Area Forecast Discussion
Issued by NWS State College, PA

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FXUS61 KCTP 211922
AFDCTP

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE STATE COLLEGE PA
222 PM EST SUN DEC 21 2014

.SYNOPSIS...
HIGH PRESSURE OVER QUEBEC WILL SHIFT EASTWARD...DELIVERING AN
INCREASINGLY MOIST EASTERLY FLOW INTO THE REGION AS A WEAK LOW
PRESSURE SYSTEM TRACKS ALONG THE MID ATLANTIC COAST. PATCHY LIGHT
PRECIPITATION WILL DEVELOP ON MONDAY...PRIMARILY RAIN...BUT A FEW
SPOTS COULD SEE SOME PATCHY FREEZING RAIN IF IT DEVELOPS EASTERLY
ENOUGH IN THE DAY. MEANWHILE...A VIGOROUS STORM SYSTEM WILL SWEEP
ACROSS THE COMMONWEALTH ON CHRISTMAS EVE DAY BRINGING WIDESPREAD
SHOWERS AND A CHANCE FOR THUNDERSTORMS ON CHRISTMAS EVE. WINDY
AND COLDER CONDITIONS WILL FOLLOW WITH SCATTERED SNOW SHOWERS
ACROSS THE WEST AND NORTH FOR CHRISTMAS DAY. DRY AND SEASONABLE
CONDITIONS ARE EXPECTED FOR FRIDAY AND SATURDAY BEFORE MORE
UNSETTLED WEATHER ARRIVES FOR THE SECOND HALF OF THE WEEKEND.

&&

.NEAR TERM /THROUGH TONIGHT/...
THE THIN STRATUS CONTINUES TO THIN FURTHER ACROSS THE CENTRAL
MOUNTAINS...WITH VISIBLE IMAGERY SUGGESTING IT MAY GIVE WAY TO AN
HOUR OR TWO OF SUNSHINE IN SOME AREAS BEFORE SUNSET. THE FAR NORTH
MAY SEE SOME BRIGHTENING BUT BKN-OVC WILL REMAIN THERE LATE THIS
AFTERNOON AND EVENING. RETURN FLOW AROUND QUEBEC SFC HIGH SHOULD
CONTINUE THIS TREND THIS EVENING...AS RETURN FLOW SOURCE AREAS
SOUTH OF THE MASON DIXON LINE HAVE ALSO CLEARED FOR THE MEANTIME.

HOWEVER...IN TIME THE COMBINATION OF INCREASING EAST FLOW AROUND
RETREATING HIGH PRESSURE OVER NEW ENGLAND AND WEAK LOW PRESSURE
DRIFTING NORTHWARD FROM THE CAROLINAS ALONG THE MID ATLANTIC COAST
WILL SHUNT WESTERN ATLANTIC LOW LEVEL MOISTURE ONSHORE LATER
TONIGHT. AS PREVIOUSLY MENTIONED...INCREASING MOISTURE...COMBINED
WITH LG SCALE FORCING AHEAD OF SHORTWAVE LIFTING OUT THE OHIO
VALLEY...IS EXPECTED TO PRODUCE SOME PRECIPITATION...BUT CONSENSUS
STILL POINTS TO LATER ON MONDAY.

&&

.SHORT TERM /MONDAY/...
AS MENTIONED IN THE NEAR TERM SECTION...INCREASING LLVL MOISTURE
IS EXPECTED TO BRING SOME SPITTY NUISANCE TYPE PRECIPITATION
ACROSS SOUTHERN AND SOUTHEASTERN AREAS ON MONDAY...MAINLY
AFTERNOON. NAM12 REMAINS AGGRESSIVE IN PUSHING MOISTURE NORTH OF
THE MASON DIXON LINE AFTER 12Z...AN OUTLIER...WHICH WOULD
INTRODUCE PRECIP TYPE ISSUES GIVEN RESIDUAL MARGINAL COLD AIR
MONDAY MORNING. CONTINUE TO DOWNPLAY THAT MODEL AND SOLUTION...AND
FAVOR A CONSENSUS LATER ARRIVAL OF LIGHT PRECIP...WHICH WOULD
STILL PRODUCE SOME PATCHY MIXED PRECIP AT THE ONSET. CONFIDENCE
STILL TOO LOW ON THIS FOR ANY ADVISORY CONSIDERATION AT THIS TIME.

DEPENDING UPON TIMING OF ARRIVAL...AREAS WHICH MAY BE MOST
SUSCEPTIBLE TO BRIEF -FZRA MON AFTERNOON WOULD BE OVER PARTS OF
THE CENTRAL AND SOUTH CENTRAL MOUNTAINS. STILL MONITORING THIS LOW
PROBABILITY EVENT AT THIS TIME...AND WILL MENTION LOW CHANCES IN
GRIDS/TEXT PRODUCTS.

&&

.LONG TERM /MONDAY NIGHT THROUGH SATURDAY/...
MOIST SERLY FLOW OFF THE ATLANTIC AND ASSOC OROGRAPHIC FORCING OF SHALLOW
MOISTURE LYR WILL LIKELY PRODUCE PATCHY LGT RAIN/DRIZZLE MON
NIGHT AND TUESDAY. SFC TEMP FIELDS OF BOTH OPER AND ENSEMBLE DATA
SUGGEST THERE COULD BE SOME LINGERING ICING CONCERNS MON NIGHT
OVER THE HIGHER TERRAIN OF CENTRAL PA.

THE MOST SIGNIFICANT FORECAST ISSUE IN THE LONG TERM REMAINS THE
EVOLUTION OF DEEP TROUGH AND ASSOC SFC LOW TO OUR WEST 23-25
DECEMBER. STRONG SOUTHERLY FLOW AHEAD OF THIS SYSTEM WILL BRING
WARM AIR INTO THE REGION TUESDAY AND WEDNESDAY. BY TUESDAY AND
WEDNESDAY MANY LOW-LEVEL TEMPERATURE AND MOISTURE FIELDS RISE WELL
ABOVE THEIR SEASONAL VALUES. THIS IMPLIES VERY WARM WEATHER...ESP
WEDNESDAY...WHEN THE SHALLOW COLD AIR SCOURS OUT EAST OF THE
MOUNTAINS.

TUESDAY THE STRONG CYCLONE...WITH SOME OF THE LOWEST SURFACE
PRESSURES IN THE EASTERN US ON 24 DECEMBER IN THE PAST 30 YEARS
WILL MOVE INTO THE GREAT LAKES. PRESSURE ANOMALIES ARE -3 TO -4
SIGMA BELOW NORMAL WITH A RETURN PERIOD ON THE ORDER OF 30 YEARS
IN SOME FORECAST SYSTEMS.

THE FRONT ASSOCIATED WITH THE CYCLONE PUSHES INTO OUR REGION
WEDNESDAY...AT THIS TIME THE 850 MB SOUTHERLY WINDS ARE FORECAST
TO BE ABOUT +3 SIGMA ABOVE NORMAL. THE STRONG SOUTHERLY FLOW...THE
HIGH PRECIPITABLE WATER...THE STRONG FRONT ALL IMPLY A FRONTAL
RAIN EVENT AND PERHAPS SOME RARE LATE DECEMBER (ELEVATED)
CONVECTION. IF THIS WERE AUTUMN OR SPRING WE MIGHT BE THINKING
QLCS. BUT SURFACE CAPE IS VIRTUALLY NON-EXISTENT BUT SHOWERY RAIN
AND PERHAPS ISOLATED THUNDER POSSIBLE.

DO NOT LIKE HAVING POPS ABOVE 60 TO 66 PERCENT AT DAY 4...BUT
GIVEN DIMINISHING SPREAD AMONGST MDL MEMBERS AND INCREASING
CONFIDENCE IN FRONTAL TIMING...WILL MAINTAIN CATEGORICAL POPS
WEDNESDAY/WED EVENING. THE PATTERN...SHARP NORTH-SOUTH FRONT TO OUR
WEST...IMPLIES MOST OF THE RAIN SHOULD FALL WITHIN 3-6 HOURS OF
THE FRONT.

THE COLD FRONT WILL LIKELY PUSH THRU THE REGION WED NIGHT...ACCOMPANIED
BY GUSTY WINDS AND LIKELY FOLLOWED BY OROGRAPHIC SNOW SHOWERS
ACROSS THE W MTNS CHRISTMAS MORNING.

CHRISTMAS EVE AND EARLY CHRISTMAS IS THE AREA OF HIGH UNCERTAINTY
AS TO WHEN THE COLD AIR COMES IN. THIS WILL DICTATE WHEN THE RAIN
COMES AND WHEN THE GUSTY WINDS SET IN. GRADIENTS ARE WHERE
FORECASTS ARE THEIR POOREST. BUT BY CHRISTMAS MORNING MOST IF NOT
ALL OF THE REGION SHOULD BE BELOW 0C AT 850 AND THE STRONG
WESTERLY WINDS SHOULD BE MIXING DOWN BEHIND THE FRONTAL BOUNDARY.
WINDY OVERNIGHT CHRISTMAS EVE INTO CHRISTMAS DAY. ANY RESIDUAL
PRECIPITATION SHOULD BE SNOW/SNOW SHOWERS AND FLURRIES.

TEMPERATURES SHOULD REBOUND FRIDAY AHEAD OF ANOTHER NORTHERN STREAM
WAVE MOVING INTO THE MID-WEST. THIS SHOULD RAISE OUR HEIGHTS AND
TEMPERATURES FRIDAY INTO EARLY SATURDAY. IT APPEARS A SERIES OF
WAVES MAY EVENTUALLY CARVE OUT A TROUGH OVER THE EAST-CENTRAL US. IF
SO...WE COULD END DECEMBER A BIT ON THE CHILLY SIDE.

&&

.AVIATION /19Z SUNDAY THROUGH FRIDAY/...
STRATOCU TRAPPED UNDER INVERSION CENTRAL AND NORTH...WHILE AN
AREA OF PARTIAL CLEARING EXTENDS FROM SC MTNS TO LOWER SUSQ
PROVIDING VFR CONDITIONS. MVFR CIGS WILL BE PREVALENT CENTRAL
AND NORTH...WHILE PARTIAL CLEARING FLIRTS WITH THE SOUTHERN TIER.

COMBINATION OF RETREATING HIGH PRESSURE OVER NEW ENGLAND AND WEAK
LOW LIFTING UP THE EAST COAST WILL BRING A GRADUALLY INCREASING
MOIST SERLY FLOW INTO CENTRAL PA LATE TONIGHT AND ESP MONDAY.
ANTICIPATE THAT WE/LL SEE REDEVELOPMENT/REINFORCEMENT OF THE
STRATOCU DECK OVER THE SPINE OF THE APPALACHIANS /FROM KJST-KBFD/
TONIGHT...POSS SPREADING INTO CENTRAL MTNS...WITH MOST POINTS EAST
REMAINING VFR.

THE INCREASING MOISTURE...COMBINED WITH LG SCALE FORCING AHEAD OF
SHORTWAVE LIFTING OUT THE OHIO VALLEY...COULD SPREAD A BIT OF
LIGHT PRECIPITATION INTO SOUTHERN PA MONDAY PM.

OUTLOOK...

MON...CIG REDUCTIONS WEST. RESTRICTIONS DEVELOPING EAST. LIGHT
RAIN POSS SOUTH...FZRA POSS WESTERN HIGHER TERRAIN. BECOMING
BREEZY.

TUE...LOW CIGS WITH DRIZZLE/LIGHT RAIN LIKELY. BREEZY.

WED...PERIODS OF RAIN ALONG WITH WIDESPREAD REDUCTIONS. WINDY
WITH LLWS.

THU...WINDY AND COLDER WITH SCT MVFR CIGS AND -SHSN. LLWS.

FRI...FAIR AND MOSTLY VFR.

&&

.CTP WATCHES/WARNINGS/ADVISORIES...
NONE.

&&

$$
SYNOPSIS...DEVOIR
NEAR TERM...DEVOIR
SHORT TERM...DEVOIR
LONG TERM...GRUMM/FITZGERALD
AVIATION...DEVOIR/RXR


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