Area Forecast Discussion
Issued by NWS State College, PA

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000 FXUS61 KCTP 181950 AFDCTP AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE STATE COLLEGE PA 250 PM EST THU DEC 18 2014 .SYNOPSIS...
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NORTHWEST WINDS WILL KEEP FLURRIES AND SCATTERED LIGHT SNOW SHOWERS OVER THE NORTH AND WEST INTO FRIDAY MORNING. HIGH PRESSURE WILL SLIDE DOWN FROM THE UPPER GREAT LAKES AND PROVIDE A GENERALLY FAIR PERIOD FOR MOST OF THE WEEKEND. AN UNSETTLED PATTERN WILL SET UP IN THE FIRST PART OF THE NEW WEEK. A WEAK LOW PRESSURE AREA COULD BRING LIGHT RAIN OR SNOW TO THE AREA ON MONDAY. THEN A SIGNIFICANT AREA OF LOW PRESSURE IS EXPECTED TO DEVELOP OVER THE MIDWEST AND UPPER GREAT LAKES. THIS WILL LIKELY PRODUCE WIDEPSREAD RAIN FOR THE STATE ON WEDNESDAY...FOLLOWED BY STRONG WEST WINDS AND SCATTERED MOUNTAIN SNOW SHOWERS FOR CHRISTMAS DAY.
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&& .NEAR TERM /THROUGH TONIGHT/...
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SEASONABLY COLD NORTHWEST FLOW ACROSS THE EASTERN LAKES WILL REMAIN IN PLACE THROUGH FRIDAY WITH THE TYPICAL DOWNWIND STRATUS ACROSS MOST OF CENTRAL PA. CYCLONIC FLOW ALOFT ON THE BACKSIDE OF NEWFOUNDLAND UPPER CUTOFF/REX BLOCKING PATTERN WILL KEEP WEAK OR NEARLY NEUTRAL COLD ADVECTION OVER THE REGION. A SOUTHERN STREAM WAVE STREAKING EASTWARD FROM THE TENNESSEE RIVER VALLEY THU AFTN WILL PRODUCE AREAS OF VERY LIGHT SNOW ACROSS THE LAUREL HIGHLANDS INTO THE SOUTH CENTRAL MOUNTAINS THIS EVENING WITH LITTLE IF ANY ACCUMULATION EXPECTED. WEST FACING SLOPES OF THE WEST CENTRAL MOUNTAINS AND LAUREL HIGHLANDS MAY SEE A LOCALIZED COATING TO AN INCH OR TWO OF SNOW AS THE UPPER AND BOUNDARY LAYER FLOW ALIGN FOR A PERIOD LATE TONIGHT INTO FRI MORNING. OTHERWISE EXPECT WIDESPREAD STRATUS TO PERSIST ACROSS MOST OF CENTRAL PA. THE SOUTHEAST WILL LIKELY SEE SOME DISSIPATION BY FRIDAY MORNING. SURFACE HIGH CENTERED OVER THE NORTHERN GREAT LAKES AND EASTERN QUEBEC WILL KEEP NORTH/NORTHWEST BOUNDARY LAYER FLOW GOING THROUGH FRI MORNING...TRANSLATING TO SFC WINDS IN THE 7 TO 11 MPH RANGE.
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&& .SHORT TERM /FRIDAY/...
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UPPER FORCING SLIDES EAST OF THE REGION EARLY FRIDAY MORNING... WITH EXPANSIVE SURFACE HIGH PRESSURE BUILDING OVER THE AREA IN THE AFTERNOON. THUS...THE AMOUNT/COVERAGE OF THE SNOW SHOWERS AND FLURRIES SHOULD DIMINISH COMPLETELY ON FRIDAY. MINS WILL STAY A LITTLE MILDER THAN CLIMO WITH CLOUDS AROUND. MAXES ON FRIDAY VERY SIMILAR TO TODAY...WITH 30S FOR THE MOUNTAINS AND NEAR 40 IN THE LOW LANDS WITH THE HELP OF SOME DOWNSLOPE WARMING.
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&& .LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/... THE FLOW REMAINS RATHER FLAT OVER THE SERN U.S. AND MID ATLANTIC STATES THIS WEEKEND. SO THE WEAK LOW WHICH HAS BEEN PROJECTED TO PASS JUST TO THE SOUTH OF THE AREA THIS WEEKEND SHOULD DO SO...AND EVEN WELL SOUTH OF THE AREA AT THAT. ALL MED RANGE GUIDANCE KEEPS THE SFC LOW ALONG A PATH FROM KATL TO KCHS. THEREFORE...WILL REMOVE ANY REMAINING POPS IN THE SAT-SUN TIME FRAME. TEMPS WILL STAY VERY SIMILAR TO WHAT WE SEE IN THE NEAR TERM...GENERALLY COOLER THAN NORMAL DURING THE DAYS. FRIDAY NIGHT SHOULD BE THE COLDEST NIGHT UNTIL AFTER CHRISTMAS WITH MINS IN THE TEENS WEST AND 20S ELSEWHERE. THE PATTERN TURNS VERY COMPLICATED/CONVOLUTED FOR THE FIRST PART OF THE WEEK. A BIG TROUGH STARTS TO DEVELOP OVER THE UPPER PLAINS/MIDWEST AND THE RESULTING HEAVE TO THE HEIGHTS OFF THE EASTERN SEABOARD WILL ALLOW ANOTHER AREA OF LOW PRESSURE TO ROLL TO THE NORTHEAST FROM THE GULF COAST ON MONDAY. THE LOW WILL STILL STAY DECENTLY FAR AWAY FROM THE LOCAL AREA. BUT THE ATLANTIC MOISTURE WILL PROBABLY AT LEAST BRUSH THE SOUTHEASTERN HALF OF THE AREA. TIMING DIFFERENCES BETWEEN THE GFS AND EC ARE ABOUT 12 HRS - WITH THE EC FASTER IN MOVING THE WAVE TO THE EAST. PRECIP TYPE AT THIS POINT LOOKS LIKE IT COULD BE LIGHT SNOW...IF IT FALLS DURING THE NIGHT. BUT A DAYLIGHT EVENT WOULD BE MORE OF A MIX OR SNOW ON THE NORTHERN END OF THE PRECIP SHIELD AND RAIN INTO THE LOWER SUSQ. POPS AT THIS POINT ARE NOT WORTH MUCH MORE THAN CHCS...DUE TO TIMING DIFFERENCES AND THE FACT THAT THE WEEKEND SYSTEM IS NOW PROGGED TO GO WELL OFF TO OUR SOUTH. THIS SYSTEM MAY END UP ROWING DOWN THE SAME STREAM...AND HAVE HEDGED FORECASTS OF THE PRECIP SHIELD TO THE SE A BIT. EASTERLY WINDS MAY PILE THE LLVL MARITIME AIR UP INTO THE CENTRAL PA MTNS FOR TUESDAY AND A DARK DAY WITH LIGHT SHOWERS OR DRIZZLE IS POSSIBLE. THE TEMPS START WARMING UP SIGNIFICANTLY ALOFT....THOUGH. THE BIG STORY IS THE EXPECTED DEVELOPMENT OF A VERY SIGNIFICANT LOW PRESSURE AREA DURING THE MID WEEK OVER THE MID MS VALLEY AND UPPER GREAT LAKES ON TUES/WED. SOME MDLS HAD PLACED A 960HPA LOW OVER LAKE HURON ON WEDNESDAY. BUT THE MOST RECENT RUNS OF THE GFS AND ECMWF PLACE A 990HPA LOW UNDER THE MAIN UPPER LOW OVER THE UPPER GREAT LAKES...AND A RAPIDLY DEEPENING BUT INITIALLY EQUAL- STRENGTH SFC LOW RUNNING UP THE PIEDMONT EAST OF THE APPALACHIANS ON WED MORNING. THIS SECOND LOW WILL THROW COPIOUS MOISTURE BACK INTO THE STATE AS IT WHIPS NORTHEAST ON CHRISTMAS EVE. THE TEMPS LOOK LIKE THEY WILL BE WARM ENOUGH FOR MAINLY RAIN OVER THE ENTIRE AREA...BUT IF THE TRACK IS SLIGHTLY MORE TO THE EAST - THE TEMPS WILL BE JUST A HAIR COLDER AND THE MOUNTAINS MAY HAVE SNOW ACCUMULATIONS. AFTER THE SECONDARY LOW ZIPS AWAY...THE CYCLONIC FLOW AROUND THE CLOSED UPPER LOW AND STRONG COLD ADVECTION WILL CERTAINLY MAKE WIDESPREAD SNOW SHOWERS OVER THE WRN MOUNTAINS...AND VERY GUSTY WINDS. TIMING DIFFERENCES WITH THE MAIN SLUG OF PRECIP ARE MINOR FOR THIS RANGE. HPC GUID OF LIKELY POPS SEEMS VERY PLAUSIBLE...AND WILL HOLD IT TO A 70 POP AT THIS POINT FOR WED DURING THE DAY. QPF IN THE GEFS PLUMES IS ON A GIANT SPREAD AT THIS POINT...BUT THE MEAN NUMBERS ARE AROUND AN INCH AND THE MEDIANS SLIGHTLY HIGHER. WILL MENTION THE STORM AND THE MOST LIKELY OUTCOMES IN THE HWO AT THIS POINT...DUE TO THE FACT THAT THE TIMING LOOKS TO BE ON ONE OF THE BUSIEST TRAVEL DAYS - DIDN/T WE JUST DO THIS A MONTH AGO - AND A SMALL CHANGE TO THE EAST IN TRACK OF THE SECONDARY LOW COULD MEAN SNOW - AT LEAST FOR THE ALLEGHENIES. && .AVIATION /18Z THURSDAY THROUGH TUESDAY/...
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LOW LVL MOISTURE TRAPPED BENEATH INVERSION AND NORTHWEST FLOW OVER THE EASTERN GLAKS WILL KEEP STRATO CU IN PLACE ACROSS MOST OF CENTRAL PA THROUGH TONIGHT. LATEST MDL SOUNDINGS POINT TOWARD PREDOMINANTLY MVFR CIGS/VSBYS AT KBFD. ELSEWHERE...MDL SOUNDINGS SUPPORT CIGS ARND 1500FT AT KJST INTO THIS EVENING...WHILE DRYING/DOWNSLOPING FLOW YIELDS PROGRESSIVELY HIGHER CIGS FURTHER EAST. A TROUGH OF LOW PRESSURE WILL PUSH THRU THE REGION TONIGHT...PERHAPS PRODUCING A PERIOD OF STEADIER LGT SNOW AT KBFD AND KJST BTWN 00Z-06Z FRIDAY WITH VSBYS IN THE 1-2SM RANGE. OUTLOOK... FRI...EARLY AM -SHSN/REDUCED VSBYS POSS AT KJST. SAT-SUN...NO SIG WX EXPECTED. MON...LOW CIGS WITH LGT RAIN/SNOW POSSIBLE.
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&& .CTP WATCHES/WARNINGS/ADVISORIES... NONE. && $$ SYNOPSIS...DEVOIR NEAR TERM...DEVOIR SHORT TERM...DEVOIR LONG TERM...DANGELO AVIATION...DEVOIR

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