FXUS61 KCTP 090000
AFDCTP

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE STATE COLLEGE PA
700 PM EST TUE DEC 8 2009

.SYNOPSIS...
STRENGTHENING LOW PRESSURE WILL TRACK NORTHEAST FROM THE SOUTHERN
HIGH PLAINS LATE THIS AFTERNOON...REACHING THE UPPER GREAT LAKES
REGION ON WEDNESDAY. THIS SYSTEM WILL BRING SEVERAL INCHES OF
SNOW...AND A WINTRY MIX OF PRECIPITATION TO MUCH OF CENTRAL
PENNSYLVANIA TONIGHT AND EARLY WEDNESDAY...FOLLOWED BY COLDER AND
WINDY CONDITIONS WEDNESDAY NIGHT AND THURSDAY. HIGH PRESSURE WILL
BUILD OVER THE STATE FRIDAY AND SATURDAY. A STORM SYSTEM MAY EDGE
NORTHEAST...JUST OFF THE CAROLINA AND MID ATLANTIC COAST LATE
SATURDAY OR EARLY SUNDAY...AND BRING SOME LIGHT SNOW TO THE REGION.

&&

.NEAR TERM /UNTIL 11 PM THIS EVENING/...
CLOUDS WILL THICKEN UP FROM THE SOUTHWEST LATE THIS AFTERNOON AND
EARLY THIS EVENING...AS AN EXTENSIVE AREA OF WARM ADVECTION PRECIP
HEADS OUR WAY. INITIALLY DEEP...SUB-FREEZING AIR WILL BE INTACT
ACROSS THE REGION...BUT WILL BE ERODED LATE TONIGHT BY A LAYER OF
MILDER AIR ALOFT SURGING NWD VIA A 60-70 KT MEAN 850-700 MB WIND
MAX.

PRECIP IN THE FORM OF SNOW/SLEET WILL DEVELOP ACROSS THE LAUREL
HIGHLANDS BETWEEN 23Z AND 01Z AND QUICKLY OVERSPREAD THE REMAINDER
OF THE REGION BEFORE MIDNIGHT. SFC TEMPS WILL SLIP 3-6 DEG F THIS
EVENING AS THE PRECIP BEGINS TO FALL INTO THE DRY BOUNDARY LAYER
AIR WITH WET BULBS IN THE MID 20S TO LOWER 30S ACROSS THE REGION
(AT 20Z). EXPECT MAINLY SNOW IN ALL AREAS (EVEN ACROSS THE LOWER
SUSQ VALLEY) RIGHT AT THE ONSET...AND A GOOD SEVERAL HOURS ACROSS
THE CENTRAL ZONES THROUGH 07Z WED.

&&

.SHORT TERM /11 PM THIS EVENING THROUGH 6 PM WEDNESDAY/...
A SEVERAL HOUR PERIOD OF VERY STRONG ISENTROPIC LIFT WILL OCCUR
JUST NORTH OF THE 850 MB WARM FRONT LATER TONIGHT...BRINGING
VARYING INTENSITY OF THE SNOW AND EVENTUALLY MIXED PRECIP AND
FREEZING RAIN/RAIN TWD DAYBREAK.

CURRENT CONFIGURATION OF THE WINTER STORM WARNING FOR 4 COUNTIES
(2 IN THE SW...AND 2 IN THE NW MTNS) CONTINUES TO LOOK IN GOOD
SHAPE BASED ON 12Z OPERATIONAL MODEL GUIDANCE AND THE LAST FEW
SREF RUNS. 

12Z DATA (GFS AND NAM) HINT AT THE POTENTIAL FOR LONGER DURATION
LOW/MID LVL SUB-FREEZING AIR ACROSS THE NRN MTNS...SOUTH AND EAST
TO THE MID SUSQ VALLEY...AND THEREFORE THE POSSIBILITY OF SLIGHTLY
HEAVIER SNOW THAN WE CURRENTLY HAVE PAINTED IN OUR GRIDS.

UP TO ONE HALF INCH OF ICE APPEARS LIKELY ACROSS THE HIGHER
TERRAIN OF CAMBRIA AND SOMERSET COUNTIES (WITH THE 09Z SREF...AND
12Z NAM/GFS SHOWING AROUND 0.50" OF ICE INVOF OF KJST)....WHILE 4
TO 6 INCHES OF SNOW TOPPED BY AROUND 0.25 OF AN INCH OF ICE SHOULD
OCCUR ACROSS MCKEAN AND POTTER COUNTIES.

AGAIN...WE SEE VERY GOOD AGREEMENT BETWEEN THE OVERNIGHT AND 12Z 
U.S. OPERATIONAL MODELS...GEM...AND EC WITH RESPECT TO 12-18 HOUR
SNOW TOTALS OF 4 TO 7 INCHES ACROSS THE NRN MTNS...WITH 2 TO 4
INCHES ACROSS THE CENTRAL SLICE OF COUNTIES...NEAR AND JUST EAST
OF THE ALLEGHENY FRONT. LITTLE CHANGE IS SEEN ON THE 12Z NAM AND
GFS RUNS.

FOR FINE TUNING PURPOSES...WILL CONTINUE TO LEAN MORE TWD THE 12Z
NAM FOR PRECIP TYPE...TIMING AND AMOUNT...COMPARED TO THE GFS
WHICH PAINTS A MUCH LARGER AREA OF HEAVY ICING ACROSS THE CENTRAL
RIDGE AND VALLEY REGION.

AREAS SOUTH AND EAST OF HARRISBURG WILL LIKELY SEE JUST AN INCH
OR TWO OF COMBINED SNOW AND SLEET...WHERE ENS PTYPES/AND JUST
ABOVE FREEZING SURFACE TEMPS DON/T INDICATE MUCH CHC OF FZRA.

MIN TEMPS IN THE U20S/L30S LIKELY TO BE REALIZED SHORTLY AFTER THE 
ONSET OF PRECIP AROUND MIDNIGHT...THEN STEADY OR VERY SLOWLY
RISING READINGS THEREAFTER UNDER WARM PATTERN ALOFT.

WARM AIR ADVECTION WILL CONTINUE WED AS A LLVL 850MB JET IS PROGGED 
ARND 40-50KTS PUSHES TEMPS NEAR 40 DEG...WHICH WILL PUSH AN 
REMAINING FZRA OVER TO RA ACROSS THE CWFA. CONFIDENCE REMAINS HIGH 
THAT A DRY WEDGE ALOFT LIFTS INTO CENTRAL PA MIDDAY WED...AND
COULD CUTOFF PRECIP FOR MUCH OF THE AFTN. 500MB TROUGH AXIS
QUICKLY PIVOTS THRU MIDDAY WED...HOWEVER IT DOESN/T APPEAR THERMAL
RIDGE DEPARTS UNTIL LATE WED. 

TRAILING COLD FRONT WILL PUSH THRU THE REGION LATE WED AFTERNOON
AND WED EVE...WITH A QUICK INCREASE IN WESTERLY WINDS. HAVE POSTED
A HIGH WIND WATCH FOR THE WRN PENN MTNS WHERE BUFKIT PROFILES SHOW
THE INCREASINGLY DEEP MIXED LAYER TAPPING 50-60 KT WINDS FROM JUST
A FEW THOUSAND FEET ABOVE THE SURFACE. SOME CONCERN THAT CURRENT
MODEL 3 HOURLY PRESSURE CHANGES OF 3-5 MB DON'T SUPPORT/HIGHLIGHT
THIS STRONG WIND POTENTIAL. IT APPEARS THAT 40-45 MPH WIND GUSTS
WILL OCCUR ACROSS THE CENTRAL AND ERN PORTIONS OF THE CWA WED
NIGHT...POSSIBLY WIND ADVISORY CRITERIA AROUND 40 KTS.  

ATTM EXPECT UPSLOPE SN/LAKE EFFECT SN NOT TO OCCUR UNTIL LATE
WED...HOWEVER RESONANCE TIME OVER THE LAKES WILL BE MINIMIZED AT
ONSET PROVIDING A CELLULAR LOOK TO PRECIP FOR NW MTNS. MODELS SHOW
THE MAIN FOCUS FOR AN INTENSE/SINGLE LES BAND TO HUG THE NW PENN
LAKE SHORE AND HEAD INTO SWRN NEW YORK WED NIGHT.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH TUESDAY/...
ACTIVE WEATHER PATTERN CONTINUES THROUGH THE PERIOD.

VERY INTENSE W/NWSTRLY FLOW WILL BE IN PLACE ACROSS THE MID-
ATLANTIC REGION THUR/FRI/SAT. STRONG UPPER JET OVER 850MB WINDS
OF 40-50KTS WILL BRING WINDY/BLUSTERY CONDITIONS TO ESPECIALLY
THE HIGHER TERRAIN WED NIGHT INTO THU...AND HIGH WIND WATCH
ALREADY IN EFFECT. WINDS DIMINISH AS THU WEARS ON...BUT COLDER AIR
BEGINS TO SLIP IN FROM THE WEST MAKING THU NIGHT INTO FRI THE
COLDEST PERIOD. LOWS THU NIGHT PROGGED TO RANGE FROM 10-20
DEGREES...AND COULD BE EVEN COLDER IN NORTHERN VALLEYS IF WINDS
DECOUPLE AND SKIES CLEAR. WESTERLY FLOW WILL BRING LAKE EFFECT
SNOW SHOWERS MAINLY TO WARREN CO /FLOW WILL DIRECT HEAVIEST SNOW
INTO W NY NORTH OF PA/ WHERE SEVERAL INCHES COULD ACCUMULATE BY
FRI.

SFC RIDGE SLIPS IN FROM THE SOUTH SAT/SUN...BUT LOOKS LIKE IT WILL
BREAK DOWN QUICKLY UNDER PROGRESSIVE FLOW. SHORTWAVE SLIDES NE
FROM GULF OF MEXICO STATES AND COULD BRING SOME LIGHT SNOW TO PA
SAT NIGHT/SUN. TEMPS REMAIN COOL THRU THE UPCOMING WEEKEND.

PATTERN TRENDS BACK TO A MORE WINTERLIKE NW FLOW WITH CONTINUED
CLIMATOLOGICAL CHANCE FOR SNOW SHOWERS IN THE HIGHER TERRAIN MON
INTO TUE...WITH A MORE INTERESTING POTENTIAL STORM BEING PROGGED
FOR WED IN BOTH THE GFS AND ECMWF.

&&

.AVIATION /00Z WEDNESDAY THROUGH SUNDAY/...
PCPN WILL LIKELY START REAL FAST AT EACH SITE...WHERE CONDITIONS
WILL DROP FROM VFR TO MVFR AND EVEN IFR IN A SHORT TIME FRAME.

PCPN TYPE WILL LIKELY START AS SNOW...BUT WILL QUICKLY MIX WITH
SLEET AND FREEZING RAIN...AS COLD AIR IS NOT REAL DEEP.

LLWS WILL BECOME AN ISSUE OVERNIGHT...GIVEN HOW STRONG THIS
LOW WILL BE.

EXPECT TEMPS TO COME UP FAST LATE TONIGHT...GIVEN THE WIND
FIELDS. ALSO IT IS NOT THAT COLD ANYWAY...SO EXPECT ALL AREAS
TO BE RAIN BY SUNRISE OR SHORTLY AFTER THIS.

WITH VERY STRONG DYNAMICS...AND SW FLOW...SOME CHC FOR JUST
A SCT DECK OF CLDS BY LATE MORNING...OR EARLY AFT...GIVEN
DRY SLOT. 

WINDS WILL SHIFT TO THE NW LATE IN THE DAY...BUT WITH SUCH
DEEP SYSTEMS...WINDS WILL TEND MORE FROM THE SW THAN OFTEN
FCST. THIS ALONG WITH A LOT OF DIRECTIONAL SHEAR...LAKE EFFECT
WILL BE SLOW TO GET INTO THE AREA IN THE TIME FRAME FROM LATE WED
INTO THU AND FRI. BEST CHC WILL BE LATER ON THU INTO FRIDAY ACROSS
THE MTS.

OUTLOOK... 
WEDNGT/THUR...MVFR/IFR NORTH AND WEST IN SNOW SHOWERS
MAINLY IN THE MOUNTAINS. STRONG WESTERLY WNDS.

FRI/FRI NIGHT...MVFR (AND LOCALIZED IFR) WEST WITH SCATTERED 
SNOW SHOWERS IN MTNS OF NORTHERN AND WESTERN PA. VFR MOST OF
CENTRAL AND EAST. STILL WINDY.

SAT/SUN...VFR...POSSIBLY DECREASING TO IFR/MVFR SHOULD A SYSTEM
ALONG THE CAROLINA COAST EDGE A LITTLE FARTHER NW...AND PROVIDE
THE REGION WITH LIGHT SNOW.

&&

.CTP WATCHES/WARNINGS/ADVISORIES...
WINTER WEATHER ADVISORY UNTIL 11 AM EST WEDNESDAY FOR PAZ004-
010>012-017>019-025>028-034>037-041-042-045-046-049>053-
056>058-063.
HIGH WIND WATCH FROM WEDNESDAY AFTERNOON THROUGH LATE 
WEDNESDAY NIGHT FOR PAZ004>006-010-011-017-024-033.
WINTER STORM WARNING UNTIL 11 AM EST WEDNESDAY FOR PAZ005-006-
024-033.

&&

$$
SYNOPSIS...LAMBERT
NEAR TERM...LAMBERT
SHORT TERM...FITZGERALD/LAMBERT
LONG TERM...RXR
AVIATION...MARTIN/LAMBERT/GARTNER