Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 262008
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
208 PM CST THU FEB 26 2015

.UPDATE...
1023 AM CST

WITH PERSISTENT BANDS OF LAKE EFFECT SNOW SHOWERS MOVING INTO
NORTHEASTERN ILLINOIS AND NORTHWESTERN INDIANA...THE LAKE EFFECT
SNOW ADVISORY HAS BEEN UPGRADED TO A LAKE EFFECT SNOW WARNING. A
LAKE EFFECT SNOW WARNING IS ALSO BEING ISSUED FOR PORTER COUNTY IN
NORTHWESTERN INDIANA. SNOW ACCUMULATIONS OF 4 INCHES TO...
LOCALLY...AS HIGH AS 8 INCHES HAVE BEEN REPORTED OVER THE CHICAGO
METRO AREA. LATEST RADAR IMAGERY INDICATES PERSISTENT BANDS OF
LAKE EFFECT SNOW SHOWERS STREAMING INLAND FROM THE LAKE WITH
PARTICULARLY HEAVY BANDS EXTENDING FROM GARY...INDIANA THROUGH FAR
EASTERN COOK AND WILL COUNTIES. ANOTHER STRONG BAND IS JUST ALONG
THE LAKESHORE FROM NEAR WAUKEGAN TO NEAR O`HARE. THESE BANDS OF
LAKE EFFECT SNOW ARE SLOWLY PROGRESSING TO THE SOUTH AND
EAST...AND ANTICIPATE THAT MUCH OF THE ACTIVITY SHOULD END OVER
LAKE COUNTY...ILLINOIS BY EARLY THIS AFTERNOON. THE ENDING TIME OF
LAKE EFFECT SNOW FOR COOK COUNTY IS A LITTLE TRICKIER DUE TO THE
GEOPOLITICAL BOUNDARIES...BUT THE GENERAL TREND SHOULD BE FOR THE
SNOW TO END FROM NORTH TO SOUTH...WITH SNOW SHOWERS LIKELY
LINGERING OVER THE FAR SERN PORTIONS OF THE COUNTY INTO THIS
EVENING. THE LATEST SHORT RANGE...HIGH RES GUIDANCE SUGGESTS THAT
A LAKE PLUME SHOULD BE PROGRESSIVE THROUGH LAKE COUNTY INDIANA
THIS EVENING...BUT COULD POSSIBLY THEN LINGER OVER PORTER COUNTY
THROUGH MUCH OF THE NIGHT TONIGHT. EXPECT THAT LAKE
COUNTY...ILLINOIS AND NORTHERN COOK COUNTY COULD SEE AN ADDITIONAL
1 TO...LOCALLY...3 INCHES OF NEW SNOWFALL. LAKE COUNTY INDIANA
SHOULD SEE AN ADDITIONAL 2 TO...LOCALLY...4 INCHES. EXPECT THAT
PORTER COUNTY SHOULD RECEIVE 4 TO...LOCALLY...8 INCHES OF LAKE
EFFECT SNOW THIS EVENING AND OVERNIGHT TONIGHT.

KREIN

&&

.SHORT TERM...
345 AM CST

THROUGH TONIGHT...

THE PRIMARY CONCERNS ARE LAKE EFFECT SNOW SHOWERS AND THEIR
IMPACTS.

THE UPPER WAVE THAT HAS BROUGHT SNOW TO THE ENTIRE AREA YESTERDAY
AFTERNOON AND NIGHT CONTINUES LIGHT SNOW EARLY THIS MORNING.
THIS WILL VERY GRADUALLY DEPART SOUTHEAST THROUGH THE MORNING WITH
DIMINISHING DEEP LAYER MOISTURE. LITTLE ADDITIONAL ACCUMULATION
IS EXPECTED WITH THIS BUT WILL LIKELY SLOW THE MORNING COMMUTE
AREAWIDE. SOME LINGERING LIGHT SNOW MAY CONTINUE ALONG AND SOUTH
OF HIGHWAY 24 AS LATE AS EARLY THIS AFTERNOON.

WHILE THE SYNOPTIC SNOW IS TAPERING...SEVERAL LAKE EFFECT SNOW BANDS
EXTEND FROM THE LAKE INTO NORTHEAST IL AND FAR SOUTHEAST WI AND HAVE
BRIEFLY BEEN HEAVY IN INTENSITY PER RADAR AND OBSERVATIONS/REPORTS.
THE CONVERGENCE IS WELL-DEFINED ON OBSERVATIONAL STREAMLINE ANALYSIS
AND DO NOT SEE MUCH CHANGE IN THIS OCCURRING THROUGH MID MORNING
BEFORE A SLOW PIVOT INTO NORTHWEST INDIANA. COLDER AIR CONTINUES
TO ADVECT INTO THE REGION ON NORTH-NORTHEAST WINDS STEEPENING
LAKE-INDUCED LAPSE RATES AND ALLOWING FOR AT LEAST SUSTAINED CLOUD
DEPTHS. OFFSETTING THIS THOUGH IS MUCH DRIER LOW- LEVEL AIR
ADVECTING INTO THE REGION...WITH DEW POINTS OF -5 TO -15 ACROSS
LOWER MICHIGAN. THE THOUGHT IS THAT THIS DRY AIR MAY COME MORE
INTO PLAY LATER TODAY AND TONIGHT...BUT FOR THE MEANTIME LAKE
EFFECT SNOW SHOWERS SHOULD CONTINUE ACROSS CHICAGO AND FAR
NORTHEAST ILLINOIS AND NORTHERN PARTS OF LAKE COUNTY INDIANA. SNOW
RATIOS FROM A COUPLE SOURCES WITH MOSTLY LAKE EFFECT SNOW HAVE
INDICATED 20:1 TYPE RATIOS WHICH IS CONCURRENT WITH THERMAL AND
MOISTURE PROFILES ON LATEST AMDAR DATA...WHICH SHOW A DEEP
ISOTHERMAL LAYER AROUND 7000 FT IN THE IDEAL SNOW GROWTH AREA.
HAVE THESE HIGHER RATIOS IN THE FORECAST WITH A SLIGHTLY BOOSTED
QPF FROM THE LOCAL 8KM ARW. ADDITIONAL AMOUNTS AFTER SUNRISE OF
ONE TO THREE INCHES ARE EXPECTED. THIS ON TOP OF THE ALREADY
FALLEN SNOW ALONG WITH THE TIMING FURTHER SUPPORTS THE ADVISORY IN
EFFECT.

AS FOR PORTER COUNTY INDIANA...THIS BECOMES MORE CHALLENGING AS HIGH
RESOLUTION MODELS REALLY AGREE ON MORE OF A ONE SNOW BAND EVOLUTION
BY LATER TODAY INTO EVENING FOR NORTHWEST INDIANA...WHICH COULD IN
THEORY MEAN A QUICK FEW INCHES OF SNOW INCLUDING INTO PORTER
COUNTY. THE MUCH DRIER SYNOPTIC AIR MASS WILL ONLY FURTHER DRY
AND DEEPEN INTO THIS EVENING...SO IT GETS A BIT TRICKY. GIVEN THE
LATER TODAY INTO EVENING TIMING...HAVE OPTED TO ALLOW DAY SHIFT
TO ASSESS ADVISORY BUT WILL HIGHLIGHT IN AN SPS FOR NOW.

FRESH SNOW COVER AND MORNING CLOUDS ALONG WITH NORTHERLY WINDS
WILL BASICALLY STIFLE ANY TEMPERATURE RISE TODAY. READINGS IN THE
TEENS WILL BE CLOSE TO A RECORD LOW MAXIMUM IN ROCKFORD...BUT AT
THIS POINT THE FORECAST IS SLIGHTLY WARMER.

FOR TONIGHT...A CLEARING SKY AND CONTINUED COLD ADVECTION WILL SET
THE STAGE FOR -15 TO -25 WIND CHILLS DEVELOPING OVERNIGHT INTO
FRIDAY MORNING. AT THIS TIME HAVE NOT ISSUED AN ADVISORY BUT IT
APPEARS DAY SHIFT MAY VERY WELL NEED ONE FOR AT LEAST NORTH
CENTRAL ILLINOIS. AS FOR ACTUAL LOW TEMPERATURES...BELOW ZERO ARE
EXPECTED AREAWIDE OUTSIDE OF DOWNTOWN CHICAGO...WITH NEGATIVE
DOUBLE DIGITS IN THE FAVORED SPOTS BECOMING MORE PROBABLE.

MTF

&&

.LONG TERM...
331 AM CST

FRIDAY AFTERNOON THROUGH WEDNESDAY...

AFTER A BITTERLY COLD START TO FRIDAY...LAKE EFFECT SNOW SHOWERS
WILL HAVE NOW SHIFTED EAST OF THE AREA AS THE SLOW MOVING UPPER
TROUGH AXIS BEGINS TO PUSH EASTWARD AS WELL...AND STRONG HIGH
PRESSURE SETTLES INTO THE REGION. 850 TEMPS WARM ONLY TO THE MID
TEENS BELOW ZERO RESULTING IN ANOTHER VERY COLD THOUGH SUNNY DAY.
HIGHS ONLY IN THE TEENS. WITH THE HIGH IN THE REGION FRIDAY
NIGHT...EXPECT CLEAR SKIES BRINGING ANOTHER BITTERLY COLD NIGHT WITH
LOWS BELOW ZERO IN MANY LOCATIONS. WITH THE HIGH NOT ENTIRELY
OVERHEAD...SOME WIND COMPONENT WILL BRING WIND CHILLS DOWN IN THE
-5 TO -20 RANGE.

THE SURFACE HIGH PUSHES OFF TO THE EAST ON SATURDAY AND WE GET INTO
RETURN FLOW ON THE BACKSIDE. 850 FLOW BECOMES DUE SOUTHERLY WHICH
WILL ALLOW THE LOW LEVEL AIR MASS TO MODERATE AND MOISTEN BUT ONLY
A LITTLE AS WE MAINTAIN AN ESE SURFACE FLOW WHICH SHOULD BE COOLER
AND DRIER. MEANWHILE ALOFT THE FLOW IS TURNING SOUTHWESTERLY OUT
AHEAD OF A DEEP UPPER LOW THAT WILL CLOSE OFF IN SOUTHERN CALIFORNIA
ALONG WITH A NORTHERN STREAM LOW/TROUGH SPREADING EAST THROUGH THE
CANADIAN PRAIRIES AND UPPER MIDWEST. HIGHS WARM INTO THE UPPER TEENS
NORTH TO LOWER 20S SOUTH WITH INCREASING CLOUDS ESPECIALLY IN THE
AFTERNOON.

LATER SATURDAY AFTERNOON AND EVENING...MORE SIGNIFICANT WARM/MOIST
ADVECTION THAT WILL BE ONGOING ACROSS THE PLAINS WILL PUSH NORTHWARD
TOWARD OUR AREA. THE STRONGER MOISTURE TRANSPORT WILL BE DIRECTED
SOUTH AND EAST OF OUR AREA BUT WILL GRAZE OUR SOUTHERN AREAS AS WELL
WITH MORE MODEST TRANSPORT FARTHER NORTH. STILL FAIRLY DEEP MOISTURE
AS SOUNDINGS ARE SATURATED THROUGH THE ENTIRE COLUMN. SEVERAL SUBTLE
SHORTWAVES WILL RIDE NORTHWESTWARD OUT AHEAD OF THE CALIFORNIA
LOW...THE FIRST ACCOMPANYING THE WARM/MOIST ADVECTION SATURDAY NIGHT
TO PRODUCE A ROUND OF SNOW THAT SHOULD INCREASE AREA WIDE FROM NORTH
TO SOUTH THROUGH THE EVENING AND ESPECIALLY OVERNIGHT.

THE BROAD WARM ADVECTION CONTINUES ON SUNDAY WITH A STRONG UPPER JET
INTENSIFYING RIGHT OVERHEAD. EXPECT CONTINUED SNOWFALL THOUGH THE
DAY WITH MORE MODERATE AMOUNTS FROM CHICAGO SOUTHWARD. HIGHEST
AMOUNTS AT THIS POINT LOOK TO BE SOUTH AND EAST INTO NW INDIANA AND
INTO CENTRAL ILLINOIS. MEANWHILE A NORTHERN STREAM TROUGH WILL BE
MOVING THROUGH THE NORTHERN PLAINS AND UPPER MIDWEST. THIS WILL
CROSS WISCONSIN SUNDAY EVENING...WITH MODEST HIGH PRESSURE BUILDING
IN QUICKLY BEHIND IT FOR QUIET CONDITIONS ON MONDAY.

BUT THE HIGH WILL BE SHORT LIVED AS THE UPPER LOW IN THE SOUTHWEST
BEGINS TO SLOWLY EJECT EASTWARD. MORE SIGNIFICANT WARM/MOIST
ADVECTION DUE TO 850 WINDS IN EXCESS OF 30KT AND PRECIPITABLE WATER
VALUES IN EXCESS OF 0.75 INCHES WILL BE ACCOMPANIED BY SOME BETTER
DYNAMICS AS THE UPPER JET STRENGTHENS TO OUR NORTH. AS MENTIONED IN
PREVIOUS DISCUSSIONS...THIS SYSTEM LOOKS TO HAVE SOME SIGNIFICANTLY
WARMER AIR WITH IT. THIS CREATES A POTENTIALLY MESSY SITUATION
DURING THE TRANSITION PERIOD AS 850 TEMPS...WHICH START TUESDAY
NIGHT AROUND -2C TO -8C FROM SOUTH TO NORTH RAPIDLY INCREASE TO +7C
SOUTH OR HIGHER TO AROUND +1 TO +3C ALONG THE WISCONSIN BORDER.
MEANWHILE SURFACE FLOW HANGS ONTO A E/SE DIRECTION THUS POTENTIALLY
DELAYING SURFACE WARMING. HAVE TRENDED WEATHER WORDING TOWARD THESE
WARMER SOLUTIONS WITH GFS/EC IN BETTER AGREEMENT IN SHOWING SNOW
TRANSITIONING TO A MIX AND THEN ALL RAIN EXCEPT IN THE FAR NORTH.
BUT CONFIDENCE IS STILL LOW AT THIS POINT GIVEN THE DYNAMICAL NATURE
OF THE SYSTEM. THE SURFACE LOW WITH THIS SYSTEM MOVES EAST TUESDAY
NIGHT ALLOWING COOLER AIR TO FILTER IN BEHIND IT. THINGS STAY ACTIVE
BEYOND THIS TIME AS WE WAIT FOR THE TROUGH IN THE WEST TO FINALLY
CROSS THE AREA.

KMD

&&

.CLIMATE...
345 AM CST

TEMPERATURES ARE FORECAST ONCE AGAIN TO BE CLOSE OR BREAK THE
FOLLOWING RECORDS THROUGH THE LATTER HALF OF THIS WEEK...INCLUDING
ONE ON FRIDAY IN ROCKFORD THAT WAS SET JUST LAST WINTER:

ROCKFORD...

TODAY FEB 26TH LOW MAXIMUM: 11 IN 1920
FRIDAY FEB 27TH LOW MAXIMUM: 15 IN 2014
SATURDAY FEB 28TH LOW: -7 IN 1962
SATURDAY FEB 28TH LOW MAXIMUM: 17 IN 1962

CHICAGO...

FRIDAY FEB 27TH LOW: -6 IN 1897
SATURDAY FEB 28TH LOW: 0 IN 1884

MTF

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 20Z...

* NORTHERLY WINDS 10 TO 14 KT BECOING NORTHWEST THIS EVENING.

KJB

//DISCUSSION...UPDATED 18Z...

THE FOCUS FOR LAKE EFFECT SNOW SHOWERS WILL GRADUALLY BEGIN TO SHIFT
EASTWARD INTO SOUTHERN COOK AND NORTHWESTERN INDIANA OVER THE NEXT
FEW HOURS. AREA SURFACE OBSERVATIONS INDICATE WINDS SHIFTING
NORTHERLY ACROSS LAKE AND NORTHERN COOK COUNTIES IN ILLINOIS. RADAR
TRENDS OVER THE PAST HOUR HAVE ALSO SHOWN A DECREASE IN INTENSITIES
ACROSS LAKE COUNTY ILLINOIS...WITH SEVERAL SITES NOW OUT OF THE SNOW
AREA. THESE SAME TRENDS SHOULD GRADUALLY SHIFT SOUTH AND EASTWARD
THROUGH THE AFTERNOON. A BETTER WELL DEVELOPED BAND OF LAKE EFFECT
SNOW SHOWERS WILL TRY TO MOVE OVER KMDW OVER THE NEXT HOUR OR
SO...AND THIS COULD RESULT IN ANOTHER BRIEF PERIOD OF LOW
VISIBILITIES...POSSIBLY DOWN TO A HALF MILE. CONDITIONS WILL
IMPROVE BY MID AFTERNOON AT KMDW...BUT AT KGYY PERIODS OF LAKE
EFFECT SNOW SHOWERS WILL CONTINUE THROUGH THE AFTERNOON...BEFORE
ABATING THIS EVENING.

AFTER THIS LAKE EFFECT FOCUS SHIFTS EAST OF THE TERMINALS A PERIOD
OF QUIET WEATHER IS EXPECTED FOR TONIGHT AND FRIDAY AS A SURFACE
RIDGE OF HIGH PRESSURE MOVES OVER CENTRAL ILLINOIS. WINDS ARE
EXPECTED TO BE RATHER LIGHT WITH A WESTERLY DIRECTION.

KJB

//ORD AND MDW CONFIDENCE...UPDATED 20Z...

HIGH WITH ALL ELEMENTS.

KJB

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

FRIDAY NIGHT...VFR. SOUTHWEST WINDS.
SATURDAY...VFR THEN SNOW WITH IFR PROBABLE SATURDAY NIGHT. SOUTH
           WINDS.
SUNDAY...SNOW WITH IFR POSSIBLE. SOUTHWEST WINDS.
MONDAY...VFR THEN LIGHT SNOW WITH IFR POSSIBLE MONDAY NIGHT. NORTH
         WINDS.
TUESDAY...WINTRY MIX LIKELY WITH IFR. SOUTHWEST WINDS.
WEDNESDAY...SLIGHT CHANCE OF SNOW AND IFR. NORTHWEST WINDS.

MTF

&&

.MARINE...
215 PM CST

THE FAR NORTHERN PORTIONS OF THE LAKE REMAINS ICE COVERED AS DO
THE SOUTHERN NEARSHORE WATERS. THERE...WAVES HAVE REMOVED FROM THE
FORECAST FOR THESE AREAS.

HIGH PRESSURE EXTENDS FROM THE NORTHERN PLAINS...WITH A CENTRAL
PRESSURE OF 30.8 INCHES...SOUTH THROUGH TEXAS AND EAST ACROSS THE
GREAT LAKES. THE CENTER OF THE HIGH WILL CONTINUE TO DROP TO THE
SOUTHEAST...TO THE UPPER MISSOURI VALLEY BY FRIDAY MORNING. WITH
RIDGE AXIS EXTENDING ACROSS LAKE MICHIGAN...A CONVERGENCE BAND HAS
SET UP WITH A PLUME OF LAKE EFFECT SNOW MOVING ACROSS THE SOUTHERN
PORTIONS OF THE LAKE. AS THE CENTER OF THE HIGH CONTINUES TO
SETTLE INTO THE OHIO VALLEY FRIDAY AND FRIDAY NIGHT...WINDS OVER
THE LAKE WILL BACK FROM NORTHERLY TO WESTERLY FRIDAY NIGHT AND
SATURDAY. AS THE HIGH MOVES OFF TO THE EAST...A WEAK COLD FRONT IS
EXPECTED TO CROSS THE UPPER MISSISSIPPI VALLEY SATURDAY
NIGHT...BACKING WINDS OVER THE LAKE TO SOUTHWESTERLY. THROUGH THIS
PERIOD...WINDS WILL BE ON A GENERAL WEAKENING TREND. THE WEAK COLD
FRONT IS EXPECTED TO SWEEP ACROSS THE LAKE SUNDAY EVENING WITH
ANOTHER ARCTIC HIGH QUICKLY FOLLOWING IN IT`S WAKE. THIS HIGH WILL
TAKE A MORE NORTHERLY TRACK...WITH THE CENTER OF THE HIGH CROSSING
THE LAKE SUNDAY NIGHT AND MONDAY...LEADING TO A SHORT PERIOD OF
LIGHT AND VARIABLE WINDS. THE HIGH SHOULD THEN PUSH EAST OF THE
LAKE MONDAY AND MONDAY NIGHT AS LOW PRESSURE DEVELOPS OVER
KANSAS/NEBRASKA IN THE LEE OF THE ROCKIES...SETTING UP A PERIOD OF
SOUTHEAST WINDS AS AN ASSOCIATED WARM FRONT EXTENDS EAST OF THE
LOW...TO THE OHIO VALLEY. THE LOW IS EXPECTED TO DEEPEN TO 29.1
INCHES AS IT TRACKS NORTHEAST AND CROSSES THE MIDDLE OF LAKE
MICHIGAN TUESDAY EVENING.

KREIN

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...LAKE EFFECT SNOW WARNING...ILZ006 UNTIL 3 PM THURSDAY.

     LAKE EFFECT SNOW WARNING...ILZ014 UNTIL 6 PM THURSDAY.

IN...LAKE EFFECT SNOW WARNING...INZ001 UNTIL 9 PM THURSDAY.

     LAKE EFFECT SNOW WARNING...INZ002 UNTIL 6 AM FRIDAY.

LM...SMALL CRAFT ADVISORY...NEARSHORE WINTHROP HARBOR TO GARY IN UNTIL 3 PM
     THURSDAY.

&&

$$

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