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AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1144 AM CST THU FEB 21 2013

DISCUSSION...
343 AM CST

MAIN CONCERN REMAINS THE DEVELOPING WINTER STORM TO AFFECT THE
REGION.

THE SYSTEM EXPECTED TO BRING THE SNOW TO THE REGION LATER TODAY HAS
FINISHED DIGGING INTO THE SOUTHERN ROCKIES...ALMOST TO THE MEXICAN
BORDER...AND WILL BEGIN TO LIFT NEWD THIS MORNING.  THE LATEST
POSITION PER SATELLITE IMAGERY IS PRETTY MUCH ON TRACK WITH RECENT
MODEL RUNS AND FEEL RELATIVELY CONFIDENT IN THE SOLUTION THAT HAS
EVOLVED OVER THE PAST COUPLE DAYS...AND WHICH REQUIRED LITTLE
ADJUSTMENT TONIGHT.

FOR THE REMAINDER OF THE NIGHT AND INTO THURSDAY MORNING...MUCH OF
THE AREA WILL START OUT WITH JUST SOME THIN MID AND HIGH CLOUD
COVER...WITH THE EXCEPTION OF FAR NERN IL/NWRN IN WHICH REMAINS
UNDER LAKE EFFECT CLOUD COVER.  THE HIGH RES LOCAL WRF RUN IS
SHOWING SOME INDICATION OF A WEAK MESO LOW FORMING OVER THE SRN TIP
OF LAKE MICHIGAN...WHICH COULD HELP ENHANCE SOME LAKE EFFECT
FLURRIES INTO SNOW SHOWERS THROUGH THE DAY TODAY...WHICH WOULD
MAINLY BE SEEN ALONG THE ILLINOIS LAKEFRONT.  IT`S POSSIBLE THAT
SOME AREAS ALONG THE LAKEFRONT COULD SEE SOME BRIEF REDUCTIONS IN
VISIBILITY OR A DUSTING IN A HEAVIER SNOW SHOWER...BUT NO
SIGNIFICANT ACCUMULATION IS EXPECTED FROM THESE SNOW SHOWERS OR
FLURRIES.

THE MORE SUBSTANTIAL SNOW IS STILL ON TRACK TO SPREAD ACROSS THE AREA
TODAY...MOVING INTO THE SWRN PORTIONS OF THE CWA BY LATE THIS
AFTERNOON AND REACHING THE ROCKFORD AND CHICAGO METRO AREA IN TIME
FOR AT LEAST THE LATTER PORTIONS OF THE EVENING RUSH HOURS.  KIND OF
FEEL LIKE A BROKEN RECORD REPEATING MYSELF FOR THE PAST FEW
NIGHTS...BUT THE SYSTEM HAS BEEN REMARKABLY WELL BEHAVED IN RUN TO
RUN CONSISTENCY...WITH THE EXCEPTION OF A FEW TIMING ADJUSTMENTS
MADE TO ACCOUNT FOR THE UPPER LOW DIGGING A LITTLE FAR SOUTHWARD
INTO THE SRN ROCKIES THAN INITIALLY ANTICIPATED.  THE LATEST
GUIDANCE IS ALSO CONSISTENT IN BRINGING A WARM WEDGE AT 7KFT INTO
THE FAR SERN PORTIONS OF THE CWA AT THE TIME OF PCPN ONSET...SO SOME
LOCATIONS...MAINLY SOUTH OF THE ILLINOIS AND KANKAKEE RIVERS WILL
LIKELY SEE A BRIEF PERIOD OF SLEET...OR A MIX OF SLEET AND SNOW
PRECEDE A CHANGE OVER TO ALL SNOW.  THE REMAINDER OF THE CWA WILL
START OUT AS ALL SNOW.  THERE WILL LIKELY BE AN INITIAL PERIOD OF
LIGHT SNOW...FOLLOWED BY 2-3 HOURS OF MODERATE SNOW WITH THE
STRONGEST WARM ADVECTION AND MAX UVV THIS EVENING BEFORE TAPERING
OFF TO SOME LIGHTER SNOW DURING THE OVERNIGHT HOURS.  DURING THE
PERIOD OF HEAVIEST SNOWFALL...1 INCH PER HOUR SNOWFALL RATES ARE
POSSIBLE...WHEN THE BULK OF THE SNOW WILL FALL.  AS THE BAND OF
STRONGEST FORCING LIFTS NWD...THE SNOW SHOULD QUICKLY TAPER OFF TO
LIGHT SNOW...WITH ONLY MINOR ADDITIONAL ACCUMULATIONS.  THE
SECONDARY CONCERN...THOUGH THE ONE WHICH MAY HAVE A GREATER IMPACT
ON TRAVEL CONDITIONS WILL BE THE TRANSITION OF THE SNOW OVER TO
FREEZING DRIZZLE. AGAIN...THE GUIDANCE IS HAS BEEN VERY CONSISTENT
BUILDING AN UPPER RIDGE FROM THE GREAT LAKES TO THE ERN
GULFMEX...WHICH WILL PUT THE BRAKES ON ANY FURTHER EWD PROGRESSION
OF THE SYSTEM AND THE UPPER LOW CENTER WILL TAKE A SHARP TURN TO THE
NORTH THROUGH KANSAS THIS EVENING AND INTO IOWA TOMORROW.  THIS WILL
LEAD TO THE MID LEVEL DRY AIR OVERSPREADING THE SATURATED LOWER
LEVELS.  WITH SFC TEMPS EXPECTED TO BE IN THE LOWER TO MIDDLE 20S
THROUGH THE NIGHT...THE SNOW WILL TRANSITION OVER TO FREEZING
DRIZZLE FOR MUCH OF THE AREA.  THE ROCKFORD AREA...AND LOCATIONS
ALONG AND WEST OF I-39 MAY ESCAPE AT LEAST SOME OF THE FREEZING
DRIZZLE AT THEY WILL BE A LITTLE CLOSER TO THE UPPER LOW
CENTER...WHICH MAY HELP MAINTAIN MORE SNOW THOUGH THE NIGHT...THOUGH
IT SHOULD STILL BE RELATIVELY LIGHT.  THE REMAINDER OF THE AREA WILL
LIKELY SEE A LEAST A LIGHT GLAZING OF ICE FROM THE FAIRLY EXTENDED
PERIOD OF FREEZING DRIZZLE LIKELY TO PERSIST THROUGH THE MORNING
RUSH HOURS.  THE FREEZING DRIZZLE POTENTIAL SHOULD END BY ARND NOON
FRIDAY AS THE LOWER LEVELS BEGIN TO WARM UP ABOVE FREEZING UNDER
PERSISTENT SWLY FLOW AS THE SYSTEM CONTINUES NEWD THROUGH WISCONSIN
FRIDAY EVENING.  THE PROGRESSION OF THE SYSTEM THROUGH WI AND INTO
THE UPPER GREAT LAKES WILL BE RELATIVELY SLOW...BLOCKED BY THE UPPER
RIDGE...SO THERE WILL ALSO BE A CHANCE FOR A PERIOD OF LIGHT SNOW
OVER THE NORTHERN PORTIONS OF THE CWA FRIDAY EVENING BEFORE THE
SYSTEM FINALLY TRACKS OFF TO THE NORTHEAST.  IT IS ANTICIPATED THAT
THIS LAST BIT OF SNOW WOULD ONLY AMOUNT TO AN ADDITIONAL TENTH OF AN
INCH OR SO.  AS FOR SNOW TOTALS...HAVE MADE LITTLE CHANGE TO THE
GOING FORECAST.  WHILE THERE WILL CERTAINLY BE ADEQUATE MOISTURE IN
PLACE DURING THE PERIOD OF STRONGEST FORCING...WITH PWATS OF 0.5 TO
0.8 INCHES...THE SNOW TO LIQUID RATIO SHOULD BE A LITTLE LOWER THAN
CLIMATOLOGICALLY NORMAL BEING A WARM ADVECTION...SOUTHERN STREAM
SYSTEM AND NOT A COLD ARCTIC SYSTEM.  SO...A WIDESPREAD AREA OF 3 TO
5 INCHES OF SNOW IS EXPECTED.

ALSO...LITTLE CHANGE IS EXPECTED THROUGH THE WEEKEND AND INTO THE
LONGER TERM FORECAST PERIOD.  THE WEEKEND IS STILL EXPECTED TO BE
QUIET WITH A WARMING TREND FROM SUNDAY AND INTO EARLY NEXT WEEK.
WITH TEMPERATURES IN THE UPPER 30S TO LOWER 40S EARLY NEXT
WEEK...MUCH OF THE SNOW WILL LIKELY MELT.  HOWEVER...ANOTHER SYSTEM
IS BEING ADVERTISED BY THE LONGER RANGE MODELS...WHICH HAS
CHARACTERISTICS AND ORIGIN SIMILAR TO THE SYSTEM TO IMPACT THE
REGION LATER TODAY.  HOWEVER...IT IS A BIT TOO EARLY TO TELL IF THIS
SYSTEM WILL BRING ANOTHER ROUND OF SNOW.

KREIN

&&

AVIATION...

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

* LOW MVFR CEILINGS CONTINUING INTO THE EVENING,

* POSSIBILITY OF SOME -SHSN OR FLURRIES INTO THE EVENING.

* STRONG EAST WINDS DEVELOPING DURING THE EVENING...WITH GUSTS
  AROUND 25 KT.

* PERIODS OF SN DEVELOPING AFTER 03 UTC...THEN POSSIBLY +SN
  BETWEEN 06 AND 10 UTC.

* POTENTIAL FOR SOME -FZDZ FRIDAY MORNING.

PAW

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

WINTER STORM APPROACHING FROM THE SOUTHWEST THIS AFTERNOON. MVFR
CEILINGS ALREADY IN PLACE AT THE TERMINALS IN ADVANCE OF THE
STORM. A FEW FLURRIES WILL CONTINUE TO BE POSSIBLE NEAR LAKE
MICHIGAN UNTIL THE MORE SIGNIFICANT SNOW WITH THIS STORM ARRIVES
THIS EVENING. TIMING HAS NOT CHANGED...STILL LOOKS LIKE SNOW
INITIALLY SPREADING INTO RFD AROUND 01Z AND THE OTHER TERMINALS
AROUND 02Z. IT APPEARS THERE WILL BE A PERIOD OF HEAVIER SNOW
LASTING A COUPLE HOURS BEGINNING AROUND 06Z WITH VISIBILITIES
DROPPING TO A HALF MILE OR LESS. THE STORM SYSTEM HAS A HISTORY OF
CONVECTION ASSOCIATED WITH THE HEAVIEST SNOW AREAS. SO THERE IS A
SLIGHT CHANCE OF THUNDER ACROSS THE AREA DURING THE PERIOD OF
HEAVIER SNOW BUT THE CHANCE IS TOO LOW TO INCLUDE IN THE TAFS.

THINGS SHOULD BEGIN TO SETTLE DOWN A BIT LATE TONIGHT INTO FRIDAY
MORNING...WITH SNOW TAPERING OFF AND VISIBILITIES IMPROVING. THERE
IS A POSSIBILITY OF THE DIMINISHING SNOW BEING MIXED WITH SOME
FREEZING DRIZZLE FRIDAY MORNING. BUT CONFIDENCE IS TOO LOW TO
INCLUDE IN THE TAFS AT THIS TIME.

PAW

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

* HIGH CONFIDENCE IN CEILING TREND...MEDIUM CONFIDENCE IN DETAILS.

* MEDIUM TO HIGH CONFIDENCE IN WIND FORECAST.

* HIGH CONFIDENCE IN SNOW TREND...MEDIUM CONFIDENCE IN TIMING AND
  DETAILS.

* LOW CONFIDENCE ON THE CHANCES OF -FZDZ ON FRIDAY.

PAW

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

FRIDAY NIGHT...CHANCE OF LIGHT SNOW EARLY...OTHERWISE
IFR...POSSIBLY IMPROVING TO MVFR LATE.

SATURDAY... MVFR CEILINGS RISING TO VFR MIDDAY.

SUNDAY... VFR.

MONDAY...VFR CEILINGS LOWERING TO MVFR LATER AFTERNOON. CHANCE RA
AND SN...MAINLY MONDAY NIGHT. STRONG EASTERLY WINDS DEVELOPING
LATE.

TUESDAY... IFR CEILINGS POSSIBLE. CHANCE OF RA OR SN.

WEDNESDAY...IFR POSSIBLE WITH A CHANCE OF -SN.

KJB

&&

MARINE...
209 AM CST

A RIDGE OF SURFACE HIGH PRESSURE...EXTENDING FROM ONTARIO
SOUTHWARD ACROSS THE WESTERN LAKES EARLY THIS MORNING...IS RESULTING
IN NORTHERLY FLOW LAKE-WIDE. HOWEVER...THE WINDS SHOULD VEER
NORTHEASTERLY...THEN EASTERLY THROUGHOUT THE DAY TODAY AS A STRONG
STORM SYSTEM MOVES ACROSS THE CENTRAL AND SOUTHERN PLAINS. THIS LOW
WILL CONTINUE NORTHEASTWARD ACROSS MISSOURI TONIGHT...AND EVENTUALLY
END UP MOVING ACROSS LAKE MICHIGAN BY FRIDAY NIGHT. IT APPEARS THE
WINDS WILL PICK UP OUT OF THE EAST UP TO 30 KT...ESPECIALLY ACROSS
THE SOUTHERN PORTION OF THE LAKE LATE THIS AFTERNOON AND THIS
EVENING. HOWEVER...THESE STRONGER WINDS WILL GRADUALLY MOVE UP THE
LAKE INTO FRIDAY AS THE LOW APPROACHES. AS THE LOW MOVES OVER THE
LAKE FRIDAY NIGHT...VARIABLE WINDS WILL SET UP...WITH
WEST-SOUTHWESTERLY WINDS DEVELOPING FAR SOUTH...LIGHT AND VARIABLE
WINDS MID LAKE...AND EASTERLY WINDS FAR NORTH.

A PERIOD OF LIGHTER WINDS ARE EXPECTED SUNDAY AND INTO MONDAY AS A
RIDGE OF HIGH PRESSURE MOVES ACROSS THE LAKES. HOWEVER...IT APPEARS
ANOTHER STRONG STORM SYSTEM WILL TRACK ACROSS THE LAKES BY THE
MIDDLE OF NEXT WEEK. THIS SYSTEM PROMISES TO PRODUCE ANOTHER PERIOD
OF STRONG EASTERLY WINDS UP AROUND 30 KT MONDAY NIGHT INTO
TUESDAY...WITH GALE FORCE WINDS POSSIBLE.

KJB

&&

LOT WATCHES/WARNINGS/ADVISORIES...
IL...WINTER WEATHER ADVISORY...ILZ019-ILZ021-ILZ023-ILZ032-ILZ033-
     ILZ039...6 PM THURSDAY TO NOON FRIDAY.

     WINTER WEATHER ADVISORY...ILZ003-ILZ004-ILZ008-ILZ010-ILZ011...8
     PM THURSDAY TO 6 PM FRIDAY.

     WINTER WEATHER ADVISORY...ILZ005-ILZ006-ILZ012-ILZ013-ILZ014-
     ILZ020-ILZ022...9 PM THURSDAY TO 6 PM FRIDAY.

IN...WINTER WEATHER ADVISORY...INZ001-INZ002...9 PM THURSDAY TO 6 PM
     FRIDAY.

     WINTER WEATHER ADVISORY...INZ010-INZ011-INZ019...6 PM THURSDAY
     TO NOON FRIDAY.

LM...SMALL CRAFT ADVISORY...ILLINOIS NEARSHORE WATERS UNTIL 4 PM FRIDAY.

     SMALL CRAFT ADVISORY...INDIANA NEARSHORE WATERS UNTIL 10 AM FRIDAY.

&&

$$