Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 210316
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
916 PM CST SAT DEC 20 2014

.UPDATE...
904 PM CST

WE STILL HAVE THE POSSIBILITY OF PATCHY FREEZING DRIZZLE
OVERNIGHT WITH THE BEST CHANCE IN OUR WESTERN AREAS.  THAT
SAID...TEMPERATURES ARE AROUND FREEZING AND NOT EXPECTED TO DROP
MUCH GIVEN THE CLOUD COVER AND WARM ADVECTION PATTERN SETTING UP.
AT THIS POINT IMPACTS DO LOOK MINIMAL...ESPECIALLY FARTHER EAST.  HAVE
KEPT TEMPS NEARLY STEADY OVERNIGHT. SOME PRECIPITATION MAY MAKE IT
CLOSER TO CHICAGO LATE TONIGHT INTO SUNDAY MORNING ONCE THE WARM
FRONT GETS CLOSER...BUT IT WOULD BE LARGELY IN THE FORM OF PATCHY
DRIZZLE. FOG MAY BE POSSIBLE AS WELL OVERNIGHT.

KMD

&&

.SHORT TERM...
247 PM CST

THROUGH SUNDAY NIGHT...

THE STREAK OF CLOUDY DAYS CONTINUES...WITH THE STUBBORN PATTERN TO
CONTINUE. AS A RESULT OF THE THICK CLOUD COVER...AND LACK OF
ORGANIZED ADVECTION OF ANY WARMER AIR AT THE SFC...TEMPS HAVE HELD
MAINLY ARND 30 TO 35 ACROSS THE BULK OF THE CWFA. SFC RIDGE
CONTINUES TO DRIFT EAST THIS AFTN...WITH A WEAK SOUTHERLY FLOW
DEVELOPING. THERE WAS A WEAK MID-LVL VORT LIFTING NORTHEAST ACROSS
SOUTHERN WISC/WESTERN LOWER MI...WHICH WAS ABLE TO PRODUCE A FEW
FLURRIES ACROSS THE FAR NORTHERN IL AREAS.

THIS AFTN/EARLY EVE LOOKS TO REMAIN QUIET/DRY...WITH A LIGHT
SOUTH/SOUTHEAST WIND. IR IMAGERY SHOWS THE NEXT WAVE ACROSS THE
CENTRAL PLAINS...WHICH IS PROGGED TO ARRIVE ACROSS NORTHWEST IL
AFTER MIDNIGHT. THERE APPEARS TO BE SOME WEAK ADVECTION OF MOISTURE
IN THE LOW-LEVELS OVERNIGHT. SOUNDINGS SUGGEST A NARROW WARM WEDGE
ALOFT WHICH COULD TRANSITION ANY PRECIP OVERNIGHT TO LIQUID. THE
FORCING/OMEGA IS RATHER WEAK...WHICH MAY LIMIT DROPLET SIZE AND
RESULT IN JUST PATCHY FREEZING DRIZZLE. TEMPS WILL LIKELY NOT MOVE
MUCH OVERNIGHT...SO HAVE HELD ONTO MID/UPR 20S OVERNIGHT.

LOCAL ARW8KM SOLUTION INDICATES THE MOISTURE TO BE RATHER MARGINAL
WITH THE SYSTEM ARRIVING OVERNIGHT/SUN MORNING...BUT PROGRESSIVE AND
LIFTING NORTH OF THE CWFA BY MIDDAY. WEAK THERMAL RIDGE AT 850MB
WOULD SUGGEST TEMPS MAY ACTUALLY WARM SUN AFTN...HOWEVER GIVEN THE
STUBBORN PATTERN WE HAVE BEEN IN WOULD LEAN TOWARDS NUDGING TEMPS
DOWN A DEGREE OR TWO. MOISTURE LAYER WILL REMAIN LOCKED IN ACROSS
THE REGION THRU SUN/SUN NGT. WITH A CONTINUED SOUTHERLY FLOW AT THE
SFC...TEMPS WILL LIKELY ONLY FALL TO ARND 30/LOW 30S.

BEACHLER

&&

.LONG TERM...
247 PM CST

MONDAY THROUGH SATURDAY...

500MB TROUGH BEGINS TO DIG INTO THE CENTRAL CONUS MON MORNING...WITH
WARM AIR STEADILY ADVECTING NORTH ACROSS THE FORECAST AREA. WHILE
THE THICK CLOUD COVER WILL PERSIST...THE PUSH OF WARM/MOIST AIR WILL
BRING TEMPS INTO THE 40S THE FAR SOUTHERN CWFA COULD SEE A FEW
LOCATIONS TOUCH THE UPR 40S MON AFTN. PRECIP WILL STEADILY PUSH EAST
AS THE UPPER LEVEL SYSTEM PROGRESSES TOWARDS THE REGION...BUT WITH
THE WARM AIR IN PLACE PRECIP SHUD FALL AS RAIN.

MON NGT THE 500MB VORT WILL SLIDE INTO THE UPPER MIDWEST...BUT THE
TROUGH WILL DIG SOUTH ACROSS THE SOUTHERN PLAINS. GUIDANCE HAS BEEN
INDICATING A SECONDARY SHORTWAVE EJECTING FROM THE TROUGH AXIS...AND
LIFTING IT NORTHEAST ACROSS THE TENN VALLEY CHRISTMAS EVE. THE
NORTHERN SFC LOW WILL BE WRAPPED UP ACROSS CENTRAL WISC TUE...WITH
THE MOIST CHANNEL FEEDING NORTH ACROSS THE FORECAST AREA THRU TUE
AFTN. THE SOUTHERN LOW WILL BE QUICKLY DEEPENING AS IT LIFTS NORTH
ACROSS THE OHIO VALLEY WED MORNING...ALLOWING WINDS TO BE ON THE
INCREASE AND HELPING TO STEADILY TRANSITION PRECIP OVER FROM RAIN TO
SNOW POST FROPA WED MORNING.

THE LAST SEVERAL CYCLES OF GUIDANCE HAVE DEMONSTRATED DIFFERENCES IN
PLACEMENT/TRACK OF THE SFC LOW WED AFTN/EVE. LATEST SOLUTION IS
PROGGED TO LIFT NORTH ACROSS NORTHERN IN AND THEN SLIGHTLY
RETROGRADE ACROSS THE SOUTHERN TIP OF LAKE MICHIGAN WED EVE. THEN BY
CHRISTMAS DAY THE TROUGH WILL BE PIVOTING EAST AND HELPING TO LIFT
THE SFC LOW NORTHEAST AWAY FROM THE REGION. SOME WRAP AROUND
MOISTURE/SNOW LOOKS FAVORABLE WED NGT/THUR MORNING...BEFORE COMING
TO AN END JUST AFTER DAYBREAK CHRISTMAS MORNING. THIS SYSTEM WILL
CONTINUE TO BE CLOSELY MONITORED.

TEMPS WILL BE RETURNING TO MORE SEASONAL CONDS...WITH HIGHS
CHRISTMAS DAY IN THE LOW/MID 30S. FLOW REMAINS RATHER PROGRESSIVE
WITH ANOTHER WEAK PUSH OF WARMER AIR RETURNING FRI AND TEMPS BACK TO
THE UPR 30S/LOW 40S. THEN FOR FRI ENSEMBLES ARE INDICATING YET
ANOTHER TROUGH BEGINNING TO DIG INTO THE CENTRAL CONUS...WITH A SFC
LOW LIFTING NORTHEAST ACROSS THE WESTERN GREAT LAKES.

BEACHLER

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 00Z...

* CIGS LOWER TO MVFR LATER TONIGHT AND POTENTIALLY TO IFR SUNDAY
* SLIGHT CHANCE OF -FZDZ OR -DZ SUNDAY MORNING

IZZI

//DISCUSSION...UPDATED 00Z...

HAVE LARGELY FOLLOWED THE TRENDS FROM PREVIOUS SET OF TAFS WITH
LOWERING CIGS TONIGHT/SUNDAY...THOUGH HAVE SPED UP THE ARRIVAL OF
MVFR CIGS TONIGHT AND LOWERED CIGS A BIT SUNDAY. EXPANSIVE AREA OF
IFR/BR/-DZ BLANKETS IOWA AND SHOULD SLOWLY SPREAD EASTWARD THROUGH
THE PERIOD. ONLY BRINGING CIGS DOWN TO LOWER END MVFR SUNDAY COULD
END UP BEING TOO OPTIMISTIC FOR ORD/MDW/GYY...WILL MONITOR TRENDS
IN THE EASTWARD EXPANSION OF IFR/LIFR TONIGHT AND WE MAY NEED TO
ADD IFR INTO THE TAFS FOR CHICAGO TERMINALS IN LATER FORECASTS. AS
WELL...IF CIGS DO DROP LOWER THEN POTENTIAL WOULD BE GREATER FOR
SOME -FZDZ TRANSITIONING TO -DZ AS TEMPS INCH UPWARD. NOT
ANTICIPATING CIGS TO LIFT MUCH FROM MORNING LOWS SUNDAY...SO IFE
CONDITIONS DEVELOP THEY COULD PERSIST THROUGH THE DAY. CERTAINLY
COULD SEE SOME OCNL GUSTS OF 15-20KT SUNDAY AFTERNOON FROM THE
SSE...BUT THINKING THE EXPECTED STRATUS DECK SHOULD LIMIT MIXING
ENOUGH TO KEEP GUSTINESS SPORADIC TO OCNL AT BEST.

IZZI

//ORD AND MDW CONFIDENCE...UPDATED 00Z...

* MEDIUM CONFIDENCE IN CIG TRENDS THROUGH SUNDAY MORNING...LOW
  CONFIDENCE IN TIMING AND HOW LOW CIGS GET
* LOW CONFIDENCE IN CIGS SUNDAY AFTERNOON
* LOW CONFIDENCE IN DZ/FZDZ POTENTIAL SUNDAY MORNING
* MEDIUM-HIGH CONFIDENCE IN WINDS

IZZI

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

MONDAY...MVFR/IFR EARLY LOWERING TO IFR/LIFR. RAIN DEVELOPING
LATE.

TUESDAY...ONCL SHRA. IFR PROBABLE.

WEDNESDAY...CHANCE SHRA/SHSN. MVFR LIKELY. IFR POSSIBLE.

THURSDAY...MVFR CIGS POSSIBLE EARLY.

FRIDAY...CHANCE OF RA/SN LATE. MVFR CIGS POSSIBLE AGAIN.

IZZI

&&

.MARINE...
203 PM CST

LIGHT SOUTHEAST TO EAST WINDS WILL INCREASE TO 10-25 KT TOMORROW
MORNING THEN TO 30 KT IN THE AFTERNOON AS THE GRADIENT TIGHTENS
BETWEEN THE DEPARTING HIGH AND THE NEXT LOW. THE LOW MOVES OVER THE
DAKOTAS SUNDAY NIGHT AND MODELS AGREE A BIT MORE IN THE PATTERN
EARLY NEXT WEEK.  HAVE MEDIUM CONFIDENCE IN THE WIND FORECAST WITH
THIS NEXT LOW.  THE LOW WEAKENS AS IT MOVES OVER WISCONSIN TUESDAY
AND A SECOND LOW FORMS OVER THE SOUTHERN MISSISSIPPI VALLEY TUESDAY
AFTERNOON. THE SECOND LOW WILL RACE NORTH AND DEEPEN REACHING
MICHIGAN WEDNESDAY. MODELS DIFFER ON HOW QUICKLY THE LOW WILL REACH
MICHIGAN.  BOTH MODELS CONTINUE TO FEATURE WESTERLY HIGH END GALES
BEHIND THE LOW WEDNESDAY AFTERNOON INTO THURSDAY.  WEST WINDS
DIMINISH THURSDAY AS THE LOW CONTINUES OFF TO THE NORTH. THE NEXT
LOW MOVES OVER THE SOUTHERN PLAINS THURSDAY AFTERNOON. GUIDANCE
DIFFERS ON THIS LOWS PATH BUT IT SHOULD IMPACT THE LAKE LATE NEXT
WEEK.

JEE

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...LMZ740...10 PM SUNDAY TO 3 AM TUESDAY.

&&

$$

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