Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 302105 AFDPHI AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE MOUNT HOLLY NJ 405 PM EST FRI JAN 30 2015 .SYNOPSIS...
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LOW PRESSURE WILL INTENSIFY INTO THE CANADA MARITIMES THROUGH SATURDAY. MEANWHILE, HIGH PRESSURE BUILDS TOWARD OUR AREA TONIGHT BEFORE SETTLING OVERHEAD LATER SATURDAY INTO SUNDAY. A LOW PRESSURE SYSTEM WILL THEN TRACK ACROSS THE MID ATLANTIC SUNDAY NIGHT AND MONDAY. HIGH PRESSURE BUILDS BACK IN AGAIN FOR TUESDAY BEFORE QUICKLY SHIFTING OFFSHORE TUESDAY NIGHT INTO WEDNESDAY, THEN A COLD FRONT PUSHES THROUGH WEDNESDAY INTO THURSDAY AS LOW PRESSURE MAY DEVELOP ALONG IT. HIGH PRESSURE THEN BUILDS FROM THE PLAINS TOWARD THE OHIO VALLEY LATE THURSDAY AND FRIDAY.
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&& .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
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ANY REMAINING SNOW SHOWERS/FLURRIES WILL END AFTER SUNSET...THEN SKIES WILL GRADUALLY CLEAR. WITH A SNOW COVER ON THE GROUND IN THE NORTH, AND WITH COLD AIR ADVECTION IN FULL FORCE...ACTUAL AIR TEMPERATURES WILL DIP DOWN TO ABOUT ZERO IN THE FAR NORTH AND INTO THE MID TEENS SOUTH. STRONG NORTHWEST WINDS 15 TO 25 MPH WITH MUCH HIGHER GUSTS...APPROACHING 50 MPH IN THE HIGHER ELEVATIONS...WIND CHILL FACTORS WILL DIP DOWN BELOW ZERO IN MANY PLACES...ESPECIALLY NORTH AND WEST OF PHILADELPHIA. IN THE HIGHER TERRAIN OF CARBON/MONROE/SUSSEX COUNTIES...WIND CHILL FACTORS IN THE RANGE OF -15 TO -20 DEGREES ARE EXPECTED...SO A WIND CHILL ADVISORY REMAINS IN EFFECT. FOR ALL AREAS...A WIND ADVISORY REMAINS IN EFFECT THROUGH THE OVERNIGHT HOURS FOR GUSTY WINDS. A BLEND OF MAV/MET GUIDANCE WAS USED TO POPULATE THE TEMPERATURE GRIDS, AND THIS BLEND WAS UNDERCUT BY A DEGREE OR TWO TO ACCOUNT FOR SNOW COVER.
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&& .SHORT TERM /6 AM SATURDAY MORNING THROUGH 6 PM SATURDAY/...
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AS HIGH PRESSURE CONTINUES TO BUILD INTO THE AREA FROM THE OHIO VALLEY...THE PRESSURE GRADIENT OVER OUR REGION WILL GRADUALLY RELAX DURING THE AFTERNOON. WHILE STRONG GUSTY WINDS WILL CONTINUE THROUGH THE MORNING HOURS...THERE WILL BE IMPROVEMENT BY LATE AFTERNOON. WITH MORNING TEMPERATURES STARTING OUT VERY COLD...EVEN FOR THIS TIME OF YEAR, WIND CHILL FACTORS WILL MAKE OUTSIDE CONDITIONS FEEL BITTER COLD. ACTUAL TEMPERATURES WILL PEAK OUT ONLY IN THE LOW TEENS UP NORTH...AND IN THE MID 20S ELSEWHERE. WITH DEWPOINT TEMPERATURES IN THE SINGLE DIGITS...RELATIVE HUMIDITIES WILL BE VERY DRY.
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&& .LONG TERM /SATURDAY NIGHT THROUGH FRIDAY/...
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THE OVERALL SYNOPTIC SETUP IS COMPRISED OF A NEGATIVE TILT TROUGH LIFTING THROUGH THE CANADIAN MARITIMES SATURDAY NIGHT AND SUNDAY, AS ENERGY ACROSS THE SOUTHWEST GENERALLY HANGS BACK BEFORE SLIDING INTO MEXICO. THE ASSOCIATED UPPER-LEVEL TROUGH SHARPENING UP SUNDAY AND MONDAY WILL DRIVE LOW PRESSURE ACROSS THE MID ATLANTIC ON MONDAY. THERE REMAINS THOUGH SOME UNCERTAINTY ON THE EXACT TRACK OF THE SURFACE LOW. THE UPPER-LEVEL TROUGH RELAXES SOME TUESDAY INTO WEDNESDAY BEFORE SHORT WAVE ENERGY IN THE NORTHERN STREAM AMPLIFIES ANOTHER TROUGH FROM THE MIDWEST AND GREAT LAKES REGION WEDNESDAY INTO THURSDAY. THIS SHOULD DRIVE A COLD FRONT EASTWARD WITH THE POTENTIAL FOR CYCLOGENESIS TO OCCUR ALONG THE FRONT AS IT REACHES OR MOVES OFF THE COAST. WE USED A MODEL BLEND APPROACH THROUGH MONDAY, THEN GENERALLY UTILIZED THE 12Z WPC GUIDANCE THEREAFTER. SOME ADJUSTMENTS WERE THEN MADE BASED ON ADDITIONAL COLLABORATION WITH OUR NEIGHBORING OFFICES AND ALSO WPC. FOR SATURDAY NIGHT AND SUNDAY...A NEGATIVE TILT TROUGH IS FORECAST TO BE LIFTING NORTHEASTWARD INTO THE CANADIAN MARITIMES DURING THIS TIME FRAME. THIS WILL TAKE A DEEPENING SURFACE LOW WITH IT. AS THE TROUGHING EASES IN THE EAST SUNDAY, HIGH PRESSURE BUILDS OVER OUR AREA BEFORE SHIFTING OFF THE SOUTHEASTERN U.S. COAST BY LATE IN THE DAY. AS THIS OCCURS, ENERGY DIVING ACROSS THE MIDWEST TO MID MISSISSIPPI VALLEY WILL AMPLIFY AN UPPER-LEVEL TROUGH. A DEVELOPING AND PROBABLY STRENGTHENING CONFLUENCE ZONE DOWNSTREAM SHOULD HELP HIGH PRESSURE BUILD ACROSS SOUTHEASTERN CANADA. ALL OF THIS WILL ASSIST IN SHARPENING A BAROCLINIC ZONE DRAPED FROM WEST TO EAST, WHERE DEVELOPING LOW PRESSURE WILL TRACK ALONG IT. BEFORE WE GET TO THAT THOUGH, SATURDAY NIGHT WILL BE RATHER COLD ALTHOUGH THE LOW-LEVEL FLOW SHOULD BE SHIFTING TO LIGHT SOUTHERLY. AS LOW PRESSURE ORGANIZES FROM THE MID MISSISSIPPI VALLEY TO THE OHIO VALLEY SUNDAY, CLOUDS ARE EXPECTED TO INCREASE ACROSS OUR AREA. THE RAMPING UP OF WAA ALOFT AND THE START OF INCREASING ISENTROPIC LIFT MAY RESULT IN SOME LIGHT SNOW DEVELOPING FROM WEST TO EAST BY LATE AFTERNOON SUNDAY. THE BULK OF THE PRECIPITATION THOUGH IS ANTICIPATED TO REMAIN TO OUR WEST DURING THE DAY SUNDAY. FOR SUNDAY NIGHT AND MONDAY...THE AMPLIFICATION OF THE AFOREMENTIONED UPPER-LEVEL TROUGH WILL CONTINUE INTO MONDAY. THIS DRIVES SURFACE LOW PRESSURE ACROSS THE MID ATLANTIC MONDAY MORNING. THE DIGGING SHORT WAVE IN THE OHIO VALLEY MAY BE SUCH TO ALLOW THE SURFACE LOW TO TRACK A BIT FARTHER NORTHWARD, AND THIS TRACK WILL BE KEY IN DETERMINING THE LOCATION OF THE HEAVIER AXIS OF PRECIPITATION ALONG WITH THE PRECIPITATION TYPES. DESPITE A FAIR AMOUNT OF SPREAD IN THE GUIDANCE REGARDING THE TRACK OF THE SURFACE LOW, THERE IS A GENERAL CONSENSUS OF A STORM AFFECTING OUR AREA. AN ACCUMULATING SNOWFALL, POTENTIALLY SIGNIFICANT, IS ANTICIPATED FOR AT LEAST PARTS OF OUR CWA. WHERE THIS OCCURS ALONG WITH A MIXED PRECIPITATION/RAIN ZONES IS LESS CERTAIN ATTM, ALTHOUGH THE TREND IS POINTING TO OUR NORTHERN ZONES HAVING THE HIGHEST SNOWFALL AMOUNTS. THE LIFT ASSOCIATED WITH THIS FEATURE SHOULD RAPIDLY INCREASE SUNDAY NIGHT INTO MONDAY FROM WEST TO EAST NORTH OF THE SURFACE LOW TRACK WHERE A TIGHTENING THERMAL GRADIENT DEVELOPS. THE ASSOCIATED FRONTOGENETIC FORCING AND WAA SHOULD RESULT IN SOME WEST-EAST BANDS OF SOME HEAVIER PRECIPITATION. THE FARTHER NORTH ONE GOES, THE BETTER CHC OF ALL SNOW. THE PICTURE BECOMES LESS CERTAIN SOUTHWARD ACROSS THE CWA AS THE INJECTION OF WARMER AIR FOR A TIME SHOULD RESULT IN A ZONE OF MIXED PRECIPITATION WITH EVEN AN AREA OF RAIN. WHERE THIS OCCURS CARRIES LOWER CONFIDENCE ATTM AS THE DIFFERENCE IN SOME OF THE MODEL GUIDANCE KEEPS THE DOOR OPEN FOR A SOME NORTHWARD ADJUSTMENT. THE NAM/SREF IS MORE AMPLIFIED, HOWEVER THERE IS MORE SUPPORT FROM THE WELL CLUSTERED GFS AND ECMWF WITH EVEN STRONGER AGREEMENT WITH THEIR ENSEMBLE MEANS. DESPITE THE LOWER CONFIDENCE, BASED ON THE GFS/ECMWF WE ADDED MORE MIXED PRECIPITATION FROM AROUND THE PHILADELPHIA METRO SOUTHWARD WITH EVEN SOME RAIN ACROSS THE DELMARVA. WE DID NOT GO AS FAR NORTH THOUGH WITH THE MIXED PRECIPITATION LIKE THE NAM WOULD SUGGEST. ANY RAIN AND MIXED PRECIPITATION SHOULD CHANGE TO SNOW BEFORE ENDING THROUGH MONDAY AFTERNOON AS COLDER AIR WRAPS INTO THE DEPARTING SYSTEM. SINCE WE ARE NOT REAL CONFIDENT ON WHERE THE MIXED PRECIPITATION ZONE MAY OCCUR, WE ELECTED TO HOLD OFF ON A WINTER STORM WATCH ATTM. OUR STORM TOTAL SNOWFALL MAP /THROUGH 7 PM MONDAY/ LEANED TOWARD THE WPC/WWD GUIDANCE AFTER EARLIER COLLABORATION, AND IT SHOULD BE NOTED THAT THIS MAP IS HIGHLY SUBJECT TO CHANGE. FOR MONDAY NIGHT...THE SURFACE LOW PULLS AWAY INTO THE CANADIAN MARITIMES DURING MONDAY NIGHT WITH HIGH PRESSURE BUILDING IN FROM THE WEST. THE ATMOSPHERE DRIES OUT PRETTY QUICKLY AND ANY LIGHT PRECIPITATION SHOULD END JUST AS FAST. CLEARING SKIES IN THE WAKE OF THE LOW ALONG WITH A FRESH SNOWPACK IN MANY AREAS WILL CONTRIBUTE TO VERY COLD OVERNIGHT TEMPERATURES. TEMPERATURES WILL PLUMMET AND WE WILL SEE TEMPS DROP INTO THE SINGLE DIGITS, BOTH PLUS AND MINUS, ACROSS THE REGION. WITH WINDS REMAINING UP THROUGH THE OVERNIGHT PERIOD, WIND CHILLS WILL BECOME A CONCERN, ESPECIALLY ACROSS THE SOUTHERN POCONOS AND NORTHWESTERN NEW JERSEY. WIND CHILL ADVISORIES MAY BE NEEDED FOR MONDAY NIGHT INTO TUESDAY MORNING ACROSS OUR NORTHERN COUNTIES. FOR TUESDAY...HIGH PRESSURE WILL BUILD ACROSS THE REGION TUESDAY WITH A SHIFT TO OUR EAST TUESDAY NIGHT. PLENTY OF COLD AIR WILL ACCOMPANY THIS HIGH PRESSURE SYSTEM WITH 850MB TEMPERATURES IN THE -10C TO -15C RANGE. MAXIMUM TEMPERATURES WILL STRUGGLE TO RISE AND GENERALLY REMAIN IN THE 20S, WITH TEENS ACROSS THE LEHIGH VALLEY, SOUTHERN POCONOS AND NORTHWESTERN NEW JERSEY. THE HIGH WILL SLIDE ACROSS THE REGION AND MOVE OFFSHORE TUESDAY EVENING. THE AIR MASS WILL MODIFY A BIT AS IT SLIDES EASTWARD AND MINIMUM TEMPERATURES WILL NOT BE AS COLD TUESDAY NIGHT AS THEY ARE ON MONDAY NIGHT. IT WILL STILL BE COLD WITH LOWS RANGING FROM THE SINGLE DIGITS ACROSS THE NORTH TO NEAR 20 ACROSS THE SOUTHERN PART OF THE FORECAST AREA. FOR WEDNESDAY AND THURSDAY...THE EXITS TO THE EAST OF THE AREA WITH A SURFACE LOW PRESSURE SYSTEM LOCATED TO THE NORTH OF THE AREA AND ANOTHER ONE DEVELOPING TO OUR SOUTH. THE NORTHERN LOW GETS PUSHED MORE TO THE NORTHEAST AND THE ATTACHED COLD FRONT LOOKS TO APPROACH THE REGION ON WEDNESDAY. THE SOUTHERN LOW MOVES TOWARD THE CAROLINA COAST AND THEN TRACKS TO THE NORTHEAST. THERE IS DIFFERING OPINIONS AMONGST THE GUIDANCE AS TO WHICH LOW WILL WIN THE BATTLE AND IN TURN WILL AFFECT THE REGION AROUND THURSDAY. WILL KEEP THE MENTION OF PRECIPITATION IN THE FORECAST AT THIS TIME AS WE MONITOR THE TRENDS OF THESE SYSTEMS OVER THE COMING DAYS. MAXIMUM TEMPERATURES WILL GENERALLY BE IN THE UPPER 20S TO UPPER 30S ACROSS THE REGION. FOR FRIDAY...HIGH PRESSURE LOOKS TO BUILD INTO THE REGION FROM THE WEST. HIGHS ON FRIDAY LOOK TO BE COLD ONCE AGAIN AS THE HIGH PUSHES TO THE EAST. TEMPERATURES INTO THE 20S CAN BE EXPECTED ACROSS THE FORECAST AREA.
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&& .AVIATION /21Z FRIDAY THROUGH WEDNESDAY/...
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THE FOLLOWING DISCUSSION IS FOR KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY AND SURROUNDING AREAS. AN AREA OF SNOW SHOWERS EXTENDING FROM SCHUYLKILL AND CARBON COUNTIES SE TO THE PHILADELPHIA AREA WILL BE MOVING THROUGH THE AREA AND AFFECTING SEVERAL OF OUR TAF SITES INTO THE EVENING HOURS. VSBYS AND CEILING HEIGHTS COULD TEMPORARILY DIP INTO THE MIFR RANGE AT TIMES. THESE CONDITIONS SHOULD IMPROVE FAIRLY QUICKLY AFTER SUNSET AS THE SNOW SHOWER ACTIVITY SHUTS DOWN. THE PRIMARY TAF SITES WITH A CONCERN ARE PHL, PNE, RDG, ILG AND TTN. THEN THE MAIN STORY SWITCHES TO THE STRONG NORTHWEST WINDS WITH GUSTS UP TO 40 KT POSSIBLE AT TIMES. THESE WINDS WILL CONTINUE THROUGH A GOOD PART OF SATURDAY. OUTLOOK... SATURDAY NIGHT...VFR. NORTHWEST TO WEST WINDS DIMINISHING TO AROUND 5 KNOTS. SUNDAY AND MONDAY...VFR TO START SUNDAY WITH INCREASING CLOUDS, THEN CEILINGS MAY LOWER TO MVFR BY SUNDAY EVENING. THE CONDITIONS SHOULD THEN QUICKLY LOWER TO IFR/LIFR SUNDAY NIGHT AND MONDAY AS A STORM MOVES ACROSS THE MID ATLANTIC. ALL SNOW LOOKS TO OCCUR FOR KABE, KRDG AND KTTN /LOWEST VISIBILITIES/, THEN SOME MIXED PRECIPITATION MAY OCCUR FOR A TIME MONDAY MORNING AT KPHL, KPNE AND KILG, WITH EVEN A CHANGE TO RAIN FOR A TIME AT KMIV AND KACY. ANY MIXED PRECIPITATION OR RAIN SHOULD CHANGE TO SNOW MONDAY AFTERNOON AS COLDER AIR ARRIVES. THE TRANSITION OF MIXED PRECIPITATION AND THE TIMING CARRIES LOWER CONFIDENCE AT THIS TIME. THERE IS THE POTENTIAL THOUGH FOR A SIGNIFICANT ACCUMULATING SNOWFALL FOR AT LEAST SOME OF OUR TERMINALS, AND AS OF NOW THE HIGHER CHANCE OF THIS IS NORTH OF KPHL. MONDAY NIGHT...IMPROVING CONDITIONS FROM MVFR TO VFR ACROSS THE TERMINALS. DIMINISHING WINDS BECOMING LIGHT OUT OF THE NORTHWEST THROUGH THE OVERNIGHT PERIOD. TUESDAY THROUGH WEDNESDAY...VFR CONDITIONS EXPECTED. LIGHT WESTERLY WINDS BECOMING SOUTHWEST ON WEDNESDAY.
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&& .MARINE...
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NO CHANGES TO THE MARINE HEADLINES. GALE WARNING CONTINUE. EXPECT GUSTS IN THE 35-40 KT THROUGH TONIGHT...AND INTO SATURDAY. SEAS IN THE 5 TO 9 FOOT RANGE CAN BE EXPECTED. CONDITIONS WILL START TO GRADUALLY IMPROVE LATER SATURDAY AFTERNOON. OUTLOOK... SATURDAY NIGHT...THE WINDS AND SEAS SHOULD QUICKLY DECREASE AS HIGH PRESSURE BUILDS IN. SUNDAY AND MONDAY...AN EAST TO NORTHEAST WIND SHOULD INCREASE LATE SUNDAY INTO MONDAY BEFORE TURNING NORTH AS LOW PRESSURE SLIDES ACROSS THE MID ATLANTIC. THIS SHOULD RESULT IN AT LEAST SMALL CRAFT ADVISORY CRITERIA BEING MET, HOWEVER AT THIS TIME THE QUICK MOVEMENT OF THE STORM SHOULD LIMIT WINDS TO LESS THAN GALE FORCE. MONDAY NIGHT THROUGH TUESDAY... SMALL CRAFT ADVISORY CONDITIONS EXPECTED THROUGH TUESDAY. WINDS WILL DECREASE AND FALL BELOW 25 KNOTS TUESDAY MORNING. SEAS WILL REMAIN ELEVATED AND FALL BELOW 5 FEET AROUND TUESDAY AFTERNOON/EVENING. . WEDNESDAY...BOTH WINDS AND SEAS SHOULD REMAIN BELOW SMALL CRAFT ADVISORY CRITERIA.
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&& .PHI WATCHES/WARNINGS/ADVISORIES...
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PA...WIND ADVISORY UNTIL 6 AM EST SATURDAY FOR PAZ054-055-060>062- 070-071-101>106. WIND CHILL ADVISORY FROM MIDNIGHT TONIGHT TO 11 AM EST SATURDAY FOR PAZ054-055. NJ...WIND ADVISORY UNTIL 6 AM EST SATURDAY FOR NJZ001-007>010- 012>027. WIND CHILL ADVISORY FROM MIDNIGHT TONIGHT TO 11 AM EST SATURDAY FOR NJZ001. DE...WIND ADVISORY UNTIL 6 AM EST SATURDAY FOR DEZ001>004. MD...WIND ADVISORY UNTIL 6 AM EST SATURDAY FOR MDZ008-012-015-019- 020. MARINE...FREEZING SPRAY ADVISORY FROM MIDNIGHT TONIGHT TO 6 PM EST SATURDAY FOR ANZ430-431-450>455. GALE WARNING UNTIL 6 PM EST SATURDAY FOR ANZ430-431-450>455.
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&& $$ SYNOPSIS...GORSE NEAR TERM...MIKETTA SHORT TERM...MIKETTA LONG TERM...GORSE/MEOLA AVIATION...GORSE/MEOLA/MIKETTA MARINE...GORSE/MEOLA/MIKETTA

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