Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

Current Version | Previous Version | Text Only | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
-- Highlight Changed Discussion --
-- Discussion containing changed information from previous version are highlighted. --
000 FXUS61 KPHI 312117 AFDPHI AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE MOUNT HOLLY NJ 417 PM EST SAT JAN 31 2015 .SYNOPSIS... STRONG LOW PRESSURE WILL CONTINUE TO TRACK INTO THE CANADIAN MARITIMES, MEANWHILE HIGH PRESSURE WILL GRADUALLY BUILD INTO OUR AREA FROM THE OHIO VALLEY INTO TONIGHT. THIS HIGH WILL THEN MOVE OFFSHORE LATE TONIGHT AND SUNDAY. BY SUNDAY NIGHT, LOW PRESSURE WILL MOVE INTO THE OHIO VALLEY THEN WILL TRACK INTO OUR REGION ON MONDAY. ANOTHER AREA OF HIGH PRESSURE WILL MOVE INTO THE REGION TUESDAY AND WEDNESDAY WITH LOW PRESSURE TRACKING JUST OFFSHORE FOR THE LATER HALF OF THE WEEK. && .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/... THE WATER VAPOR IMAGERY SHOWS A STRONG UPPER-LEVEL LOW OFF THE NEW ENGLAND COAST AND INTO THE CANADIAN MARITIMES, ASSOCIATED WITH A NEGATIVELY TILTED TROUGH. ANOTHER UPPER-LEVEL LOW IS SLIDING SOUTHEASTWARD ACROSS NORTHERN MEXICO, MEANWHILE A SHORT WAVE IS DIVING SOUTHEASTWARD ACROSS THE NORTHERN ROCKIES. AT THE SURFACE, STRONG LOW PRESSURE IS MOVING INTO THE CANADIAN MARITIMES AND HIGH PRESSURE IS CENTERED ACROSS THE SOUTHEASTERN STATES BUT EXTENDS NORTHWARD ACROSS THE EASTERN GREAT LAKES. AS THE UPPER-LEVEL TROUGH ACROSS THE EAST RELAXES, HIGH PRESSURE WILL MOVE EASTWARD AND SETTLE OVER OUR AREA LATER TONIGHT. THIS WILL ALLOW THE PRESSURE GRADIENT TO LESSEN AND THE WINDS ARE EXPECTED TO DIMINISH QUICKLY THIS EVENING. WE SHOULD DECOUPLE IN MANY PLACES, ESPECIALLY THOSE AREAS THAT STILL HAVE SNOW COVER. A COLD FRONT THOUGH WILL BE DROPPING TOWARD OUR AREA LATE TONIGHT FROM THE NORTHWEST. THIS WILL SET THE STAGE FOR A DEVELOPING THERMAL GRADIENT ALOFT THAT BECOMES ORIENTATED FROM WEST TO EAST. THE DEVELOPMENT OF WAA UP THE OHIO VALLEY WILL ALSO INCREASE THE CLOUDS SOME MAINLY LATE TONIGHT ESPECIALLY ACROSS OUR WESTERN ZONES. AS FOR THE LOW TEMPERATURES, WE USED A BLEND OF THE NAM/GFS MOS BUT THEN ADJUSTED THIS DOWN A BIT ACROSS THE TYPICALLY COLDER LOCALES AND ALSO WHERE SNOW COVER REMAINS. && .SHORT TERM /6 AM SUNDAY MORNING THROUGH 6 PM SUNDAY/... A SHORT WAVE SLIDING EAST-SOUTHEASTWARD FROM THE MIDWEST TO MID-MISSISSIPPI VALLEY AREA WILL SHARPEN AN UPPER-LEVEL TROUGH. IT IS THIS FEATURE THAT HAS BEEN MOSTLY DRIVING THE CHANGES SEEN IN THE MODEL GUIDANCE, BUT THE TREND HAS BEEN FOR A SHARPER TROUGH AND THEREFORE A NORTHWARD TRACK OF THE SURFACE LOW. THERE WILL BE DOWNSTREAM WAA OCCURRING ALOFT ALTHOUGH THE SURFACE FLOW WILL INITIALLY BECOME SOUTHERLY. THE EXISTING SNOW COVER MAY SLOW THE BOUNDARY LAYER WARMING TO SOME EXTENT SUNDAY, HOWEVER AN INCREASE IN CLOUDS SHOULD ALSO SLOW THE WARMING. THE MODEL GUIDANCE IS IN PRETTY GOOD AGREEMENT IN QUICKLY DEVELOPING ISENTROPIC LIFT UP AND OVER THE DEVELOPING 850 MB THERMAL GRADIENT. THIS SHOULD RESULT IN SOME LIGHT SNOW EXPANDING EASTWARD WITH TIME, AND THIS SHOULD BE OVERSPREADING AREAS MAINLY WEST OF INTERSTATE 95 BY LATE AFTERNOON. THE LOW LEVELS SHOULD INITIALLY BE RATHER DRY, THEREFORE WE DID NOT INCREASE THE POPS ALL THAT FAST EASTWARD DURING THE DAY. IT APPEARS THAT ANY SNOW ACCUMULATIONS BY EVENING WILL BE LIGHT. THE WARMING ALOFT WILL PROBABLY RESULT IN SOME RAIN/SLEET ACROSS OUR SOUTHERN ZONES, IF PRECIPITATION OCCURS THERE BY LATE AFTERNOON. AS FOR THE HIGH TEMPERATURES, WE USED A BLEND OF THE NAM/GFS MOS OVERALL. WE ARE EXPECTING A MILDER DAY COMPARED TO TODAY. && .LONG TERM /SUNDAY NIGHT THROUGH SATURDAY/... MOST OF THE PREVIOUS WATCH HAS BEEN CONVERTED TO A WARNING, MOSTLY FOR SNOW, WHILE WE HAVE EXPANDED THE WATCH FARTHER SOUTH TO COVER THE UNCERTAINTY INVOLVED WITH THE PROGRESSION OF THE RAIN/SNOW LINE. THIS IS STILL A HIGHLY UNCERTAIN FORECAST SOUTH OF THE LEHIGH VALLEY, EVEN WITHIN 36 HOURS OF THE START. SUNDAY EVENING - MONDAY...VERY COMPLEX SYSTEM AND FORECAST TAKING SHAPE ACROSS OUR REGION LATE SUNDAY EVENING THROUGH MONDAY AFTERNOON. NOTICED THAT THE GFS/EC, WHEN COMPARED TO THE NAM INITIALIZED JUST A TAD STRONGER WITH THE MAIN SHORTWAVE ACROSS THE PAC-NW. THE STRONGER SCENARIO THESE MODELS SHOW, AS THE SHORTWAVE CONTINUES TO AMPLIFY HEADING SOUTHWARD, CAUSES THE SURFACE INFLECTION TO BE DISPLACED FARTHER NORTH AS IT APPROACHES OUR REGION. THIS HAS BEEN NOTED OVER THE PAST COUPLE OF DAYS WITH A CONTINUED NORTHWARD TRACK OF THE SURFACE LOW...NOT MUCH BLOCKING TO SUPPRESS THE LOW FARTHER SOUTH. FOR OUR SENSIBLE WEATHER SUNDAY NIGHT INTO MONDAY. FRONT END THETA-E AND WAA AHEAD OF THE APPROACHING LOW SHOULD OVERSPREAD LIGHT SNOW ACROSS MOST OF THE REGION NORTH OF ILG. THE BEST MESOSCALE LIFT/BANDING POTENTIAL LOOKS TO OCCUR BETWEEN 6-12Z ACROSS THE NORTHERN ZONES. AT THIS POINT THE BETTER LIFT IN THE DGZ SHOULD BE CONFINED TO NORTH OF TTN-RDG LINE...BEFORE THE WARM SOUTHERLY AIR REALLY GETS CRANKING AND WARMS THE COLUMN ENOUGH. THERE IS A DECENT CHANCE THAT THE POCONOS STAY ALL SNOW THROUGH THE DURATION OF THE EVENT WITH SOME MIXING AS FAR NORTH AS THE LEHIGH VALLEY. +3 TO +5 850MB TEMPERATURES INDICATE A LACK OF SLEET POTENTIAL EARLY MONDAY MORNING SOUTH OF A ABE-SMQ LINE WITH THE PHILADELPHIA METRO AREA SOUTHWARD ALL RAIN. SURFACE TEMPERATURES TOO ARE VERY TRICKY IN THIS SETUP AS MOST OF THE GUIDANCE WANTS TO BLAST THE SURFACE TEMPERATURES ABOVE FREEZING BY EARLY MONDAY MORNING. CURRENT THINKING IS THAT THEY ARE NOT ACCOUNTING FOR THE AMOUNT OF SNOWPACK CURRENTLY IN PLACE ACROSS SOUTHEAST PENNSYLVANIA AND CENTRAL NEW JERSEY. THIS WAS THE MAIN REASON THAT WE EXPANDED THE WATCH...COMBINATION OF FRONT-END SNOW FOLLOWED BY A PERIOD OF FREEZING RAIN. WE CAN ALWAYS GO TO AN ADVISORY IF NEEDED BUT DID NOT WANT TO JUMP THE GUN IN CASE THE ICE SIGNAL BECOMES EVEN STRONGER. BY LATER MONDAY AFTERNOON THE SYSTEM SHOULD BE QUICKLY PULLING TO OUR NORTHEAST ALLOWING THE COLUMN TO COOL WEST TO EAST WITH ANOTHER CHANGEOVER BACK TO SNOW SHOWERS. TUESDAY - WEDNESDAY: BEHIND THE SYSTEM, A PRETTY POTENT BLAST OF COLD COMES INTO THE REGION BY TUESDAY. 925 AND 850 MB BOTH WILL BE WELL BELOW -10C COUPLED WITH STRONG NORTHWESTERLY WINDS. THIS QUICKLY MODIFIES BY WEDNESDAY AHEAD OF ANOTHER COLD FRONTAL BOUNDARY WITH A RETURN TO SOUTHERLY WINDS. SNOWCOVER LEADS TO GREATER UNCERTAINTY IN TEMPERATURES. SOME MIXING ON TUESDAY MORNING COULD ALLOW FOR SLIGHTLY WARMER OVERNIGHT LOWS BEFORE THE HIGH PRESSURE SETTLES IN ON TUESDAY. WEDNESDAY NIGHT THROUGH FRIDAY NIGHT: POTENTIAL EXISTS FOR AN AREA OF LOW PRESSURE TO DEVELOP IN THE GULF OF MEXICO AND TRACK NORTHEAST ALONG THE COLD FRONT WHICH PASSES THROUGH WEDNESDAY NIGHT. LIMITED TIME TO FOCUS ON THIS PERIOD BUT LEFT IN CHANCE OF SNOW THURSDAY INTO FRIDAY. THE ECMWF IS FURTHEST EAST WITH THE GFS, CMC AND UKMET HINTING AT THIS THREAT. && .AVIATION /21Z SATURDAY THROUGH THURSDAY/... THE FOLLOWING DISCUSSION IS FOR KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY AND SURROUNDING AREAS. REST OF THIS AFTERNOON...VFR. NORTHWEST WINDS 10-20 KNOTS WITH GUSTS TO 25 KNOTS, DIMINISHING TOWARD EVENING. TONIGHT...VFR, WITH SOME INCREASE IN MID TO HIGH LEVEL CLOUDS LATE. WEST-NORTHWEST WINDS QUICKLY DIMINISHING TO AROUND 5 KNOTS IN THE EVENING, THEN LOCALLY BECOMING SOUTHWESTERLY. SUNDAY...VFR TO START WITH INCREASING CLOUDS. THE CEILINGS MAY LOWER TO MVFR AT KABE AND KRDG TOWARD 18Z. SOME LIGHT SNOW IS POSSIBLE IN THE AFTERNOON WEST OF THE KPHL METRO. THE TIMING OF THIS IS OF LOWER CONFIDENCE AT THIS TIME, THEREFORE IT WAS NOT INCLUDED. LIGHT SOUTHWEST WINDS MAY BECOMING LIGHT AND VARIABLE AT MOST TERMINALS, THEN TURN SOUTHEAST BY EVENING. OUTLOOK... SUNDAY NIGHT - MONDAY...WIDESPREAD MVFR/IFR IN AREAS OF RAIN/SNOW/SLEET. EXPECTING MOSTLY SNOW AT RDG/ABE WITH SNOW TURNING TO RAIN/SLEET AT PHL/ILG/TTN/PNE...RAIN AT MIV/ACY BY MONDAY MORNING. COULD BE A PERIOD OF FREEZING RAIN IN SOUTHEAST PENNSYLVANIA INTO CENTRAL NEW JERSEY MONDAY MORNING. LINGERING MOISTURE MONDAY AFTERNOON SHOULD TURN INTO SNOW SHOWERS. TUESDAY - WEDNESDAY...VFR. GUSTY NORTHWEST WINDS TUESDAY DECREASE ON WEDNESDAY. WINDS TURN MORE TOWARDS THE SOUTHWEST BY WEDNESDAY AHEAD OF THE NEXT POTENTIAL SYSTEM. && .MARINE... HIGH PRESSURE WILL BUILD INTO THE AREA THROUGH TONIGHT, THEN TO OUR EAST SUNDAY. THIS WILL ALLOW THE PRESSURE GRADIENT TO RELAX AND THE WINDS TO DIMINISH. THE DOWNWARD TREND IN THE WINDS CONTINUES TO OCCUR, AND THEREFORE WE WILL REPLACE THE GALE WARNING WITH A SMALL CRAFT ADVISORY. THIS WILL RUN THROUGH 03Z ON DELAWARE BAY AND THEN 05Z ON THE COASTAL WATERS AS SEAS WILL TAKE A LITTLE LONGER TO SUBSIDE. THE AVAILABLE BUOY DATA INDICATES THAT WHILE FREEZING SPRAY IS OCCURRING THIS AFTERNOON, THE RATE OF ACCRETION HAS REALLY DROPPED OFF. THEREFORE, WE WILL CANCEL THE FREEZING SPRAY ADVISORY. THE WINDS WILL BE LIGHTER SUNDAY AND TURN MORE SOUTHERLY. OUTLOOK... SUNDAY NIGHT: SUB SCA SEAS AND WINDS WITH SEAS BUILDING TOWARD SUNRISE MONDAY. MONDAY THROUGH TUESDAY: SCA SEAS THROUGH TUESDAY MORNING AND POSSIBLE NORTHWEST GALE GUSTS LATE MONDAY AFTERNOON AND NIGHT. TUESDAY NIGHT AND WEDNESDAY: SUB SCA SEAS AND WINDS. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...WINTER STORM WARNING FROM 7 PM SUNDAY TO 1 PM EST MONDAY FOR PAZ054-055-060>062. WINTER STORM WATCH FROM SUNDAY EVENING THROUGH MONDAY AFTERNOON FOR PAZ101-103-105. NJ...WINTER STORM WARNING FROM 7 PM SUNDAY TO 1 PM EST MONDAY FOR NJZ001-007-008. WINTER STORM WATCH FROM SUNDAY EVENING THROUGH MONDAY AFTERNOON FOR NJZ009-010-012. DE...NONE. MD...NONE. MARINE...SMALL CRAFT ADVISORY UNTIL MIDNIGHT EST TONIGHT FOR ANZ450>455. SMALL CRAFT ADVISORY UNTIL 10 PM EST THIS EVENING FOR ANZ430- 431. && $$ SYNOPSIS...GAINES/GORSE NEAR TERM...GORSE SHORT TERM...GORSE LONG TERM...GAINES/HEAVENER AVIATION...GAINES/GORSE/HEAVENER MARINE...GAINES/GORSE/HEAVENER

USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.