Area Forecast Discussion
Issued by NWS State College, PA

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000 FXUS61 KCTP 211922 AFDCTP AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE STATE COLLEGE PA 222 PM EST SUN DEC 21 2014 .SYNOPSIS...
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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.
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&& .NEAR TERM /THROUGH TONIGHT/...
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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.
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&& .SHORT TERM /MONDAY/...
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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.
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&& .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/...
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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.
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&& .CTP WATCHES/WARNINGS/ADVISORIES... NONE. && $$ SYNOPSIS...DEVOIR NEAR TERM...DEVOIR SHORT TERM...DEVOIR LONG TERM...GRUMM/FITZGERALD AVIATION...DEVOIR/RXR

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