Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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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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