FXUS61 KCTP 241137
AFDCTP
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE STATE COLLEGE PA
637 AM EST THU DEC 24 2009
.SYNOPSIS...
A STRONG LOW PRESSURE SYSTEM OVER THE MIDWEST WILL PUSH RAIN INTO
THE REGION CHRISTMAS DAY. SUB-FREEZING AIR TRAPPED AT THE SURFACE
WILL CREATE POTENTIALLY ICY CONDITIONS ACROSS ALMOST ALL OF
CENTRAL PENNSYLVANIA...ESPECIALLY THE LAUREL HIGHLANDS. MOST
LOCATIONS WILL TURN TO PLAIN RAIN BY FRIDAY EVENING. COLD AIR WILL
MOVE IN LATE SATURDAY AND SUNDAY...AND LAST THROUGH MOST OF NEXT
WEEK.
&&
.NEAR TERM /THROUGH TONIGHT/...
PER LATEST FOG ENHANCED IR PICS...LOW STRATUS OVER THE NORTH HAS
CONTINUED TO CREEP SLOWLY TO THE SOUTH. WINDS AT THE CLOUD LEVEL
WILL CONTINUE TO VEER ONLY SLIGHTLY FM N TO NE TODAY. WILL
THEREFORE MAKE THE SKY COVER VERY PESSIMISTIC THIS MORNING WITH
ONLY A SLOW DECREASE IN CLOUDS PAINTED IN THE CENTRAL COS AS WEAK
MIXING TAKES PLACE. THE NE MAY STAY CLOUDY ALL DAY. CLOUDS WILL
KEEP MAXES BELOW FREEZING FOR THE NORTH...AND EVEN THE SUNNY
VALLEYS IN THE SOUTH WILL STRUGGLE TO GET INTO THE M30S.
&&
.SHORT TERM /FRIDAY THROUGH SATURDAY/...
STORM OVER THE CENTRAL CONUS IS OBVIOUSLY THE CENTER OF ATTENTION
THIS MORNING. MAIN CHANGES TO PREV FCST WAS TO CONTINUE THE TREND
OF THE PAST FEW DAYS IN SLOWING THE ARRIVAL OF THE RAIN UNTIL
SUNRISE IN THE SW...AND UNTIL NOON ALONG A BFD-UNV-THV LINE. NERN
SECTIONS MAY NOT SEE MEAS RAIN BEFORE 4 PM.
LLVL FLOW CONTINUES TO VEER TO THE E/SE TONIGHT AND DRAG THE USUAL
LOW CRUD INTO THE REGION FM THE ATLANTIC. AS IS NORMAL...THE
INVERSION WILL KEEP THE MSTR VERY SHALLOW...AND THE CLOUDS MAY
GENERATE SOME PATCHY DZ LATE TONIGHT. THE TROUBLE WITH THIS WOULD
BE THAT TEMPS WILL BE AOB FZG PRETTY MUCH ALL NIGHT OVER THE
ENTIRE AREA...AND COULD RESULT IN FZDZ. ATTM EXPECT COVERAGE TO BE
VERY SPOTTY/PATCHY...BUT IF IT LOOKS MORE WORRISOME...AN ADVSRY
STARTING LATE TONIGHT IS POSSIBLE. WATCH WAS CONSIDERED FOR THE
LAURELS BUT NOT 50PCT SURE THEY WILL GET A QUARTER OF AN INCH OF
ICE BEFORE THE EXPECTED CHANGE. CONFIDENCE IS HIGH THAT WE WILL
EVENTUALLY NEED AN ADVSRY ALMOST AREA-WIDE FOR CHRISTMAS DAY FOR
FZRA. BUT...A THIRD PD ADVSRY IS A BEYOND MY COMFORT LEVEL TO
MAKE THAT DEFINITE OF A CALL.
THE DEEPER MSTR CLIMBS INTO THE W ON A 30-35KT SRLY LLVL JET LATE
TONIGHT AND SPREADS SW-NE DURING THE DAY ON CRISTMAS. THE TEMPS
ALOFT ARE VERY WARM...AND THIS SHOULD MAKE FOR LIQUID. BUT...THE
SHALLOW COLD AIR AT THE SFC IN THE MORNING WILL CONTINUE TO BE
REINFORCED BY ERLY FLOW EAST OF THE MTNS DUE TO COLD AIR DAMMING
SET UP. SET UP IS NOT ENTIRELY CLASSIC...WITH THE SFC HIGH A
LITTLE TOO FAR TO THE N. BUT THE STRENGTH OF THE HIGH AT 1038MB+
CORE OVER QUE WILL SUPPLY THE COLD AIR. THIS WILL MAKE FREEZING
RAIN THE DOMINANT PTYPE EARLY ON IN THE EVENT. HOWEVER...THE
AMOUNTS WILL BE LIGHT UNTIL FRIDAY NIGHT...WHEN THE TEMPS SHOULD
BE MILD ENOUGH FOR ONLY PLAIN RAIN. EXPECT A GRADUAL
CHANGEOVER TO PLAIN RAIN DURING THE DAYLIGHT HRS ON CHRISTMAS.
THIS WILL END THE THREAT FOR ICING...BUT TRAVEL - ESP OVER THE
LAURELS WHERE PRECIP STARTS AROUND SUNRISE - WILL BE SLIPPERY AND
PERHAPS HAZARDOUS.
RAIN BECOMES LIGHTER DURING THE DAY ON SAT AS DRIER AIR MOVES IN
FROM TOP DOWN...AND TEMPS DROP BEHIND THE PASSAGE OF A VERY COMPLEX
FRONTAL SYSTEM. EXPECT THE HEAVIEST OF THE RAIN TO BE FRI NIGHT
AND EARLY SAT. UP TO AN INCH OF LIQUID IS POSSIBLE ACROSS THE SRN
HALF...AND COULD PUSH A FEW OF THE SMALLER TRIBS TO CAUTION
STAGES. THE INITIAL RAIN SHOULD BE ABSORBED BY THE DEEP
SNOWPACK...BUT THEN START TO MELT IT AS DEWPOINTS CLIMB AND RUNOFF
WILL INCREASE. THE GRADUAL NATURE OF THIS OPERATION WILL KEEP
THREAT FOR FLOODING VERY MINIMAL. DO EXPECT SOME SMALL STREAM OR
CLOGGED-STORM-DRAIN PROBLEMS TO ARISE...THOUGH.
COLDER AIR WILL BEGIN TO MIX WITH/CHANGE THE RAIN SHOWERS TO SNOW
SHOWERS OVER THE W SAT. LITTLE OR NO ACCUMS ARE EXPECTED BEFORE
SAT NITE THOUGH.
&&
.LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/...
AS THE PRIMARY SFC LOW WEAKENS OVR THE MIDWEST...A NEW SECONDARY
LOW WILL FORM ALONG THE COLD/OCCLD FNT OVR THE NC/VA PIEDMONT BY
26/12Z. CONCEPTUALLY I COULD SEE A CHANGE OVER TO SNOW ACROSS THE
WEST/NORTH LATE SATURDAY ON THE BACKSIDE OF THE LOW AS IT RIDES
UP THE MID ATLANTIC AND SRN ENG COASTS. MDLS SHOW THE CORE OF THE
COLDER AIR ALOFT BLASTING INTO THE AREA FROM THE WEST SAT
NGT...LKLY CHANGING ANY PCPN OVER TO SNOW/SHWRS. THE SHARP
GRADIENTS AND LLVL WIND FIELDS COULD MAKE FOR STRONG/GUSTY POST
FNTL WINDS.
COLD/BRISK CYC FLOW WILL PRODUCE LES OVER THE ALLEGHENIES INTO
EARLY NEXT WEEK. HIGH PRES SHOULD BRING SOME IMPROVEMENT BY THE
MIDDLE OF NEXT WEEK. LONG RANGE MDL MEANS SHOW SOME VERY COLD AIR
BUILDING UP OVR NW CANADA...WHICH COULD BE TAPPED SOMETIME EARLY
NEXT YEAR.
&&
.AVIATION /12Z THURSDAY THROUGH MONDAY/...
LOW LVL MOISTURE TRAPPED BENEATH SUBSIDENCE INVERSION RESULTING IN
AN AREA OF STRATOCU ACROSS NORTHERN PA EARLY THIS MORNING. CURRENT
CIGS RUNNING FROM ARND 500FT OVR THE HIGH TERRAIN...TO ARND
1500FT IN THE VALLEYS. ACROSS SOUTHERN PA...CLEAR SKY/CALM WIND
HAS ALLOWED AREAS OF RADIATION FOG TO FORM.
LATEST MESOSCALE MODEL DATA SUGGESTS ANY LOW LVL MOISTURE IN THE
FORM OF LOW CLDS OR FOG SHOULD MIX OUT BY LATE MORNING...GIVING
WAY TO VFR CONDS FOR THE REST OF TODAY. LOW LVL EASTERLY FLOW WILL
EVENTUALLY DRAW ENOUGH ATLANTIC MOISTURE INLAND TO PRODUCE IFR
CIGS EARLY CHRISTMAS MORNING. INTENSE LLVL JET IS POISED TO PUSH
INTO CENTRAL PA AIRFIELDS EARLY FRI...WITH 45-55KTS JUST OFF THE
DECK ARND 1400FT AGL AND MAY PRODUCE LLVL WS.
OUTLOOK...
FRI...WIDESPREAD IFR CONDS LIKELY/LLWS POSSIBLE. FREEZING RAIN
POSSIBLE CENTRAL MTNS FRI AFTN.
SAT...WIDESPREAD IFR CONDS WITH AFTN IMPROVEMENT POSSIBLE.
SUN-MON...MVFR/IFR WEST W/SHSN. MAINLY VFR EAST.
&&
.CTP WATCHES/WARNINGS/ADVISORIES...
NONE.
&&
$$
SYNOPSIS...DANGELO
NEAR TERM...DANGELO
SHORT TERM...DANGELO
LONG TERM...STEINBUGL
AVIATION...FITZGERALD
