Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 232347
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
547 PM CST TUE DEC 23 2014

.SHORT TERM...
309 PM CST

SYNOPSIS...

A POWERFUL LOW PRESSURE SYSTEM WILL IMPACT NORTHERN AND CENTRAL
ILLINOIS AND NORTHWEST INDIANA ON CHRISTMAS EVE. DESPITE QUITE
MARGINAL TEMPERATURES FOR SNOW...THE STORM IS EXPECTED TO PRODUCE
INTENSE ENOUGH PRECIPITATION TO CAUSE INITIAL RAIN OR A MIX TO
CHANGE OVER TO HEAVY AT TIMES WET SNOW. A FAIRLY NARROW BAND OF
INTENSE SNOW IS EXPECTED. IN THIS AREA...SIGNIFICANT ACCUMULATIONS
POTENTIALLY RESULTING IN SUBSTANTIAL TRAVEL IMPACTS ON A VERY BUSY
TRAVEL DAY MAY OCCUR. THIS WILL INCLUDE AT LEAST PART OF THE HEART
OF THE CHICAGO METRO. WHILE CONFIDENCE REMAINS ON THE LOWER SIDE
WITH THIS COMPLEX SYSTEM...FELT IT WAS PRUDENT TO ISSUE A WINTER
STORM WATCH FOR MUCH OF THE CHICAGO METRO WHERE THE MOST SIGNIFICANT
IMPACTS COULD OCCUR. HAVE ALSO ISSUED A WINTER WEATHER ADVISORY
SURROUNDING THE WATCH AREA...WHERE HAZARDOUS TRAVEL IS EXPECTED BUT
SLIGHTLY LOWER SNOWFALL AMOUNTS.

WINTER STORM...TONIGHT THROUGH WEDNESDAY NIGHT...

SURFACE ANALYSIS SHOWS DEVELOPING LOW PRESSURE AREA ALREADY DOWN TO
JUST UNDER 1000 MB NEAR NEW ORLEANS. THERE IS A SUBSTANTIAL AMOUNT
OF CONVECTION NEAR THE GULF COAST AND THE TORNADO WATCH IN EFFECT
THERE SHOWS THE AMOUNT OF ENERGY AND MOISTURE ASSOCIATED WITH THIS
DEVELOPING SYSTEM. MOIST PLUME SURGING NORTHWARD IS QUITE EVIDENT ON
WATER VAPOR IMAGERY AND WILL CONTRIBUTE TO ANOMALOUSLY HIGH PWAT
VALUES ALSO FAVORABLE FOR HEAVY PRECIPITATION. MUST NOTE THAT
THE AMOUNT OF CONVECTION NEAR THE GULF MAY BE PLAYING SOME HAVOC
WITH UPPER LEVEL LOW EVOLUTION AND SFC LOW TRACK FORECASTS AMONGST
THE VARIOUS MODELS..PARTICULARLY THE NON- CONVECTION ALLOWING
GLOBAL MODELS.

AS THE SURFACE LOW TRACKS NORTHWARD TONIGHT...SHARPLY DIGGING
UPPER/MID LEVEL TROUGH WILL INTENSITY THANKS TO NEARLY 140 KT
UPPER JET SAMPLED BY 12Z RAOB AT GJT ROUNDING THE BASE OF THE
TROUGH. INTENSIFICATION OF MID LEVEL LOW WILL ALSO TRANSLATE TO
DEEPENING OF SFC LOW AS IT LIFTS NORTHWARD TOWARD THE OHIO VALLEY
BY DAYBREAK AND THEN LOWER MICHIGAN BY LATER IN THE DAY DOWN TO
SUB 990 MB. THIS TRACK IS TYPICALLY FAVORABLE FOR HEAVY SNOW
SOMEWHERE IN OUR AREA. THE HUGE QUESTION MARK CONTINUES TO BE THE
QUITE MARGINAL THERMAL PROFILES AND ALSO THE VERY MILD
TEMPERATURES THIS AFTERNOON THAT ARE CAUSING WARM ROAD SURFACES.

ALSO STILL HAVE A QUESTION ON SPEED/TIMING OF SFC LOW LIFTING
NORTHWARD...WITH GFS ON FASTEST SIDE AND LIKELY TOO FAST...WHILE 12Z
NAM WAS LIKELY A SLOW/DEEP OUTLIER. FURTHERMORE...TYPICALLY RELIABLE
ECMWF SHIFTED EASTWARD TODAY...WHICH LOWERS CONFIDENCE SOME...BUT IN
COLLAB WITH WPC...FELT EC WAS A FAR EAST OUTLIER.

WITH ALL THIS BEING SAID...THE RAPIDLY DEEPENING CIRCULATION IS
EXPECTED TO FORM A VERY IMPRESSIVE TROWAL WRAPPING AROUND
IT...WITH VERY INTENSE 850-700 MB FRONTOGENESIS AND OMEGA WITHIN
THIS TROWAL/DEFORMATION AXIS. INITIAL PRECIP SPREADING NORTHWARD
TOWARD DAYBREAK IS LIKELY TO BE MAINLY RAIN OR A MIX...AND HAVE
PRETTY GOOD CONFIDENCE IN THIS. THE KEY TIMEFRAME WILL COME AS THE
MORNING PROGRESSES ONWARD...WITH RESPECT TO DYNAMIC COOLING
RESPONSE FROM THE INTENSE UPWARD MOTION IN THE DEFORMATION
AXIS/TROWAL. ALSO AIDING IN THIS WILL BE THE EXISTING VERY STEEP
LAPSE RATES ABOVE 600 MB PLUS PRESENCE OF NEGATIVE EPV ABOVE THE
FGEN...WHICH ALSO MAKE LIGHTNING STRIKES A NON-ZERO POSSIBILITY.
FEEL THAT THE HIGHER RES GUIDANCE HAS A BETTER HANDLE ON THE
DYNAMIC COOLING POTENTIAL AND THE HEAVY PRECIP RATES WILL FORCE A
FLIP OVER TO A VERY HEAVY WET SNOW WITH TIME AND TEMPERATURES
GETTING FORCED DOWN CLOSER TO FREEZING.

UNFORTUNATELY FOR HOLIDAY TRAVELERS...MUCH OF THE 12Z DATA WITH THE
EXCEPTION OF THE ECMWF PLACED THE CHICAGO METRO IN THE CROSS HAIRS
OF THE BEST BANDING SIGNAL. CURRENT EXPECTATION IS FOR RAIN/MIX TO
CHANGE TO SNOW FROM SOUTHWEST TO NORTHEAST...GENERALLY MID/LATE
MORNING INTO EARLY AFTERNOON. ASSUMING CHANGEOVER OCCURS AS WE
EXPECT...AREAS WITHIN THE BAND COULD SEE 1-2"+ PER HOUR SNOWFALL
RATES DURING THE EARLY TO MID AFTERNOON...POTENTIALLY MAKING TRAVEL
NEARLY IMPOSSIBLE. THESE RATES WILL ALSO OVERCOME THE WARM ROAD
TEMPERATURES TO ENABLE ACCUMS ON SURFACES. WHILE HEAVY SNOW RATES
WILL LIKELY RESULT IN PERIODS OF VISIBILITY 1/4 MILE OR LESS...GUSTY
NORTH-NORTHWEST WINDS COULD CAUSE VISIBILITY ISSUES EVEN IN LIGHTER
SNOW RATES DESPITE WET NATURE OF THE SNOW. THE LOCATIONS THAT SEE
THE HEAVIEST SNOW COULD VERY WELL BE NARROWER THAN WHAT IS
DEPICTED IN THE GRIDS. HOWEVER...AS NOTED IN SYNOPSIS...FELT IT
WAS PRUDENT TO ISSUE AN IMPACT BASED WATCH FOR THE CHICAGO METRO
GIVEN THE CIRCUMSTANCES. WHILE MOST LOCATIONS LIKELY WILL NOT SEE
6" OF SNOW...SOMEWHERE WITHIN THE WATCH AREA COULD END UP WITH
6"+...WITH GENERALLY 2 TO 5 INCHES IN ADVISORY AREAS. THE SYSTEM
WILL LIFT OFF TO THE NORTHEAST QUICKLY DURING THE EVENING...SO
MUCH OF THE ACCUMULATING SNOW SHOULD BE OVER BY THEN.

OVERALL CONFIDENCE...MEDIUM.

THE MODERATE TO HEAVY SNOW ACCUMULATIONS ARE CONSIDERED A MORE
LIKELY SCENARIO. HOWEVER...THE BUST POTENTIAL WITH THIS SYSTEM
REMAINS HIGH GIVEN LINGERING UNCERTAINTY EVEN AT THIS LATE HOUR WITH
MARGINAL BOUNDARY LAYER CONDITIONS AND TRACK OF THE SURFACE
LOW/HEAVIEST PRECIP. CONTINUE TO STAY TUNED FOR THE LATEST
UPDATES.

RC

THURSDAY THROUGH FRIDAY NIGHT...

HIGH PRESSURE OVER THE GULF COAST WILL STRENGTHEN AS IT BUILDS
NORTHEAST TO THE MID ATLANTIC AND CAROLINAS...AS LOW PRESSURE OVER
THE SOUTH CENTRAL PLAINS LIFTS NORTHEAST TO THE WESTERN LAKES BY
FRIDAY AFTERNOON...AND THEN INTO QUEBEC SATURDAY MORNING. THE
GRADIENT WILL LIKELY REMAIN STRONG ENOUGH FOR SOME GUSTY SOUTHERLY
WINDS BOTH THURSDAY AND FRIDAY. WITH THIS SOUTHERLY FLOW...HIGH
TEMPS LIKELY TO REACH THE MID/UPPER 30S FRIDAY AND INTO THE 40S ON
FRIDAY. HOWEVER...HIGHS LIKELY TO BE IMPACTED BY LOCATION AND AMOUNT
OF SNOWFALL FROM WEDNESDAY AND DIDN/T TRY TO PIN DOWN COOLER AREAS
IN THE GRIDS JUST YET.

LOW LEVEL MOISTURE CONTINUES TO SPREAD ACROSS THE AREA FRIDAY INTO
FRIDAY NIGHT AS THE DEPTH OF THE SATURATED LAYER STEADILY INCREASES
AND REMAINS WELL ABOVE FREEZING. THUS SOME POTENTIAL FOR SPRINKLES
OR SHOWERS OR PERHAPS SOME DRIZZLE EXISTS. OPTED NOT TO INCLUDE ANY
MENTION YET BUT WILL NEED TO BE WATCHED. BY EVENING...COLDER AIR
WILL BE SLOWLY SPREADING INTO THE WESTERN CWA AHEAD OF A COLD FRONT
AND INCLUDED A MIX OF RAIN/SNOW AFTER MIDNIGHT. CMS

&&

.LONG TERM...
328 PM CST

SATURDAY THROUGH TUESDAY...

UPPER LEVEL SHORTWAVE TROUGH AND ASSOCIATED SURFACE COLD FRONT WILL
EXIT THE LOCAL AREA EARLY SATURDAY.  LINGERING PATCHY LIGHT RAIN AND
SNOW WILL TRANSITION TO A CHANCE OF LIGHT SNOW CONTINUING SATURDAY
EVENING AS COLDER AIR ADVECTS SOUTH AND EAST VIA A BRISK NORTHWEST
FLOW.  LARGE SPRAWLING HIGH PRESSURE OVER THE PLAINS THEN BUILDS
EAST INTO THE AREA SUNDAY WITH TEMPERATURES CLOSE TO SEASONAL
AVERAGES. AT LEAST PARTIAL SUNSHINE EXPECTED WILL GIVE THE APPEARANCE
OF A NICER DAY DURING A MONTH WHEN SUNSHINE HAS BEEN A RARE
COMMODITY.

MEANWHILE SHORTWAVE AMPLIFYING IN THE SOUTHERN STREAM HELPS ORGANIZE
SURFACE LOW PRESSURE ALONG THE LOUISIANA GULF COAST SUNDAY.  KEPT A
LOW POP CHANCE OF LIGHT SNOW MENTIONED IN THE FAR SOUTHEAST COUNTIES
ON MONDAY AS THIS LOW LIFTS NORTHEAST TO THE CENTRAL APPLICATIONS.
A COLD FRONT ASSOCIATED WITH A NORTHERN STREAM LOW MOVING ACROSS THE
PLAINS TO EASTERN LAKE SUPERIOR BY TUESDAY USHERS IN A REINFORCING
SHOT OF SEASONALLY CHILLY AIR ON FORECAST DAY 7... MOISTURE LOOKS
VERY LIMITED BUT MAY SEE A FLURRY OR TWO AS DECENT COLD ADVECTION
SQUEEZES OUT WHATEVER MOISTURE IS AVAILABLE.

ED F

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 00Z...

* RAIN OVERSPREADING THE TERMINALS EARLY TO MID MORNING.
  PRECIPITATION TRANSITIONING TO A MODERATE AND AT TIMES HEAVY
  SNOW FROM LATE MORNING TO EARLY AFTERNOON...AND CONTINUING INTO
  THE EVENING.
* CIGS/VSBY DROPPING TO IFR/LIFR. VLIFR PSBL.
* GUSTY N/NNE WINDS EARLY WEDNESDAY BECOMING NORTHWEST IN THE
  AFTERNOON.

BMD

//DISCUSSION...UPDATED 00Z...

A TRICKY FORECAST ON TAP WITH PERIODS OF HEAVY SNOW POSSIBLE AROUND
MIDDAY WEDNESDAY BEING THE PRIMARY CONCERN. THIS EVENING...QUIET
WEATHER IS EXPECTED AS LOW PRESSURE LIFTS ACROSS WISCONSIN AND
FILLS. SOUTHWEST WINDS AHEAD OF THIS LOW WILL TRANSITION TO
NORTHERLY OVERNIGHT AS A STRONGER LOW PRESSURE SYSTEM DEEPENS AND
LIFTS ACROSS THE OHIO VALLEY. AS THIS LOW CONTINUES INTO LOWER
MICHIGAN THROUGH THE DAY WEDNESDAY...NORTHEASTERN ILLINOIS AND
NORTHWEST INDIANA WILL FALL IN A FAVORABLE LOCATION FOR HEAVY BANDS
OF SNOW TO DEVELOP...THOUGH DETAILS IN PRECISELY WHERE THESE BANDS
WILL NOT BE CLEAR UNTIL TOMORROW. THE LATEST MODEL GUIDANCE TO ROLL
IN THIS AFTERNOON HAS TRENDED SLIGHTLY FARTHER EAST RESULTING IN
DPA/ORD BEING ON THE WESTERN PERIPHERY OF THE AREA OF CONCERN.
FURTHER COMPLICATING ISSUES...MODELS ARE STRUGGLING TO LOCK ONTO A
CONSISTENT THERMAL PROFILE WHICH WOULD ALLOW US TO BETTER HANDLE
WHEN RAIN WILL TRANSITION OVER TO SNOW. HAVE MAINTAINED A PROB30
DURING THE LATE MORNING HOURS TO HEDGE FOR AN EARLIER TRANSITION TO
SNOW AS THE BEST FRONTOGENETIC FORCING OCCURS EARLIER IN THE DAY AND
MAY ALLOW THE COLUMN TO COOL QUICKER THAN GUIDANCE SUGGESTS. ONCE
THE TRANSITION OCCURS THOUGH...EXPECT SNOW TO QUICKLY BEGIN
ACCUMULATING...WITH RATES OF 1-2 INCHES POSSIBLE AT TIMES WITHIN THE
HEAVIEST BANDS...WHEREVER THEY MAY FORM. IF A HEAVY BAND DOES
DEVELOP OVER THE TERMINALS...THE LATEST TAF IS LIKELY TOO
CONSERVATIVE WITH RESPECT TO VSBY AS LESS THAN A QUARTER MILE WOULD
BE LIKELY.

BMD


//ORD AND MDW CONFIDENCE...UPDATED 00Z...

* MEDIUM-HIGH CONFIDENCE IN PERIODS OF RAIN TRANSITIONING TO
  SNOW...BUT LOW-MEDIUM CONFIDENCE IN TIMING TRANSITION AND LOW
  CONFIDENCE IN BAND OF HEAVIEST SNOW.
* MEDIUM-HIGH CONFIDENCE IN CIG TRENDS...MEDIUM IN VSBY
  TRENDS...THOUGH LOW-MEDIUM ON TIMING.

BMD

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

THURSDAY...MVFR/IFR IMPROVING TO VFR.
FRIDAY...MVFR/IFR POSSIBLE.
SATURDAY THROUGH MONDAY...MVFR POSSIBLE.

BMD

&&

.MARINE...

243 PM...LOW PRESSURE OVER CENTRAL WISCONSIN WILL DISSIPATE
TONIGHT AS IT LIFTS NORTHEAST. EASTERLY FLOW PERSISTS ACROSS THE
FAR NORTHERN PART OF THE LAKE. AS THIS LOW WEAKENS...SPEEDS WILL
SLOWLY DIMINISH THIS EVENING AND LIKELY BECOME LIGHT AND VARIABLE
FOR A SHORT TIME TONIGHT. ANOTHER LOW OVER LOUISIANA WILL MOVE TO
NORTHEAST INDIANA BY WEDNESDAY AFTERNOON AS IT DEEPENS. NORTHERLY
WINDS WILL STEADILY INCREASE WEDNESDAY MORNING INTO WEDNESDAY
AFTERNOON. THE LOW WILL CONTINUE TO NEAR LAKE HURON WEDNESDAY
EVENING AND HOW FAST THE LOW MOVES...ITS TRACK AS WELL AS HOW MUCH
IT DEEPENS WILL ALL DETERMINE HOW STRONG WIND SPEEDS/GUSTS BECOME
OVER LAKE MICHIGAN. IT APPEARS A PERIOD OF GALE FORCE WINDS ARE ON
TRACK...THOUGH DURATION COULD BE SHORTER...ESPECIALLY ON THE SOUTH
END OF THE LAKE. OPTED TO UPGRADE THE CURRENT GALE WATCH TO A GALE
WARNING BUT ADDITIONAL TWEAKS TO TIMING AND SPEEDS ARE POSSIBLE...
ESPECIALLY OVER THE NORTHERN END OF THE LAKE...WHERE SPEEDS COULD
BE STRONGER.

THIS LOW WILL CONTINUE NORTHEAST INTO QUEBEC THURSDAY...AS A RIDGE
OF HIGH PRESSURE MOVES ACROSS THE OHIO VALLEY. THIS WILL SHIFT WINDS
WESTERLY THEN SOUTHWESTERLY BY THURSDAY NIGHT...THOUGH SPEEDS WILL
LIKELY REMAIN IN THE 15-25 KT RANGE. ANOTHER LOW WILL MOVE FROM THE
SOUTH/CENTRAL PLAINS THURSDAY NIGHT TO THE NORTHERN LAKES FRIDAY.
SOUTHERLY FLOW WILL PERSIST AHEAD OF THIS LOW ON FRIDAY...THEN SHIFT
WEST/NORTHWEST BEHIND A COLD FRONT LATE FRIDAY NIGHT INTO SATURDAY.
CMS

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...WINTER WEATHER ADVISORY...ILZ005-ILZ006-ILZ011-ILZ019-ILZ032-
     ILZ033-ILZ039...9 AM WEDNESDAY TO 9 PM WEDNESDAY.

     WINTER STORM WATCH...ILZ012-ILZ013-ILZ014-ILZ020-ILZ021-ILZ022-
     ILZ023...9 AM WEDNESDAY TO 9 PM WEDNESDAY.

IN...WINTER STORM WATCH...INZ001...9 AM WEDNESDAY TO 9 PM WEDNESDAY.

     WINTER WEATHER ADVISORY...INZ002-INZ010...3 PM WEDNESDAY TO 9 PM
     WEDNESDAY.

LM...GALE WARNING...LMZ080-LMZ669-LMZ671-LMZ673-LMZ675-LMZ740-LMZ741-
     LMZ742-LMZ743-LMZ744-LMZ745-LMZ777-LMZ779-LMZ870-LMZ872-
     LMZ874-LMZ876-LMZ878...NOON WEDNESDAY TO 3 AM THURSDAY.

     GALE WARNING...LMZ261-LMZ362-LMZ364-LMZ366-LMZ563-LMZ565-LMZ567-
     LMZ868...3 PM WEDNESDAY TO 6 AM THURSDAY.

&&

$$

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