Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 201753
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1153 AM CST SAT DEC 20 2014

.SHORT TERM...
302 AM CST

TODAY THROUGH SUNDAY...

THE MAIN CONCERNS DURING THE SHORT TERM WILL BE WILL BE CENTERED
AROUND THE POTENTIAL FOR SOME FREEZING DRIZZLE ACROSS PORTIONS OF
NORTHERN ILLINOIS LATE TONIGHT INTO EARLY SUNDAY MORNING.

SURFACE HIGH PRESSURE CONTINUES TO BE IN PLACE ACROSS THE GREAT
LAKES REGION EARLY THIS MORNING. HOWEVER...ATTENTION FOR TODAY IS
FOCUSED ON A MID LEVEL DISTURBANCE...CURRENTLY NOTED IN THE WATER
VAPOR IMAGERY ACROSS SOUTHWESTERN IOWA. THIS DISTURBANCE WILL SHIFT
EASTWARD ACROSS CENTRAL AND NORTHERN ILLINOIS THROUGH EARLY THIS
AFTERNOON. OVERALL...MODEL SOUNDINGS DO NOT APPEAR ALL THAT
FAVORABLE FOR ANY LIGHT SNOW. HOWEVER...THERE HAVE BEEN SOME REPORTS
OF LIGHT FLURRIES ACROSS IOWA EARLY THIS MORNING...SO I HAVE OPTED
TO KEEP THE MENTION OF SOME FLURRIES GOING ACROSS THE AREA THROUGH
EARLY AFTERNOON.

WARM AIR ADVECTION/ISENTROPIC ASCENT WILL INCREASE
ACROSS MUCH OF THE MIDWEST AND GREAT LAKES REGION LATER TONIGHT INTO
SUNDAY MORNING IN RESPONSE TO AN INCREASING SOUTHERLY WIND FIELD
AHEAD OF A DEVELOPING STORM SYSTEM ACROSS THE NORTHERN HIGH PLAINS.
IT APPEARS THAT THIS WARM AIR ADVECTION WILL BE ENOUGH TO RESULT IN
THE DEVELOPMENT OF SOME FREEZING DRIZZLE OR LIGHT SNOW. MODEL
SOUNDINGS...ESPECIALLY FROM THE NAM SUGGEST CLOUD HEIGHTS WILL ONLY
REACH TO AROUND -7 DEGREES CELSIUS...SUGGESTING THE POSSIBILITY OF A
DEARTH OF ICE CRYSTALS IN THE CLOUDS. AS SUCH...THIS INCREASES THE
THREAT FOR FREEZING DRIZZLE VERSES LIGHT SNOW. OVERALL...THIS WILL
BE A VERY LIGHT EVENT...AND APPEARS TO MAINLY BE CONFINED ACROSS
NORTH CENTRAL ILLINOIS...THOUGH PORTIONS OF THE CHICAGO AREA COULD
ALSO EXPERIENCE SOME. THE PRIMARY TIMING FOR THIS ACTIVE LOOKS TO
BE AFTER MIDNIGHT TONIGHT THROUGH AROUND MID MORNING ON SUNDAY.
THEREAFTER...SURFACE TEMPERATURES SHOULD WARM ABOVE FREEZING ENDING
THE THREAT OF FREEZING PRECIP.

KJB

&&

.LONG TERM...
347 AM CST

MONDAY THROUGH FRIDAY...

NOT MUCH HAS CHANGED IN THE EXPECTATION OF A TRANSITIONING WEATHER
PATTERN ACROSS THE CONUS SUPPORTIVE OF SEVERAL PERIODS OF INCLEMENT
WEATHER THROUGH MUCH OF THE UPCOMING HOLIDAY WEEK. THEREFORE...THE
MAIN CONCERNS DURING THE LONG TERM PERIOD WILL FOCUS AROUND THE
CHANCES AND TIMING OF RAINFALL MONDAY INTO TUESDAY...THEN THE
POTENTIAL FOR A SIGNIFICANT WINTER STORM SOMEWHERE ACROSS THE GREAT
LAKES REGION INTO CHRISTMAS EVE.

A VERY IMPRESSIVE NORTHERN PACIFIC JET STREAK IS EXPECTED TO DEVELOP
EASTWARD ONSHORE ACROSS THE PACIFIC NORTHWEST BY EARLY SUNDAY...THEN
BEGIN TO DIG A SIGNIFICANT UPPER LEVEL TROUGH ACROSS THE CENTRAL
CONS BY MID WEEK. MODEL GUIDANCE CONTINUES IN GOOD AGREEMENT IN
TRACKING AN INITIAL AREA OF LOW PRESSURE EASTWARD FROM THE NORTHERN
HIGH PLAINS TO THE UPPER MISSISSIPPI VALLEY BY TUESDAY. THIS WOULD
KEEP ALL OF NORTHERN ILLINOIS/NORTHWESTERN INDIANA IN THE WARM
SECTOR THROUGH EARLY TUESDAY...THUS ASIDE FROM A SMALL POSSIBILITY OF
THE PRECIP BEGINNING AS A MIX OF SLEET AND RAIN AT ONSET  EARLY
MONDAY...IT APPEARS THAT WE WILL BE IN FOR A RAIN EVENT ACROSS THE
ENTIRE AREA MONDAY AFTERNOON INTO MONDAY NIGHT.

THINGS BEGIN TO GET REALLY TRICKY LATE TUESDAY AND INTO CHRISTMAS
EVE...AS MODEL GUIDANCE IS INSTANT IN SECONDARY VORT MAX DIGGING
SOUTH AND ACROSS THE LOWER MISSISSIPPI VALLEY BY TUESDAY
NIGHT...LEADING TO A VERY AMPLIFIED AND INCREASING NEGATIVELY TILTED
UPPER TROUGH ACROSS THE CENTRAL CONUS. THIS IS A CLASSIC UPPER LEVEL
PATTERN SUPPORTIVE OF TREMENDOUS ATMOSPHERIC DYNAMICS AND HENCE
RAPID AND DEEP CYCLOGENESIS...SOMEWHERE ACROSS THE GREAT LAKES
REGION. IN FACT...THIS COULD RESULT IN "BOMB CYCLOGENESIS"...WHICH IS
DEFINED AS A DEEPENING RATE OF THE LOWS CENTRAL PRESSURE OF 24 MB IN
AS MANY HOURS. MODEL GUIDANCE CONTINUES TO BE IN SOME DISAGREEMENT IN
WHERE EXACTLY THIS STORM SYSTEM WILL TRACK INTO CHRISTMAS EVE.
HOWEVER...SUPPORT IS IN PLACE TO SUGGEST A GOOD POTENTIAL OF A
STRONG STORM SYSTEM ACROSS THE REGION DURING THIS PERIOD.
UNFORTUNATELY...THE ACTUAL TRACK OF THE STORM WILL BE THE DIFFERENCE
IN IF WE END UP WITH LITTLE OR NO SNOW...OR A SIGNIFICANT WINTER
STORM ACROSS THE AREA. THE LIKELIHOOD OF VERY STRONG WINDS WITH SUCH A
DEEP LOW COULD ALSO RESULT IN SIGNIFICANT BLOWING SNOW...IF IT DOES
OF COURSE SNOW THIS FAR WEST. THEREFORE...THIS STORM BEARS
SIGNIFICANT ATTENTION IN THE COMING DAYS.

FOLLOWING THIS SYSTEM...YET ANOTHER STORM SYSTEM COULD TAKE AIM ON
THE AREA BY FRIDAY. HOWEVER...UNCERTAINTY WITH THIS STORM IS
EXTREMELY HIGH...GIVEN THAT THE ACTUAL EVOLUTION OF THE FIRST STORM
SYSTEM WILL NEED TO BE IRONED OUT. OVERALL THOUGH IT LOOKS LIKE WE
ARE ENTERING INTO AN ACTIVE WEATHER PERIOD THROUGH THE REST OF THE
FORECAST PERIOD.

KJB

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 18Z...

* MVFR CIGS RETURNING THIS EVENING OR OVERNIGHT.
* PATCHY FREEZING DRIZZLE EARLY SUNDAY.

BMD

//DISCUSSION...UPDATED 18Z...

HIGH PRESSURE REMAINS OVER THE REGION TODAY RESULTING IN LIGHT
FLOW...HOWEVER WINDS WILL TREND MORE SOUTHERLY THOUGH THE TAF PERIOD
AS THE HIGH DRIFTS EAST OF THE AREA. MVFR CIGS HAVE SCATTERED AND
DRIFTED EAST OF THE TERMINALS THIS MORNING...BUT EXPECT A RETURN OF
THE MVFR THIS EVENING OR OVERNIGHT...THOUGH SPECIFIC TIMING REMAINS
UNCERTAIN. OVERNIGHT...WARM AIR ADVECTION INCREASES JUST OFF THE
DECK AND MAY ALLOW FOR SOME LIGHT FREEZING DRIZZLE TO DEVELOP WITH
SURFACE TEMPS JUST BELOW FREEZING. FORCING IS BEST OVER NORTH
CENTRAL ILLINOIS SO WILL MAINTAIN PROB30 MENTION AT RFD ONLY FOR
NOW. WINDS WILL GRADUALLY INCREASE THROUGH THE DAY SUNDAY AS THE
GRADIENT TIGHTENS AHEAD OF LOW PRESSURE APPROACHING FROM THE
NORTHWEST.

BMD

//ORD AND MDW CONFIDENCE...UPDATED 18Z...

* MEDIUM CONFIDENCE IN CIG TRENDS BUT LOW IN TIMING.
* LOW CONFIDENCE IN FREEZING DRIZZLE.

BMD

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

SUNDAY NIGHT...DRY...MAINLY VFR.

MONDAY...RAIN AND MVFR LIKELY. BREEZY SOUTH-SOUTHEAST WINDS.

TUESDAY...CHANCE OF RAIN. MVFR POSSIBLE. SOUTH WINDS BECOMING WEST.

WEDNESDAY...CHANCE OF SNOW. MVFR/IFR POSSIBLE. NORTHWEST WINDS
BECOMING GUSTY FROM THE WEST.

THURSDAY...DRY. MAINLY VFR. GUSTY WEST WINDS DIMINISHING.

FRIDAY...CHANCE OF RAIN OR SNOW. MVFR POSSIBLE. LIGHT EAST WINDS
BECOMING WEST.

RATZER

&&

.MARINE...
137 AM CST

THE MAIN MARINE FORECAST CONCERNS THIS MORNING FOCUS ON A COUPLE OF
PERIODS OF STRONG WINDS OVER THE NEXT SEVERAL DAYS...SOUTH AND
SOUTHEAST WINDS 25-30 KT DEVELOPING SUNDAY AND CONTINUING THROUGH
MONDAY EVENING...THEN THE POTENTIAL FOR WEST HIGH GALES LATE
WEDNESDAY THROUGH THURSDAY...CHRISTMAS EVE AND CHRISTMAS DAY.

IN THE NEAR TERM...WEATHER IS QUITE TRANQUIL ACROSS LAKE MICHIGAN
THIS MORNING AS DIFFUSE HIGH PRESSURE WAS DRIFTING SLOWLY EAST
ACROSS THE REGION. THE RIDGE AXIS WILL SHIFT EAST OF THE LAKE THIS
AFTERNOON ALLOWING CURRENT VARIABLE WINDS TO BECOME SOUTHERLY. SOUTH
WINDS WILL GRADUALLY INCREASE LATE TONIGHT AND SUNDAY AS AN AREA OF
LOW PRESSURE DEVELOPS SOUTHEAST IN THE LEE OF THE NORTHERN ROCKIES
WITH A CORRESPONDING TIGHTENING OF THE SURFACE PRESSURE GRADIENT
BETWEEN THE LOW AND THE HIGH TO EAST OF THE LAKES. MODEL FORECAST
SOUNDINGS SUPPORT WINDS 20-25 KT SUNDAY AFTERNOON INCREASING TO 30
KT SUNDAY NIGHT/MONDAY...BECOMING A BIT MORE SOUTHEAST LATE MONDAY
AS THE SURFACE LOW MOVES INTO THE UPPER MIDWEST.

THE SURFACE LOW IS EXPECTED TO FILL A BIT ACROSS THE REGION TUESDAY
AS IT OCCLUDES. MODEL FORECASTS CONTINUE TO DEPICT A COMPLEX
EVOLUTION BY MID-WEEK HOWEVER...WITH A SECOND AREA OF LOW PRESSURE
FARTHER SOUTH ALONG THE COLD FRONT DEEPENING AND WRAPPING NORTHWARD
INTO THE GREAT LAKES WEDNESDAY AND THURSDAY. SPREAD REMAINS AMONGST
MODEL FORECASTS OF THE EXACT TRACK AND DEPTH OF THIS LOW...BUT IT
DOES APPEAR THAT A DEEPENING SUB-980 MB LOW WILL TRACK NORTH ACROSS
THE GREAT LAKES REGION CHRISTMAS EVE INTO CHRISTMAS DAY. SUCH A LOW
PASSING EAST OF LAKE MICHIGAN SUPPORTS THE POTENTIAL FOR THE
DEVELOPMENT OF WESTERLY GALES ON THE LAKE...PERHAPS HIGH GALES ABOVE
40 KT...FROM WEDNESDAY AFTERNOON OR EVENING THROUGH THE DAY
THURSDAY.

RATZER

&&

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

$$

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