY. SECONDARY ARCTIC FRONT ASSOCIATED WITH
THIS CLIPPER SHOULD MOVE THROUGH THE AREA BY TUES EVENING AS OLD MAN
WINTER ESSENTIALLY DRY HEAVES MORE ARCTIC AIR INTO THE AREA FOR
LATER TUES NIGHT INTO WEDNESDAY. WHERE EVER SUNDAY`S SYSTEM LEAVES A
SNOW COVER COULD RESULT IN TEMPS A BIT COLDER THAN WHAT GOING
FORECAST HAS...BUT WITH MEDIUM RANGE MODELS CONSISTENTLY TRENDS A
BIT LESS INTENSE WITH EACH SUCCESSIVE COLD SNAP AS IT APPROACHES
DECIDED NOT TO GET CARRIED AWAY WITH LOWERING TEMPS TOO MUCH.

IZZI

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 09Z...

* NONE.

LENNING

//DISCUSSION...UPDATED 06Z...

COULD ALMOST GO WITH ONE LINE TAFS FOR STEADY NORTHWEST WINDS AND
A MID LEVEL VFR CLOUD DECK AHEAD OF AN APPROACHING DOME OF HIGH
PRESSURE...BUT WANTED TO CONVEY A TURN TO MORE OF A NNW
DIRECTION FOR THIS AFTERNOON AND THUS MORE OF A CROSSWIND ON THE
EAST-WEST RUNWAYS.

THIS PATTERN PERSISTS UNTIL LATE SATURDAY WHEN THE CENTER OF THE
HIGH PASSES JUST NORTH OF THE AREA AND TURNS WINDS EAST OFF THE
LAKE FOR SATURDAY NIGHT AND SUNDAY.

LENNING

//ORD AND MDW CONFIDENCE...UPDATED 09Z...

* HIGH CONFIDENCE FOR ALL ELEMENTS.

LENNING

//OUTLOOK FOR ORD/MDW FOR 12Z SATURDAY-12Z THURSDAY...UPDATED 00Z...

SATURDAY...VFR.

SUNDAY...CHANCE OF SNOW...ESPECIALLY LATE. IFR CONDITIONS
DEVELOPING LATE.

MONDAY...CHANCE OF SNOW WITH MVFR OR LOWER EARLY.

TUESDAY AND WEDNESDAY...MAINLY VFR.

KJB

&&

.MARINE...
300 AM CST

COLD WESTERLY FLOW WILL PERSIST ACROSS THE LAKE THROUGH SATURDAY
DUE TO THE INFLUENCE OF LOW PRESSURE OVER QUEBEC AND A BROAD AREA
OF HIGH PRESSURE SPREADING INTO THE NORTHERN PLAINS AND UPPER
MIDWEST. A NEW LOW PRESSURE SYSTEM EXPECTED TO DEVELOP SUNDAY
OVER THE LOWER MISSISSIPPI VALLEY WILL LIFT INTO THE CENTRAL GREAT
LAKES MONDAY. NORTH OF THE LOW WINDS TURN EAST LATE SUNDAY AND
THEN GO WEST AGAIN MONDAY AFTER THE LOW LIFTS ACROSS THE LAKE.

THE COMBINATION OF COLD STRONG WINDS AND RELATIVELY HIGH WAVES
WILL SUPPORT FREEZING SPRAY ACROSS MUCH OF THE LAKE TONIGHT AND
ACROSS THE NORTHERN HALF TOMORROW.

LENNING

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...NEARSHORE GARY TO MICHIGAN CITY IN UNTIL 10 AM FRIDAY.

&&

$$

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