🚀 go-pugleaf

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1149 AM CST WED FEB 6 2013

DISCUSSION...
THURSDAY EVENING...
945 AM CST

BASED ON OUTPUT FROM 06Z RUN OF GFS/GFS-ENSEMBLE AND GEM PLUS THE
12Z RUN OF THE NAM...MODELS PROG THE TRACK OF THE SURFACE LOW
OVERNIGHT AND THU TO BE SOMEWHERE BETWEEN 100 AND 200 MI FURTHER
S. BASED ON THESE MORE SOUTHERN TRACKS WARM AIR ADVECTION WOULD
NOT OCCUR AS FAR N ACROSS NORTHERN IL AND INTO SOUTHERN WI. THIS
RESULTS IN SIGNIFICANT CHANGES IN PRECIP TYPES ACROSS THE
FORECAST AREA AS WELL AS POTENTIAL SNOWFALL ACCUMULATIONS.

CURRENT THINKING IS THAT DURING THE OVERNIGHT WE WILL SEE MAINLY
SNOW TRANSITIONING TO SNOW...SLEET AND FREEZING RAIN NORTH AND A
FREEZING RAIN AND SLEET MIX S. THU MORNING THE S SHOULD WARM ENOUGH
FOR JUST RAIN...WHILE THE NORTH IS EXPECTED TO HAVE SNOW AND SLEET
AND A CHANCE OF FREEZING RAIN. THE NORTH WILL BE SNOW AND SLEET IN
THE AFTERNOON WHILE THE I-80 CORRIDOR MAY CHANGE TO A SNOW AND SLEET
MIX WHILE AREAS TO THE SOUTH REMAIN JUST RAIN.

TOTAL SNOWFALL ACCUMULATION FROM OVERNIGHT TONIGHT THROUGH THU
EVENING COULD BE IN THE 3 TO 6 INCH RANGE N OF I-88...2 TO 4
INCHES BETWEEN I-88 AND I-80...1 TO 2 INCHES S OF I-80 AND 1 TO 2
INCHES SOUTH OF THE KANKAKEE AND ILLINOIS RIVERS.

THESE ADJUSTMENTS ARE PRELIMINARY AS EXAMINATION AND EVALUATION
OF MORE NEW MODEL OUTPUT AND GUIDANCE AS IT COMES IN THIS MORNING
IS STILL NEEDED TO ATTEMPT TO NARROW THE DIFFERENCES BETWEEN
POTENTIAL SURFACE LOW TRACKS AND FURTHER REFINEMENT OF TIMING...
LOCATION AND AMOUNTS OF THE VARIOUS PRECIP TYPES.

A WINTER WINTER STORM WATCH WILL BE ISSUED FOR LAKE AND MCHENRY
COUNTY WHERE 6+ INCHES OF SNOW ARE POSSIBLE. A WINTER WEATHER
ADVISORY MAY BE NEEDED FOR THE AREAS FROM AROUND I-88 AND I-80.

TRS

//PREV DISCUSSION...
338 AM CST

ANTI-CYCLONIC FLOW HAS PUSHED OVERHEAD AND ALLOWED FOR A STEADY
EROSION OF THE CLOUDS ACROSS THE CWFA EARLY THIS MORNING. IT WAS A
RACE TO SEE IF THE DRY AIR WOULD ARRIVE PRIOR TO THE CLEARING...AND
IT APPEARS THE DRY AIR HAS WON. A WEAK BOUNDARY CONTINUES TO PUSH
SOUTH ACROSS THE CWFA STRETCHING FROM STERLING TO KANKAKEE...AS DEW
PTS EARLY THIS MORNING HAVE FALLEN INTO THE LOWER TEENS ACROSS THE
CENTRAL/NORTHERN SECTIONS OF THE CWFA BEHIND THE BOUNDARY. IN
ADDITION WINDS HAVE TURNED NORTH/NORTHEAST. WITH A THINNING CLOUD
COVER OVERHEAD...TEMPS WILL QUICKLY RADIATE PRIOR TO SUNRISE INTO
THE LOW/MID TEENS. A FEW LOCATIONS COULD DIP INTO THE SINGLE DIGITS
ACROSS FAR NORTHCENTRAL ILLINOIS.

THEN FOR THE REMAINDER OF THE DAY PARTLY CLOUDY SKIES WILL ALLOW FOR
A BREAK IN THE PLETHORA OF CLIPPERS THAT HAVE ZEROED IN ON NORTHERN
IL/NORTHWEST IN THE PAST SEVERAL DAYS. THERMAL TROUGH ALOFT WILL
LINGER THROUGH THIS AFTN...WHICH WILL LIMIT HOW WARM SFC TEMPS
BECOME TODAY. 2M TEMP PLUMES CONTINUE TO POINT TOWARDS TEMPS WARMING
INTO THE LOW/MID 30S. A FEW GUIDANCE MEMBERS SUGGEST ALONG AND SOUTH
OF A PONTIAC TO FOWLER LINE THAT TEMPS COULD PUSH TO THE UPR 30S
THIS AFTN. WITH THE LACK OF SNOW FURTHER SOUTH...AND WINDS TURNING
SOUTH THIS AFTN...THIS MAY BE FEASIBLE.

CONFIDENCE...HIGH.

TONIGHT THROUGH THURSDAY...

WITH SFC RIDGE STEADILY DRIFTING EAST...HIGH/MID LVL MOISTURE WILL
STEADILY ADVECT EAST FROM A DEVELOPING SHORTWAVE ACROSS THE
CENTRAL/SOUTHERN PLAINS. 500MB FLOW REMAINS FLAT...HOWEVER A BUCKLE
IS POISED TO DEVELOP LATE TONIGHT ACROSS THE CENTRAL PLAINS
ASSOCIATED WITH THE NEXT SYSTEM. 850MB THERMAL TROUGH QUICKLY LIFTS
NORTHEAST AS TEMPS ALOFT ARE PROGGED TO WARM TO ARND 0 TO 2 DEG C BY
03Z...HOWEVER A WEDGE OF DRY AIR APPEARS TO LINGER BASED OFF OF
LATEST GUIDANCE SOUNDINGS. THUS ANY PRECIP WILL MOST LIKELY OCCUR
AFTER MIDNIGHT. WITH THE THICKENING CLOUD COVER THIS EVENING...IT IS
UNCLEAR HOW MUCH COOLING THE SFC WILL SEE. THUS HAVE TEMPS HOLDING
IN THE UPR 20S TO ARND 30 DEG. A NICE INVERSION SHOULD SET THE STAGE
IN THE NEAR SFC ENVIRONMENT FOR P-TYPE CONCERNS EARLY THUR MORNING.

AS MOISTURE ARRIVES ACROSS THE WEST AFT MIDNIGHT...SOUNDINGS
INDICATE A WARM WEDGE ALOFT OF 1 TO 2 DEG WHILE THE SFC WILL BE
SUB-FREEZING. THIS MOST LIKELY WILL PARTIALLY MELT THE
DENDRITES...RESULTING IN SLEET/SNOW AT THE ONSET. THEN AS WARM AIR
CONTINUES TO ADVECT NORTH THE ENVIRONMENT SHOULD PUSH THIS OVER FROM
A PARTIAL MELT TO A FULL MELT...THUS POINTING TOWARDS FREEZING
RAIN/ICE FROM THE 09Z TO 15Z TIMEFRAME. SOLUTIONS CONTINUE TO SHOW
SOME DIFFERENCES WITH RESPECT TO THE PLACEMENT OF THE SFC LOW/850
LOW ALONG WITH HOW FAR NORTH THE WARM AIR WILL SURGE. THIS POSES
EVEN GREATER CHALLENGES IN NAILING DOWN THE P-TYPE. SREF/GEFS
MEMBERS HAVE A 60-80% PROB THAT IN THIS 9-15Z TIMEFRAME THE PRECIP
WILL FALL AS FZRA...WITH ABOUT A 50% PROB OF 0.1" OF ICE ACCUM. AT
THIS TIME HAVE OPTED TO HOLD OFF ON ANY HEADLINES.

THE SNOW PLUME PROBS ARE MUCH LOWER ACROSS THE CENTRAL SECTIONS OF
THE CWFA AT 20-30%. THE FURTHER NORTH ALONG THE ILLINOIS/WISCONSIN
STATELINE THESE P-TYPE PROBS BECOME INVERTED...AND IT APPEARS STRONG
FGEN WILL HOVER IN THIS AREA INTO SOUTHERN WISC. THUS A SHARP
TRANSITION LINE IS EXPECTED. TEMPS WILL SLOWLY WARM AT THE
SFC...THUS A PERIOD OF FZRA WILL MOST LIKELY CONTINUE THRU DAYBREAK
INTO MID-MORNING ACROSS THE HEART OF THE CWFA. THEN TEMPS SHOULD
PUSH INTO THE MID 30S. ACROSS THE SOUTHERN SECTIONS OF THE CWFA
TEMPS COULD PUSH TO 40-42 DEG BY EARLY AFTN.

PWAT VALUES WITH THIS SYSTEM ALSO SURGE TO JUST UNDER 1" WHICH IS
NEARLY 3-4 SIGMA ABOVE CLIMO. THIS COUPLED WITH STRONG MERIDIONAL
FLOW FROM SOUTH TO NORTH...WILL USHER IN A DECENT SHOT OF QPF.
ENSEMBLE MEMBERS ARE SIMILAR TO OPERATIONAL GFS/EC IN PAINTING ABOUT
A 80% PROB OF 0.4" ACROSS THE CENTRAL/NORTHERN CWFA THUR. FURTHER
SOUTH/SOUTHEAST SECTIONS WILL SEE LIGHTER PRECIP TOTALS.

500MB TROUGH AXIS STEADILY PIVOTS EAST AND ARRIVES ACROSS THE REGION
THUR AFTN...WITH THE FRONTAL PASSAGE EXPECTED TO OCCUR LATE THUR
AFTN. THIS WILL BRING COOLER AIR BACK INTO THE REGION THUR NIGHT AND
ALLOW ANY PRECIP THAT LINGERS TO TRANSITION BACK TO SNOW. TEMPS THUR
NIGHT WILL FALL INTO THE LOW/MID 20S...TO POSSIBLY THE UPR 20S
ACROSS THE FAR SOUTHEAST SECTIONS OF THE CWFA.

CONFIDENCE IN TEMPERATURES...HIGH.

CONFIDENCE IN FREEZING RAIN...MEDIUM/HIGH.

FRIDAY THROUGH TUESDAY...

500MB TROUGH AXIS WILL CONTINUE TO PIVOT EAST EARLY FRI...WITH A
MID-LVL RIDGE PROGGED TO START DEVELOPING ACROSS THE CENTRAL CONUS.
FURTHER UPSTREAM A TROUGH WILL CONTINUE TO DIG INTO THE FOUR-CORNERS
REGION WITH YET ANOTHER RIDGE OFF THE PAC-NW COAST.

THIS WILL HELP KICK WARMER AIR NORTH FROM THE WESTERN GULF FOR THE
UPCOMING WEEKEND THROUGH MONDAY. TEMPS FOR THE WEEKEND COULD BE
FLIRTING WITH 40 DEG...POSSIBLY THE MID/UPR 40S FOR AREAS ALONG AND
SOUTH OF A LA SALLE TO KANKAKEE LINE BY SUN. THE WILDCARD WILL BE A
POTENTIAL RAIN EVENT AS GUIDANCE HAS CONTINUED TO POINT TOWARDS A
ROBUST SYSTEM DEVELOPING SUN ACROSS THE SOUTHERN PLAINS...ADVECTING
WESTERN GULF MOISTURE NORTH THROUGH THE OHIO VALLEY INTO THE GREAT
LAKES. AT THIS TIME HAVE OPTED TO KEEP MENTION OF CONVECTION OUT OF
THE FORECAST FOR SUN/MON.

ENSEMBLES THEN SUGGEST THE RIDGING WILL BEGIN TO RELAX FOR EARLY
NEXT WEEK AS THE TROUGH OVER THE FOUR-CORNERS REGION SLOWLY PIVOTS
EAST TOWARDS THE SOUTHERN ROCKIES. THIS COUPLED WITH SOME LOWERING
OF THE HEIGHTS ACROSS THE GREAT LAKES...COULD ALLOW FOR TEMPS TO
RETURN TO MORE SEASONAL CONDITIONS TOWARDS THE MIDDLE OF NEXT WEEK.

CONFIDENCE...MEDIUM/HIGH.

BEACHLER

&&

AVIATION...

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

* SOUTHERLY WINDS THIS AFTERNOON AND EVENING.

* MIXED PRECIPITATION DEVELOPING TOWARDS DAWN THURSDAY MORNING.
  PRECIPITATION TRANSITIONING TO MOSTLY SNOW AT ORD AND MDW BY MID
  DAY.

* CEILING/VISIBILITY DROPPING TO MVFR WHEN PRECIPITATION BEGINS
  AND THEN TO IFR DURING THE MORNING HOURS.

PAW

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

HIGH PRESSURE CENTERED OVER NORTHERN IL IS GIVING US A BREAK TODAY
FROM THE RECENT STRING OF WINTER WEATHER. BUT THE BREAK WILL BE
SHORT AS THE NEXT SYSTEM ARRIVES LATE TONIGHT INTO THURSDAY. THIS
MORNINGS MODEL RUNS TAKE THE CENTER OF THE SYSTEM MUCH FURTHER TO
THE SOUTH...WHICH MEANS THE ASSOCIATED COLD AIR WILL ALSO BE
FURTHER TO THE SOUTH THAN PREVIOUSLY THOUGHT. THE TRANSITION AREAS
FROM SNOW TO FREEZING RAIN AND SLEET AND FROM THIS MIXED
PRECIPITATION TO RAIN ARE EXPECTED TO BE ACROSS NORTHERN IL AND
NORTHWEST IN...AND ARE LIKELY TO BE ADJUSTED AS THE SYSTEM GETS
CLOSER. CONDITIONS WILL DETERIORATE WITH THE PRECIPITATION...WITH
IFR LIKELY BY MID TO LATE MORNING.

PAW

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

* HIGH CONFIDENCE IN ALL FORECAST ELEMENTS FOR TODAY.

* LOW TO MEDIUM CONFIDENCE IN PRECIPITATION TYPE AND TIMING FOR
  THURSDAY.

* MEDIUM COMFIDENCE IN MVFR/IFR CONDITIONS.

PAW

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

THURSDAY NIGHT...LINGERING FLURRIES OR SNOW SHOWERS WITH MVFR
CONDITIONS EARLY.

FRIDAY...VFR...WX NIL.

SATURDAY...VFR...WX NIL EARLY.  CHANCE -SHSN/MVFR LATE.

SUNDAY...RAIN LIKELY. MVFR LIKELY...IFR POSSIBLE.

MONDAY...CHANCE OF RAIN OR SNOW. MVFR POSSIBLE.

TUESDAY...VFR...WX NIL.

KREIN

&&

MARINE...
310 AM CST

LOW PRESSURE OVER SOUTHERN QUEBEC WILL MOVE NORTHEAST TO NOVA
SCOTIA THIS MORNING WILE HIGH PRESSURE OVER WISCONSIN AND
ILLINOIS MOVES EAST ACROSS THE GREAT LAKES TODAY AND STRENGTHENS
AS IT MOVES EAST TO SOUTHERN QUEBEC BY THURSDAY. MEANWHILE...A
WEAK TROUGH OF LOW PRESSURE WILL DEVELOP OVER THE CENTRAL PLAINS
WEDNESDAY AND TRACK TO THE UPPER MIDWEST ON THURSDAY. A WEAK COLD
FRONT ASSOCIATED WITH THIS TROUGH WILL CROSS LAKE MICHIGAN
THURSDAY EVENING. BY FRIDAY EVENING...THIS LOW WILL MERGE WITH
DEEPENING LOW PRESSURE OFF THE MID ATLANTIC COAST...WHILE A RIDGE
OF HIGH PRESSURE EXTENDS FROM QUEBEC...THROUGH THE UPPER GREAT
LAKES...TO THE MIDDLE MISSISSIPPI VALLEY.

KREIN

&&

LOT WATCHES/WARNINGS/ADVISORIES...
IL...WINTER STORM WATCH...ILZ005-ILZ006...6 AM THURSDAY TO 9 PM
     THURSDAY.

IN...NONE.
LM...NONE.
&&

$$