Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 170810
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
210 AM CST WED DEC 17 2014

.SHORT TERM...
313 PM CST

TONIGHT THROUGH FRIDAY...

FILLING SURFACE LOW WAS OFF TO OUR EAST ACROSS THE CENTRAL GREAT
LAKES THIS AFTERNOON...BENEATH WEAKENING UPPER LEVEL LOW WHICH IS IN
THE PROCESS OF MERGING WITH STRONG DIGGING SHORT WAVE OVER THE UPPER
MISSISSIPPI VALLEY. THIS SECOND WAVE IS PROGGED TO SHEAR SOUTHEAST
ACROSS THE REGION TO THE EAST-SOUTHEAST AS IT REDEVELOPS INTO
REMNANT UPPER TROUGH TO THE EAST. VISIBLE SATELLITE IMAGERY SHOWS
MOST OF THE DEEPER CLOUD COVER HAS MOVED OFF TO THE EAST OF THE
FORECAST AREA...ALONG WITH THE BRUNT OF THE RADAR DETECTABLE
PRECIPITATION. EXPECT SOME PATCHY LIGHT RAIN/DRIZZLE AND PERHAPS A
FEW WET FLURRIES TO GRADUALLY END ACROSS THE AREA OVER THE NEXT FEW
HOURS AS MID-LEVELS CONTINUE TO DRY OUT...WITH 12Z RAOBS FROM
UPSTREAM LOCATIONS SUCH AS ABR AND OAX INDICATIVE OF MUCH DRIER AIR
ABOVE THE BOUNDARY LAYER. OTHERWISE...BLUSTERY WEST-NORTHWEST WINDS
WILL CONTINUE TO PULL COLDER AND DRIER AIR INTO THE AREA TONIGHT
WITH WINDS SLOW TO DIMINISH DUE TO LINGERING SURFACE TROUGHING
ACROSS THE UPPER MIDWEST. THOUGH REMAINING MOISTURE WILL BE SHALLOW
IT APPEARS CLOUD COVER WILL LINGER AT LEAST INTO EARLY WEDNESDAY
BEFORE STRONGER SUBSIDENCE CAN ERODE AND SCATTER CLOUDS MORE
EFFECTIVELY. DESPITE CLOUDS...UPSTREAM TEMPS ACROSS EASTERN IA ARE
ONLY IN THE LOW TO MID 20S AT THIS HOUR...SETTING THE STAGE FOR
CHILLIER TEMPS TONIGHT.

SHEARED MID-LEVEL VORTICITY AXIS FROM SHEARING UPPER MIDWEST SHORT
WAVE REMAINS OVER AREA WEDNESDAY..THOUGH HEIGHT RISES DO DEVELOP AS
THE NEW UPPER LOW DEEPENS OVER NEW ENGLAND. THIS ALLOWS THE GRADIENT
TO WEAKEN MORE QUICKLY ESPECIALLY DURING THE AFTERNOON AND
CONTINUING WEDNESDAY NIGHT AS SURFACE HIGH PRESSURE RIDGE MOVES
SLOWLY INTO WESTERN AND CENTRAL ILLINOIS. GUIDANCE DOES DEPICT A
SHORT WAVE APPROACHING FROM THE SOUTHERN PLAINS/LOWER MISSOURI
VALLEY AREA WEDNESDAY NIGHT...THOUGH THIS WAVE DAMPENS SIGNIFICANTLY
AS IT MOVES INTO THE UPPER RIDGE ACROSS THE MIDWEST. MODEL FORECAST
SOUNDINGS AND TIME-HEIGHT PLOTS INDICATE INCREASING MID-HIGH LEVEL
MOISTURE MAINLY AFTER MIDNIGHT WEDNESDAY NIGHT ACROSS OUR SOUTHWEST.
GUIDANCE (NAM...GFS...ECMWF AND SREF) DOES PRODUCE SOME LIGHT QPF
ACROSS CENTRAL ILLINOIS THURSDAY AS THIS WAVE CONTINUES TO
DEAMPLIFY...THOUGH FORECAST SOUNDINGS INDICATE LOW LEVELS INITIALLY
DRY AND SATURATION APPEARS INCOMPLETE MAKING PRECIP AT THE SURFACE A
LOW PROBABILITY. WEAKENING SURFACE HIGH PRESSURE RIDGE EVENTUALLY
MOVES INTO THE AREA THURSDAY NIGHT AND FRIDAY...ROUNDING OUT A FEW
DAYS OF MAINLY DRY AND SEASONABLY COOLER DAYS.

RATZER

&&

.LONG TERM...
313 PM CST

SATURDAY THROUGH TUESDAY...

ATTENTION TURNS TO A PAIR OF APPROACHING SHORT WAVE TROUGHS IN A
SOMEWHAT SPLIT FLOW REGIME. GUIDANCE HAS TRENDED WEAKER WITH THE
SOUTHERN STREAM DISTURBANCE...WHICH MOVES OUT ACROSS THE SOUTHERN
PLAINS AND GULF COAST STATES THROUGH SATURDAY. DECENT AGREEMENT IN
MODEL SOLUTIONS OF A MORE DIGGING NORTHERN STREAM WAVE COMING OUT OF
THE NORTHERN ROCKIES FRIDAY AND MOVING INTO THE LOWER MISSOURI AND
LOWER OHIO VALLEYS ON SATURDAY. WHILE MAIN SURFACE LOW PRESSURE
DEVELOPS WITH SOUTHERN SYSTEM ALONG THE GULF COAST...NORTHERN STREAM
FORCING DOES INDUCE AN INVERTED TROUGH INTO THE OHIO VALLEY SATURDAY
WHICH CONTINUES TO SUPPORT THE POTENTIAL FOR SOME LIGHT SNOW FROM
SOUTHERN/CENTRAL ILLINOIS INTO NORTHERN INDIANA...POSSIBLY INCLUDING
THE SOUTHEASTERN PART OF OUR CWA AS THE UPPER TROUGH MOVES OVERHEAD.
GUIDANCE STILL FOCUSES GREATER QPF SOUTHEAST OF THE FORECAST AREA...
BUT PRUDENT TO MAINTAIN SLIGHT CHANCES FOR SOME LIGHT ACCUMULATING
SNOW SOUTHEAST OF ABOUT A PONTIAC-VALPARAISO LINE. SURFACE HIGH
PRESSURE RIDGING QUICKLY BUILDS IN BEHIND THIS SYSTEM SATURDAY NIGHT
INTO SUNDAY. TEMPS DO MODERATE TO NORMAL FOR THE WEEKEND...GENERALLY
MID 20S AT NIGHT TO LOW/MID 30S BY DAY.

12Z GFS AND ECMWF RUNS HAVE COME INTO GOOD AGREEMENT IN BRINGING A
MORE AMPLIFIED UPPER TROUGH INTO THE CENTRAL CONUS SUNDAY NIGHT
THROUGH TUESDAY...WITH A STRONGER EMBEDDED SHORT WAVE MOVING ACROSS
THE UPPER MIDWEST LATE MONDAY-TUESDAY WITH DEEPENING LOW PRESSURE
PASSING TO OUR NORTH. THIS RESULTS IN WARMING TEMPERATURES ON
STRENGTHENING SOUTH WINDS...WITH THE POTENTIAL OF RAIN/SNOW CHANGING
TO RAIN BY LATE MONDAY AND CONTINUING TUESDAY AHEAD OF THE
APPROACHING COLD FRONT AS THE LOW MOVES ACROSS THE NORTHERN LAKES.
SOME DETAIL AND SPEED DIFFERENCES DO EXIST BETWEEN MODELS AND MUCH
COULD CHANGE BETWEEN NOW AND MONDAY...THOUGH IT DOES APPEAR THAT
COLDER AIR WOULD SPREAD INTO THE REGION LATER TUESDAY/TUESDAY NIGHT
WITH PRECIPITATION TAPERING OFF AS IT MIXES/CHANGES TO LIGHT SNOW.
COLDER AIR WOULD THEN ARRIVE IN TIME FOR THE CHRISTMAS HOLIDAY.

RATZER

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 06Z...

* GUSTY NORTHWEST WINDS THRU WEDNESDAY AFTERNOON.
* MVFR CIGS THRU WEDNESDAY AFTERNOON. CMS

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

MVFR CIGS REMAIN LOCKED IN ACROSS MUCH OF THE REGION AND FORECAST
SOUNDINGS SUGGEST THESE MAY STAY TRAPPED UNDER AN INVERSION
THROUGH MUCH OF WEDNESDAY. CONFIDENCE IS FAIRLY LOW BUT OPTED TO
CONTINUE BKN CIGS UNTIL EVENING. NORTHWEST WINDS 12-16KTS WILL
SLOWLY DIMINISH WEDNESDAY MORNING INTO THE AFTERNOON. GUSTS ARE
STILL IN THE LOWER/MID 20KT RANGE AND THESE TOO WILL SLOWLY
DIMINISH THRU THE DAY...BUT LIKELY NOT COMPLETELY UNTIL LATE IN
THE AFTERNOON. CMS

//ORD AND MDW CONFIDENCE...UPDATED 06Z...

* MEDIUM FOR CIGS THRU THE PERIOD.
* HIGH FOR WINDS THRU THE PERIOD. CMS

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

* THURSDAY THROUGH MONDAY...DRY/VFR.

KJB

&&

.MARINE...

210 AM...LOW PRESSURE NEAR LAKE ONTARIO WILL MOVE EAST AND BE
ABSORBED BY LOW PRESSURE MOVING NORTH ALONG THE NEW ENGLAND COAST.
THIS LOW WILL THEN SLOWLY LIFT NORTHEAST AND DEEPEN OVER THE NEXT
FEW DAYS. A LARGE AREA OF HIGH PRESSURE EXTENDING FROM MANITOBA
INTO THE CENTRAL PLAINS WILL SLOWLY BUILD EAST ACROSS THE LAKES
REGION THURSDAY AND FRIDAY. NORTHWEST WINDS TO 30 KTS IN BETWEEN
THESE TWO SYSTEMS WILL GRADUALLY DIMINISH TONIGHT INTO THURSDAY.
AS THE HIGH DRIFTS ACROSS THE LAKES REGION...THERE WILL BE A
PERIOD OF LIGHT PERHAPS EVEN CALM WINDS FRIDAY NIGHT INTO SATURDAY
NIGHT UNTIL THE HIGH MOVES EAST SUNDAY AND SUNDAY NIGHT AND THE
FLOW TURNS BACK SOUTHERLY. CMS

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...NEARSHORE WINTHROP HARBOR TO GARY IN
     UNTIL 10 AM WEDNESDAY.

     SMALL CRAFT ADVISORY...NEARSHORE GARY TO MICHIGAN CITY IN
     UNTIL 10 AM THURSDAY.

&&

$$

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