FXUS61 KCTP 232137
AFDCTP
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE STATE COLLEGE PA
437 PM EST WED DEC 23 2009
.SYNOPSIS...
LOW PRESSURE AND A POOL OF VERY COLD AIR ALOFT WILL DROP SOUTHEAST
TO THE SOUTHERN NEW ENGLAND COAST TONIGHT. A RIDGE OF HIGH
PRESSURE WILL BRIEFLY BUILD ACROSS THE REGION LATE TONIGHT THROUGH
THURSDAY BEFORE A STRONG LOW PRESSURE SYSTEM OVER THE MIDWEST
CREATES POTENTIALLY ICY CONDITIONS FOR EARLY IN THE DAY ON
CHRISTMAS. THE PRECIPITATION FROM THIS STORM SHOULD TURN TO PLAIN
RAIN IN MOST PLACES BY FRIDAY EVENING...AND LAST INTO SATURDAY.
&&
.NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...
UPPER LOW OVER SOUTHERN VT/NH LATE THIS AFTERNOON WILL SPIRAL THE
TAIL END OF SOME QUITE COLD...850 MB AIR (-10 TO -14C) ACROSS OUR
NERN ZONES THROUGH EARLY TONIGHT. THIS FEATURE WILL BE ACCOMPANIED
BY A SHALLOW/BKN LAYER OF STRATO CU NEAR THE NEW YORK BORDER (AND
PERHAPS AS FAR SE AS SCHUYLKILL COUNTY OVERNIGHT). CLOUD LAYER
TEMPS AROUND -13C WILL SUPPORT OCCASIONAL FLURRIES THERE.
ELSEWHERE...MILDER TEMPS ALOFT AND A SLOWLY VEERING BOUNDARY LAYER
FLOW TO THE NORTH...THEN NE TWD DAYBREAK THURSDAY WILL LEAD TO
MAINLY CLEAR SKIES AND COLD TEMPS (RANGING FROM NEAR 10F ACROSS
SOME OF THE NORMALLY COLD SPOTS IN THE NRN AND WRN MTNS....TO THE
TEENS ELSEWHERE).
&&
.SHORT TERM /6 AM THURSDAY MORNING THROUGH THURSDAY NIGHT/...
THE CALM BEFORE SANTA...I MEAN THE STORM...WILL OCCUR DURING THE
DAY THURSDAY INTO CHRISTMAS EVE AS A RIDGE OF HIGH PRESSURE AT
THE SFC/ALOFT DRIFTS NE ACROSS PENN.
LIGHT NORTHEAST TO EAST WINDS WILL ACCOMPANY SUNSHINE...MIXING
WITH SOME THIN HIGH CLOUDS AT TIMES THURSDAY.
HIGH TEMPS FOR THURSDAY WILL BE FAIRLY UNIFORM ACROSS THE CWA...IN
THE U20S TO L30S.
LLVL WINDS WILL INCREASE FROM THE EAST CHRISTMAS EVE...THEN VEER
TO THE SOUTHEAST INTO EARLY CHRISTMAS MORNING. THIS WILL HELP TO
SPREAD A LAYER OF STRATUS NWWD FROM THE WRN ATLANTIC VIA
UPSLOPE/ISENTROPIC LIFT WITHIN THE 285-290K THETA CHANNEL.
MAIN CONCERN HEADING INTO PRIME SLEIGH TIME EARLY CHRISTMAS DAY
CONTINUES TO BE THE THREAT FOR -FZRA ACROSS THE LAURELS...AND
FZDZ ELSEWHERE (GIVEN THE SFC TEMPS ONLY SLOWLY RISING FROM THE
LOWER TO MID 20S...AND CLOUD TEMPS OF -4 TO -6C). EXPECT ONLY A
FEW TO SVRL HUNDREDTHS OF AN INCH OF LIQ EQUIV...BUT JUST ENOUGH
TO POTENTIALLY CAUSE SIGNIF TRAVEL PROBLEMS FROM BLACK ICE.
THE TIMING OF THE THICKENING CLOUD COVER VERY LATE AT
NIGHT...ALONG WITH A STRONG NERLY AGEOSTROPHIC COMPONENT TO THE
LLVL WIND WILL "LOCK IN" THE COLD AIR EAST OF THE ALLEGHENIES.
THE MAIN QUESTION FOR EARLY FRIDAY MORNING IS HOW FAST THE LIGHT
QPF WILL SPREAD NE INTO THE REGION. NAM CONTINUES TO BE SEVERAL
HOURS (OR MORE) SLOWER THAN THE 12Z GFS. 09Z SREF APPEARS TO BE A
GOOD COMPROMISE AT THIS POINT...WITH IT DISPLAYING THE VERY LIGHT
FZRA EARLY FRIDAY...THEN SLOWLY TRANSITIONING TO RAIN/FZRA DURING
CHRISTMAS DAY.
&&
.LONG TERM /FRIDAY THROUGH WEDNESDAY/...
MID/UPR LVL PHASING OF SHRT/WV ENERGY DROPPING SEWD THRU THE NRN
ROCKIES AND LIFTING NEWD FM THE SRN PLAINS WILL RESULT IN AN
ANOMALOUSLY DEEP-LAYERED LOW BECOMING ESTABLISHED OVER THE NATIONS
MIDSECTION AT THE START OF THE PD. IMPRESSIVE 12 HR HEIGHT FALLS
AND MID LVL JET STREAK WILL AID IN CONSIDERABLE INTENSIFICATION OF
A SFC CYCLONE MOVG FM THE ARKLATEX TO NEAR THE IA/MO BORDER BY
25/12Z.
MEANWHILE FURTHER EAST...A LARGE DOME OF HIGH PRES WILL BUILD
ACROSS ERN CANADA AND WEDGE DOWN THE EAST SIDE OF THE APPLCHNS.
THE COLD/DRY...SHALLOW AIR MASS NEAR THE SFC WILL BE OVERRUN BY A
PLUME OF RICH LLVL MSTR/PWS ASSOC WITH A 50-60KT SELY LLJ IN
ADVANCE OF APPROACHING COLD FRONT. AS THIS MSTR ENCOUNTERS THE
SHALLOW SUB-FREEZING AIR IN THE COLD SECTOR...THE POTENTIAL WILL
EXIST FOR SIGNIFICANT ICING VIA FREEZING RAIN FROM THE LAUREL
HIGHLANDS INTO THE INTERIOR CENTRAL MTNS.
THE COLD AIR WILL BE MAINTAINED/ENHANCED BY A 25-35KT LLVL
AGEOSTROPHIC WIND AND EVAP COOLING AS THE ATM SATURATES AND COLUMN
COLLAPSES FROM THE TOP DOWN. IT IS NOT INCONCEIVABLE THAT PCPN
COULD START OUT AS A BRIEF PD OF SNOW...OR MIX WITH SNOW AS
INTENSITY OF PCPN INCREASES. BUT THE HIGH PROBABILITY OUTCOME IS
THAT FREEZING RAIN WILL BE THE DOMINANT PTYPE. A MANUAL BLEND OF
THERMAL PROFILES AND GRIDDED QPFS RESULTED IN ICE ACCUMS BTWN ONE
TENTH AND ONE QUARTER INCH /0.1-0.25"/ ACROSS THE LAUREL HIGHLANDS
WITH LESSER AMNTS /0.01-0.1"/ FURTHER TO THE NORTHEAST OVR THE
NORTH-CENTRAL MTNS. WE CONSIDERED A WATCH FOR THE POTENTIAL OF
MEETING WARNING CRITERIA FOR ICE /0.25"/ ACROSS CAMBRIA/SOMERSET
COS...BUT ULTIMATELY DECIDED TO LET LATER SHIFTS CONTINUE TO
REVIEW THE NEAR/SHORT TERM MESOSCALE DATA AND POST APPROPRIATE
FLAGS IF NECESSARY. BROAD UPPER LEVEL DIVERGENCE...HI AVAILABLE
MSTR AND STG SELY LLJ WILL LKLY BRING WDSPRD 0.50-1.00" 24HR QPF
AMTNS TO THE AREA ENDING 12Z SAT.
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
NORTH ON 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 /21Z WEDNESDAY THROUGH MONDAY/...
VFR CONDITIONS WILL PERSIST THROUGH THE FIRST HALF OF THURSDAY NIGHT
ACROSS MOST OF CENTRAL PENN. THE LONE EXCEPTION WILL BE SOME
PERIODS OF MVFR CIGS (TONIGHT) DUE TO SHALLOW...BKN-OVC STRATO CU
DRIFTING SE FROM LAKES ERIE AND ONTARIO.
A WIDESPREAD DECK OF IFR TO MVFR CIGS (WITH VFR TO MVFR VSBYS IN
FZDZ) WILL DEVELOP ACROSS THE REGION LATE THURSDAY NIGHT INTO
FRIDAY MORNING...AS THE LOW LEVEL EASTERLY FLOW VEERS ABOUT 30 DEG
(TO THE SOUTHEAST) AND STRENGTHENS.
LLWS WILL DEVELOP AT ALL TAF SITES VERY LATE THURSDAY NIGHT AND
CONTINUE RIGHT INTO SATURDAY AS THE SOUTHEASTERLY FLOW JUST A FEW
TO SVRL KFT AGL INCREASES TO 45 TO 55 KTS LATE FRIDAY INTO SAT.
WIDESPREAD LOW CIGS (IFR...THROUGH VLIFR ON THE RIDGES) AND MVFR
TO OCCASIONALLY IFR VSBYS WILL OCCUR FOR FRIDAY AFTERNOON INTO
SATURDAY.
OUTLOOK...
FRI...POOR FLYING CONDS. WIDESPREAD IFR TEMPO LIFR/VLIFR WEST/CENTRAL.
MVFR EAST. -FZRA/FZDZ EARLY THEN RA AND (AREAS OF FZRA PERSISTING
ACROSS THE CENTRAL AND EAST).
SAT...MVFR/SCT IFR. SHSN WEST.
SUN...AND MONDAY MVFR/IFR WEST W/SHSN. MAINLY VFR...WITH PATCHY
MVFR EAST.
&&
.CTP WATCHES/WARNINGS/ADVISORIES...
NONE.
&&
$$
SYNOPSIS...LAMBERT
NEAR TERM...LAMBERT
SHORT TERM...LAMBERT
LONG TERM...STEINBUGL
AVIATION...LAMBERT/MARTIN
