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AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1228 PM CST WED DEC 19 2012

DISCUSSION...
11 AM CST

HAVE EXPANDED THE WINTER STORM WATCH TO INCLUDE NORTH OF THE I-80
CORRIDOR...INCLUDING CHICAGOLAND AND FAR NORTHWEST INDIANA. THIS
WATCH IS MUCH MORE TOWARD IMPACT-BASED BETWEEN THE COMBINATION OF
SNOWFALL RATES AND WIND DURING THURSDAY EVENING AS OPPOSED TO
ACTUALLY SNOWFALL AMOUNTS. THE FORECAST ESSENTIALLY HAS REMAINED
THE SAME.

THE 12Z GUIDANCE IS VERY CONSISTENT AMONGST ITSELF OF THE UPPER
LEVEL AND SURFACE LOW PATHS AND MAGNITUDES...WITH THE LOW PASSING
OVER CHICAGO DURING THE 1-4 PM PERIOD. ANOMALOUSLY WARM AND MOIST
AIR IS BROUGHT NORTHWARD AHEAD OF THIS SYSTEM TONIGHT INTO THU.
GIVEN THE SYNOPTICS OF THIS SYSTEM...IT BECOMES SIGNIFICANTLY
WRAPPED UP THURSDAY AFTERNOON WITH THE COLD AIR WRAPPING EASTWARD
ACROSS CENTRAL IL AND THE WARM AIR LIKELY HOLDING IN PLACE ACROSS
CHICAGO. PROJECTED NAM MAXIMUM COLUMN WET BULB AND ASSOCIATED
POINT SOUNDINGS INDICATE PRECIPITATION WILL CHANGE OVER TO SNOW
SOUTHWEST AND SOUTH OF CHICAGO BEFORE IT WILL IN CHICAGO DURING
LATER THURSDAY AFTERNOON...AND EVEN QUICKER THAN THE FAR NORTH
METRO. THE HEAVIER SNOW HOWEVER WILL STILL BE FAVORED ACROSS THE
NORTHERN HALF OF THE AREA AS THE FORCING IS INCREDIBLY STRONG IN
THE MID AFTERNOON THROUGH THE EVENING...ASSOCIATED WITH THE
DEFORMATION AREA...INSTABILITY PRESENT ALIGNED WITH A FAVORED SNOW
GROWTH ZONE...AND STRONG DEEP CONVERGENCE. COBB DATA FROM BOTH THE
NAM AND GFS INDICATE A FEW HOUR WINDOW OF EXPLOSIVE SNOWFALL RATES
/2 PLUS INCHES PER HOUR/ MOVING GRADUALLY FROM WEST TO EAST OVER
NORTHERN IL FROM 3-10 PM...MOVING ACROSS CHICAGO DURING THE LATTER
PART OF THAT TIME. THIS COALESCES WITH THE STRONGEST WIND
PERIOD...WHICH STILL ON THE 12Z GUIDANCE FAVORS 50+ MPH GUSTS FOR A
SEVERAL HOUR WINDOW. BECAUSE OF THESE HIGHER END SNOWFALL RATES
AND WINDS...HAVE EXPANDED THE WATCH EAST AND SOUTH. AT LEAST A
HIGHER END ADVISORY LOOKS LIKELY...BUT THE POTENTIAL FOR MORE
SIGNIFICANT IMPACTS IS THERE...INCLUDING TEMPORARY BLIZZARD
CONDITIONS IN THE WESTERN PORTION OF THE WATCH AND WITHIN THE
CURRENT WARNING.

MTF/RC

&&

AVIATION...

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

* EAST WINDS WITH GUSTS 20-25 KT TONIGHT.

* RAIN ARRIVES ARND MIDNIGHT WITH A PERIOD OF MOD/HEAVY RAIN PSBL
  MID THURSDAY MORNING/EARLY AFTERNOON.

* CIGS QUICKLY FALLING TO IFR/LIFR OVERNIGHT.

* ISOLATED-SCATTERED TS PSBL LATE THURSDAY MORNING/EARLY
  AFTERNOON...OCNL WIND GUSTS TO 30 KT PSBL IF A TS PASSES OVER
  THE AIRFIELD.

* RAIN CHANGES OVER TO SNOW ARND 00Z WITH VSBYS LOWERING.
  EXPECTING WET LARGE FLAKES AT FIRST POSSIBLY MIXED WITH RAIN.

* NORTHWEST WINDS WITH GUSTS ARND 50 KT LIKELY TOMORROW EVENING.

JEE

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

WEAK HIGH PRESSURE IS CENTERED OVER SOUTHERN LAKE MICHIGAN AND
SLOWLY DRIFTING EASTWARD. EXPECTING THE LIGHT AND VARIABLE WINDS
TO BECOME SOUTHEAST THEN EAST BEHIND THE HIGH. CIGS HAVE FOR THE
MOST PART RISEN TO VFR OVER THE TERMINALS WITH SPOTS OF MVFR
MOVING THROUGH. ODDLY ENOUGH NW INDIANA...WESTERN IL....AND
WISCONSIN ARE STILL HOLDING ON TO THE MVFR OR LOWER CIGS. THIS
SUGGESTS THAT CIGS COULD EASILY FALL BACK TO IFR/LIFR TONIGHT.

THE NEXT SYSTEM IS CURRENTLY OVER NORTHEASTERN NEW MEXICO...AND
WILL MOVE INTO NORTHERN MISSOURI/CENTRAL IL LATE TONIGHT.
EXPECTING RAIN TO MOVE INTO RFD THIS EVENING. WITH INCREASED
COLUMN MOISTURE EXPECTING CIGS TO QUICKLY FALL INTO THE IFR/LIFR
RANGE. THE UPPER LEVEL GRADIENT WILL TIGHTEN AS THE LOW
APPROACHES...THEREFORE ALSO EXPECTING WINDS TO INCREASE WITH GUSTS
ARND 25 KT OVERNIGHT. WINDS WILL SLOWLY BECOME SE THROUGHOUT THE
DAY IN RESPONSE TO THE LOW MOVING INTO NORTHERN IL. THE LOW BEGINS
TO RAPIDLY DEEPEN LATE THURSDAY MORNING/AFTERNOON WHICH COULD
RESULT IN A FEW MODERATE TO HEAVY RAIN SHOWERS.

REMOVED GUSTS FOR A FEW HOURS ARND NOON TOMORROW AS THE LOW PASSES
DIRECTLY OVER NE IL. SINCE THE LOW WILL CONTINUE TO DEEPEN AT THIS
TIME...UPWARD FORCING WILL BE VERY STRONG. THINKING THE FORCING
COULD RAISE A FEW PARCELS ABOVE THE LOW LEVEL WARM
LAYER...RESULTING IN ISOLATED-SCATTERED THUNDERSTORMS. CONFIDENCE
IS LOW IN BOTH THE OCCURRENCE AND LOCATION OF TS SINCE THEY WILL BE
SO ISOLATED.

THEN WINTER ARRIVES. RAIN WILL CHANGE OVER TO SNOW FROM WEST TO
EAST AS THE LOWS COLD FRONT SWINGS THROUGH. MIXED PRECIP WITH
HEAVY WET SNOW IS LIKELY FOR A COUPLE OF HOURS DURING THE CHANGE
OVER. LOOKING AT RFD TO CHANGE OVER ARND 21Z...AND THE EASTERN
TERMINALS ARND 00Z. EXPECTING VSBYS TO FALL QUICKLY WITH THE
ADDITION OF LARGE SNOWFLAKES. COULD EVEN SEE A LITTLE
THUNDERSNOW...BUT ALSO HAVE LOW CONFIDENCE IN TIMING AND LOCATION
OF TSSN DUE TO HOW ISOLATED IT WILL LIKELY BE. BUT ONCE AGAIN WITH
THE DYNAMIC LOW DEEPENING OVERHEAD...FORCING WILL BE PLENTIFUL.

LASTLY...WINDS WILL BE HOWLING BEHIND THE COLD FRONT. WINDS SHIFT
TO OUT OF THE NORTHWEST WITH SUSTAINED SPEEDS ARND 30 KT AND GUSTS
ARND 45-50KT.

TO RECAP...GUSTY E WINDS BCMG SE...IFR/LIFR CIGS...AND RAIN MOVING
IN TONIGHT. RAIN AND IFR/LIFR CIGS EXPECTED ALL DAY THURSDAY WITH
BRIEF PERIODS OF HEAVY RAIN AND/OR TS PSBL. RAIN CHANGES OVER TO
SNOW FROM WEST TO EAST THURSDAY AFTERNOON/EVENING WITH A FEW HOURS
OF A RAIN/WET SNOW MIX PSBL. VSBY DROPS TO IFR/LIFR WITH SNOW AND
WINDS BECOME NW 30G45KT TOMORROW EVENING.

JEE

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

* HIGH CONFIDENCE IN ALL FORECAST TRENDS.

* LOW CONFIDENCE IN TS OCCURRENCE.

* MEDIUM-HIGH CONFIDENCE IN TIMING OF RAIN AND RASN...HIGH
  CONFIDENCE IN OCCURRENCE.

* MEDIUM CONFIDENCE IN CIG...WIND SHIFT...AND WIND GUST TIMING.

* MEDIUM CONFIDENCE IN CIG HEIGHTS TONIGHT/THURS MORNING.

JEE

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

* THURSDAY NIGHT...VERY WINDY WITH GUSTS 35+ KT IN THE EVENING.
  SNOW ENDING OVERNIGHT. BLOWING SNOW. LIFR IMPROVING TO MVFR.
* FRIDAY...GUSTY NORTHWEST WINDS. MVFR IMPROVING TO VFR.
* SATURDAY THROUGH MONDAY...DRY. VFR.
* TUESDAY...CHANCE OF SNOW.

&&

MARINE...

411 AM...HIGH END STORM FORCE WINDS TO 60 KT NOW EXPECTED LATE
THURSDAY AFTERNOON INTO THURSDAY NIGHT.

LOW PRESSURE OVER SOUTHEAST COLORADO WILL MOVE EAST ACROSS THE
CENTRAL PLAINS TODAY THEN TURN NORTHEAST TONIGHT AS IT MOVES
TOWARD CHICAGO/SOUTHERN LAKE MICHIGAN THURSDAY AFTERNOON. THE LOW
WILL BE DEEPENING AS IT APPROACHES AND EAST GALES WILL DEVELOP
FIRST OVER SOUTHERN LAKE MICHIGAN OVERNIGHT AND THEN ACROSS THE
ENTIRE LAKE INTO THURSDAY MORNING. WINDS WILL THEN TURN NORTHEAST
AND EVENTUALLY NORTH OVER THE NORTHERN TWO THIRDS OR SO OF THE
LAKE AS THE LOW PASSES INTO SOUTHERN LOWER MICHIGAN...WHICH WILL BE
SOMETIME LATE THURSDAY AFTERNOON OR VERY EARLY THURSDAY EVENING.
IT IS THIS TIME PERIOD WHERE NORTHWEST WINDS WILL QUICKLY INCREASE
TO STORM FORCE WITH GUSTS NOW EXPECTED AS HIGH AS 60 KT. THE LOW
WILL SHIFT EAST TOWARD DETROIT BY FRIDAY MORNING. THE STRONGEST
WINDS MAY ONLY LAST 3-5 HOURS...BUT IT WILL STILL TAKE MOST OF THE
NIGHT AND INTO FRIDAY MORNING FOR THE GRADIENT TO RELAX ENOUGH FOR
WINDS TO DROP INTO HIGH END GALES. THE LOW WILL THEN MERGE WITH
ANOTHER LOW MOVING UP THE MID ATLANTIC ON FRIDAY...WHICH WILL SLOW
THE OVERALL MOVEMENT EAST AND WITH HIGH PRESSURE MOVING SOUTHEAST
ACROSS THE PLAINS...EXPECTED A LONGER DURATION OF GALE FORCE WINDS
INTO FRIDAY EVENING AT LEAST. WINDS WILL SLOWLY DIMINISHING FRIDAY
NIGHT INTO SATURDAY AS THE RIDGE SHIFTS EAST...BUT ALSO WEAKENS.
CMS

&&

LOT WATCHES/WARNINGS/ADVISORIES...
IL...WINTER STORM WARNING...ILZ003-ILZ004-ILZ008-ILZ010...9 AM
     THURSDAY TO MIDNIGHT FRIDAY.

     WINTER STORM WATCH...ILZ005-ILZ011-ILZ019...NOON THURSDAY TO 6
     AM FRIDAY.

     WINTER STORM WATCH...ILZ006-ILZ012-ILZ013-ILZ014-ILZ020-ILZ021-
     ILZ022...6 PM THURSDAY TO 6 AM FRIDAY.

IN...WINTER STORM WATCH...INZ001-INZ002...6 PM THURSDAY TO 6 AM
     FRIDAY.

LM...GALE WARNING...LMZ080-LMZ567-LMZ669-LMZ671-LMZ673-LMZ675-LMZ740-
     LMZ741-LMZ742-LMZ743-LMZ744-LMZ745-LMZ777-LMZ779-LMZ868-
     LMZ870-LMZ872-LMZ874-LMZ876-LMZ878...3 AM THURSDAY TO 1 PM
     THURSDAY.

     STORM WARNING...LMZ080-LMZ567-LMZ669-LMZ671-LMZ673-LMZ675-LMZ740-
     LMZ741-LMZ742-LMZ743-LMZ744-LMZ745-LMZ777-LMZ779-LMZ868-
     LMZ870-LMZ872-LMZ874-LMZ876-LMZ878...1 PM THURSDAY TO 4 PM
     FRIDAY.

     GALE WARNING...LMZ261-LMZ362-LMZ364-LMZ366-LMZ563-LMZ565...8 AM
     THURSDAY TO 6 PM THURSDAY.

     STORM WARNING...LMZ261-LMZ362-LMZ364-LMZ366-LMZ563-LMZ565...6 PM
     THURSDAY TO 4 PM FRIDAY.

     SMALL CRAFT ADVISORY...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-
     LMZ745...8 PM WEDNESDAY TO 3 AM THURSDAY.

&&

$$