Area Forecast Discussion
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000
FXUS63 KLOT 261636
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1036 AM 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 15Z...

* IFR/LIFR SNOW SHOWERS THROUGH LATE MORNING AT ORD AND THROUGH
  EARLY TO MID AFTERNOON AT MDW. POSSIBLY BRIEF 1/2SM
  VISIBILITY...WITH HIGHER PROBABILITY AT MDW.

* NORTH-NORTHEAST WINDS WITH AT LEAST OCCASIONAL GUSTS OF 15-20 KT
  THROUGH THE MORNING. WINDS TURNING NORTH-NORTHWEST DURING THE
  AFTERNOON. THE TRANSITION TO A NORTHWESTERLY WIND MAY NOT OCCUR
  UNTIL LATE AFTERNOON/EARLY EVENING.

* MVFR CIGS IN LAKE EFFECT SNOW WITH OCCASIONAL IFR VERTICAL
  VISIBILITY IN HEAVIER SNOW SHOWERS.

//DISCUSSION...UPDATED 12Z...

LAKE EFFECT SNOW SHOWERS WILL CONTINUE THIS MORNING INTO NORTHEAST
ILLINOIS AND NORTHWEST INDIANA. THE CONDITIONS THAT WERE OBSERVED
OVERNIGHT BASICALLY WILL PERSIST AT ORD AND MDW THROUGH 15Z TO 16Z
OR SO...WITH TEMPORARY LIFR VISIBILITY IN THE MODERATE TO BRIEFLY
HEAVY SNOW BANDS. BECAUSE THESE BANDS ARE SMALL IN
WIDTH...TEMPORARY CONDITIONS ARE THE NATURE AND CONTINUE TO GO
THAT ROUTE WITH THE TAF. THE LOW-LEVEL WIND FLOW WILL ORIENT MORE
DUE NORTH BY LATER MORNING AND EARLY AFTERNOON...STEERING THE BULK
OF THE LAKE EFFECT ACTIVITY INTO NORTHWEST INDIANA. HAVE THE
LOWEST VISIBILITY AT GYY MORE SO DURING THAT TIME...ALTHOUGH THEY
COULD HAVE THAT LOW OF VISIBILITY EVEN EARLIER THIS MORNING.

THE EXACT TIMING OF WHEN THE WINDS SHIFT MORE NORTH-NORTHWEST AT
ORD IS CHALLENGING...AS THE LAKE EFFECT SNOW BANDS ARE PARTLY
DICTATING THAT. SO CONFIDENCE ON THAT IS LOW.

BY AFTERNOON...DRIER AIR WILL WORK INTO ORD AND MDW ENDING THE
LAKE EFFECT SNOW THREAT. WINDS WILL EASE IN SPEED AND SLOWLY BACK
TONIGHT.

//ORD AND MDW CONFIDENCE...UPDATED 15Z...

* HIGH IN SNOW SHOWERS THROUGH AT AROUND MIDDAY AT ORD AND THROUGH
  EARLY AFTERNOON AT MDW. MEDIUM-HIGH IN TEMPORARY LIFR VISIBILITY
  AT MDW...LOW AT ORD.

* HIGH ON WINDS BEING MORE NORTHEAST THAN DUE NORTH THROUGH MIDDAY.
  LOW ON TIMING OF WIND SHIFT TO NORTH-NORTHWEST.

* MEDIUM-HIGH ON CIGS.

//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...
259 AM CST

THE LAKE WILL GENERALLY REMAIN BETWEEN LOW PRESSURE SYSTEMS
THROUGH THE WEEKEND. PRESENTLY LAKE EFFECT SNOW SHOWERS...SOME OF
THEM HEAVY...WILL CONTINUE ACROSS THE SOUTHERN HALF OF THE LAKE
THROUGH TONIGHT. ASSOCIATED WITH THESE BANDS ARE SOME GUSTY WINDS
TO 30 KT. HAVE ISSUED A SMALL CRAFT ADVISORY FOR THE ILLINOIS AND
PART OF THE INDIANA NEARSHORE FOR THIS REASON. ICE COVERAGE OF
COURSE WILL DAMPEN MOST IF NOT ALL WAVE ACTION THERE.

HIGH PRESSURE WILL SLIDE JUST SOUTH OF LAKE MICHIGAN FRIDAY NIGHT
AND SLOWLY EXPAND UP TO THE EASTERN LAKES ON SATURDAY. A LOW
PRESSURE TROUGH BETWEEN NORTHERN AND SOUTHERN STREAM LOW PRESSURE
SYSTEMS WILL INCH ITS WAY OVER THE LAKE LATE SATURDAY NIGHT INTO
SUNDAY. A COLD FRONT WILL BASICALLY DEVELOP WITH THIS AND MOVE
OVER THE LAKE LATER SUNDAY SHIFTING WINDS BACK TO THE NORTHWEST.
LOW PRESSURE IS EXPECTED TO MOVE NORTHEAST NEAR OR OVER THE GREAT
LAKES REGION SOMETIME TUESDAY OR TUESDAY NIGHT WHICH COULD BRING
GUSTIER WINDS...BUT TOO MUCH UNCERTAINTY WITH THIS PATTERN IN THE
EXTENDED FORECAST TO HAVE MUCH CONFIDENCE.

MTF

&&

.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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