Area Forecast Discussion
Issued by NWS State College, PA

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000 FXUS61 KCTP 221525 AFDCTP AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE STATE COLLEGE PA 1025 AM EST MON DEC 22 2014 .SYNOPSIS... HIGH PRESSURE WILL RETREAT NORTHEAST INTO THE CANADIAN MARITIMES EARLY THIS WEEK...AS A WEAK LOW PRESSURE SYSTEM LIFTS UP THE MID ATLANTIC COAST. AN INTENSIFYING STORM SYSTEM WILL TRACK WEST OF PENNSYLVANIA WEDNESDAY NIGHT...PUSHING A COLD FRONT THROUGH THE STATE. ANOTHER LESS POTENT COLD FRONT IS LIKELY TO IMPACT THE REGION BY NEXT WEEKEND. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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15Z UPDATE** LOW CLOUDS LIFTING CENTRAL MOUNTAINS AT MID MORNING WITH DEEPER MOISTURE STARTING TO POOL SOUTH OF THE MASON DIXON LINE...IN RESPONSE TO INCREASING MARITIME INFLUENCE IN INCREASING EASTERLY/ONSHORE FLOW BETWEEN CAPE HATTERAS LOW PRESSURE AND NORTHERN NEW ENGLANHIGH PRESSURE. THE INCREASING MOISTURE...COMBINED WITH LG SCALE FORCING AHEAD OF SHORTWAVE LIFTING OUT THE SE STATES...COULD SPREAD A BIT OF LIGHT RAIN INTO SOUTHERN PA THIS AFTN. BLEND OF LATEST MDL GUIDANCE SUPPORTS CHC POPS ACROSS THE SOUTHERN COUNTIES THIS AFTN. FOR THE DAY...THE MAIN CONCERN FOCUSES ON POTENTIAL OF SOME ICING LATER ON... GIVEN COLD AIR DAMMING AND LLVL AGEOSTROPHIC NLY FLOW LOCKING THE COLD AIR IN. LATEST HRRR AND 4KM HRW-ARW DIFFER SIGNIFICANTLY ON ARRIVAL OF PRECIP INTO SOUTH CENTRAL PA. LATEST HRW-ARW HAS COME INTO LINE WITH PREVIOUS HRRR HOWEVER...IN DELAYING PRECIP UNTIL AFTER 20Z...AND MOST OF THAT BEING RAIN OVERF THE LOWER SUSQ RIVER VALLEY. THUS...LATEST HI RES RUNS...FORECAST SOUNDINGS AND ENS PTYPES SUGGEST THE BEST ICING CHANCE LATE THIS AFTERNOON...STARTING NEAR THE MASON DIXON LINE MID TO LATE AFTERNOON...AND LIFTING NORTHWARD INTO THE LAURELS AND SOUTH CENTRAL MOUNTAINS BEFORE DARK. STILL NO ADVY TO BE POSTED AT THIS TIME DUE TO THE LOW CERTAINTY OF MEASURABLE FZRA...BUT CONFIDENCE INCREASING FROM ADAMS COUNTY WESTWARD THAT AN ADVISORY MAY BE NEEDED FROM ADAMS COUNTY WESTWARD MID TO LATE AFTERNOON. WILL MONITOR TRENDS AND ISSUE ACCORDINGLY.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM TUESDAY/...
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MOIST SERLY FLOW OFF THE ATLANTIC AND ASSOC OROGRAPHIC FORCING OF SHALLOW MOISTURE LYR WILL LIKELY PRODUCE PATCHY LGT RAIN/DRIZZLE TONIGHT AND TUESDAY. BEST CHC OF MEASURABLE PRECIP SHOULD COME BTWN 00Z-06Z THIS EVENING WITH BENEFIT OF LG SCALE FORCING AHEAD OF MID LVL SHORTWAVE. SFC TEMP FIELDS OF BOTH OPER AND ENSEMBLE DATA SUGGEST THERE COULD BE ICING CONCERNS OVER THE HIGHER TERRAIN OF CENTRAL PA. HOWEVER...TEMPS WILL BE VERY MARGINAL WITH ICING MOST LIKELY ON THE RIDGETOPS. ALSO...NOT ENTIRELY SURE MEASURABLE PRECIP WILL EVEN REACH FAR ENOUGH WEST TO CREATE A CONCERN. WILL THEREFORE CONTINUE TO MENTION THREAT OF ICY ROADS IN THE HWO AND AWAIT 12Z GUIDANCE TO GET A BETTER HANDLE ON THIS EVENT. ANY ICING THREAT SHOULD END BY DAYLIGHT TUESDAY...WHEN ALL PLUME MEMBERS RISE ABV FREEZING. RAINFALL AMTS TUESDAY SHOULD BE VERY LIGHT...WITH SHALLOW MOISTURE IN MDL SOUNDINGS IMPLYING OCNL DRIZZLE RATHER THAN RAIN.
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&& .LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/... THE MOST SIGNIFICANT FORECAST ISSUE IN THE LONG TERM REMAINS THE EVOLUTION OF DEEP TROUGH AND ASSOC SFC LOW TRACKING WEST OF PA WED/WED NITE. STRONG SOUTHERLY FLOW AHEAD OF THIS SYSTEM SHOULD SCOUR OUT LINGERING SHALLOW COLD AIR EAST OF THE APPALACHIANS. LOW-LEVEL TEMPERATURE AND MOISTURE FIELDS RISE WELL ABOVE THEIR SEASONAL VALUES...IMPLYING IT WILL BE A BALMY CHRISTMAS EVE FOR ALL. ALL MED RANGE GUIDANCE TIMES THE TRAILING COLD FRONT THRU CENTRAL PA WED NITE. A STRONG SOUTHERLY LL JET AND ASSOC PWAT ANOMALIES ABOUT +3 SIGMA ABOVE NORMAL ARE PROGGED BY GEFS TO PASS THRU THE AREA IN ADVANCE OF THE FRONT WED NITE. THIS STRONG SOUTHERLY FLOW...HIGH PRECIPITABLE WATER AND SHARP CDFRONT ALL IMPLY A FRONTAL RAIN EVENT WITH PERHAPS SOME RARE LATE DECEMBER (ELEVATED) CONVECTION. HAVE ADDED CHC OF THUNDER BASED ON EXPECTED STRONG LG SCALE FORCING IN ADVANCE OF UPPER TROF AND PRESENCE OF SOME MEAGER MDL CAPES. INTENSIFYING SFC LOW PASSING NORTH OF PA IS A FAVORABLE PATTERN FOR STRONG WINDS ACROSS CENTRAL PA DURING THE COLD SEASON...SO STILL MONITORING THE POTENTIAL OF STRONG WINDS COINCIDENT AND FOLLOWING PASSAGE OF CDFRONT LATE WED NITE AND THURSDAY. WILL MENTION POTENTIAL IN HWO. COLDER AIR WORKING IN BEHIND COLD FRONT WILL LIKELY PRODUCE OROGRAPHIC SNOW SHOWERS ACROSS THE ALLEGHENY PLATEAU ON CHRISTMAS DAY. HOWEVER...AIR MASS NOT LOOKING PARTICULARLY COLD AND SFC TEMPS LIKELY TO BE ABV FREEZING...LIMITING THE POTENTIAL OF ACCUMS. TEMPERATURES SHOULD REBOUND FRIDAY AHEAD OF ANOTHER NORTHERN STREAM WAVE MOVING INTO THE MID-WEST. ECENS OUTPUT SUGGESTS THIS SHORTWAVE WILL LIFT NORTH OF PA NEXT WEEKEND...PUSHING A DYING COLD FRONT INTO THE REGION. && .AVIATION /15Z MONDAY THROUGH FRIDAY/...
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COMBINATION OF RETREATING HIGH PRES OVR THE CANADIAN MARITIMES AND WEAK LOW LIFTING UP THE MID ATLANTIC COAST WILL SPREAD AN INCREASINGLY MOIST SERLY FLOW INTO CENTRAL PA ON TODAY. LOWER CIGS LIFTING NORTHWARD THIS MORNING IN RESPONSE TO DEEPENING LLVL MOISTURE AND ONSHORE FLOW. THE INCREASING MOISTURE...COMBINED WITH LG SCALE FORCING AHEAD OF SHORTWAVE LIFTING OUT THE SE STATES...COULD SPREAD A BIT OF LIGHT RAIN INTO SOUTHERN PA LATE THIS AFTN. MAIN CONCERN WILL BE POTENTIAL OF SOME LIGHT ICING...GIVEN COLD AIR DAMMING SIGNATURE AND IMPLIED LOW-LEVEL AGEOSTROPHIC NORTHERLY FLOW HOLDING LOW-LEVEL COLD AIR IN. OPER MDL SOUNDINGS AND ENS PTYPES SUGGEST THE BEST CHC OF LATE DAY LIGHT -FZRA WOULD BE OVR THE LAUREL HIGHLANDS AND HIGHER TERRAIN OF THE SC MTNS...WITH A BETTER CHANCE TONIGHT ACROSS THE CENTRAL MTNS. OUTLOOK... TUE...LOW CIGS WITH DRIZZLE/LIGHT RAIN LIKELY. BECOMING BREEZY MAINLY ACROSS HIGHER TERRAIN. LLWS POSS N/W. WED...PERIODS OF RAIN ALONG WITH WIDESPREAD REDUCTIONS. BREEZY. LLWS LIKELY. 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...FITZGERALD NEAR TERM...DEVOIR/FITZGERALD SHORT TERM...FITZGERALD LONG TERM...FITZGERALD AVIATION...RXR

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