Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 310306
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
906 PM CST FRI JAN 30 2015

.UPDATE...

847 PM...EVENING UPDATE...NO CHANGES PLANNED TO CURRENT WINTER
STORM WATCH. WHILE THE 00Z NAM IS ROLLING IN WITH NO SIGNIFICANT
CHANGES FROM 12Z/18Z NAM RUNS...WILL ALLOW MIDNIGHT SHIFT TO
EVALUATE THE FULL SUITE OF 00Z GUIDANCE BEFORE MAKING ANY UPGRADE
DECISIONS AS WELL AS AMOUNTS.

IN THE SHORT TERM...MAINLY CLEAR SKIES AND LIGHT WINDS HAVE
ALLOWED TEMPS TO DIP INTO THE TEENS IN SOME LOCATIONS...BUT AS THE
GRADIENT SLOWLY TIGHTENS FROM WEST TO EAST...ALONG WITH SOME MID/
HIGH LEVEL CLOUD COVER...TEMPS WILL LIKELY LEVEL OFF AND POSSIBLY
BECOME STEADY OVERNIGHT. CMS

&&

.SHORT TERM...
1128 AM CST

TONIGHT THROUGH SATURDAY MORNING...

WITH HIGH PRESSURE OVERHEAD...MINIMAL CLOUD COVER COVERS MUCH OF
THE REGION THIS AFTERNOON. THE EXCEPTION BEING NORTHWEST
INDIANA...WHICH REMAINS UNDER LAKE EFFECT CLOUDS AND FLURRIES.
TEMPERATURES HAVE SLOWLY WARMED INTO THE MID/UPR 20S...WITH A FEW
LOCATIONS SOUTH OF PONTIAC/WATSEKA POSSIBLY TOUCHING 30 DEGREES
LATER THIS AFTERNOON.

SFC RIDGE WILL STEADILY SLIDE EAST OVERNIGHT...WITH LIGHT LLVL FLOW
TURNING WESTERLY TO SOUTHWESTERLY. MINIMAL CLOUD COVER OVERNIGHT
SHOULD ALLOW TEMPS TO STEADILY RADIATE INTO THE MID/UPR TEENS.

SATURDAY...
MAIN FOCUS WILL BE ON TIMING OF MOISTURE RETURNING ALONG WITH THE
DEVELOPING WINTER STORM. FORTUNATELY IT APPEARS THE TIMING WILL BE
AFTER THE SATURDAY PERIOD. LLVL FLOW WILL REMAIN
SOUTHWESTERLY...WITH STEADY SATURATION FROM ALOFT TO THE SFC.
THERMAL TEMPS ALOFT WILL RISE TO ARND 0DEG C BY MIDDAY...BUT AS
MOISTURE ARRIVES ALOFT...EXPECT TEMPS TO SLOWLY COOL. SOUNDINGS
STILL ADVERTISE SOME DRY AIR IN THE MID LEVELS. TEMPS COULD WARM
INTO THE LOW/MID 30S...POSSIBLY BRIEFLY TOUCHING 40 DEGREES IN THE
FAR SOUTHERN CWFA BEFORE CLOUDS THICKEN WITH MOISTURE ARRIVAL.

BEACHLER

317 PM CST...

WINTER STORM...SATURDAY AFTERNOON THROUGH EARLY MONDAY...

HEADLINES: HAVE CONTINUED WITH WINTER STORM WATCH FOR ENTIRE CWA
SATURDAY EVENING THROUGH LATE SUNDAY NIGHT/EARLY MONDAY. PART OF
THE EVOLUTION OF THE STORM IS TIED TO THE TRACK AND STRENGTH OF
NORTHERN STREAM ENERGY MOVING ONTO THE BRITISH COLUMBIA COAST THIS
MORNING THAT WAS ONLY PARTIALLY SAMPLED BY THE UPPER AIR NETWORK.
THUS WHILE CONFIDENCE IS INCREASING ON A VERY SIGNIFICANT WINTER
STORM...CONFIDENCE IS NOT HIGH ENOUGH YET (80%+) IN TOTAL AMOUNTS
TO ISSUE A WARNING.

WE ARE STILL LOOKING AT A LONG DURATION PRIMARILY MODERATE SNOW
EVENT. HOWEVER...IT MUST BE NOTED THAT THE MUCH OF THE GUIDANCE
HAS TRENDED STRONGER AND FARTHER NORTH WITH THE SURFACE AND
MIDLEVEL LOWS THAT WILL IMPACT THE AREA. ENERGY FROM SOUTHWEST
CLOSED LOW WILL EJECT AND INTERACT WITH A STRONG NORTHERN WAVE
DIVING SOUTHEAST TOWARD THE REGION FROM THE NORTHERN PLAINS. THIS
WILL RESULT IN INTENSIFYING WARM ADVECTION/ISENTROPIC ASCENT IN
ADVANCE OF AN INITIALLY WEAK SURFACE LOW OVER THE SOUTHERN PLAINS
SATURDAY EVENING AND NIGHT. AT THE SAME TIME...COLDER AIR WILL
OOZE SOUTHWARD BEHIND COLD FRONT EMANATING FROM STRONG ARCTIC
HIGH PRESSURE SPREADING SOUTHEAST FROM THE CANADIAN PRAIRIES.

LONG PERIOD OF WARM ADVECTION DRIVEN SNOWS STARTING MAINLY SATURDAY
EVENING...BUT POSSIBLY AS EARLY AS THE LATE AFTERNOON IN THE FAR
SOUTHWEST CWA AS TOP DOWN SATURATION OCCURS. BOUNDARY LAYER
CONDITIONS WILL BE A BIT MARGINAL IN THESE AREAS...BUT MAX WET BULB
TEMPS ARE JUST SLIGHTLY ABOVE FREEZING...SO ANY RA/SN MIX THAT
OCCURS SHOULD QUICKLY TRANSITION TO SNOW...BUT WITH LOWER RATIOS
CLOSER TO 10:1. CAVEAT TO THIS WILL BE IF SYSTEM AND 850 MB LOW
TRACK CONTINUES NORTHWARD SHIFT ON GUIDANCE...PARTS OF SOUTHERN
CWA MAY FLIRT WITH MARGINAL THERMAL PROFILES LATER INTO SATURDAY
EVENING.

SNOW WILL OVERSPREAD THE AREA FROM WEST-SOUTHWEST TO EAST
NORTHEAST DURING SATURDAY EVENING...WITH CONDITIONS DETERIORATING
OVERNIGHT. WHILE SNOWFALL RATES WILL GENERALLY BE MODERATE (1/2"
TO 1"/HR)...STRENGTHENING 700 MB FGEN LAYING OUT ROUGHLY ALONG
I-80 BY LATE SATURDAY NIGHT INTO EARLY SUNDAY MORNING COULD RESULT
IN OCCASIONAL HEAVY RATES (1-2"/HR). AS SURFACE LOW TRACKS FROM
NORTHERN/CENTRAL MO TO CENTRAL IL DURING THE DAY ON SUNDAY...THE
DEFORMATION AXIS OF THE STRENGTHENING SURFACE AND MID LEVEL LOWS
WILL LINGER INTO LATE SUNDAY...KEEPING LIGHT TO MODERATE SNOW
GOING. IN ADDITION...850 MB LOW TRACK IS PROGGED TO TAKE NEARLY
IDEAL TRACK FOR SIGNIFICANT SNOW IN THE CWA FROM CENTRAL IL INTO
CENTRAL INDIANA. FINALLY...ON SUNDAY NIGHT...WHILE SNOW WILL TAPER
OFF FOR MUCH OF NORTH CENTRAL IL...IMPROVING THERMODYNAMICS AND
NORTH-NORTHEAST FLOW DOWN THE LENGTH OF LAKE MICHIGAN IS EXPECTED
TO RESULT IN LAKE ENHANCEMENT TRANSITIONING TO TRUE LAKE EFFECT
SNOW. BUFKIT SOUNDINGS ARE INDICATING IN EXCESS OF 200 J/KG OF
LAKE INDUCED CAPE. WHILE INVERSION HEIGHTS WILL BE SOMEWHAT
MARGINAL...LIFT WILL BE THROUGH DGZ AND ALSO HIGHER-RES GUIDANCE
IS INDICATING VERY STRONG CONVERGENCE DEVELOPING. HAVE BUMPED UP
POPS TO LIKELY INTO NE IL AND EXTREME NW INDIANA...BUT CONFIDENCE
IS LOW ON EXACTLY HOW MUCH SNOW WILL BE ADDED BY LAKE EFFECT INTO
EARLY MONDAY.

DESPITE LONG DURATION PRIMARILY MODERATE SNOW...HAZARD POSED BY
THIS STORM IS LIKELY TO BE MAGNIFIED BY STRENGTHENING PRESSURE
GRADIENT SUNDAY MORNING INTO SUNDAY NIGHT. TRACK OF LOW FOLLOWED
BY ARCTIC HIGH SPREADING SOUTHEAST IN ITS WAKE WILL RESULT IN
STRONG ISALLOBARIC PRESSURE FALLS FOLLOWED BY PRESSURE RISES. IF
SOME OF THE STRONGER SURFACE LOW SOLUTIONS ON GUIDANCE COME TO
FRUITION...THIS WILL BE EVEN MORESO THE CASE. DURING THIS
TIMEFRAME...SNOW WILL BE INCREASINGLY FLUFFY...SO BLOWING SNOW DUE
TO NORTHEAST WIND GUSTS OF 30-35 MPH COULD COMBINE WITH FALLING
SNOW TO RESULT IN NEAR BLIZZARD CONDITIONS IN OPEN AREAS. ON THE
IMMEDIATE LAKEFRONT...WIND GUSTS FROM THE FRICTIONLESS LAKE COULD
EVEN TOUCH 40 MPH...POTENTIALLY RESULTING IN NEAR BLIZZARD
CONDITIONS THERE. CONVERGENT LAKE BAND COULD ALSO RESULT IN EVEN
SLIGHTLY STRONGER WINDS ON THE LAKEFRONT SUNDAY NIGHT INTO EARLY
MONDAY MORNING...KEEPING THE THREAT OF SIGNIFICANTLY REDUCED
VISIBILITY GOING WHILE WINDS DIMINISH FOR THE REST OF THE AREA. AS
A FINAL ADDITIONAL NOTE ON THE LAKEFRONT...INCREASING CONCERN FOR
AT LEAST MINOR LAKESHORE FLOODING IN NE IL BY SUNDAY EVENING DUE
TO WAVES BUILDING TO 10 TO 14 FT. HAVE NOT ISSUED A LAKESHORE
FLOOD WATCH...BUT MAY NEED TO CONSIDER A LAKESHORE FLOOD ADVISORY.

OVERALL...CONTINUE TO HAVE HIGHEST CONFIDENCE IN GREATER THAN 8
INCH SNOW AMOUNTS IN 24 HOURS ROUGHLY ALONG/SOUTHEAST OF A LINE
FROM WINTHROP HARBOR TO FORRESTON IL. LOCALLY HIGHER AMOUNTS IN
EXCESS OF A FOOT ARE POSSIBLE NEAR LAKE MICHIGAN AND ALONG/SOUTH
OF INTERSTATE 88. NORTHWEST OF THAT LINE...INCLUDING THE ROCKFORD
METRO...POTENTIAL EXISTS FOR GREATER THAN 6 INCHES...BUT
BLOWING/DRIFTING SNOW IS LIKELY TO CAUSE SIGNIFICANT TRAVEL
IMPACTS EVEN IF STRICT WARNING CRITERIA OF 8 INCHES IN 24 HOURS IS
NOT MET.

RC

&&

.LONG TERM...
317 PM CST

MONDAY MORNING THROUGH THURSDAY...

TRANSITORY AND WEAKENING SURFACE HIGH PRESSURE MOVES THROUGH THE
AREA ON MONDAY. AS THIS OCCURS LOW LEVEL FLOW WHICH INITIALLY STILL
BE SLIGHTLY EAST OF NORTH WILL TURN NORTHWESTERLY THROUGH THE DAY.
LAKE EFFECT SNOW SHOWERS WILL BE WINDING DOWN DURING THE MORNING
PERIOD AS SIGNIFICANT DRYING IN THE 850-700 LAYER OCCURS AND WINDS
BACK TO NORTHWESTERLY...WITH ANY ACTIVITY EAST OF THE AREA BY MID
AFTERNOON. DESPITE SUNNY SKIES...COLD NORTH WINDS AND A FRESH
SNOWPACK WILL MEAN HIGHS IN THE TEENS.

NO REST FOR THE WEARY...AS THE FLOW REMAINS ACTIVE IN FOR MID WEEK.
FORTUNATELY THE INITIAL COLD ON MONDAY WILL BE SHORT LIVED AS THE
STRONG ARCTIC HIGH WEAKENS AND SHIFTS EAST MONDAY NIGHT. WNW FLOW
ALOFT WILL PERSIST OVER THE AREA WITH FAST MOVING PACIFIC SYSTEM
PASSING THROUGH THE REGION...AND A BAROCLINIC ZONE DRAPED SOMEWHERE
OVER THE REGION AS WELL. THIS SETS THE STAGE FOR QUICK
MOVING...BUT GENERALLY WEAKER SYSTEMS...BUT LOOKS LIKE THE
POTENTIAL FOR PERIODS OF MOSTLY LIGHT SNOW. THE FIRST MOVES THROUGH
MONDAY NIGHT WITH SOME VERY LIGHT SNOW POSSIBLE. A SECOND SYSTEM IS
ON ITS HEELS FOR TUESDAY EVENING WHICH PRESENTS MORE OF A SMALL
P-TYPE CONCERN. AFTERWARDS...A STRENGTHENING ARCTIC HIGH WILL MOVE
INTO THE NORTHERN PLAINS ON WEDNESDAY. AHEAD OF THE HIGH A COLD
FRONT MAY PROVIDE THE FOCUS FOR ADDITIONAL SNOW...FOLLOWED BY
ANOTHER GLANCING BLOW OF ARCTIC AIR THURSDAY INTO FRIDAY WITH HIGHS
IN THE TEENS AND LOWS SURROUNDING BOTH SIDES OF THE ZERO DEGREE
MARK...WIND CHILLS POSSIBLY GETTING NEARING -10 OR SO THURSDAY
NIGHT.

KMD

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 03Z...

* NONE THROUGH MID SATURDAY AFTERNOON.

* CIGS LOWER TO MVFR LATE SATURDAY AFTERNOON.

* SNOW ARRIVES EARLY SATURDAY EVENING BRINGING LOWERING CIGS AND
  IFR VSBY.

* SOUTHWEST WINDS TURN NORTHEAST SATURDAY EVENING.

MDB

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

HIGH PRESSURE IS MOVING EAST ACROSS THE AREA THIS EVENING WITH
WINDS BECOMING STEADIER FROM THE SOUTHWEST. A LAKE INFLUENCE HAS
TURNED WINDS OFF THE LAKE BETWEEN NORTHEAST AND SOUTHEAST NEAR THE
LAKE BUT THIS WILL BE FADING IN FAVOR OF SOUTHWESTERLY WINDS.
SOUTHWEST WINDS WILL COME UP TOWARDS 10 KT SATURDAY MORNING THEN
EASE UP LATE IN THE DAY AS THE PRESSURE GRADIENT SLACKENS.

CONDITIONS WILL DETERIORATE SATURDAY EVENING AS LOW PRESSURE
MOVES ACROSS THE CENTRAL PLAINS. CIGS WILL STEADILY LOWER LATE IN THE
DAY AND LIKELY BE MVFR BY LATE AFTERNOON. SNOW WILL SPREAD IN FROM
THE SOUTHWEST DURING THE EARLY TO MID EVENING AND CONTINUE THROUGH
SUNDAY. PEAK SNOW INTENSITY LOOKS TO BE SATURDAY NIGHT THROUGH
MIDDAY SUNDAY BEFORE EASING UP SUNDAY AFTERNOON...WITH 0.5 TO 1
INCH PER HOUR ACCUMULATION RATES EXPECTED. VSBY WILL LIKELY FALL
TO LESS THAN 1/2SM AT TIMES AS WELL. WINDS WILL TURN NORTHEASTERLY
SATURDAY EVENING AND INCREASE IN SPEED SUNDAY MORNING WITH GUSTS
NEAR 30 KT POSSIBLE. LAKE EFFECT/ENHANCEMENT BECOMES A CONCERN FOR
ORD/MDW/DPA/GYY LATE SUNDAY AFTERNOON INTO SUNDAY EVENING AS WINDS
TURN NORTHERLY. IF THIS MATERIALIZES SNOWFALL RATES WOULD INCREASE
AGAIN FOR A TIME WITH THE FOCUS TRANSITIONING FROM NE ILLINOIS TOWARD
NW INDIANA SUNDAY NIGHT INTO MONDAY MORNING.

MDB

//ORD AND MDW CONFIDENCE...UPDATED 03Z...

* HIGH CONFIDENCE IN THE FORECAST THROUGH SATURDAY AFTERNOON.

* HIGH CONFIDENCE IN CIGS LOWERING TO MVFR LATER SATURDAY AFTERNOON.

* HIGH CONFIDENCE IN SNOW ARRIVING BY MID SATURDAY EVENING. MEDIUM
  CONFIDENCE IN SPECIFIC TIMING. MEDIUM-HIGH CONFIDENCE IN QUICK
  REDUCTION TO IFR VSBY IN SNOW.

* MEDIUM-HIGH CONFIDENCE IN TIMING OF WINDS SHIFTING FROM
  SOUTHWEST TO NORTHEAST SATURDAY EVENING.

MDB

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

SUNDAY...SNOW THROUGH THE DAY...TAPERING OFF OVERNIGHT. MVFR
CIGS/VIS WITH PERIODS OF IFR CONDITIONS EXPECTED...ESPECIALLY
DURING THE MORNING.

MONDAY...VFR/WX NIL EARLY. SLIGHT CHANCE SNOW OVERNIGHT. MVFR
CIGS/VIS LIKELY.

TUESDAY AND WEDNESDAY...CHANCE OF SNOW. PERIODS OF MVFR CIGS/VIS
LIKELY.

THURSDAY...VFR/WX NIL.

MDB

&&

.MARINE...
139 PM CST

A BRIEF REPRIEVE FOR THE LAKE WILL BE SHORT LIVED AS MODERATELY STRONG
HIGH PRESSURE OVERHEAD SHIFTS EASTWARD TONIGHT. WEAK LOW PRESSURE IN
THE CANADIAN PRAIRIES WILL PUSH EAST TO LAKE SUPERIOR TONIGHT
SHIFTING WINDS TO SOUTHWESTERLY. STRONG ARCTIC HIGH PRESSURE IN
SASKATCHEWAN WILL PUSH INTO INTO THE NORTHERN PLAINS BY SUNDAY
AFTERNOON. MEANWHILE...LOW PRESSURE WILL EJECT FROM THE ROCKIES
SATURDAY NIGHT AND MOVE NORTHEAST THROUGH THE OHIO VALLEY SUNDAY
AFTERNOON AND EVENING...WHICH WILL ALLOW A COLD FRONT TO MOVE DOWN
THE LAKE. THIS TRACK COULD CREATE NORTHEASTERLY GALE CONDITIONS ON
THE LAKE SUNDAY AFTERNOON AND EVENING...ESPECIALLY ON THE SOUTH
HALF. THE WEAKENING HIGH MOVES OVER THE LAKE MONDAY THEN SHIFTS
EAST MONDAY NIGHT. ANOTHER WEAK LOW PASSES OVER MID LAKE AROUND
TUESDAY NIGHT...THEN HIGH PRESSURE REBUILDS INTO THE PLAINS AND
WILL SEND ANOTHER COLD FRONT OVER THE LAKE INTO THURSDAY.

KMD

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...WINTER STORM WATCH...ILZ003-ILZ004-ILZ008-ILZ010-ILZ011-ILZ019-
     ILZ020-ILZ021-ILZ032-ILZ039...6 PM SATURDAY TO MIDNIGHT
     MONDAY.

     WINTER STORM WATCH...ILZ005-ILZ006-ILZ012-ILZ013-ILZ014-ILZ022-
     ILZ023-ILZ033...9 PM SATURDAY TO 6 AM MONDAY.

IN...WINTER STORM WATCH...INZ001-INZ002-INZ010-INZ011-INZ019...9 PM
     SATURDAY TO 6 AM MONDAY.

LM...GALE WATCH...LMZ080-LMZ671-LMZ673-LMZ675-LMZ740-LMZ741-LMZ742-
     LMZ743-LMZ744-LMZ745-LMZ777-LMZ779-LMZ872-LMZ874-LMZ876-
     LMZ878...NOON SUNDAY TO MIDNIGHT MONDAY.

&&

$$

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