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Area Forecast Discussion
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000
FXUS61 KPHI 021358
AFDPHI

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MOUNT HOLLY NJ
958 AM EDT WED SEP 2 2015

.SYNOPSIS...
HIGH PRESSURE REMAINS NEAR THE AREA TODAY AND TONIGHT, THEN A 
BACKDOOR COLD FRONT SLIDES ACROSS OUR REGION THURSDAY NIGHT INTO 
FRIDAY. HIGH PRESSURE WILL BUILD ACROSS THE NORTHEAST AND INTO THE 
MID ATLANTIC LATER FRIDAY AND SATURDAY, BEFORE WEAKENING SOME SUNDAY 
AND MONDAY. A COLD FRONT MAY THEN ARRIVE LATE TUESDAY.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...

HIGH PRESSURE WAS CENTERED OVER THE OH AND TN VALLEYS THIS MORNING. 
THE FOG CONTINUES TO LIFT DURING THE MID MORNING AND ALL SITES 
EXCEPT FOR VAY ARE NOW REPORTING VISIBILITY ABOVE 1SM. HENCE, THE 
DENSE FOG ADVISORY WAS ALLOWED TO EXPIRE AT 9 AM. EXPECT FOG AND 
STRATUS TO DISSIPATE BEFORE 11 AM.

IT WILL ONCE AGAIN FEEL LIKE A SUMMER DAY OUT THERE WITH THE HEAT 
AND HUMIDITY BEING THE MAIN STORYLINE TODAY. WESTERLY RETURN FLOW 
AROUND THE PERIPHERY OF THE SOUTHEAST US RIDGE WILL CONTINUE TO DRAW 
WARM AIR INTO THE MID ATLANTIC. BASED ON FULL MIXING, H8 TEMPS OF 16-
18C, WILL YIELD HIGH TEMPERATURES SIMILAR TO YESTERDAY...IN THE
LOW 90S TO PERHAPS MID 90S IN PHILA (EXCEPT COOLER ACROSS THE
HIGHLANDS AND ALONG THE COAST). INLAND OF THE SEA-BREEZE FRONT
EXPECT DEWPOINTS TO MIX DOWN A BIT TOWARD THE MID 60S, YIELDING
HEAT INDICES 2-3 DEGREES ABOVE THE DRY BULB TEMPERATURE.

SOME OF THE HI-RES MODEL GUIDANCE HIGHLIGHT TWO AREAS FOR POTENTIAL 
CONVECTIVE DEVELOPMENT THIS AFTERNOON: 1) EASTERN MD WHERE A WEAK 
MID-LEVEL SHORTWAVE TROUGH WILL APPROACH THE CHESAPEAKE BAY AND 2) 
IN SOUTHERN NJ/SOUTHERN DE ALONG A SEA-BREEZE FRONT. MODEL
SOUNDINGS SHOW A MID-LEVEL SUBSIDENCE INVERSION SIGNIFICANT ENOUGH
TO CAP DEEPER CONVECTION (EXCEPT FOR THE NAM) BUT THERE IS ENOUGH
MOISTURE TRAPPED BELOW THE INVERSION (AS EVIDENT BY THE MORNING
FOG) FOR AN ISOLATED SHOWER OR TWO TO DEVELOP IN THESE AREAS.
MOST AREAS WILL STAY DRY THOUGH.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM THURSDAY/...
WITH THE SAME AIR MASS STILL IN PLACE TONIGHT, IT LOOKS LIKE ANOTHER 
ROUND OF PATCHY FOG IS LIKELY OVERNIGHT AS ONCE AGAIN RADIATIONAL 
COOLING WILL LEAD TO LOWS NEAR THE DEW POINT TEMPERATURES.

&&

.LONG TERM /THURSDAY THROUGH TUESDAY/...
THE SYNOPTIC SETUP IS COMPRISED OF A WEAKENING TROUGH ACROSS THE 
EAST THURSDAY WHICH LIFTS OUT DURING FRIDAY AS A RIDGE TO THE WEST 
SLIDES EASTWARD. THIS RIDGE IS FORECAST TO REMAIN IN PLACE OVER THE 
WEEKEND, THEN IT SHOULD FLATTEN OUT EARLY NEXT WEEK AS A SHORT WAVE 
TROUGH SLIDES ACROSS THE NORTHERN TIER OF STATES. OVERALL, THIS 
PATTERN SHOULD TEND TO FAVOR WARMTH AND DRYNESS ALTHOUGH A BRIEF 
BREAK OCCURS DUE TO A BACKDOOR COLD FRONT. THE REGION NEEDS RAIN, 
HOWEVER IT LOOKS LIKE THE CHCS ARE RATHER LOW GIVEN NO ORGANIZED 
SYSTEM MOVING THROUGH. WE USED A MODEL/CONTINUITY BLEND FOR THURSDAY 
INTO SATURDAY, WITH A 00Z WPC GUIDANCE/CONTINUITY BLEND THEREAFTER. 
SOME ADJUSTMENTS WERE THEN MADE AFTER ADDITIONAL COLLABORATION WITH 
OUR NEIGHBORING OFFICES.

FOR THURSDAY AND FRIDAY...AS AN UPPER-LEVEL TROUGH GRADUALLY LIFTS 
OUT OF THE EAST AND RIDGING MOVES EASTWARD FROM THE GREAT LAKES AND 
PLAINS, SURFACE HIGH PRESSURE WILL BUILD INTO NEW ENGLAND. THIS 
SHOULD SEND A BACKDOOR COLD FRONT THROUGH OUR AREA LATE THURSDAY 
INTO FRIDAY. SOME SHORT WAVE ENERGY EMBEDDED WITHIN THE LINGERING 
TROUGH COMBINED WITH SOME CONVERGENCE MAY RESULT IN SOME CONVECTION 
THURSDAY FROM SOUTH TO NORTH. THIS IS NOT ANTICIPATED TO BE ALL THAT 
WIDESPREAD AS THE LIFT GENERALLY LOOKS WEAK. ANY CONVECTION ON 
FRIDAY SHOULD BE SHIFTING SOUTHWESTWARD AS SURFACE HIGH PRESSURE 
BUILDS DOWN FROM NEW ENGLAND. THIS WILL RESULT IN THE LOW-LEVEL FLOW 
TURNING FROM THE EAST OR NORTHEAST, AND WITH MORE OF AN OCEAN 
INFLUENCE A COOLER AIRMASS IS ANTICIPATED. AS OF NOW, IT APPEARS 
THAT THE DRY AIR MAY WIN OUT AND THEREFORE DESPITE THE ONSHORE FLOW 
WE SHOULD CLEAR THE CLOUDS OUT FROM NORTH TO SOUTH DURING THE COURSE 
OF FRIDAY. IF THE TRANSITION IS DELAYED OR IS NOT CLEAN, THEN A 
PERIOD OF LOW CLOUDS AND FOG MAY BE POSSIBLE WITH THE ONSET OF THE 
ONSHORE FLOW. SINCE THE PROSPECTS OF SHOWERS AND THUNDERSTORMS IS ON 
THE LOW SIDE DURING THIS TIME FRAME, WE KEPT THE POPS AT OR BELOW 
LOW CHC. THERE DOES APPEAR TO BE A WINDOW WHERE BREEZY CONDITIONS 
DEVELOP ESPECIALLY CLOSER TO THE COAST FRIDAY.

FOR SATURDAY AND SUNDAY...AS THE UPPER-LEVEL RIDGE BUILDS ACROSS THE 
AREA, SURFACE HIGH PRESSURE OVER THE REGION SHOULD EASE EASTWARD. 
THIS WILL ALLOW THE ONSHORE FLOW TO WEAKEN AND EVENTUALLY TURN 
SOUTHERLY. THERE COULD BE SOME RESIDUAL CLOUDINESS ACROSS THE FAR 
SOUTHERN ZONES TO START SATURDAY DEPENDING ON THE DRY AIR PUSH, 
OTHERWISE A FAIRLY QUIET WEEKEND IN THE WEATHER DEPARTMENT. AS WE 
LOSE THE ONSHORE INFLUENCE, DAYTIME TEMPERATURES ARE ANTICIPATED TO 
REBOUND FAIRLY QUICKLY WITH SOME INLAND AREAS NEARING 90 DEGREES 
SUNDAY.

FOR MONDAY AND TUESDAY...A SHORT WAVE TROUGH SLIDING ACROSS THE 
NORTHERN TIER OF STATES SHOULD FLATTEN THE RIDGE. THIS WILL ALSO 
FORCE HIGH PRESSURE MORE OFFSHORE, AND A WEAK COLD FRONT MAY MOVE 
THROUGH LATE TUESDAY. THE BULK OF THE ENERGY LOOKS TO TRACK WELL TO 
OUR NORTH TUESDAY, THEREFORE THE CHCS FOR CONVECTION AS THE FRONT 
ARRIVES IS LESS CERTAIN. WE THEREFORE HAVE A DRY MONDAY FORECAST AND 
THEN BRING IN SOME SLIGHT CHC POPS DURING TUESDAY. ONCE AGAIN AHEAD 
OF THIS FRONT, VERY WARM TO HOT CONDITIONS LOOK TO OCCUR.

&&

.AVIATION /14Z WEDNESDAY THROUGH SUNDAY/...
THE FOLLOWING DISCUSSION IS FOR KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY AND SURROUNDING AREAS.

THE FOG IS QUICKLY LIFTING MID MORNING. ANY LINGERING MVFR VSBYS
SHOULD IMPROVE TO VFR BY 15Z. VFR THIS AFTERNOON AND EVENING.

LIGHT AND VARIABLE WINDS THIS MORNING WILL BECOME W-NW THIS
AFTERNOON BUT ONLY ABOUT 5 KT. THIS WILL ALLOW FOR A SEA-BREEZE
FRONT TO MOVE INLAND ONCE AGAIN THIS AFTERNOON. THUS HAVE
INCLUDED A SHIFT TO SOUTHEASTERLY NOT ONLY AT THE COASTAL (KACY
AND KMIV) BUT ALSO KPHL, KPNE, AND KTTN, WHICH SAW THE SEA BREEZE
YESTERDAY. THE SEA BREEZE SHOULD DISSIPATE NEAR OR SHORTLY AFTER
SUNSET, LEADING TO MOSTLY LIGHT AND VARIABLE WINDS OVERNIGHT.

FOG COULD REDEVELOP AGAIN TONIGHT OVERNIGHT, ESPECIALLY FOR THE
COASTAL TAF SITES.

OUTLOOK...
THURSDAY AND FRIDAY...VFR OVERALL, ALTHOUGH LOCAL FOG/STRATUS IS 
POSSIBLE TO START EACH MORNING. A FEW SHOWERS OR THUNDERSTORMS ARE 
POSSIBLE ESPECIALLY LATER THURSDAY AND INTO FRIDAY. THE WINDS SHIFT 
TO NORTHEASTERLY BEHIND A FRONT AND MAY TURN A BIT GUSTY FRIDAY 
ESPECIALLY CLOSER TO THE COAST.

SATURDAY AND SUNDAY...MOSTLY VFR AS HIGH PRESSURE BUILDS IN FROM NEW 
ENGLAND. EAST TO NORTHEAST WINDS MAY GUST TO 20 KNOTS SATURDAY, THEN 
DIMINISH AT NIGHT AND TURN SOUTHEASTERLY DURING SUNDAY.

&&

.MARINE...
WINDS AND SEAS SHOULD REMAIN WELL BELOW SMALL CRAFT ADVISORY 
CRITERIA, WITH PEAK WIND GUSTS ONLY EXPECTED TO BE NEAR 15KT, AND 
SEAS SHOULD STAY AROUND OR BELOW 2 FT.

OUTLOOK...
THURSDAY...THE CONDITIONS ARE ANTICIPATED TO BE BELOW SMALL CRAFT 
ADVISORY CRITERIA AS A BACKDOOR COLD FRONT STARTS TO ARRIVE LATE. A 
WIND SHIFT TO THE EAST AND NORTHEAST AT NIGHT FROM NORTH TO 
SOUTH IS ANTICIPATED.

FRIDAY AND SATURDAY...SEAS BUILD TO SMALL CRAFT ADVISORY CRITERIA ON 
THE OCEAN WITH AN EASTERLY FLOW BEHIND A BACKDOOR COLD FRONT DURING 
THE DAY FRIDAY AND CONTINUE INTO SATURDAY. THE WIND GUSTS LOOK TO 
REACH SMALL CRAFT ADVISORY CRITERIA ACROSS MUCH OF THE AREA DURING 
THE COURSE OF FRIDAY, THEN DIMINISH DURING SATURDAY.

SUNDAY...SEAS ARE ANTICIPATED TO HAVE SUBSIDED BELOW SMALL CRAFT 
ADVISORY CRITERIA AS HIGH PRESSURE BUILDS IN.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...NONE.
NJ...NONE.
DE...NONE.
MD...NONE.
MARINE...NONE.

&&

$$
SYNOPSIS...GORSE
NEAR TERM...KLEIN
SHORT TERM...JOHNSON
LONG TERM...GORSE
AVIATION...GORSE/JOHNSON/KLEIN
MARINE...GORSE/JOHNSON



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