Area Forecast Discussion
Issued by NWS Chicago, IL

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000
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...
COMING SOON.

&&

.AVIATION...

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

* MVFR CEILINGS LIKELY BECOMING VFR BEFORE SUNRISE. EXPECTED TO
  LOWER TO MVFR AGAIN THIS EVENING.

* LIGHT/VARIABLE WINDS BECOMING LIGHT SOUTH-SOUTHWEST BY MIDDAY.

RATZER

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

WEAK SURFACE HIGH PRESSURE RIDGE MOVING SLOWLY EAST ACROSS THE
AREA THIS MORNING. CALM OR LIGHT/VARIABLE WINDS AND MVFR CLOUDS
TRAPPED BENEATH SHALLOW INVERSION PERSISTING. SURFACE OBS INDICATE
MVFR CEILINGS ERODING FROM THE SOUTHWEST HOWEVER...WITH THE
POTENTIAL FOR CONDITIONS TO BECOME VFR AT TAF SITES DURING THE
PRE-DAWN HOURS. QUESTION IS HOW LONG VFR LASTS...AS EXTENSIVE AREA
OF MVFR/IFR CEILINGS WEST OF THE MISSISSIPPI RIVER WILL EVENTUALLY
SPREAD INTO THE AREA TONIGHT. KRFD BEING FARTHER WEST LOOKS TO BE
BACK INTO MVFR CONDITIONS PERHAPS AS EARLY AS MID-LATE MORNING.
CEILINGS WILL LIKELY LOWER INTO IFR RANGE LATE THIS EVENING INTO
EARLY SUNDAY...WITH THE POTENTIAL FOR SOME FLURRIES OR FREEZING
DRIZZLE DEVELOPING SUNDAY MORNING.

CALM OR LIGHT/VARIABLE WINDS LESS THAN 5 KT WILL BECOME SOUTH-
SOUTHWEST AT 5 KT BY EARLY AFTERNOON AND REMAIN LIGHT SOUTH
THROUGH TONIGHT.

RATZER

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

* MEDIUM IN CEILING TRENDS AND VFR DURATION TODAY.

* HIGH IN WIND TRENDS.

RATZER

//OUTLOOK FOR ORD/MDW FOR 12Z SUNDAY-12Z FRIDAY...UPDATED 00Z...

SUNDAY...MVFR CIGS.  SOUTH WINDS.

MONDAY...MVFR CIGS. BRIEF PERIOD OF RAIN OR MIXED RAIN AND SNOW.

TUESDAY...MVFR CIGS. CHANCE OF RAIN.

WEDNESDAY...MVFR CIGS. BLUSTERY - GUSTY WEST WINDS. CHANCE OF
SNOW.

THURSDAY...VFR.

ED F

&&

.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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