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AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1134 AM CST WED FEB 20 2013

DISCUSSION...

954 AM...MORNING UPDATE...BUMPED POPS TO LIKELY FOR LAKE EFFECT
SNOW SHOWERS OVER NORTH CENTRAL PORTER COUNTY WITH KVPZ/KMGC
REPORTING VISIBILITIES IN THE 3-5SM RANGE AND KPPO ALMOST DOWN TO
1SM. THIS WILL LIKELY PERSIST INTO THE AFTERNOON AND EXPECT SOME
MINOR ACCUMULATIONS PERHAPS LOCALLY UP TO AN INCH. AS WINDS TURN
MORE NORTHERLY TONIGHT...THIS ACTIVITY WILL SHIFT WEST AND MAY
NEED TO EVENTUALLY BUMP CURRENT FLURRIES MENTION TO AT LEAST A
CHANCE OF SOME SNOW SHOWERS FOR LAKE COUNTY INDIANA AND SOUTHEAST
COOK COUNTY. CMS

//PREV DISCUSSION...
328 AM CST

FOR THE REMAINDER OF THE NIGHT AND INTO WEDNESDAY...THE PRIMARY
CONCERN WILL BE TEMPERATURES.  AS ARCTIC HIGH PRESSURE SPREADS
ACROSS THE AREA...SKIES HAVE CLEARED AND WINDS HAVE DROPPED OFF
CONSIDERABLY.  HOWEVER...A PERSISTENT 10 MPH BREEZE AND A CURRENT
LACK OF SNOW COVER HAS LIMITED RADIATIONAL COOLING PREVENTED
TEMPERATURES FROM DROPPING MUCH THROUGH THE NIGHT.  AT 300AM
CST...TEMPERATURES ACROSS THE AREA ARE HANGING IN AROUND
10F...EXCEPT FOR LOCATIONS WEST OF THE FOX RIVER VALLEY...WHERE
TEMPS HAVE DROPPED INTO THE MIDDLE SINGLE DIGITS.  COMBINED WITH THE
10 MPH WINDS...WIND CHILL READING RANGE FROM 5 TO 10 DEGREES BELOW
ZERO.  EXPECT THAT BY DAYBREAK...TEMPS AND WIND CHILL READINGS
SHOULD BE ABLE TO DROP A FEW MORE DEGREES...WITH THE COOLEST
AREA...AROUND THE ROCKFORD AREA...SEEING TEMPERATURES BOTTOM OUT
JUST ABOVE 0F...WITH WIND CHILLS AROUND -15F TO START THE DAY.  WITH
AMPLE SUNSHINE AND WEAKER WINDS EXPECTED TODAY...CONDITIONS WILL BE
ABLE TO REBOUND FROM THE RAW...BLUSTERY WEATHER OBSERVED TUESDAY.
THE ONE AREA OF MINOR CONCERN MORE FOR LATER TONIGHT WILL BE FOR
INCREASING LAKE EFFECT CLOUD COVER AND A CHANCE FOR SOME SCT
FLURRIES NEAR THE LAKE WITH WINDS TURNING THROUGH NORTHERLY TO
EASTERLY AS THE HIGH PRESSURE BUILDS TO THE EAST.  THERE IS A CHANCE
THAT SOME FLURRIES MAY PERSIST INTO EARLY THURSDAY MORNING.

THE PRIMARY WEATHER CONCERN REMAINS THE DEVELOPING WINTER STORM THAT
HAS BEEN DISCUSSED FOR THE PAST SEVERAL DAYS.  THE MODEL GUIDANCE
REMAINS IN RELATIVELY GOOD AGREEMENT ON HANDLING THE SYSTEM...BUT
ONE AREA OF AGREEMENT IS ALSO THE TREND TO SLOW DOWN THE PROGRESSION
OF THE SYSTEM.  THE LATEST MODEL RUNS CONTINUE THE TREND OF DIGGING
THE MAIN UPPER LOW A LITTLE FARTHER SOUTH INTO THE DESERT SOUTHWEST
BEFORE THE SYSTEM LIFTS NEWD INTO THE CENTRAL PLAINS.
OTHERWISE...THERE HAS BEEN LITTLE CHANGE TO THE EVOLUTION OF THE
SYSTEM...AT LEAST IN THE MODEL WORLD.  THE LATEST SATELLITE IMAGERY
SHOWS THE SYSTEM DIGGING SOUTHWARD ALONG THE CENTRAL CALIFORNIA
COAST...WHICH IS RIGHT ON TRACK WITH THE MODEL INITIALIZATIONS AND
INITIAL TRACKS.  SO...FEEL THAT THE GUIDANCE IS GENERALLY ON TRACK
WITH THE SYSTEM...AND FEEL RELATIVELY CONFIDENT THAT THE LATEST
ADJUSTMENTS TO THE GOING FORECAST...MAINLY TO DELAY THE ONSET OF
SNOW BY 3 TO 6 HOURS...PUTS US ON COURSE FOR WHAT IS STILL SHAPING
UP TO THE THE MOST WIDESPREAD SYNOPTIC SNOW OF THE SEASON FOR
NORTHERN ILLINOIS AND NORTHWEST INDIANA.  OTHERWISE...HAVE MADE
LITTLE CHANGE TO THE GOING FORECAST.  THE GENERAL SCENARIO WILL BE
FOR A WIDESPREAD WARM ADVECTION SNOW...TO SPREAD FROM SOUTHWEST TO
NORTHEAST ACROSS THE AREA...WITH PCPN MOVING INTO THE PONTIAC AREA
BY LATE THURSDAY AFTERNOON.  THE GUIDANCE IS STILL INDICATING THE
POTENTIAL FOR A PERIOD OF SLEET TO MIX WITH THE INITIAL SNOW AS A
WEDGE OF WARMER AIR ALOFT AT ARND 7KFT RIDES OVER THE SUB-FREEZING
LOWER LEVELS.  THE WARM WEDGE MAY BE SUFFICIENTLY DEEP ENOUGH THAT
IT MAY TAKE A LITTLE LONGER FOR THE ATMOSPHERIC COLUMN TO COOL
ENTIRELY BELOW FREEZING...AND SOME SLEET MAY REACH AS FAR NORTH AS
THE SOUTH CHICAGO SUBURBS THURSDAY EVENING.  OTHERWISE...THE SNOW
SHOULD OVERSPREAD THE AREA THROUGH THURSDAY EVENING AND INTO THE
NIGHT.  BY EARLY FRIDAY MORNING...THE MID LEVEL DRY PUNCH SHOULD
PUSH OVER THE AREA...LARGELY SHUTTING OFF THE SNOW-MAKING PROCESS AS
THE SATURATED LAYER OF THE ATMOSPHERE WILL BE WELL BELOW THE
DENDRITIC GROWTH ZONE.  THERE COULD BE ENOUGH RESIDUAL UVV TO ALLOW
FOR A LITTLE SNOW TO MIX WITH THE FREEZING DRIZZLE BY THE TIME OF
THE DRY-SLOTTING...BUT ANTICIPATE THAT BY FRIDAY MORNING...THE BULK
OF ANY REMAINING PCPN SHOULD BE LIGHT FREEZING DRIZZLE WITH LITTLE
ADDITIONAL SNOW.  AS FOR TOTAL AMOUNTS...ANTICIPATE A WIDESPREAD
AREA OF 3 TO 4 INCHES OVER THE CWA...WITH LOCATIONS ALONG AND WEST
OF THE I-39 CORRIDOR POSSIBLE SEEING UPWARDS OF 5 INCHES...WHILE THE
NWRN INDIANA COUNTIES SEEING AROUND 2 TO 3 INCHES.  THERE IS STILL
THE POTENTIAL FOR THE LIGHT FREEZING DRIZZLE TO LINGER UNTIL MID-DAY
WHEN TEMPERATURES WILL FINALLY REACH ABOVE FREEZING OVER MUCH OF THE
AREA.  SO...ON TOP OF THE SNOW...THERE IS THE LIKELIHOOD OF A
GLAZING OF ICE.  ALL THINGS CONSIDERED...STILL FEEL THAT A WINTER
WEATHER ADVISORY WILL LIKELY BE REQUIRED.

FOLLOWING THE PASSAGE OF THE UPPER LOW...THE HIGH AMPLITUDE...
QUICKLY PROGRESSIVE PATTERN WILL CONTINUE THROUGH THE WEEKEND AND
INTO EARLY NEXT WEEK.  SATURDAY SHOULD STILL BE ON THE COOLER SIDE
AS THE EXITING UPPER LOW WILL STILL HAVE SOME INFLUENCE ON THE LOCAL
AREA...BUT MORE SIGNIFICANT WARMING IS EXPECTED FOR SUNDAY AND INTO
EARLY NEXT WEAK AS UPPER RIDGING BUILDS ACROSS THE AREA.  AT THIS
POINT...IT LOOKS LIKE THAT HIGHS ON MONDAY AND TUESDAY SHOULD RANGE
FROM THE UPPER 30S OVER THE ROCKFORD AREA TO THE MIDDLE 40S OVER
NWRN INDIANA.

KREIN

&&

AVIATION...

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

* NORTHWEST WINDS THROUGH THE DAY. OCCASIONAL GUSTS TO 12-15 KT
  POSSIBLE.

* POTENTIAL FOR LAKE-EFFECT LOW CLOUDS AND SNOW SHOWERS LATE
  TONIGHT INTO THURSDAY MORNING.

PAW

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

A HIGH PRESSURE RIDGE OVER NORTHERN IL AND NORTHWEST IN TODAY WILL
MOVE TO THE EAST TONIGHT. WINDS BEHIND THE RIDGE WILL BE LIGHT
AND WILL GRADUALLY SHIFT TO THE NORTH AND THEN NORTHEAST THROUGH
THE NIGHT. NORTHEAST WINDS WILL ALLOW LOWER CEILINGS OVER LAKE
MICHIGAN TO SPREAD INTO ORD...MDW AND GYY LATE TONIGHT. SNOW
SHOWERS OR FLURRIES ARE POSSIBLE WITH THESE LOWER CEILINGS...EVEN
THOUGH THIS IS NOT AN IDEAL SETUP FOR LAKE-EFFECT SNOW.

ON THURSDAY WINDS WILL BE INCREASING AS THE NEXT SYSTEM APPROACHES
FROM THE SOUTHWEST WITH SIGNIFICANT PRECIPITATION. AT THE PRESENT
TIME IT APPEARS PRECIPITATION WILL BEGIN AFTER THE END OF THE
18Z TAF PERIOD.

PAW


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

* HIGH CONFIDENCE IN ALL FORECAST ELEMENTS FOR TODAY.

* MEDIUM CONFIDENCE IN LAKE-EFFECT POTENTIAL LATE TONIGHT INTO
  THURSDAY.

PAW

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

THURSDAY NIGHT...SNOW DEVELOPING BY MID TO LATE EVENING. THE SNOW
COULD BE HEAVY AT TIMES. PERIODS OF IFR/LIFR LIKELY.

FRIDAY...IFR/LIFR POSSIBLE WITH FZDZ OR LIGHT SNOW.

SATURDAY...MVFR CIGS POSSIBLE.

SUNDAY...VFR.

MONDAY...CHANCE OF SNOW OR RAIN. MVFR/IFR POSSIBLE. STRONG
EASTERLY WINDS POSSIBLE.

TUESDAY...CHANCE OF SNOW OR RAIN. MVFR/IFR POSSIBLE.

KJB

&&

MARINE...
214 AM CST

A POTENT AREA OF SURFACE HIGH PRESSURE...EXTENDING FROM ONTARIO
SOUTHWARD ACROSS THE UPPER MIDWEST...WILL CONTINUE TO BUILD EASTWARD
TOWARDS THE WESTERN LAKES TODAY AND TONIGHT AS LOW PRESSURE MOVES
EAST NORTHEASTWARD ACROSS QUEBEC. AS THIS OCCURS...THE WINDS WILL
GRADUALLY BECOME NORTHERLY LAKE-WIDE BY TONIGHT. WIND SPEEDS WILL
REMAIN ELEVATED TO AROUND 35 KT GALES NORTH TO AROUND 30 KT SOUTH.
DUE TO THE FACT THAT THE THREAT FOR PROLONGED GALE FORCE WINDS
APPEARS TO HAVE ENDED ACROSS THE SOUTHERN HALF OF LAKE MICHIGAN...I
HAVE OPTED TO END THE WARNING EARLY. HOWEVER...I WILL LEAVE THE
WARNING IN PLACE FOR THE NORTH HALF THROUGH THIS EVENING...ALTHOUGH
THIS MAY ALSO BE ABLE TO BE CANCELLED BY LATE THIS AFTERNOON. NO
CHANGES WERE MADE TO THE FREEZING SPRAY HEADLINES AS THE CURRENT
ENDING TIMES APPEAR GOOD.

THE WINDS ACROSS THE LAKE WILL GRADUALLY SHIFT NORTHEASTERLY BY
THURSDAY MORNING...THEN EASTERLY BY THURSDAY EVENING IN RESPONSE TO
ANOTHER STORM SYSTEM EXPECTED TO DEVELOP ACROSS THE SOUTHERN PLAINS
BY THURSDAY...THEN MOVE NORTHEASTERN ACROSS MISSOURI...TO NEAR
SOUTHERN LAKE MICHIGAN BY FRIDAY EVENING. IT APPEARS WINDS ON THE
SOUTHERN HALF OF THE LAKE COULD ONCE AGAIN INCREASE TO 25 TO 30 KT
OUT OF THE EAST LATE THURSDAY INTO FRIDAY. THIS FETCH WILL CONTINUE
TO PRODUCE LARGE WAVES INTO THE NEAR SHORE WATERS FOR AN EXTENDED
PERIOD OF TIME.

ONCE THIS SYSTEM PASSES...THE WINDS ACROSS THE LAKE SHOULD ABATE TO
20 KT OR LESS FOR MUCH OF THE WEEKEND. HOWEVER...THE WEATHER PATTERN
ACROSS THE GREAT LAKES REGION IS EXPECTED TO BECOME ACTIVE AGAIN BY
EARLY NEXT WEEK...WITH ANOTHER POTENT STORM POSSIBLY AFFECTING THE
REGION FOR MONDAY INTO TUESDAY. THIS SYSTEM COULD PRODUCE ANOTHER
PERIOD OF STRONG WINDS ACROSS LAKE MICHIGAN...WITH GALE FORCE WINDS
POSSIBLE.

KJB

&&

LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...HEAVY FREEZING SPRAY WARNING...LMZ261-LMZ362-LMZ364-LMZ366-
     LMZ563-LMZ565-LMZ567-LMZ868 UNTIL 9 PM WEDNESDAY.

     GALE WARNING...LMZ261-LMZ362-LMZ364-LMZ366-LMZ563-LMZ565-LMZ567-
     LMZ868 UNTIL 9 PM WEDNESDAY.

     HEAVY FREEZING SPRAY WARNING...LMZ080-LMZ669-LMZ671-LMZ673-
     LMZ675-LMZ777-LMZ779-LMZ870-LMZ872-LMZ874-LMZ876-LMZ878
     UNTIL NOON WEDNESDAY.

     SMALL CRAFT ADVISORY...NEARSHORE WATERS
     UNTIL 3 PM THURSDAY.

&&

$$