Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 260953
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
353 AM CST THU FEB 26 2015

.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.

MTF

&&

.LONG TERM...
331 AM CST

FRIDAY NIGHT 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 LATER THIS WEEK...INCLUDING ONE IN ROCKFORD SET
JUST LAST WINTER:

ROCKFORD...

THURSDAY 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 09Z...

* IFR/LIFR SNOW SHOWERS THROUGH AT LEAST MID TO LATE MORNING.
  POSSIBLY BRIEF 1/4SM VISIBILITY NAMELY AT MDW.

* NORTH-NORTHEAST WINDS WITH AT LEAST OCCASIONAL GUSTS OF 15-20 KT
  THROUGH THE MORNING.

* MVFR CIGS WITH OCCASIONAL IFR CIGS IN HEAVIER SNOW SHOWERS.

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

THE UPPER LEVEL WAVE THAT HAS BROUGHT WIDESPREAD SNOW TO NORTHERN
ILLINOIS AND NORTHWEST INDIANA WILL GRADUALLY SHIFT EASTWARD
THROUGH THE MORNING HOURS. WHILE THAT WILL QUICKLY BECOME LESS OF
A FACTOR...THE SETUP FOR LAKE EFFECT SNOW SHOWERS WILL CONTINUE TO
IMPROVE. THERMAL PROFILES FROM AMDAR DATA INDICATE AROUND 8000 FT
IN THE IDEAL SNOW GENERATION AREA WITHIN THE DEVELOPING LAKE
EFFECT CONVERGENCE...AND HAVE ALREADY SEEN RADAR TRENDS AND
OBSERVATIONS/REPORTS SUPPORTING LOCALLY HEAVY SNOW. SO HAVE
TRENDED MORE PESSIMISTIC WITH THE TAF...WITH TEMPORARY LIFR
CONDITIONS IN SNOW SHOWERS. BRIEF 1/4SM VISIBILITY CERTAINLY IS
POSSIBLE IN THE HEAVIER SNOW BANDS AND ALREADY HAS BEEN BRIEFLY
OBSERVED AT MDW. VERTICAL VISIBILITY HAS NOT DROPPED BELOW 800 FT
THUS FAR...AND WOULD NOT EXPECT TO SEE THAT GET MUCH LOWER...BUT
STILL IMPACTS WILL BE FELT AT THE AIRPORTS DUE TO QUICK BURSTS OF
LOWER VISIBILITY AND SNOW ACCUMULATION.

GIVEN THE SMALL SCALE NATURE OF SPECIFIC SNOW BANDS AND THE
CHALLENGE TO PREDICT IF THIS EVENT WILL REMAIN AS SEVERAL SNOW
BANDS OR DEVELOP INTO ONE OR TWO MORE FOCUSED ONES BY
DAYBREAK...CONFIDENCE ON EXACT TIMING OF LOWER VISIBILITY IS LOW
AND THUS FEEL IT PRUDENT TO REMAIN WITH LONGER TEMPO TIMES. ALSO
FOR SNOW ACCUMULATIONS AT THE AIRPORTS...BEST FORECAST IS THAT
ADDITIONAL ACCUMULATIONS AFTER 06Z OF ONE TO TWO AND A HALF INCHES
FOR MDW AND MAYBE A LITTLE HIGHER AT GYY MAINLY COMING LATER THIS
MORNING...WITH AROUND ONE INCH OR A LITTLE MORE AT ORD.
CONFIDENCE IS LOW-MEDIUM ON THESE ACCUMULATIONS GIVEN THE NARROW
NATURE OF THE BANDING.

THE LAKE EFFECT SNOW BANDS WILL PIVOT MORE INTO NORTHWEST INDIANA
BY EARLY AFTERNOON AND GRADUALLY EASE EASTWARD INTO EVENING.
CLOUDS IN THE WAKE SHOULD LIFT AND SCATTER ACROSS NORTHERN
ILLINOIS...WITH RFD EXPERIENCING THIS IMPROVEMENT ALREADY THIS
MORNING.

NORTH-NORTHEAST WINDS AT ORD AND MDW WILL PIVOT MORE NORTHERLY IN
ASSOCIATION WITH THE LAKE EFFECT CONVERGENCE. WHEN EXACTLY THIS
HAPPENS IS CHALLENGING TO SAY...BUT IT ALREADY SEEMS TO BE BEING
OBSERVED AS OF 06Z AT ORD...SO NEAR DUE NORTH WINDS MAY BE MORE
COMMON AT ORD THROUGH MUCH OF THIS MORNING.

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

* HIGH IN SNOW SHOWERS THROUGH AT LEAST 14Z-16Z AT ORD AND THROUGH
  THE MORNING AT MDW. MEDIUM-HIGH IN VISIBILITY ALTERNATING
  BETWEEN MAINLY IFR AND LIFR ESPECIALLY AT MDW.

* HIGH ON NORTH-NORTHEAST WINDS AT MDW THROUGH AT LEAST MID
  MORNING. LOW-MEDIUM AT ORD WHERE THEY ARE LIKELY TO BE CLOSER TO
  DUE NORTH AT TIMES THROUGH THE MORNING. HIGH ON WIND SPEEDS AND
  OCCASIONAL GUSTS.

* MEDIUM-HIGH ON CIGS AND THAT THEY WILL REMAIN ABOVE 700 FT.

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

FRIDAY...VFR.

SATURDAY...VFR THEN LIGHT SNOW AND IFR POSSIBLE SATURDAY NIGHT.

SUNDAY...LIGHT SNOW LIKELY...IFR POSSIBLE.

MONDAY...VFR THEN LIGHT SNOW AND IFR POSSIBLE MONDAY NIGHT.

TUESDAY...SNOW LIKELY...IFR POSSIBLE.

MTF/RC

&&

.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 ADVISORY...ILZ006-ILZ014 UNTIL NOON THURSDAY.

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

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

&&

$$

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