Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 210910
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
310 AM CST SUN DEC 21 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 09Z...

* MVFR CEILINGS DEVELOPING PRE-DAWN AND LIKELY LINGERING THROUGH
  THE PERIOD.

* POTENTIAL FOR A PERIOD OF FOG/LIGHT FREEZING DRIZZLE THIS
  MORNING...THOUGH BETTER POTENTIAL WEST/NORTHWEST OF CHICAGO
  TERMINALS.

RATZER

//DISCUSSION...UPDATED 09Z...

AREA OF MVFR CIGS HAS MOVED ACROSS CHICAGO TERMINALS...THOUGH
SURFACE OBS UPSTREAM TO THE SOUTHWEST INDICATE CIGS RISING BACK TO
VFR AND EXPECT THIS TREND TO CONTIUE FOR THE NEXT FEW HOURS. MORE
SOLID MVFR CIG/VIS AND DZ/FZDZ JUST OFF TO THE WEST/NORTHWEST
INCLUDING KRFD.

RATZER

PREVIOUS DISCUSSION FROM 06Z...

LIGHT SOUTHWEST FLOW JUST ABOVE THE SURFACE IS ADVECTING LOW LEVEL
MOISTURE STEADILY NORTHEASTWARD ACROSS THE REGION...WITH EXPANSIVE
AREA OF MVFR/IFR/LIFR EXTENDING UPSTREAM TO THE WEST AND SOUTHWEST
OF NORTHERN ILLINOIS TERMINALS. MAIN MOIST AXIS AND LOWEST
CONDITIONS ALONG WITH FOG AND DRIZZLE/FREEZING DRIZZLE/FLURRIES
LIFTING FROM IOWA INTO NORTHWEST ILLINOIS...SETTING UP LIKELY IFR
CONDITIONS EARLY THIS MORNING FOR KRFD. FARTHER EAST...CHICAGO
AREA TERMINALS EXPECTED TO DETERIORATE TO MVFR BUT LOWER
CONDITIONS SHOULD REMAIN JUST OFF TO THE WEST/NORTHWEST. GIVEN
EXTENSIVE UPSTREAM CONDITIONS WOULD EXPECT LOW CLOUDS TO LINGER
THROUGH MUCH OF THE PERIOD AS SURFACE LOW PRESSURE DEVELOPING OVER
THE NORTHERN PLAINS MAINTAINS SOUTHWEST FLOW ALOFT.

SOUTH WINDS AROUND 5 KT WILL LIKELY BECOME A LITTLE EAST OF SOUTH
IN 170-160 DEGREE RANGE THROUGH THE DAY WITH SPEEDS INCREASING
INTO THE 10-15 KT RANGE. MODEL SOUNDINGS SUGGEST SOME GUSTS ABOVE
15 KT POSSIBLE LATER THIS AFTERNOON/TONIGHT...THOUGH SHALLOW MIXED
LAYER ARGUES AGAINST PREVAILING GUST MENTION.

RATZER

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

* MEDIUM-LOW CONFIDENCE IN CEILING TRENDS THIS MORNING.

* LOW CONFIDENCE IN CEILING TRENDS THIS AFTERNOON/TONIGHT.

* LOW CONFIDENCE IN MVFR FOG AND DZ/FZDZ POTENTIAL THIS MORNING.

* MEDIUM-HIGH CONFIDENCE IN WINDS.

RATZER

//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...
240 AM CST

MAIN MARINE CONCERNS CONTINUE TO FOCUS ON STRENGTHENING
SOUTH-SOUTHEAST WINDS TODAY THROUGH LATE MONDAY...AND POTENTIAL FOR
GUSTY NORTHWEST-WEST WINDS...POSSIBLY GALES...FROM LATE WEDNESDAY
INTO THURSDAY.

SURFACE HIGH PRESSURE RIDGE MOVED EAST OF THE LAKE ON SATURDAY
ALLOWING WINDS TO BECOME SOUTHERLY. THE SURFACE PRESSURE GRADIENT
WILL SLOWLY TIGHTEN ACROSS THE UPPER MIDWEST AND WESTERN LAKES TODAY
AS AN AREA OF LOW PRESSURE MOVES SOUTHEAST FROM THE LEE OF THE
CANADIAN ROCKIES...WITH SOUTH-SOUTHEAST WINDS INCREASING INTO THE
25-30 KT RANGE FROM LATER THIS AFTERNOON INTO MONDAY AS THE LOW
APPROACHES FROM THE NORTHERN PLAINS. THE IS EXPECTED TO OCCLUDE AND
BEING TO FILL LATE MONDAY NIGHT...ALLOWING THE GRADIENT TO WEAKEN
AND WINDS TO DIMINISH EARLY TUESDAY. THE REMNANT SURFACE TROUGH WILL
CROSS THE LAKE LATE TUESDAY NIGHT OR EARLY WEDNESDAY...WITH LIGHT
WINDS SHIFTING TO THE WEST-NORTHWEST.

FORECAST MODEL SOLUTIONS FOR THE SURFACE PATTERN FROM WEDNESDAY
AFTERNOON THROUGH THURSDAY HAVE BECOME MORE DIVERGENT OVER THE PAST
FEW RUNS...WITH QUITE A SPREAD IN THE LOCATION TRACK AND DEPTH OF A
SECOND SURFACE LOW WHICH IS EXPECTED TO DEVELOP SOMEWHERE ACROSS THE
OHIO OR TENNESSEE RIVER VALLEYS. OVERALL...GUIDANCE INDICATING A
WEAKER LOW DURING THIS PERIOD...THOUGH MODELS ARE OBVIOUSLY
STRUGGLING WITH DEVELOPMENT OF DIGGING SOUTHERN STREAM JET STREAK
AND THE RESULTING UPPER TROUGH AND DOWNSTREAM SURFACE LOW
DEVELOPMENT. WITH SUCH DIVERGENT SOLUTIONS HAVE AVOIDED MAKING LARGE
SWINGS IN THE FORECAST...AND HAVE HELD ON TO THE GENERAL SCENARIO OF
PREVIOUS FORECAST...THOUGH WITH SLIGHTLY WEAKER WIND FIELD DURING
THE LATE WEDNESDAY-THURSDAY PERIOD. ADJUSTMENTS WILL UNDOUBTEDLY BE
REQUIRED AS SUITE OF MODELS DEVELOP BETTER CONVERGENCE/AGREEMENT IN
THEIR SOLUTIONS.

RATZER

&&

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

&&

$$

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