🚀 go-pugleaf

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1215 PM CST TUE DEC 18 2012

DISCUSSION...
1103 AM CST

MORNING UPDATE ON IMPENDING WINTER STORM...

GOING FORECAST LOOKS GOOD AND NO MIDDAY CHANGES PLANNED TO EXISTING
HEADLINES. TRACK GUIDANCE AMONGST NEW 12Z OPERATIONAL RUNS HAS A
FAIRLY TIGHT CLUSTERING...WITH CENTER OF THE INTENSE LOW PRESSURE
PASSING NEAR OR PROBABLY JUST SOUTH OF CHICAGO OVER EXTREME
NORTHWEST INDIANA BY MID DAY THURS..GIVE OR TAKE A FEW HOURS.
THERE IS SOME ENSEMBLE SPREAD...OWING TO THE MIDLEVEL WAVE THAT WILL
BE RESPONSIBLE FOR OUR STORM JUST NOW BEGINNING TO MAKE LANDFALL IN
CA. WITH A BETTER SAMPLING BY 00Z RAOB CYCLE THIS EVENING...SOME
ADDITIONAL TRACK ADJUSTMENTS ARE POSSIBLE IF NOT LIKELY LEADING UP
TO THE EVENT.

THE 12Z NAM CAME IN SLIGHTLY FASTER AND THUS QUICKER CHANGEOVER TO A
PERIOD OF POTENTIALLY HEAVY WET SNOW. THIS WOULD HAVE SIGNIFICANT
IMPLICATIONS ON THE THURS PM RUSH...AS THERE WOULD BE A LONGER
PERIOD OF FALLING SNOW COUPLED WITH STRONG GUSTY NORTHWEST WINDS AND
DROPPING TEMPERATURES. THE 12Z GFS...ON THE OTHER HAND...WOULD
LIKELY KEEP EVEN THE NW PORTION OF THE CWA AS MAINLY RAIN THROUGH
ABOUT MID DAY THURSDAY AND NOT CHANGE OVER TO SNOW IN CHICAGOLAND
UNTIL LATE AFTERNOON/EARLY EVENING. A 12Z GFS LIKE SCENARIO MAY LEAD
TO SLIGHTLY LESS HAZARDOUS ROAD CONDITIONS FOR THE THURS PM
COMMUTE...ESPECIALLY FOR CHICAGO. NONETHELESS...IT HAS A MORE
INTENSE WIND FIELD WITH POTENTIAL FOR GUSTS OVER 50 MPH WHICH WOULD
MAKE FOR A WIND WHIPPED WET SNOW ONCE CHANGEOVER OCCURS AND STILL
LIKELY RESULT IN SIGNIFICANT TRAVEL IMPACTS.

GIVEN THAT THE DIFFERENCES IN GUIDANCE THAT ARE WITHIN THE REALM OF
MODEL NOISE...THEY WILL NOT HAVE A BIG INFLUENCE ON SNOWFALL TOTALS
WITH THIS SYSTEM...CONFIDENCE ON SNOWFALL AMOUNTS WILL REMAIN LOW TO
MEDIUM. HOWEVER I CONTINUE TO FAVOR THE NORTHWEST CWA WHERE THE
WATCH IS IN EFFECT FOR THE HIGHEST TOTALS AND BIGGEST COMBINED
WIND/SNOW IMPACT FROM THIS STORM. WILL CONTINUE TO EXAMINE THE NEED
FOR ANY EXPANSION OF THE GOING HEADLINES.

RC/IZZI

//PREV DISCUSSION...
402 AM CST

WINTER STORM WATCH NOW IN EFFECT FOR NORTH CENTRAL IL THURSDAY AND
THURSDAY EVENING.

FORECAST FOCUS LIES PRIMARILY WITH TWO PRECIPITATION PRODUCING
SYSTEMS TO AFFECT OUR AREA THROUGH THURSDAY. FIRST WILL BE A LIGHT
SNOW/RAIN PRODUCER TODAY MAINLY NORTH OF THE I-80 CORRIDOR...AND THE
OTHER BEING THE POWERFUL WINTER STORM WHICH WILL BRING RAIN/SNOW AND
50 MPH WINDS TO THE AREA BY THURSDAY. HAVE ELECTED TO ISSUE A WINTER
STORM WATCH FOR PORTIONS OF NORTH CENTRAL IL FOR THURSDAY AND
THURSDAY EVENING FOR A COMBINATION OF SNOW AND BLOWING SNOW
CONDITIONS.

EARLY MORNING WATER VAPOR IMAGERY CLEARLY SHOWS TODAYS WEATHER MAKER
AS A WELL-DEFINED SHORT WAVE OVER SOUTH DAKOTA AND NEBRASKA. SURFACE
LOW PRESSURE REFLECTION WAS NOTED OVER NORTHEAST NEBRASKA...WITH AN
IMPRESSIVE AREA OF SURFACE PRESSURE FALLS OVER WESTERN IOWA. THIS
SHORT WAVE AND SURFACE LOW COMBINATION WILL CONTINUE TO RACE EAST
INTO NORTHERN IL THIS AFTERNOON. THOUGH THE VORT IS PROGGED TO
BECOME MORE SHEARED WITH TIME...HIGH-RES GUIDANCE INDICATES
INCREASING FRONTOGENETIC FORCING FROM NORTHEAST IA EASTWARD ACROSS
NORTHERN IL/FAR SOUTHERN WI REGION DURING THE LATE MORNING AND
AFTERNOON HOURS AND APPEARS SUPPORTIVE OF A PERIOD OF SOME LIGHT
PRECIP ACROSS FAR NORTHERN IL/NORTHWEST IN. FORECAST SOUNDINGS
INDICATE MARGINAL THERMAL PROFILES...LIKELY INITIALLY LIGHT SNOW BUT
LIKELY BEING VERY WET OR MIXING WITH RAIN ESPECIALLY SOUTH OF I-88.
LITTLE ACCUMULATION ANTICIPATED...THOUGH THE NORTHERN TIER OF IL
COUNTIES COULD PERHAPS PICK UP A COUPLE TENTHS OF AN INCH OF WET
SNOW WHERE STRONGER BANDING OCCURS. SHORT WAVE AND SURFACE LOW
CONTINUE TO TRACK QUICKLY OFF TO THE EAST EARLY THIS EVENING...WITH
ANY LINGERING PRECIPITATION ENDING RAPIDLY FROM WEST TO EAST.

TURNING ATTENTION TO THE MAJOR WINTER STORM SYSTEM WEDNESDAY NIGHT
THROUGH THURSDAY NIGHT...MODELS CONTINUE TO MERGE ON THE TRACK OF
THE SURFACE LOW LIFTING NORTHEAST ACROSS IL AND PASSING VERY CLOSE
TO CHICAGO BY 18Z THURSDAY. 00Z ECMWF HAS COME INTO LINE WITH
GEM/GFS/UKMET SOLUTIONS...WHICH INCREASES CONFIDENCE IN THIS SET OF
SOLUTIONS. WITH THIS STORM TRACK...THERMAL PROFILES ARE EXPECTED TO
BE WARM ENOUGH TO SUPPORT RAIN ACROSS THE ENTIRE CWA FROM WEDNESDAY
NIGHT INTO EARLY THURSDAY MORNING...WITH SOME POTENTIAL FOR A
RAIN/SNOW MIX OVER FAR NORTHWEST IL AT ONSET LATE WEDNESDAY
AFTERNOON. THICKNESS VALUES THEN RAPIDLY DIMINISH FROM WEST TO EAST
THURSDAY...INDICATING A CHANGE OVER SNOW ACROSS NORTH CENTRAL IL
DURING THE MORNING...AND ACROSS THE REMAINDER OF THE AREA DURING THE
AFTERNOON HOURS. SNOWFALL ACCUMULATION OF SEVERAL INCHES IS LIKELY
ACROSS NORTH CENTRAL IL WHERE THE DEFORMATION AXIS RESIDES...WITH
AMOUNTS LIKELY TO DIMINISH FAIRLY QUICKLY TO THE SOUTHEAST. CURRENT
EXPECTATIONS ARE THAT SNOWFALL MAY APPROACH 6 INCHES ACROSS THE FAR
NORTHWEST COUNTIES OF THE CWA...TAPERING TO PERHAPS 1-2 INCHES
ACROSS THE SOUTHEAST.

MOST IMPORTANTLY HOWEVER...IS THAT THE SURFACE LOW WILL BE DEEPENING
RAPIDLY THURSDAY AFTERNOON AND EVENING AS IT MOVES ACROSS THE AREA
AND INTO CENTRAL LOWER MICHIGAN THURSDAY NIGHT. FORECAST SOUNDINGS
CONTINUE TO DEPICT WEST-NORTHWEST WINDS GUSTING TO 45-50 MPH BY
THURSDAY AFTERNOON AND CONTINUING INTO THE EVENING BEFORE GRADUALLY
DIMINISHING LATE THURSDAY NIGHT. REGARDLESS OF SNOWFALL AMOUNTS...
THIS COMBINATION OF WIND AND SNOW WILL LIKELY CAUSE SIGNIFICANT
BLOWING AND DRIFTING...AS WELL AS REDUCTIONS IN VISIBILITY.
CONDITIONS WILL LIKELY BE PARTICULARLY BAD ACROSS NORTH CENTRAL
IL...WHERE GREATER SNOWFALL RATES AND ACCUMULATIONS AS WELL AS
HIGHER SNOW/LIQUID RATIOS WILL COMPOUND THE BLOWING/DRIFTING. NOT
OUT OF THE QUESTION THAT SOME OF THESE AREAS COULD SEE NEAR-BLIZZARD
CONDITIONS FOR A TIME LATE THURSDAY MORNING THROUGH THURSDAY
AFTERNOON. FOR THIS REASON...A WINTER STORM WATCH IS BEING ISSUED
FOR AREAS FROM MCHENRY AND DEKALB COUNTIES WESTWARD.

SNOW IS EXPECTED TO TAPER OFF QUICKLY FROM WEST TO EAST THURSDAY
EVENING AS THE STORM LIFTS AWAY FROM THE AREA. WEST-NORTHWEST WIND
FIELDS SHOULD FOCUS POST-STORM LAKE EFFECT SNOW SHOWERS LARGELY EAST
OF PORTER COUNTY. DRY BUT COLD CONDITIONS ARE THEN EXPECTED THROUGH
THE COMING WEEKEND...WITH HIGHS GENERALLY IN THE UPPER 20S/LOW 30S
AND LOWS GENERALLY IN THE TEENS/LOW 20S.

RATZER

&&

AVIATION...

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

* PERSISTENCE OF MVFR CIGS THROUGH EARLY AFTERNOON.
* MVFR CIGS AND POSSIBLY IFR CIGS DEVELOPING AGAIN THIS
  EVENING/TONIGHT.
* SSE-S WINDS SHIFTING TO SSW EARLY EVENING THEN W-NW DURING THE
  LATER EVENING AND LATE NIGHT.
* FOG POTENTIAL OVERNIGHT INTO WEDNESDAY MORNING.

TRS

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

BAND OF PRECIPITATION...MAINLY LIGHT SNOW...EXTENDS FROM THE
EASTERN MN-IA BORDER E INTO SW WI THEN SE ACROSS SE WI...EXTREME
NE IL AND ACROSS SOUTHERN LAKE MI TO NW IN. ONLY SOME VERY LIGHT
SNOWFALL REPORTED WITH JUST TRACE AMOUNTS. AS WE PROGRESS THROUGH
EARLY AFTERNOON THIS BAND WHICH IS ALIGNED WITH THE BAROCLINIC
ZONE WILL CONTINUE TO PUSH NE AND E OF THE LOCAL AREA AS THE
SURFACE AND 850 HPA LOWS CONTINUE TO MOVE E ACROSS FAR NORTHERN
IL DURING THE AFTERNOON AND EARLY EVENING SHIFTING BEST WARM AIR
ADVECTION...ISENTROPIC UPGLIDE AND OMEGA OFF TO THE NE. OUTSIDE
OF A FEW LEFTOVER FLURRIES IN THE EARLY AFTERNOON NO ADDITIONAL
PRECIPITATION ANTICIPATED REST OF THE DAY AND THROUGH THE NIGHT.

SUBSIDENCE IN WAKE OF THE SHORT WAVE TROUGH MOVING E ACROSS IA IS
RESULTING IN CLEARING OF CLOUDS. THERE IS AN EXTENSIVE AREA OF
IFR CONDITIONS DUE TO BOTH LOW CEILINGS AND RESTRICTED
VISIBILITIES FROM E CENTRAL AND SW IA NW TO THE EASTERN DAKOTAS.
TRACK OF LOW LEVEL CYCLONIC CIRCULATION SUGGESTS THESE CLOUDS
WILL SPREAD SE ACROSS AND THEN E ACROSS W CENTRAL AND NW IL LATE
AFTERNOON AND EARLY THIS EVE...CONTINUING E TO THE LOCAL AREA
DURING THE LATER EVENING AND OVERNIGHT. DO EXPECT THEN TO LIFT
ENOUGH TO RISE INTO LOW END MVFR RANGE AS LOW TO MID 30S DEW
POINTS WILL BE ADVECTING INTO A SOMEWHAT WARMER AREA.

WEAK HIGH PRESSURE DROPPING ACROSS THE AREA BEHIND THE DEPARTING
LOW WILL RESULT IN LIGHT AND VARIABLE WINDS FROM LATE TONIGHT AND
INTO WED MORNING. MOIST LOW LEVELS AND DECREASING DEW POINT
DEPRESSIONS DURING THE OVERNIGHT THROUGH EARLY MORNING SUGGEST
WIDESPREAD BR.

TRS

$$