Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 011830
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1230 PM CST SUN FEB 1 2015

.UPDATE...

WE HAVE INCREASED SNOWFALL AMOUNTS SOME THIS AFTERNOON...ESPECIALLY
ACROSS NORTHERN ILLINOIS AND FAR NORTHWESTERN INDIANA. A VERY
IMPRESSIVE SYSTEM CONTINUES TO DEVELOP ACROSS THE CENTRAL CONUS THIS
MORNING IN RESPONSE TO A STRONG PV ANOMALY OVER THE CENTRAL PLAINS.
AS THIS STORM CONTINUES TO INTENSIFY THIS AFTERNOON...MODEL GUIDANCE
SUGGESTS THAT A BAND OF STRONG FRONTOGENESIS WILL DEVELOP IN AN
EMERGING DEFORMATION ZONE ASSOCIATED WITH THE 700 MB LOW...RIGHT
ACROSS NORTHERN ILLINOIS. THIS ALSO LOOKS TO OCCUR DURING THE
PERIOD OF BEST STRONG MID/UPPER LEVEL Q-VECTOR DIVERGENCE
ASSOCIATED WITH THE MID/UPPER LEVEL SYSTEM. ALL IN ALL THIS POINTS
TO STRONG DYANAMICS SUPPORTIVE OF PERIODS OF HEAVY SNOW THROUGH
THE DAY TODAY...AND LINGERING THIS EVENING. IT APPEARS THE
HEAVIEST AMOUNTS WILL EXCEED A FOOT IN MANY AREAS...WITH A FOOT
AND A HALF NOT OUT OF THE QUESTION FOR NORTHERN ILLINOIS.

THE OTHER CONCERN IS BLIZZARD CONDITIONS...WE ARE UPGRADING THE REST
OF NORTH CENTRAL ILLINOIS TO A BLIZZARD WARNING. NORTHEASTERLY
WINDS WILL ONLY INCREASE THROUGH THE AFTERNOON...WITH GUSTS
POTENTIAL UP AROUND 40 KT AT TIMES GIVEN THE STRONG GRADIENT SETTING
UP ON THE BACK SIDE OF THIS SYSTEM. THIS HAS ME SERIOUSLY CONCERNED
THAT AS THE SNOW CONTINUES TO BECOME MORE POWDERY THROUGH THE
DAY...THAT WHITE OUT CONDITIONS WILL BE OBSERVED ACROSS NORTHERN
ILLINOIS. THE OTHER CONCERN IS FOR DOWNED TREE BRANCHES AND POWER
LINES. THE SNOW THAT HAS FALLEN THUS FAR ACROSS THE AREA HAS BEEN A
STICKY WET SNOW AND HAS PRODUCED SIGNIFICANT ACCUMULATIONS ON TREES
AND POWER LINES. THE INCREASING WINDS COULD BRING DOWN POWER
LINES...LEADING TO SOME ISOLATED POWER OUTAGES.

NEW UPDATES SHOULD BE OUT SHORTLY.

KJB

&&

.SHORT TERM...
347 AM CST

THROUGH MONDAY...

HAVE BEEN SWAMPED THIS MORNING AND AM RUNNING BEHIND...SO GOING TO
KEEP DISCUSSION BRIEF THIS MORNING AND JUST HIT THE IMPORTANT
CHANGES. THE BIGGEST CHANGES TO GOING FORECAST IS THE UPGRADE TO
A BLIZZARD WARNING FOR PORTIONS OF NE IL/CHICAGO METRO AND TO
LOWER SNOW AMOUNTS AND ADD MIXED PRECIP BACK INTO GRIDS FAR SE
CWA.

GUIDANCE HAS CONTINUED TO TREND NORTH AND STRONGER WITH THE TRACK OF
THIS SYSTEM AND COMPARING GUIDANCE WITH SFC OBS AND BEST SFC
PRESSURE FALLS IT WOULD SEEM TO SUPPORT THE FARTHER NORTH TRACK THAT
THE RAP IN PARTICULAR HAS LATCHED ONTO. THE SOMEWHAT DEEPER LOW
ALONG WITH THE VERY STRONG ARCTIC HIGH SPREADING SOUTHEAST WILL
RESULT IN VERY STRONG WINDS DEVELOPING THIS AFTERNOON. INCREASING
LOW LEVEL COLD AIR ADVECTION AND SFC HEIGHT RISES FAVOR PRETTY GOOD
MIXING IN THE BOUNDARY LAYER...WHICH IS WHAT ALL MODEL FORECAST
SOUNDINGS ARE ADVERTISING. TAKING EVEN THE BEST CASE SCENARIO AND
ONE OF THE WEAKEST MODELS BUFKIT MOMENTUM TRANSFER VALUES SUPPORT
4-7 HOURS OF WIND GUSTS BETWEEN 35-40 MPH LATER THIS AFTERNOON AND
EVENING...WITH MORE AGGRESSIVE GFS/RAP SUPPORTING EVEN STRONGER
WINDS OVER A LONGER DURATION. SNOW HAS BEEN WET THUS FAR BUT WITH
TIME SHOULD SEE THE TRANSITION TOWARD DRIER/MORE BLOWABLE SNOW THIS
AFTERNOON FROM NORTH TO SOUTH AS THE COLUMN COOLS AND THE SNOW
LIQUID CONTENT DECREASES. BIGGEST UNCERTAINTY REVOLVES AROUND
REGARDING UPGRADE TO BLIZZARD WARNING WAS THE SNOW LIQUID CONTENT
AND PROBABILITY AND CONFIDENCE IS CERTAINLY HIGHER THE FARTHER NORTH
FROM I-80 THAT YOU GET...BUT EVEN AS FAR SOUTH AS I-80 FELT FAIRLY
CONFIDENT THAT WHITE OUT CONDITIONS COULD DEVELOP LATE THIS
AFTERNOON AND ESPECIALLY THIS EVENING. FARTHER WEST ACROSS NORTH
CENTRAL ILLINOIS FEEL THAT A BLIZZARD WARNING COULD EASILY BE
JUSTIFIED...BUT IN COORDINATION WITH MKX/DVN HAVE OPTED TO HOLD OFF
FOR NOW AS SNOW INTENSITY COULD BE DECREASING AS WINDS PICK UP.
QUITE POSSIBLE DAY SHIFT MAY NEED TO RE-ACCESS THIS AND AN UPGRADE
TO BLIZZARD WARNING FOR RFD AND POINTS WEST COULD OCCUR ON THE DAY
SHIFT.

OTHER BIG CHANGE TO FORECAST WAS TO ADD SLEET AND THEN DRIZZLE TO
THE FAR SOUTHEAST CWA. SFC TEMPS HAVE BEEN A BIT WARMER AND THE
FARTHER NORTH TRACK OF THE SFC LOW SHOULD KEEP LOW LEVEL WAA GOING
INTO EARLY AFTERNOON WITH AT LEAST THE THREAT OF MIXED
PRECIP...POSSIBLY TRANSITIONING OVER TO JUST DRIZZLE/SNIZZLE AS MID
LEVEL DRY SLOT OVERSPREADS THAT AREA REMOVING ICE NUCLEI FROM THE
CLOUDS LATE THIS AFTERNOON INTO THIS EVENING. THE WETTER SNOW SOUTH
OF I-80 SHOULD RESULT IN LESS SEVERE AND LESS IMPACTFUL
BLOWING/DRIFTING SNOW. GIVEN THE MARGINAL TEMPS AND POTENTIAL MIX
PRECIP HAVE CUT BACK ON ACCUMS IN THE FAR SOUTHEAST COUNTIES OF THE
CWA.

ACCUMULATING LAKE EFFECT POTENTIAL ACTUALLY LOOKS SOMEWHAT LESS
IMPRESSIVE THAN IT DID LAST NIGHT...AND IT WAS ALREADY LOOKING
MARGINAL. SOME SYSTEM SNOWS SHOULD LINGER INTO THIS EVENING BUT LAKE
CONTRIBUTIONS LOOK MINOR WITH LOW INVERSION HEIGHTS AND DECREASING
BOUNDARY LAYER RH AS THE THERMODYNAMICS IMPROVE EVER SO SLIGHTLY.
ANY LAKE ENHANCEMENT TO SNOW TOTALS SHOULD END UP BEING TRIVIAL.

FINALLY...GOING SNOWFALL TOTALS LOOK RIGHT ON TARGET AND MADE VERY
FEW CHANGES OTHER THAN IN SE AS NOTED ABOVE. A RESPECTABLY LARGE
SWATH OF A FOOT OR MORE OF SNOW STILL LOOKS LIKELY AND PROBABLY
CENTERED RELATIVELY CLOSE TO THE I-80 TO I-88/290 CORRIDORS.
ACTUALLY THINKING THAT MAX SNOWFALL TOTALS IN THE CWA COULD END UP
BEING ON THE HIGH END OF THE 10-14 INCH RANGE WITH LOCALIZED TOTALS
POTENTIALLY PUSHING A FOOT AND A HALF IF THE DEFO BAND IS INDEED AS
IMPRESSIVE AS SOME OF THE GUIDANCE WOULD SUGGEST. WHILE SNOWFALL
TOTALS WILL BE VERY IMPRESSIVE OVER MOST OF THE CWA...IT IS
IMPORTANT TO REMEMBER THE BLIZZARD WARNING IS ISSUED NOT FOR
SNOWFALL AMOUNTS BUT RATHER FOR VISIBILITY AND WINDS. THE CRITERIA
FOR BLIZZARD WARNING IS 3+ HOURS OF LESS THAN 1/4SM VSBY IN FALLING
AND/OR BLOWING SNOW ALONG WITH SUSTAINED WINDS OR FREQUENT GUSTS
OVER 35 MPH.

IZZI

&&

.LONG TERM...

MONDAY NIGHT THROUGH SATURDAY...

134 AM...FORECAST CONCERNS INCLUDE A CHANCE OF LIGHT SNOW MONDAY
NIGHT INTO TUESDAY...THEN A BETTER CHANCE FOR ACCUMULATING LIGHT
SNOW TUESDAY NIGHT INTO WEDNESDAY MORNING.

A RIDGE OF HIGH PRESSURE WILL BE MOVING SOUTHEAST OF THE AREA
MONDAY EVENING AS A WEAK WAVE MOVES ACROSS THE REGION MONDAY NIGHT.
FAIRLY LIGHT QPF FROM ALL THE MODELS WITH THIS WAVE BUT PLENTY COLD
ENOUGH FOR THE LOW QPF AMOUNTS TO POTENTIALLY FLUFF UP TO A HALF
INCH MONDAY NIGHT. WHILE TEMPS BY THIS TIME WILL LIKELY BE IN THE
SINGLE DIGITS OR LOW TEENS...CLOUD COVER AND WEAK WARM AIR ADVECTION
MAY ALLOW TEMPS TO HOLD STEADY OR RISE A FEW DEGREES OVERNIGHT.

NEXT WAVE QUICKLY SHIFTS SOUTHEAST ACROSS THE AREA BY TUESDAY
EVENING WITH QPF AMOUNTS BETWEEN ONE AND TWO TENTHS. THIS HAS BEEN
A FAIRLY CONSISTENT FEATURE ON THE MODELS AND IF THIS MATERIALIZES
AS ADVERTISED...A FEW INCHES OF SNOW WILL BE POSSIBLE. TIMING AND
LOCATION ALSO ARE A BIT UNCERTAIN...EARLIER COULD IMPACT THE TUESDAY
EVENING RUSH...LATER COULD IMPACT THE WEDNESDAY MORNING RUSH. BUT
GIVEN THE POTENTIAL FOR SOME ACCUMULATION...OPTED TO BUMP POPS UP TO
LIKELY FOR TUESDAY NIGHT.

HIGH PRESSURE THEN SETTLES SOUTH ACROSS THE MIDWEST AND LAKES REGION
WEDNESDAY INTO THURSDAY AND WITH IT...SOME VERY COLD AIR. WITH A
DEEP SNOWPACK BY THAT TIME AND DIMINISHING WINDS WEDNESDAY NIGHT
INTO THURSDAY...EXPECT MANY LOCATIONS TO DROP BELOW ZERO FOR LOWS
THURSDAY MORNING. THE AIRMASS MODIFIES/WARMS SOME THURSDAY BUT
STILL EXPECTING LOWS IN THE LOWER SINGLE DIGITS THURSDAY EVENING...
POSSIBLY BELOW ZERO BUT WARM AIR ADVECTION BY MORNING WITH
INCREASING SOUTHERLY WINDS MAY PUT THE BRAKES ON HOW LOW TEMPS FALL
THURSDAY NIGHT. STRONG SOUTHERLY WINDS CONTINUE INTO SATURDAY WITH
WARMING TEMPS...THAT WILL LIKELY BE MUTED SOME BY THE SNOWPACK. CMS

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 18Z...

* SNOW THROUGH TONIGHT. LIGHT LAKE EFFECT SNOW EARLY MONDAY MAINLY
  FOR MDW. PREVAILING VIS AT OR UNDER 1SM THROUGH THIS EVENING.

* NELY WINDS GUSTING TO 30-35 KT LATE THIS AFTERNOON/EARLY
  EVENING.

* BLOWING AND DRIFTING SNOW LIKELY THIS AFTERNOON AND TONIGHT.

* PERIODS OF VIS 1/4SM OR LESS THIS AFTERNOON WHEN GUSTS INCREASE
  TO 30 KT AND HIGHER.

* IFR-LIFR CIGS THROUGH LATE EVENING.

KMD

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

STEADY SNOWFALL CONTINUES THROUGH THE AFTERNOON AND EVENING. THE
SNOW HAS BEEN WET TO THIS POINT...BUT FROM NORTH TO SOUTH IT WILL
SHIFT TO A DRIER SNOW AS THE AFTERNOON AND EVENING GO ON.
VISIBILITIES HAVE BEEN LARGELY STEADY AROUND 1/2 MILE...AND WHILE
FLUCTUATIONS IN THIS WILL OCCUR FROM TIME TO TIME...EXPECT 1/2 TO
3/4 VSBYS TO GENERALLY BE THE RULE. AS THE WINDS RAMP UP LATER THIS
AFTERNOON AND EVENING...FEEL THAT THERE WILL BE OCCASIONAL 1/4 MILE
VISIBILITIES AS THE SNOW GETS A BIT MORE BLOWABLE...BUT STILL NOT
CONFIDENT IN THIS CONDITION PREVAILING. WILL HAVE TO MONITOR AS THE
WINDS RAMP UP. IF THIS WERE TO OCCUR...IT WOULD BE THE LATE
AFTERNOON AND EARLY EVENING PERIOD WHEN THE SURFACE LOW MOVES A BIT
FARTHER NORTH AND EAST. THINK THAT THE 22Z-02Z TIME FRAME WOULD BE
THE BEST BET FOR THIS. WINDS HOLD IN THE 050 DIRECTION THEN BACK
THROUGH THE EVENING TO 020 BY MID EVENING...THEN NNW OVERNIGHT.

SHOULD SEE SOME SLIGHTY IMPROVEMENT IN VSBY AND DECREASE IN SNOWFALL
FROM 4Z-6Z...AND AFTER 6Z SNOWFALL SHOULD DECREASE SIGNIFICANTLY
THOUGH NOT END...AND WINDS SHOULD START A MORE SIGNIFICANT
DOWNWARD TREND.  EXPECT LIGHT LAKE EFFECT SNOW SHOWERS OVERNIGHT.
MOST TERMINALS EXCEPT MDW AND GYY SHOULD BE DRY BY DAYBREAK WITH A
WEAKENING WESTERLY WIND.

KMD


//ORD AND MDW CONFIDENCE...UPDATED 18Z...

* MEDIUM TO HIGH CONFIDENCE IN WX/VIS TRENDS.

* MEDIUM CONFIDENCE IN CIG TRENDS.

* MEDIUM CONFIDENCE IN TIMING OF IMPROVING CIG/VIS.

* MEDIUM TO HIGH CONFIDENCE IN WIND TRENDS.

KMD

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

MONDAY NIGHT...VFR/WX NIL EARLY. CHANCE SNOW WITH MVFR CIGS/VIS
LATE.

TUESDAY...SLIGHT CHANCE SNOW EARLY WITH MVFR CIGS/VIS. VFR/WX NIL
AFTERNOON.  SNOW LIKELY WITH IFR CIG/VIS OVERNIGHT.

WEDNESDAY...SLIGHT CHANCE SNOW WITH MVFR CIG/VIS EARLY. VFR/WX NIL
OVERNIGHT.

THURSDAY...VFR/WX NIL.

FRIDAY...VFR/WX NIL.

SATURDAY...SLIGHT CHANCE SNOW WITH MVFR CIG/VIS POSSIBLE.

KREIN

&&

.MARINE...
315 AM CST

THE SFC LOW OVER THE PLAINS LOOKS TO BE DEEPENING A LITTLE FASTER
AND LOWER WHILE THE HIGH PRESSURE RIDGING OVER THE UPPER GREAT
LAKES IS TRENDING A LITTLE STRONGER. THIS COMBINATION WITH STRONG
COLD ADVECTION SETTING UP OVER THE LAKE TODAY...HAVE EXPANDED THE
GALE WARNING OVER THE ENTIRE OPEN WATERS OF LAKE MICHIGAN AS WELL
AS EXTENDING IN TIME...STARTING A LITTLE EARLIER...AT 15Z THIS
MORNING AND GOING A LITTLE LATER...TO 09Z TONIGHT. ALSO...HAVE
INCREASED GALES TO 40KT OVER THE SOUTHERN PORTIONS OF THE LAKE
WHERE THE PRESSURE GRADIENT AND PRESSURE FALLS ARE STRONGEST. AS
THE WINDS PICK UP TO GALE FORCE AND MUCH COLDER AIR FILTERS OVER
THE LAKE LATER TODAY...HEAVY FREEZING SPRAY WILL BE LIKELY...SO
HAVE ALSO ISSUED A HEAVY FREEZING SPRAY WARNING FROM 15Z THIS
MORNING TO 09Z TONIGHT. THE COLDER AIR WILL SPREAD OVER THE
NORTHERN PORTIONS OF THE LAKE EARLIER THAN THE SOUTHERN
PORTIONS...SO IT IS POSSIBLE THAT THE HEAVY FREEZING SPRAY
POTENTIAL MAY NOT REACH THE SOUTHERN PORTIONS OF THE LAKE UNTIL
THIS AFTERNOON...BUT WENT WITH ONE SEGMENT TO THE WARNING TO
COINCIDE WITH THE GALE WARNING TIMING. THE LOW SHOULD PUSH EAST
INTO THE MID ATLANTIC REGION BE MONDAY AFTERNOON...WHILE HIGH
PRESSURE SPREADS ACROSS THE MISSISSIPPI RIVER INTO THE WESTERN
GREAT LAKES REGION. THIS WILL LEAD TO SUBSIDING WINDS AND
WAVES...ESPECIALLY AS WEAK HIGH PRESSURE PRESSURE CROSSES THE LAKE
MONDAY NIGHT AND TUESDAY.

KREIN

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...LAKESHORE FLOOD ADVISORY...ILZ006-ILZ014 UNTIL 10 AM MONDAY.

     BLIZZARD WARNING...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-
     ILZ011-ILZ012-ILZ013-ILZ014-ILZ019-ILZ020-ILZ021-ILZ022
     UNTIL MIDNIGHT MONDAY.

     WINTER STORM WARNING...ILZ023-ILZ032-ILZ033-ILZ039 UNTIL
     MIDNIGHT MONDAY.

IN...BLIZZARD WARNING...INZ001 UNTIL MIDNIGHT MONDAY.

     WINTER STORM WARNING...INZ002-INZ010-INZ011-INZ019 UNTIL
     MIDNIGHT MONDAY.

LM...GALE WARNING...LMZ080-LMZ261-LMZ362-LMZ364-LMZ366-LMZ563-LMZ565-
     LMZ567-LMZ669-LMZ671-LMZ673-LMZ675-LMZ740-LMZ741-LMZ742-
     LMZ743-LMZ744-LMZ745-LMZ777-LMZ779-LMZ868-LMZ870-LMZ872-
     LMZ874-LMZ876-LMZ878 UNTIL 3 AM MONDAY.

     HEAVY FREEZING SPRAY WARNING...LMZ080-LMZ261-LMZ362-LMZ364-
     LMZ366-LMZ563-LMZ565-LMZ567-LMZ669-LMZ671-LMZ673-LMZ675-
     LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-LMZ745-LMZ777-LMZ779-
     LMZ868-LMZ870-LMZ872-LMZ874-LMZ876-LMZ878 UNTIL 3 AM MONDAY.

&&

$$

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