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AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1158 AM CST MON FEB 18 2013

DISCUSSION...
1017 AM CST

FOR MORNING UPDATE...

HAVE MADE SOME MINOR ADJUSTMENTS TO BUMP TEMPS UP 1-3 DEGREES OR SO
ACROSS THE AREA THIS AFTERNOON AND TO TWEAK SKY COVER FOR A LITTLE
THINNER CLOUD COVER THIS MORNING. HAVE ALSO SLOWED PRECIP ARRIVAL
UNTIL THIS AFTERNOON.

LOW PRESSURE WAS ANALYZED OVER NORTHERN MINNESOTA AT 15Z...WITH
COLD FRONT AND ASSOCIATED LOW PRESSURE TROUGH EXTENDING
SOUTH-SOUTHWEST THROUGH THE PLAINS. SFC PRESSURE FALLS SPREADING
THROUGH THE GREAT LAKES/MIDWEST HAVE TIGHTENED THE GRADIENT
OVERNIGHT AND HAVE RESULTED IN GUSTY SOUTH WINDS NEAR 30 MPH AT THIS
HOUR. WITH THE STRONG WARM ADVECTION...TEMPS HAVE SURGED INTO THE
UPPER 40S/LOW 50S ALREADY ACROSS CENTRAL IL...AND VARIOUS MODEL LOW
LEVEL THERMAL PROGS SUGGEST HIGHS WILL LIKELY END UP RANGING FROM
UPPER 40S NEAR THE IL/WI BORDER TO THE LOW/MID 50S ACROSS THE
CENTRAL AND SOUTHERN PARTS OF THE CWA...WITH QUICK RISES THIS
MORNING FOLLOWED BY MORE STEADY READINGS BY MIDDAY/EARLY AFTERNOON.

OTHERWISE...LOCAL AND UPSTREAM 12Z SOUNDINGS INDICATE FAIRLY DRY AIR
IN PLACE ACROSS THE REGION...WITH MOISTURE IN THE SFC-850 LAYER
STILL WELL UPSTREAM FROM MISSOURI INTO THE SOUTHERN PLAINS/LOWER
MISSISSIPPI VALLEY. WHILE LITTLE PRECIP NOTED UPSTREAM IN REGIONAL
RADAR MOSAIC...SUSPECT RAIN WILL BLOSSOM FAIRLY QUICKLY BY EARLY
AFTERNOON AS MOISTURE CONTINUES TO STREAM NORTHEAST AHEAD OF THE
COLD FRONT. THEREFORE EXPECTATION IS THAT PRECIP WILL HOLD OFF UNTIL
EARLY AFTERNOON AND LATER FOR MUCH OF THE CWA.

ONE OTHER CONCERN IS THAT WINDS COULD APPROACH ADVISORY CRITERIA FOR
A TIME TODAY...PARTICULARLY OVER SOUTHEAST PARTS OF THE CWA WHERE
WARMER TEMPS/BETTER MIXING/LATER CLOUD THICKENING WILL OCCUR. WHILE
WIND FORECAST CURRENTLY JUST BELOW CRITERIA...WILL CONTINUE TO
MONITOR WIND TRENDS INTO THIS AFTERNOON ACROSS EAST CENTRAL
IL/NORTHWEST AND WEST CENTRAL INDIANA FOR POTENTIAL ADVISORY.

RATZER

//PREV DISCUSSION...
403 AM CST

THE WEATHER FOR THE NEXT SEVERAL DAYS WILL BE A REAL ROLLER COASTER
RIDE WITH PERIODS OF RAIN...SNOW...WARM...AND COLD AS A HIGH
AMPLITUDE...VERY PROGRESSIVE PATTERN SETS UP OVER NORTH AMERICA.

ON THE LOCAL SCALE...TEMPERATURES HAVE BEEN STEADY OR SLOWLY
CLIMBING THROUGH THE NIGHT AS STRONG WARM ADVECTION BUILDS IN
ADVANCE OF THE APPROACHING STRONG COLD FRONT. SELY WINDS HAVE ALSO
BEEN ON THE INCREASE.  THERE IS SOME CONCERN THAT WIND SPEEDS MAY
REACH ADVISORY CRITERIA OVER LOCATIONS SOUTH OF THE KANKAKEE AND
ILLINOIS RIVERS...BUT THERE IS A STRONG POSSIBILITY THAT INCREASING
CLOUD COVER AND RAIN MAY MOVE INTO THE CWA TOO QUICKLY TO ALLOW THE
STRONGER WINDS AT THE TOP OF THE BOUNDARY TO MIX DOWN TO THE SFC.
SO...WILL NOT ISSUE A HIGH WIND ADVISORY AT THIS TIME.  AS FOR THE
AFOREMENTIONED RAIN...HAVE MADE LITTLE CHANCE TO THE GOING SHORT TERM
FORECAST AND STILL EXPECT THAT RAIN SHOULD BEGIN TO OVERSPREAD THE
AREA FROM SOUTH TO NORTH...WITH THE RAIN BEGINNING IN THE PONTIAC
AREA BY AROUND NOON AND THEN QUICKLY SPREADING TO THE NORTH.  GIVEN
THAT THE SYSTEM WILL TRACK EASTWARD RATHER QUICKLY...THE AXIS OF
HEAVIEST PCPN SHOULD SHIFT INTO EAST-CENTRAL ILLINOIS AND
NORTHWESTERN INDIANA BY LATE AFTERNOON OR EARLY EVENING.  FOR THE
ROUGHLY 15 HOUR PERIOD OF PCPN POTENTIAL TODAY THROUGH THIS
EVENING...EXPECT UPWARDS OF A QUARTER INCH OF TOTAL RAIN EAST OF A
LINE FROM VALPARAISO TO WATSEKA...WITH AMOUNTS TAPERING OFF TO ABOUT
A TENTH OF AN INCH OF SO OVER A CORRIDOR FROM CHICAGO TO PONTIAC AND
ONLY PERHAPS A FEW HUNDREDTHS OF AN INCH ALONG THE I-30
CORRIDOR...INCLUDING THE ROCKFORD AREA.  WITH THE STRONG WARM
ADVECTION TODAY...STILL EXPECT THAT TEMPERATURES SHOULD EASILY REACH
INTO THE UPPER 40S AND LOWER 50S.  THE WARMER CONDITIONS WILL
QUICKLY COME TO AN END OVERNIGHT AS THE STRONG COLD FRONT PUSHES
THROUGH THE REGION.  THE PCPN SHOULD TAPER OFF TO SOME LIGHT SNOW OR
A RAIN/SNOW MIX OVER NWRN INDIANA AS THE PCPN EXITS THE CWA.

TUESDAY WILL BE MUCH COOLER FOLLOWING THE PASSAGE OF THE FIRST COLD
FRONT WITH HIGHS RANGING FROM THE UPPER TEENS OVER THE ROCKFORD AREA
TO THE MIDDLE 20S TO THE SOUTH. WITH ARCTIC HIGH PRESSURE BUILDING
ACROSS THE REGION TUESDAY NIGHT...CLOUD COVER AND WINDS WILL
DECREASE SHARPLY...ALLOWING FOR TEMPERATURES TO DROP OFF INTO THE
SINGLE DIGITS.  FOR WEDNESDAY INTO THURSDAY...THERE WILL BE LITTLE
TEMPERATURE VARIATION AS THE COOLER AIR REMAINS PARKED OVER THE
MIDWEST.

THE NEXT MAJOR CONCERN IS SHAPING UP FOR THURSDAY WITH THE PASSAGE
OF THE MUCH DISCUSSED SYSTEM THAT ORIGINATED IN THE GULF OF ALASKA.
WHILE THE LONGER RANGE MODEL GUIDANCE HAS BEEN RATHER CONSISTENT
WITH THE LARGE SCALE EVOLUTION OF THE SYSTEM...AS WE APPROACH CLOSER
TO THE EVENT...THE MODELS HAVE BEEN CONVERGING ON A VERY CONSISTENT
SOLUTION WITH THE UPPER LOW LIFTING OUT OF THE 4 CORNERS REGION
WEDNESDAY EVENING ANS THEN QUICKLY REACHING THE CENTRAL PLAINS BY
THURSDAY AFTERNOON.  UNFORTUNATELY...AS FAR AS THE SNOW-CROWS ARE
CONCERNED...A BLOCKING RIDGE IS BEING ADVERTISED TO AMPLIFY OVER THE
CENTRAL GREAT LAKES TO SERN COAST...EFFECTIVELY STOPPING THE SYSTEM
IN IT`S TRACKS AND FORCES IT TO TAKE A SHARP TURN TO THE
NORTHEAST...INTO THE UPPER MISSOURI VALLEY BY THURSDAY EVENING.  THE
SFC REFLECTION OF THE SYSTEM...CONSEQUENTLY...TAKES A MORE NORTHERLY
TRACK...LIFTING FROM SWRN KS THURSDAY MORNING TO NORTHERN MISSOURI
BY THURSDAY NIGHT...WHILE WEAKENING CONSIDERABLY.  WHILE IN SITU
TEMPERATURES WILL CERTAINLY BE LOW ENOUGH FOR A FAIRLY EXTENDED
PERIOD OF ACCUMULATING SNOW...THE TRACK OF THE SFC LOW WILL NOT BE
CONDUCIVE TO HEAVY SNOW.  THERE IS STILL A BIT OF UNCERTAINTY AS TO
THE EXACT TRACK OF THE SFC LOW...THE SYSTEM IS SHAPING UP TO BE THE
MOST WIDESPREAD ACCUMULATING SNOW EVENT OF THIS WINTER.  AT THIS
POINT...WIDESPREAD AMOUNTS OF 4 TO 5 INCHES IS NOT OUT OF THE
QUESTION.  THE PCPN SHOULD TAPER OFF TO SOME LIGHT SNOW SHOWERS...OR
MORE LIKELY...FREEZING DRIZZLE AS A MID-LEVEL DRY SLOW OVERRUNS THE
SATURATED LOWER LEVELS.  AS THIS SYSTEM IS STILL 3 DAYS OFF...THERE
MAY YET BE SOME ADJUSTMENTS TO THE FORECAST NECESSARY...SO STAY
TUNED FOR FURTHER UPDATES.

FOR THE LONGER TERM FORECAST PERIOD...FROM FRIDAY THROUGH
MONDAY...THE GENERALLY TREND SHOULD BE A WARMING TREND AS THE
SNOW-MAKER WEAKENS AS IT PASSES THROUGH THE GREAT LAKES REGION AND
UPPER RIDGING BUILDS OVER THE CNTRL PLAINS AND GRADUALLY PROGRESSES
EWD WHALE NEW TROUGHING DROPS OUT OF THE ROCKIES.  TEMPERATURES
SHOULD GENERALLY NEAR TO SLIGHTLY ABOVE SEASONAL NORMAL LEVELS
WITH LITTLE CHANCE FOR PCPN.

KREIN

&&

AVIATION...

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

* S WIND GUSTS OF 25 TO 30 KT THIS AFTERNOON.

* -RA/MVFR DEVELOPING BY MID AFTERNOON WITH IFR CIGS PROBABLE
  THIS EVE.

* SHIFT TO STRONG WEST WINDS IN THE 0230Z-0400Z PERIOD.

* EXTENDED PERIOD OF WIND GUSTS TO 35 KT AFTER THAT POINT THROUGH
  TUE AFTERNOON.

* SNOW SHOWERS IN THE AREA TUE MORNING.


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

QUITE A BIT GOING ON IN THE FORECAST WITH RAIN AND LOWER CLOUDS
MOVING IN THIS AFTERNOON FOLLOWED BY A MARKED WIND SHIFT WITH A
COLD FRONT THIS EVE THEN A PROLONGED PERIOD OF GUSTY WESTERLY
WINDS. TWO SEPARATE LOW PRESSURES ACROSS WESTERN WI AND CENTRAL MO
EARLY THIS AFTERNOON WILL CONSOLIDATE ACROSS NORTHERN IL BY EARLY
EVE WITH A PATH OF THE LOW NEAR CHICAGO. IMMEDIATELY BEHIND THIS
FEATURE A SHARP COLD FRONT WILL PUSH EAST THROUGH THE REGION WITH
A LARGE PRESSURE CHANGE AND TEMPERATURE DROP AIDING IN STRONG
WESTERLY WIND SPEEDS FROM FRONTAL PASSAGE ALL THE WAY INTO TUE
AFTERNOON. GUSTS TO 40 KT CANNOT BE RULED OUT THROUGH THU MORNING.

AHEAD OF THE LOW THIS AFTERNOON...ATMOSPHERIC MOISTENING WILL BE
TAKING PLACE. SATELLITE AND SHORT TERM MODEL TRENDS INDICATE THIS
SHOULD RAPIDLY INCREASE BETWEEN 20Z AND 22Z...WHERE CLOUD BASES
WILL LOWER TO MVFR AND RAIN/LIGHT SHOWERS TO BEGIN SOON
AFTERWARD. IFR CIGS ARE LIKELY AROUND AND NEAR THE LOW PASSAGE
TIME. ITS POSSIBLE THESE END UP BEING MORE TEMPORARY THAN
PREVAILING...BUT CONFIDENCE IN OCCURRENCE IS MEDIUM TO A LITTLE
BETTER. AS COLDER AIR WRAPS INTO THE AREA BEHIND THE SYSTEM COLD
FRONT...RAIN SHOULD MAKE A FAIRLY QUICK TRANSITION TO SNOW.
HOWEVER...THE PERIOD OF TIME FROM TRANSITION TO END OF APPRECIABLE
PRECIPITATION IS LIKELY TO ONLY BE SHORT ESPECIALLY FOR THE
CHICAGO AREA TAF SITES. SNOW SHOWERS ARE THEN PROBABLE AS THE
UPPER LEVEL LOW PRESSURE GRAZES THE AREA VERY LATE TONIGHT
THROUGH TUE MORNING. SOME TEMPORARY IFR VISIBILITY IN THIS PERIOD
IS CERTAINLY POSSIBLE.


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

* HIGH CONFIDENCE IN WIND TRENDS FOR A LARGE PORTION OF THE TAF
  INCLUDING WIND SHIFT TO WESTERLY TIMING. WITHIN 01Z TO 04Z
  SPEEDS MAY TEMPORARILY DROP QUITE A BIT AS LOW PASSES OVER...BUT
  CONFIDENCE IN THIS IS FAIRLY LOW.

* MEDIUM CONFIDENCE IN -RA AND MVFR TIMING.

* MEDIUM CONFIDENCE IN IFR TIMING AND DURATION.

* MEDIUM CONFIDENCE IN SNOW SHOWERS AROUND THE AREA TUE MORNING.


//OUTLOOK FOR ORD/MDW FOR 00Z WEDNESDAY-12Z MONDAY...UPDATED 18Z...

TUESDAY NIGHT...BECOMING VFR.

WEDNESDAY...VFR.

THURSDAY...SNOW DEVELOPING IN THE LATE AFTERNOON/EARLY EVENING
WITH FZDZ POSSIBLE LATER IN THE NIGHT. IFR CONDITIONS LIKELY BY
EVENING. STRONG EAST WINDS LIKELY.

FRIDAY...IFR/LIFR POSSIBLE WITH DZ/-RA.

SATURDAY...MVFR CIGS POSSIBLE.

SUNDAY...VFR

MTF

&&

MARINE...
318 AM CST

IT APPEARS THAT A SIGNIFICANT GALE EVENT WILL DEVELOP ACROSS THE
LAKE...ESPECIALLY ACROSS THE NORTHERN HALF OF THE LAKE ON TUESDAY
AFTERNOON AND EVENING.

STRONG SOUTHERLY WINDS...WITH GALES AROUND 35 KT WILL BE THE MAIN
STORY THROUGH THE DAY TODAY AS AN AREA OF LOW PRESSURE MOVES ACROSS
WISCONSIN AND DEEPENS LATER TODAY. THIS LOW WILL EVENTUALLY MOVE
ACROSS THE NORTHERN PORTION OF THE LAKE THIS EVENING. ALTHOUGH THERE
WILL LIKELY BE A SHORT 4 TO 6 HOUR PERIOD WHEN THE WINDS WILL
DIMINISH BELOW GALES...I OPTED TO MAKE IT SIMPLE AND EXTEND THE GALE
WARNING FOR THE OPEN WATERS THROUGH TUESDAY EVENING FOR THE SOUTH
HALF AND THROUGH WEDNESDAY MORNING FOR THE NORTH HALF. ONCE THE LOW
SHIFTS ACROSS THE LAKE LATER THIS EVENING A STRONG COLD FRONT WILL
SWING ACROSS THE SOUTHERN END OF THE LAKE. THE WINDS WILL QUICKLY
TURN WESTERLY AND INCREASE TO GALES OVERNIGHT TONIGHT. I EXPECT THE
STRONGEST WINDS UP TO AROUND 40 KT TO DEVELOP FIRST ACROSS THE
SOUTHERN END OF THE LAKE LATER TONIGHT. HOWEVER...ON TUESDAY...VERY
STRONG GALES UP TO 45 KT LOOK TO DEVELOP ACROSS THE NORTHERN HALF OF
THE LAKE IN RESPONSE TO STRONG PRESSURE RISES AND COLD AIR ADVECTION
ALONG THE BACK SIDE OF THE DEPARTING AREA OF LOW PRESSURE. THE LOW
WILL BE SLOW TO MOVE AWAY...THUS THE GALES TO 35 KT LOOK TO CONTINUE
INTO WEDNESDAY MORNING ACROSS THE FAR NORTHERN PORTION OF THE
LAKE.

FOR THE NEAR SHORES...STRONG SOUTHERLY WINDS WILL ALSO BE THE RULE
TODAY. IT IS POSSIBLE THAT WINDS COULD REACH GALE FORCE AT TIMES
THIS AFTERNOON...ESPECIALLY ACROSS THE INDIANA SHORES. HOWEVER...I
OPTED TO CONTINUE THE SMALL CRAFT FOR TODAY...AND WILL MENTION AN
OCCASIONAL GALE FORCE GUST. I DID ISSUE A GALE WARNING FOR LATE
THIS EVENING THROUGH THE DAY ON TUESDAY AS WESTERLY WINDS WILL
LIKELY REACH GALE FORCE IN THE WAKE OF THE COLD FRONTAL
PASSAGE.

THE OTHER CONCERN FOR BOTH THE OPEN WATERS AND THE NEAR SHORES WILL
BE HEAVY FREEZING SPRAY ON TUESDAY. THE STRONG WINDS AND HIGH WAVES
WILL COMBINE WITH VERY COLD AIR TEMPERATURES...IN THE TEENS TO
PRODUCE HEAVY ICING POTENTIAL ON SHIPS. THIS APPEARS TO BE A GOOD
BET. HOWEVER...I HELD ON TO THE WATCH...AS THE MAIN CONCERN INITIAL
WILL BE THE STRONG GALE FORCE WINDS. LATER FORECASTS WILL LIKELY
NEED TO UPGRADE TO A HEAVY FREEZING SPRAY WARNING.

KJB

&&

LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...NEARSHORE WATERS
     UNTIL 10 PM MONDAY.

     GALE WARNING...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-LMZ745...10 PM
     MONDAY TO 9 PM TUESDAY.

     HEAVY FREEZING SPRAY WATCH...LMZ080-LMZ261-LMZ362-LMZ364-LMZ366-
     LMZ563-LMZ565-LMZ567-LMZ669-LMZ671-LMZ673-LMZ675-LMZ740-
     LMZ741-LMZ742-LMZ743-LMZ744-LMZ745-LMZ777-LMZ779-LMZ868-
     LMZ870-LMZ872-LMZ874-LMZ876-LMZ878...NOON TUESDAY TO NOON
     WEDNESDAY.

     GALE WARNING...LMZ080-LMZ669-LMZ671-LMZ673-LMZ675-LMZ777-LMZ779-
     LMZ870-LMZ872-LMZ874-LMZ876-LMZ878 UNTIL 9 PM TUESDAY.

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

&&

$$