Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 201153
AFDLOT

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

.SHORT TERM...
FXUS63 KLOT 200903
AFDLOT

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

...SHORT TERM...TODAY THROUGH SUNDAY...
302 AM CST

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...
FXUS63 KLOT 200947
AFDLOT

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

.SHORT TERM...
FXUS63 KLOT 200903
AFDLOT

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

...SHORT TERM...TODAY THROUGH SUNDAY...
302 AM CST

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

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 12Z...

* MVFR CEILINGS SCATTERING/RAISING TO VFR THIS MORNING... THEN
  LIKELY LOWERING TO MVFR AGAIN TONIGHT.

* POTENTIAL FOR SOME LIGHT FREEZING DRIZZLE SUNDAY MORNING.

RATZER

//DISCUSSION...UPDATED 12Z...

DIFFUSE SURFACE HIGH PRESSURE RIDGE DRIFTING EAST ACROSS TERMINALS
EARLY THIS MORNING. LIGHT WINDS AND SHALLOW INVERSION CONTINUE TO
PLAGUE THE REGION WITH EXTENSIVE MVFR CEILINGS...THOUGH SIZABLE
AREA OF VFR ACROSS WESTERN/CENTRAL IL HAS BEEN DEVELOPING SLOWLY
NORTHEAST WITH TIME OVERNIGHT...AND WILL LIKELY ALLOW FOR
SCATTERING OF MVFR CIGS AT AREA TERMINALS THIS MORNING. MODEL
FORECAST GUIDANCE INDICATES AREA WILL LIKELY REMAIN ALONG VFR/MVFR
INTERFACE TO THE EAST...SO HOUR TO HOUR DETAILS OF CEILING DETAILS
A LITTLE UNCERTAIN. CEILINGS EXPECTED TO GRADUALLY LOWER BACK INTO
MVFR RANGE TONIGHT...WITH THE POTENTIAL FOR SOME LIGHT
PRECIPITATION IN THE FORM OF FREEZING DRIZZLE TOWARD SUNDAY
MORNING ESPECIALLY ACROSS KRFD AREA.

SURFACE RIDGE AXIS APPEARS TO BE PASSING EAST OF ORD/MDW AT 12Z...
WITH LIGHT SOUTHWEST WINDS TO GRADUALLY BECOME SOUTHERLY LATE
TODAY AND TONIGHT.

RATZER


//ORD AND MDW CONFIDENCE...UPDATED 12Z...

* MEDIUM CONFIDENCE IN CEILING TRENDS.

* LOW CONFIDENCE IN -FZDZ SUNDAY MORNING.

RATZER

//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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