Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 240755
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
155 AM CST MON NOV 24 2014

.UPDATE...

FOR EVENING UPDATE...

ONLY MINOR ADJUSTMENTS MADE TO GOING FORECAST TONIGHT...PRIMARILY
TO ADD AREAS OF FOG ALONG THE LAKE AND ACROSS PARTS OF NORTHERN
ILLINOIS OVERNIGHT.

COMPLEX UPPER PATTERN CONTINUES TO EVOLVE ACROSS THE REGION THIS
EVENING AS A SERIES OF MID-LEVEL SHORT WAVES ROTATE THROUGH THE
LARGER-SCALE UPPER TROUGH OVER THE CENTRAL AND EASTERN PARTS OF
NORTH AMERICA. AT THE SURFACE...AN AREA OF LOW PRESSURE WITH A
COUPLE OF INDIVIDUAL LOW CENTERS WAS OVER IL AT 02Z/8 PM CST...
WITH STRONG (6 MB/3 HRS) SURFACE PRESSURE FALLS ACROSS MUCH OF THE
MIDWEST AND SOUTHERN GREAT LAKES REGION. THE LATEST MODEL GUIDANCE
CONTINUES TO CONSOLIDATE A SURFACE LOW OVER FAR NORTHEAST IL OVER
THE NEXT SEVERAL HOURS...LIFTING IT NORTHEAST AND DEEPENING IT
OVERNIGHT AND EARLY MONDAY AS IT PULLS AWAY FROM THE AREA.

REGIONAL RADAR MOSAICS INDICATE A LARGE SHIELD OF RAIN ACROSS THE
EASTERN HALF OF ILLINOIS AND MUCH OF THE GREAT LAKES REGION
PRIMARILY ASSOCIATED WITH A STRONG SHORT WAVE LIFTING NORTHEAST
CROSS THE OHIO VALLEY. ANOTHER AREA OF RAIN AND EMBEDDED THUNDER
WS NOTED BACK ACROSS THE LOWER MISSOURI VALLEY WITH ANOTHER STRONG
WAVE. RAIN MAY TAPER OFF TO OCCASIONAL LIGHT RAIN OR DRIZZLE
ACROSS SOME AREAS WITH A BIT OF A BREAK BETWEEN THE SOLID RAIN
WITH THESE SYSTEMS...THOUGH A PERSISTENT DRIZZLE WILL LIKELY MAKE
IT SEEM LIKE ITS RAINING MOST OF THE TIME ANYWAYS. IN ADDITION TO
THE RAIN...HIGH DEWPOINT AIR WRAPPING NORTHWESTWARD AHEAD OF THE
SURFACE LOW WAS PRODUCING FOG ACROSS SOUTHERN LAKE MICHIGAN WHERE
LAKE WATER TEMPS WERE IN THE MID-40S...AND AFFECTING AREAS ALONG
THE ILLINOIS SHORE. WILL ALSO LIKELY SEE SOME PATCHES OF FOG WHERE
PRECIP DIMINISHES TO DRIZZLE OVERNIGHT...ESPECIALLY IN BAGGY
PRESSURE GRADIENT AND WEAKER WINDS IN VICINITY OF THE SURFACE LOW.

WEST WINDS WILL KICK IN EARLY MONDAY MORNING AS THE LOW PULLS
NORTH AND A COLD FRONT PUSHES EAST ACROSS THE AREA. THIS SHOULD
HELP PUSH SOME OF THE FOG OUT EARLY IN THE MORNING. COLDER AIR AND
WIND GUSTS TO 40 MPH BECOME AN ISSUE DURING THE DAY...ALONG WITH
PRECIPITATION CHANGE FROM LIQUID TO FROZEN AS THE COLUMN COOLS.
POTENTIAL FOR A PERIOD OF MODERATE/LOCALLY HEAVY WET SNOW EXISTS
OVER NORTH CENTRAL/PARTS OF NORTHEAST IL WHERE SOUTHERN PERIPHERY
OF DEFORMATION BAND PASSES. CONFIDENCE NOT SUPER HIGH WITH AMOUNTS
AT THIS POINT...THOUGH NO CHANGES TO WSW HEADLINES AT THIS TIME.

RATZER

&&

.SHORT TERM...
244 PM CST

UNSEASONABLY WARM/MOIST AIRMASS CONTINUES TO ADVECT NORTH ACROSS THE
REGION THIS AFTN...AND LOOKS TO CONTINUE THROUGH TONIGHT. THE MAIN
CONCERN FOR THE OVERNIGHT WILL BE ON RAINFALL TOTALS AND IF THIS
WILL POSE ANY HYDRO CONCERNS.

STRONG WARM AIR ADVECTION THIS AFTERNOON HAS ALLOWED SFC TEMPS TO
WARM INTO THE 50S ACROSS MUCH OF THE FORECAST AREA WITH DEW POINTS
APPROACHING THE UPPER 40S/ARND 50. THE AXIS OF THE HEAVIER RAINFALL
THIS AFTN HAS SHIFTED TO THE NORTHWEST AND STRETCHES FROM CENTRAL
WISC SOUTHWEST THROUGH FAR EASTERN IOWA. HAVE NUDGED TEMPS OVERNIGHT
UP A TAD...AND EXPECT TEMPS TO REMAIN NEARLY CONSISTENT UNTIL CLOSER
TO DAYBREAK MON WHEN THE FRONTAL BOUNDARY BEGINS TO APPROACH THE
NORTHWEST CWFA. GUIDANCE CONTINUES TO SUGGEST THE LOW DROPPING
SOUTHEAST ACROSS THE UPPER MIDWEST AND THE SECONDARY ONE LIFTING
NORTH FROM THE SOUTHWEST...WILL CONVERGE ACROSS SOUTHWEST LOWER
MICHIGAN JUST BEFORE DAYBREAK MON. THE COLD FRONT WILL BE NEARING
THE WESTERN CWFA ARND DAYBREAK...AND STEADILY SLIDE EAST THRU THE
MORNING. ASSOCIATED WILL BE A SHARP THERMAL TROUGH THAT WILL CAUSE
TEMPS TO STEADILY FALL THROUGHOUT THE MORNING/AFTN. SO THE DAY MAY
START MILD...BUT BY MIDDAY THAT WILL CERTAINLY NOT BE THE CASE WITH
TEMPS GENERALLY IN THE UPR 20S/LOW 30S. PRECIP WILL BE QUICKLY
TRANSITIONING OVER FROM RAIN TO SNOW.

AS THE SNOW BEGINS MIDDAY ACROSS THE NORTHWEST...GUIDANCE HAS BEEN
RELATIVELY CONSISTENT THAT AT ONSET RATIOS WILL HOVER ARND 8:1 TO
9:1...THEN AS THE COLDER AIR ADVECTS IN THOSE RATIOS SHUD STEADILY
CLIMB TO 14:1-17:1 BY LATE MON AFTN/EARLY EVE. COMPOUNDING THE SETUP
WILL BE STEADILY INCREASING PRESSURE GRADIENT THAT WILL CREATE GUSTY
WINDS NEARING 30-35MPH. THERE IS A NARROW WINDOW WHERE SNOWFALL
RATES COULD NEAR 1-2"/HR FOR A COUPLE HOURS...MAINLY BETWEEN 2-4PM
CST AND MAINLY WEST OF A LASALLE TO CRYSTAL LAKE LINE. HAVE BUMPED
SNOW TOTALS UP TO 2-4" AND COULD SEE IN PORTIONS OF WINNEBAGO COUNTY
3-5" CLOSER TO THE WISC STATELINE IN NORTHERN WINNEBAGO COUNTY.

ELSEWHERE SNOW WILL BEGIN TO ACCUMULATE...BUT EXPECTING MUCH LESS
WITH PERHAPS AN INCH IN CHICAGO AND LESSER TOTALS FURTHER SOUTH.
500MB TROUGH AXIS PIVOTS OVERHEAD MON EVE...AND WILL SLOWLY BRING AN
END TO THE ACCUMULATING SNOW. THE BETTER MOISTURE WILL QUICKLY PEEL
OFF TO THE NORTHEAST LATE MON NGT...HOWEVER ENOUGH OMEGA/MOISTURE TO
KEEP MENTION OF LGT SNOW SHOWERS MON NGT.

OVERNIGHT LOWS MON DIP FALL TO THE LOW 20S TO MID 20S IN THE
SOUTHEAST.

BEACHLER

&&

.LONG TERM...
244 PM CST

TUESDAY THROUGH WEDNESDAY...
LARGE MID-LVL TROUGH WILL BE IN PLACE TUE/WED WITH A WEAK SFC RIDGE
LIFTING NORTH AND BRINGING A GRADUAL EROSION TO THE CLOUDS TUE
NGT/WED. UNFORTUNATELY THIS MAY BE SHORT LIVED AS GUIDANCE SUGGESTS
A QUICK SHORTWAVE WILL DROP SOUTH FROM CENTRAL CANADA INTO THE
NORTHERN PLAINS EARLY WED...AND APPROACH THE FORECAST AREA MIDDAY
WED. THIS SYSTEM COULD BRING ANOTHER PERIOD OF LIGHT SNOW WITH SOME
MINOR ACCUMULATIONS.

TEMPS TUE WILL REMAIN IN THE 20S...HOWEVER WITH SOME THINNING IN THE
CLOUDS FOR WED TEMPS COULD REBOUND INTO THE LOW/MID 30S. BUT IT WILL
HINGE UPON THE ARRIVAL OF THE NEXT MID-LVL WAVE AND CLOUD COVER.

THURSDAY THROUGH SATURDAY...
ENSEMBLES IN THE LONGER TERM CONTINUES TO POINT TOWARDS A TROUGH
CARVED OUT ACROSS THE GREAT LAKES/TENN VALLEY FOR THE SECOND HALF OF
THE WEEK...AND THEN BEGINS TO FLATTEN FOR THE SECOND HALF OF THE
WEEKEND. AT THIS TIME GUIDANCE SUGGESTS DRY AIR AND A SFC RIDGE
OVERHEAD FOR THANKSGIVING DAY. TEMPS WILL GENERALLY BE IN THE 20S
FOR THE BULK OF THE EXTENDED.

BEACHLER

&&

.AVIATION...

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

* IFR CIGS PERSIST THROUGH THE NIGHT AND MAY LOWER TO 300-400 FT.

* STEADY RAIN TURNS TO DRIZZLE OVERNIGHT WITH STEADIER RAIN
  RETURNING AFTER DAYBREAK/MID MORNING.

* IFR VSBY LIKELY OVERNIGHT WITH OCCASIONAL VSBY BELOW 3/4SM.
  HIGHER LIKELIHOOD FOR ORD THAN MDW.

* LIGHT/VARIABLE WINDS OVERNIGHT SHIFT WEST MONDAY MORNING THEN
  SOUTHWEST WITH GUSTS TO 30 KT WITH A PERIOD OF 35 KT GUSTS
  POSSIBLE MIDDAY/EARLY AFTERNOON.

* RAIN CHANGES TO SNOW MIDDAY/EARLY AFTERNOON...SOME LIGHT
  ACCUMULATION POSSIBLE...ESPECIALLY LATER IN THE DAY.

* IFR CIGS LIFTING TO MVFR LATE THIS AFTERNOON

KMD

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

BAND OF LIGHT/OCCASIONALLY MODERATE RAIN IS LIFTING NORTHWEST
THROUGH THE TERMINALS EARLY THIS MORNING WITH WIDEPREAD IFR CIGS
AND LARGELY IFR/LIFR VSBYS WITH A SMALL CORRIDOR IN THE SOUTHWEST
FROM KIIK TO KMDW SOUTHEASTWARD. EXPECT VSBYS TO DROP ACROSS THE
AREA AS THE FIRST ROUND OF RAIN DISSIPATES SOME AND LIGHTER WINDS
CONTINUE.

THE NEXT BATCH OF PRECIPIATION IS FORECAST TO MOVE THROUGH JUST
AFTER DAYBREAK WITH INITIAL PRECIPITATION TYPE AS -RA. EXPECT CIGS
TO REMAIN IFR BUT PROBABLY LIFT SOME AS WEST WINDS BEGIN TO PICK
UP WITH LOW PRESSURE MOVING NORTHEASTWARD WITH -RASN MIX AROUND
15Z AT KRFD AND A FEW HOURS LATER IN THE CHICAGO AREA...TURNING TO
ALL -SN. MOISTURE WIL BE DECREASING FROM SOUTH TO NORTH AS THE
TRANSITION FROM -RA TO -SN OCCURS. THE HIGHEST LIKELIHOOD FOR
ACCUMS IS WEST OF THE MAIN CHICAGO TERMINALS TOWARD ROCKFORD...BUT
SOME ACCUMS ARE CERTAINLY STILL PLAUSIBLE EVERYWHERE. LOOKS LIKE A
MIX OF SNOW SHOWERS OR POSSIBLY MORE FLURRIES OVERNIGHT WITH AN
IFR TRANSITION TO MVFR.

KMD


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

* HIGH IN IFR CONTINUING WITH SOME LOWERING OVERNIGHT.

* HIGH CONFIDENCE IN STEADIER RAIN DIMINISHING TO DRIZZLE
  OVERNIGHT WITH STEADIER RAIN RETURNING AFTER DAYBREAK/MID
  MORNING.

* MEDIUM-HIGH CONFIDENCE IN VSBY FALLING TO IFR OVERNIGHT. LOW-
  MEDIUM CONFIDENCE IN POTENTIAL TO GO BELOW 1SM...HIGHER
  CONFIDENCE FOR ORD.

* HIGH CONFIDENCE IN WIND TRENDS TONIGHT BUT MEDIUM CONFIDENCE IN
  DIRECTION/SPEED DETAILS.

* HIGH CONFIDENCE IN WIND FORECAST MONDAY AND 30+ KT GUSTS. MEDIUM
  CONFIDENCE IN SPORADICALLY REACHING 35 KT OR SO.

* HIGH CONFIDENCE IN RAIN CHANGING TO SNOW BUT LOW CONFIDENCE IN
  HOW MUCH PRECIP COVERAGE THERE WILL BE THROUGH THE
  AFTERNOON/EVENING.

* HIGH CONFIDENCE ON CIG TRENDS...LOW-MEDIUM ON WHEN IFR LIFTS TO
  MVFR AND HOW LONG MVFR LASTS.

KMD

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

TUESDAY...MVFR CIGS POSSIBLE WITH A FEW FLURRIES IN THE MORNING.
WEST WINDS.

WEDNESDAY...CHANCE OF SNOW. MVFR/IFR PSBL. SSE WINDS TURNING WEST.

THURSDAY...SCHC SNOW...OTHERWISE VFR. BREEZY NORTHWEST WINDS.

FRIDAY...SCHC SNOW. SSW WINDS.

SATURDAY...VFR. WEST WINDS.

MDB

&&

.MARINE...
154 AM CST

DEEPENING LOW PRESSURE IS PASSING OVER THE LAKE EARLY THIS MORNING
BRINGING A LULL IN WINDS AND WAVES. THE LOW WILL CONTINUE NORTHEAST
INTO SOUTHWEST QUEBEC LATER THIS AFTERNOON THEN REMAIN STEADY AS IT
PUSHES OFFSHORE OVER NOVA SCOTIA. A STRONG SOUTHWEST TO NORTHEAST
ORIENTED PRESSURE GRADIENT WILL REACH THE SOUTH TIP OF THE LAKE BY
15Z...THEN SPREAD OVER THE REMAINDER OF THE LAKE IN THE AFTERNOON.
GALES LOOK SOLID FOR THE SOUTH HALF OF THE THE LAKE...BUT DUE TO A
WEAKENING PRESSURE GRADIENT WITH TIME TODAY AND A MODEST HIGH
BUILDING INTO THE CANADIAN PRARIES AND UPPER MIDWEST...FEEL THAT
WINDS WILL LIKELY PEAK JUST UNDER GALES FOR THE NORTH HALF. A FEW
GALE FORCE GUSTS ARE POSSIBLE...ESPECIALLY LATE AFTERNOON/EARLY
EVENING.

WEAK HIGH PRESSSURE EXTENDING FROM THE NORTH TIP OF LAKE SUPERIOR TO
THE OHIO VALLEY PASSES OVER THE LAKE TUESDAY AFTERNOON. ANOTHER
WEAKENING LOW EMERGING FROM THE CANADIAN ROCKIES WILL PASS JUST
SOUTH OF THE TIP OF LAKE MICHIGAN WITH THE REMNANT INVERTED TROUGH
CLEARING THE LAKE BY THURSDAY MORNING. THIS WILL BE FOLLOWED BY WEAK
HIGH PRESSSURE FOR A FEW DAYS.

KMD


&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...WINTER WEATHER ADVISORY...ILZ003-ILZ004-ILZ008-ILZ010-
     ILZ011...NOON MONDAY TO MIDNIGHT TUESDAY.

     WINTER WEATHER ADVISORY...ILZ005-ILZ006-ILZ012...3 PM MONDAY TO
     MIDNIGHT TUESDAY.

IN...NONE.
LM...GALE WARNING...LMZ080-LMZ669-LMZ671-LMZ673-LMZ675-LMZ740-LMZ741-
     LMZ742-LMZ743-LMZ744-LMZ745-LMZ777-LMZ779-LMZ870-LMZ872-
     LMZ874-LMZ876-LMZ878...9 AM MONDAY TO 3 AM TUESDAY.

&&

$$

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