Area Forecast Discussion
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000
FXUS63 KLOT 232043
AFDLOT

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

.UPDATE...
1037 AM CST

CHRISTMAS EVE STORM UPDATE...

THE LATEST AVAILABLE 12Z GUIDANCE HAS HONED IN PRETTY CLOSELY ON A
LOW PRESSURE TRACK FROM FROM CENTRAL KY TO NEAR IND TO CENTRAL OR
SOUTHWEST LOWER MI ON WEDNESDAY. THIS GIVES US MORE CONFIDENCE IN
WHERE THE HEAVIEST SWATH OF PRECIPITATION WILL FALL IN THE INTENSE
DEFORMATION AXIS. THERE IS A STILL A SIGNIFICANT TIMING
DISCREPANCY BETWEEN THE NAM ON THE SLOW END OF THE GUIDANCE
ENVELOPE AND 12Z GFS ON THE FASTEST END...AS IT HAS CONSISTENTLY
SHOWN FOR SEVERAL MODEL CYCLES. SO CONFIDENCE IS LOWER ON TIMING
AND DURATION OF HEAVY PRECIP...WITH THE GFS SUGGESTING THAT THE
BULK OF THE PRECIPITATION FALLS BETWEEN 06Z AND 18Z...WHILE THE
NAM FAVORS THE HEAVIEST PRECIPITATION OVER THE CHICAGO METRO AND
POINTS EAST FROM THE LATE MORNING UNTIL THE LATE AFTERNOON.

THE BIGGEST QUESTION MARK REMAINS WITH THE VERY MARGINAL BOUNDARY
LAYER CONDITIONS FOR SNOW AT PRECIP ONSET AND THROUGH MUCH OF THE
EVENT. PRECIPITATION IS EXPECTED TO START AS RAIN THEN FLIP OVER
TO SNOW. THE THINKING IS THAT THE GFS IS NOT HANDLING THE DYNAMIC
COOLING OFFERED BY THE INTENSE UPWARD MOTION IN THE HEART OF THE
FRONTOGENETICALLY DRIVEN DEFORMATION BAND...AND THUS REMAINS TOO
WARM WITH NEAR SFC THERMAL PROFILES AND MOST OF THE PRECIP FALLS
IN THE FORM OF RAIN/MIX. THE FORECAST WILL LEAN IN FAVOR OF
DYNAMIC COOLING OFFSETTING THE INITIALLY MARGINAL
CONDITIONS...WHICH IS CAPTURED WELL BY THE 12Z NAM. HOWEVER...IT
WILL LIKELY BE A VERY LOW SNOW TO LIQUID RATIO SNOW AND THE HUGE
SNOWFALL AMOUNTS ON THE NAM COBB OUTPUT ARE VERY LIKELY OVERDONE.

WOULD LIKE TO SEE THE REST OF THE 12Z GUIDANCE BEFORE MAKING ANY
HEADLINE DECISIONS...WHICH WILL LIKELY BE MADE BY THE EARLY
AFTERNOON. OVERALL THINKING REMAINS THAT CONFIDENCE IS GROWING IN
A HIGH IMPACT SNOW EVENT ACROSS PART OF THE CWA TOMORROW...WHICH
IS BECOMING MORE LIKELY TO INCLUDE THE CHICAGO METRO. THE SPS HAS
BEEN UPDATED.

RC

&&

.SHORT TERM...

TODAY...
331 AM CST

STACKED UPPER/SURFACE LOW PRESSURE WAS OVER IOWA EARLY THIS
MORNING...WITH SURFACE OCCLUSION/WARM FRONT EXTENDING EAST AND
SOUTHEAST ACROSS NORTHERN ILLINOIS INTO WEST CENTRAL INDIANA. AREA
OF STEADIER MODERATE RAIN WAS LIFTING NORTH OF THE IL/WI BORDER...
WITH ONLY SPOTTY SHOWERS OR SOME LINGERING DRIZZLE EXPECTED THIS
MORNING WITHIN SYSTEM DRY SLOT. UPPER LOW IS PROGGED TO WEAKEN AND
SHEAR NORTHEAST ACROSS THE NORTHERN GREAT LAKES THROUGH THIS
AFTERNOON...WITH ITS SURFACE LOW ALSO MOVING NORTHEAST OF THE AREA
AND ALLOWING SOUTHERLY WINDS TO MAINTAIN RELATIVELY MILD WEATHER
ACROSS THE FORECAST AREA WITH TEMPS GENERALLY HOLDING IN THE 40S.
MAY SEE SOME BREAKS IN THE CLOUDS AS SKIES HAVE CLEARED UPSTREAM
ACROSS MISSOURI...THOUGH WILL LIKELY SEE REDEVELOPMENT OF LOWER
CLOUDS IN MOIST LOW LEVEL ENVIRONMENT AND WEAK WEAK TROUGHING
ALOFT.

RATZER

TONIGHT THROUGH WEDNESDAY NIGHT...
323 AM CST

HIGH IMPACT WINTER STORM WITH HEAVY WET SNOW POSSIBLE CHRISTMAS EVE.

GENERALLY SPEAKING...WE ARE IN A MUCH BETTER PLACE WITH RESPECT TO
MODEL AGREEMENT THIS MORNING THAN WE WERE LAST NIGHT OR EVEN 12
HOURS AGO. HOWEVER...THERE REMAIN SOME DIFFERENCES WITH RESPECT TO
THE EXACT TRACK AND SPEED/TIMING OF THE SYSTEM. OTHER FORECAST
BUGABOO IS THE MARGINAL LOW LEVEL THERMAL PROFILES DURING THE HEART
OF THE EVENT.

WHILE THERE ARE STILL DIFFERENCES...ONE THING THE COLLECTION OF
MODEL GUIDANCE HAS NOW COME AROUND TO IS THE IDEA OF AN EXPLOSIVELY
DEEPENING CYCLONE DEVELOPING TRACKING NORTH FROM THE GULF COAST
TODAY TO LOWER MICHIGAN BY CHRISTMAS EVE NIGHT. EVENING UPPER AIR
ANALYSIS DEPICTED 170KT 250MB JET SAMPLED BY 00Z RAOB AT SALT LAKE
CITY...WITH THIS JET DIVING DOWN THE BACK SIDE OF THE LONG WAVE
TROUGH AND PROGGED TO SHARPEN/AMPLIFY SOUTHERN FLANK OF THE TROUGH
OVER THE SOUTHERN PLAINS TODAY. THE UPPER JET EMERGES FROM THE BASE
OF THE TROUGH WEDNESDAY GUIDANCE DEPICTS A RAPIDLY DEEPENING CYCLONE
WITH CIRCULATION PROGGED TO CLOSE OFF ALL THE WAY UP TO 300MB.

THE RAPID DEEPENING AND QUICK MOVEMENT OF THE MID/UPPER LEVEL LOW
WILL LIKELY RESULT IN EXTREMELY IMPRESSIVE LARGE SCALE FORCING WITH
AN IMPRESSIVE TROWAL/DEFORMATION ZONE LIKELY TO DEVELOP ON THE
WESTERN FLANK OF THE CYCLONE. VERY STRONG LOW/MID LEVEL
FRONTOGENESIS WILL OCCUR WITHIN THE TROWAL AND COINCIDENT WITH THE
RAPIDLY STRENGTHENING CYCLONE WILL PROMOTE STRONG VERTICAL
CIRCULATION. GUIDANCE ALSO SUGGESTS WEAKLY STABLE TO UNSTABLE
CONDITIONS ALOFT WITHIN THIS TROWAL WITH NEGATIVE EPV AND STEEP
MID-HIGH LEVEL LAPSE RATES...WHICH GIVEN THE STRONG FRONTOGENESIS
WOULD TEND TO FAVOR A NARROWER BUT INTENSE PRECIPITATION BAND.
GFS/WRF-NAM BOTH INDICATE THE PRESENCE OF SOME WEAK UPRIGHT
INSTABILITY WHICH GIVEN THE FORCING COULD ALLOW FOR UPRIGHT
CONVECTION WITHIN THE DEFO BAND AND POSSIBLY A COUPLE LIGHTNING
STRIKES.

WHILE GUIDANCE IS IN PRETTY GOOD AGREEMENT ON THE ABOVE...THERE ARE
A COUPLE OF KEY AREAS OF LINGERING UNCERTAINTY. FIRST...MODEL
BOUNDARY LAYER THERMAL PROFILES ARE FORECAST TO BE QUITE MARGINAL
FOR SNOW AND EVEN MORE MARGINAL FOR ACCUMULATING SNOW. DESPITE THE
MARGINAL THERMAL PROFILES...AM FAIRLY CONFIDENT THAT DYNAMIC COOLING
WITHIN THE DEFO BAND WILL LIKELY EASILY COOL THE BOUNDARY LAYER
SUFFICIENTLY TO SUPPORT ACCUMULATING SNOW...PARTICULARLY WHERE THE
HEAVIEST PRECIP OCCURS. THE 02/07/13 SYSTEM IS A PRIME EXAMPLE OF A
CASE OF VERY STRONG DYNAMIC COOLING THAT WAS VERY POORLY HANDLED BY
THE MODELS AND RESULTED IN NEARLY 10 INCHES OF SNOW WHERE MODELS
GUIDANCE FORECAST SFC TEMPS IN THE MID TO UPPER 30S.

THE BIGGER UNCERTAINTY AND WHAT IS KEEPING ME FROM JUMPING OUT WITH
A WINTER STORM WATCH AT THIS TIME IS THE TRACK DIFFERENCES IN THE
MODELS. WHILE THEY ARE NOT HUGE AND NOT SURPRISING GIVEN THE RAPIDLY
DEEPENING NATURE OF THE SYSTEM...THEY ARE SUBSTANTIAL ENOUGH TO MAKE
IT TOUGH TO PINPOINT THE PORTION OF OUR CWA MOST AT RISK. GIVEN
COMBINATION OF STRONG FRONTOGENESIS...INSTABILITY...AND MARGINAL
THERMAL PROFILES IT SEEMS LIKELY THE AREAL EXTENT OF HEAVY SNOW
WOULD NOT BE LARGE WITH POTENTIALLY SHARP CUT OFF ON BOTH THE
WESTERN AND EASTERN FLANKS OF THE SNOW BAND...SIMILAR TO WHAT WAS
SEEN IN THE 12/01/2006 SYSTEM WHICH IS ONE OF THE STRONGEST ANALOGS
FOR THIS SYSTEM AND PRODUCED LOCALLY OVER A FOOT OF SNOW.

THE CURRENT SNOWFALL FORECAST MAP WAS CREATED BLENDING VARYING
MODEL TRACK SOLUTIONS TOGETHER WHICH YIELD SNOWFALL AMOUNTS THAT
WILL LIKELY BE TOO LOW WHERE THE HEAVIEST SNOW FALLS AND TOO HIGH
OUTSIDE OF THAT BAND...BUT LIKELY THE BEST COMPROMISE THAT CAN BE
DONE FROM THIS DISTANCE GIVEN WHAT LOOKS TO BE A MESOSCALE HEAVY
SNOW THAT WILL SET UP. WHERE THE HEAVY SNOW BAND SETS UP...THINK IT
IS PRETTY LIKELY THAT VSBYS OF 1/4SM OR LESS WITH HEAVY WET SNOW ARE
LIKELY WITH SNOWFALL RATES >/= 2 INCHES PER HOUR POSSIBLE. THE PEAK
INTENSITY OF THE SNOW LOOKS TO COINCIDE WITH THE DAYTIME HOURS WHEN
MANY PEOPLE WILL LIKELY BE TRAVELING...SO ASSUMING THE BAND SETS UP
AS EXPECTED...IT COULD MAKE TRAVEL EXTREMELY DIFFICULT TOMORROW
AFTERNOON. SOME MODELS ARE AS FAR WEST AS NW IL INTO RFD (PARALLEL
GFS AND WRF-NMM) RANGING TO AS FAR EAST AS NORTHWEST INDIANA (00Z
ECMWF). WOULD TEND TO FAVOR THE MORE WESTERN SOLUTIONS AS RAPIDLY
DEEPENING CYCLONES TYPICALLY TEND TO END UP FARTHER WEST THAN
PROGGED WHICH COULD FAVOR CHICAGO AREA OR POINTS JUST WEST FOR THE
HEAVY SNOW THREAT. BUT WITH A STILL RELATIVELY LARGE SPREAD IN TRACK
SOLUTIONS WOULD BE DIFFICULT TO PIN POINT A SPECIFIC AREA FOR A
WINTER STORM WATCH WHEN IN REALITY THE HEAVY (6"+) SNOW AMOUNTS
COULD BE CONFINED TO <25MI WIDE SWATH. AFTER COORDINATION
INTERNALLY...WITH NEIGHBORING WFOS...AND WPC HAVE OPTED TO JUST HIT
THE THREAT HARD IN UPDATED SPS AND HOPEFULLY MORNING MODEL RUNS WILL
NARROW DOWN THE THREAT AREA ENOUGH TO ALLOW HEADLINES TO BE ISSUED
BY MIDDAY.

IZZI

&&

.LONG TERM...
331 AM CST

THURSDAY THROUGH MONDAY...

DEEPENING LOW PRESSURE FROM CHRISTMAS EVE STORM LIFTS QUICKLY AWAY
FROM THE REGION THURSDAY MORNING...WITH BUILDING UPPER RIDGE ALOFT
BETWEEN DEPARTING LOW AND UPSTREAM TROUGH OVER THE WESTERN CONUS.
GUIDANCE IN REASONABLE AGREEMENT IN SENDING A LEAD SHORT WAVE
NORTHEAST TOWARD THE UPPER MIDWEST ON FRIDAY...WITH SURFACE LOW
PRESSURE TRACKING TO OUR NORTHWEST FROM IOWA TO THE UPPER PENINSULA
OF MICHIGAN. SOUTHERLY FLOW DEVELOPS IN ADVANCE OF THIS SYSTEM AND
ITS TRAILING COLD FRONT...WITH TEMPERATURES SURGING BACK INTO THE
40S FRIDAY. MODEL QPF IS HIGHEST FARTHER SOUTH AHEAD OF THE COLD
FRONT ACROSS THE LOWER OHIO VALLEY...THOUGH IT DOES LOOK LIKE A
POTENTIAL FOR SOME LIGHT RAIN OR RAIN/SNOW MIX TO DEVELOP ACROSS THE
AREA FRIDAY NIGHT...WITH SOME LINGERING LIGHT SNOW OR SNOW SHOWERS
LINGERING ACROSS THE SOUTHEASTERN CWA SATURDAY AS COLDER AIR SPREADS
IN ON BLUSTERY WEST-NORTHWEST WINDS.

STRONG HIGH PRESSURE THEN BUILDS SOUTHEAST ACROSS THE PLAINS AND
UPPER MIDWEST INTO SUNDAY...THOUGH ECMWF MAINTAINS A LITTLE LIGHT
PRECIPITATION ALONG THE RETREATING BAROCLINIC ZONE TO OUR SOUTHEAST
AND HAVE KEPT SOME LOW POPS FOR LIGHT SNOW SATURDAY NIGHT AND SUNDAY
FOR SOUTH AND EAST OF THE CHICAGO AREA. THE REMNANT WESTERN CONUS
UPPER TROUGH THEN MOVES EAST ACROSS THE SOUTHERN PLAINS AND LOWER
MISSISSIPPI/OHIO VALLEYS SUNDAY NIGHT. DRY NORTHERLY FLOW
AROUND THE UPPER MIDWEST HIGH...WITH GENERALLY DRY AND SEASONABLY
COLD WEATHER SUNDAY INTO MONDAY.

RATZER

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 20Z...

* BURST OF HEAVY WET SNOW LIKELY WEDNESDAY FROM MIDDAY THROUGH THE AFTERNOON.

* A PERIOD OF VLIFR CONDITIONS POSSIBLE WEDNESDAY MIDDAY-AFTERNOON WITH
  HEAVY WET SNOW.

* EXACT LOCATION OF BAND OF HEAVIEST SNOW REMAINS UNCERTAIN. SNOW
  AMOUNTS WILL DROP OFF SHARPLY TO EITHER SIDE.

ED F

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

COMPLICATED WEATHER SITUATION UNFOLDING FOR WEDNESDAY WITH
POTENTIAL FOR VERY LOW CIGS AND VSBYS ASSOCIATED WITH BAND OF
HEAVY WET SNOW. FIRST... SOUTHWEST WINDS WILL CONTINUE TO GUST
20-25 KT THIS AFTERNOON WITH LOW CLOUDS LURKING JUST TO THE
NORTHWEST OF ORD/MDW AS LOW PRESSURE TRACKS NE ACROSS WISCONSIN
REACHING NORTHERN LAKE MICHIGAN TONIGHT. WINDS WILL SLACKEN AS THE
LOW PULLS AWAY TONIGHT WITH CIGS LOWERING OVERNIGHT.

MEANWHILE... POWERFUL LOW PRESSURE MOVING FROM THE OHIO VALLEY TO
LOWER MICHIGAN BY LATE DAY WEDNESDAY WILL LIKELY BRING RAIN
CHANGING TO HEAVY WET SNOW QUICKLY WEDNESDAY MID-LATE MORNING WITH
THE POTENTIAL FOR SNOWFALL RATES OF 1 TO LOCALLY OVER 2 INCHES PER
HOUR WITH VERY LOW CIGS AND VSBYS POSSIBLE. WHILE MODELS TRENDING MORE
CONSISTENT ON OVERALL SCENARIO... DIFFERENCES REMAIN THAT CAN MAKE
THE DIFFERENCE IN EXACTLY WHERE THE NARROW BAND OF HEAVIEST SNOW
THAT THIS SYSTEM IS CAPABLE OF WILL SET UP. COMING FORECASTS
WILL SEEK TO FURTHER REFINE THE EXACT LOCATION OF THIS
POTENTIALLY HIGHLY IMPACTFUL EVENT.

ED F

//ORD AND MDW CONFIDENCE...UPDATED 20Z...

* MEDIUM TO HIGH ON CIG TRENDS AND WINDS THIS AFTERNOON/EVENING.

* MEDIUM TO HIGH ON POTENTIAL FOR SNOW ON WEDNESDAY...BUT LOW ON
  ONSET TIMING AND TIMING OF CHANGE OVER FROM RA TO SN... AS WELL
  AS HOW POOR CONDITIONS MAY DETERIORATE TO.

ED F

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

WEDNESDAY NIGHT...SNOW ENDING IN THE EVENING. IFR OR LOWER
IMPROVING.

THURSDAY...CHANCE OF MVFR CIGS...MAINLY EARLY.

FRIDAY...CHANCE OF MVFR CIGS. STRONG SOUTH WINDS POSSIBLE.

SATURDAY AND SUNDAY...CHANCE OF MVFR CIGS.

MONDAY...MAINLY VFR.

KJB

&&

.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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