Area Forecast Discussion
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000
FXUS63 KLOT 260935
AFDLOT

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

.MESOSCALE DISCUSSION...1200 AM CST

SEMI-ORGANIZED MODERATE TO AT TIMES HEAVY LAKE EFFECT SNOW
SHOWERS HAVE BEEN STREAMING INTO NORTHEAST ILLINOIS OVER THE PAST
COUPLE HOURS. THIS HAS BECOME THE MORE IMPACTING PHENOMENON AS THE
SYNOPTIC SNOW FROM THE UPPER LEVEL SHORT WAVE EASE SOUTHEAST.

REPORTS AT MIDNIGHT INDICATE SNOW OF AROUND AN INCH TO INCH AND A
HALF HAVE BEEN OBSERVED IN THE PAST TWO-THREE HOURS IN COOK
COUNTY. RADAR ECHOES OF 25-30 DBZ HAVE CORRELATED WITH VISIBILITY
OF ONE HALF TO AT TIMES ONE QUARTER MILE WITH SOME OF THIS LAKE
EFFECT SNOW ACTIVITY...INDICATING AT LEAST TEMPORARY HEAVY
RATES UNDER THESE SHOWERS. ON THE LARGE SCALE...LOW-LEVEL DRY AIR
IS ADVANCING SOUTHWARD...BUT NOT SURE IF THIS WILL HAVE MUCH
IMPACT ON THE LAKE EFFECT SNOW SHOWERS AS LAKE-INDUCED EQUILIBRIUM
LEVELS ARE ACTUALLY FORECAST TO IMPROVE WITH THE COOLING AIR MASS.
IN ADDITION...AMDAR DATA INDICATES ISOTHERMAL PROFILES OF AROUND
8000 FT THICK IN THE IDEAL SNOW GROWTH AREA WHERE THIS LAKE EFFECT
CONVERGENCE IS STREAMING INTO NORTHEAST IL. WITH MESOSCALE
CONVERGENCE LIKELY TO HOLD INTO NORTHEAST ILLINOIS THROUGH MID-
MORNING OR SO...MAY END UP NEEDING A LAKE EFFECT SNOW ADVISORY FOR
COOK COUNTY SIMPLY FOR THE AT TIMES HEAVIER RATES INTO THE MORNING
COMMUTE AND ADDITIONAL ACCUMULATIONS THAT COULD NEAR THREE INCHES
IF A BAND OR BANDS HOLD IN ANY ONE PLACE FOR A COUPLE/FEW HOURS OR
SO. WILL CLOSELY MONITOR OBSERVATIONAL TRENDS AND SNOWFALL REPORTS
AND ESTIMATES FROM ANY LATE NIGHT REPORTS. WILL ALSO NEED TO
ASSESS IF SUCH AN ADVISORY WILL BE NEEDED TODAY FOR LAKE AND
PORTER COUNTIES IN INDIANA.

MTF

&&

.SHORT TERM...

THROUGH THURSDAY NIGHT...

UPDATED OVERNIGHT SHORT TERM DISCUSSION FORTHCOMING. SEE MESOSCALE
DISCUSSION ABOVE FOR LAKE EFFECT SNOW DETAILS.

DISCUSSION FROM 300 PM...FORECAST CONCERNS/CHALLENGES ARE SNOWFALL
AMOUNTS THRU THURSDAY MORNING AND LAKE EFFECT POTENTIAL/AMOUNTS
OVERNIGHT THRU THURSDAY EVENING.

SNOW CONTINUES TO OVERSPREAD NORTHWEST IL REDUCING VISIBILITIES TO
UNDER A MILE AT TIMES AND THIS WILL CONTINUE MOVING EAST/SOUTHEAST
ACROSS THE REST OF THE AREA DURING THE MID/LATE AFTERNOON. WITH
TIME...THE BEST FORCING SAGS SOUTH ACROSS THE SOUTHERN CWA INTO
CENTRAL IL THIS EVENING THUS EXPECT AMOUNTS TO BE HEAVIEST ACROSS
THE SOUTHWEST CWA WHERE STORM TOTAL 2-4 INCHES ARE POSSIBLE...
WITH LESSER AMOUNTS TO THE NORTH...IN THE 1-3 INCH RANGE. CHANGES
TO THIS EVENING WERE TO INCREASE TO CAT POPS AND ALSO BUMP UP QPF
AMOUNTS SOME...GENERALLY IN THE ONE TO TWO TENTHS RANGE. THE NET
AFFECT OF THIS IS THAT STORM TOTAL AMOUNTS ARE NOW ON THE HIGHER
END OF THE ABOVE RANGES. ITS POSSIBLE SOME AREAS IN THE SOUTHWEST
CWA MAY REACH 4 INCHES...BUT CONFIDENCE IS LOW ENOUGH TO HOLD OFF
ON ANY ADVISORIES FOR THOSE SOUTHWEST COUNTIES WITH AN SPS
CURRENTLY OUT HIGHLIGHTING THESE HIGHER AMOUNTS.

WHILE THE BULK OF THE ACCUMULATING SNOW WILL LIKELY FALL THIS
EVENING...THERE WILL LIKELY REMAIN VERY LIGHT SNOW/FLURRIES THRU
MUCH OF THE NIGHT INTO THURSDAY MORNING...AS IT FINALLY ENDS FROM
THE NORTHWEST MIDDAY/EARLY THURSDAY AFTERNOON.

HOWEVER...NORTHEAST WINDS HAVE ALREADY ALLOWED SOME LAKE EFFECT
SNOW TO DEVELOP ACROSS SOUTHEAST WI AND THIS NORTHEAST FLOW WILL
FIRST ADD SOME LAKE ENHANCEMENT THIS EVENING AND THEN TRANSITION
TO LAKE EFFECT SNOW SHOWERS OVERNIGHT INTO THURSDAY MORNING.
THERMAL PROFILES SHOW A FAIRLY GOOD SET UP FOR LAKE EFFECT AND
HIGH RES GUIDANCE WOULD SUGGEST A POSSIBLE LAKE EFFECT PLUME
AFFECTING LAKE COUNTY IL OVERNIGHT...THEN EASTERN COOK COUNTY
THURSDAY MORNING AS IT SHIFTS INTO NORTHWEST INDIANA. BUT
CONFIDENCE REGARDING DURATION AND LOCATION IS FAIRLY LOW. AREAS
NEAR THE LAKE MAY END UP WITH AN ADDITIONAL 1-2 INCHES OF LAKE
EFFECT OVERNIGHT INTO THURSDAY MORNING. AS THE BAND SHIFTS INTO
NORTHWEST INDIANA THURSDAY AFTERNOON...DECENT CONVERGENCE SETS UP
BY THURSDAY EVENING SO SOME CONCERN THAT IF A BAND CAN PERSIST FOR
ANY LENGTH OF TIME...ADDITIONAL ACCUMULATION WOULD POSSIBLE...
MAINLY EAST OF GARY.

AFTER TEMPERATURES REACHED THE LOWER/MID 20S NORTH TO NEAR 30
SOUTH TODAY...THEY WILL FALL THROUGH THE TEENS THIS EVENING AND
POSSIBLY REMAIN STEADY IN THE LOWER/MID TEENS ACROSS THE SOUTH AND
NEAR THE LAKE OVERNIGHT...FALLING INTO THE SINGLE DIGITS ACROSS
THE NORTHWEST CWA BY MORNING. BUT NOT MUCH RECOVERY IS EXPECTED
FOR THURSDAY HIGHS...HANGING OUT MAINLY IN THE TEENS.

HIGH PRESSURE WILL THEN APPROACH THE REGION THURSDAY NIGHT INTO
FRIDAY MORNING WITH CLEARING SKIES AND DIMINISHING WINDS. THIS
WILL ALLOW TEMPS TO DROP WELL BELOW ZERO...PERHAPS 10 BELOW IN THE
OUTLYING COLD SPOTS. AND DESPITE THE LIGHTER WINDS...WIND CHILLS
COULD DIP TO ADVISORY LEVELS AS COLD AS 25 BELOW FRIDAY MORNING.
CMS

&&

.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...

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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