Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 140309 AFDPHI AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE MOUNT HOLLY NJ 1109 PM EDT SUN JUL 13 2014 .SYNOPSIS... HIGH PRESSURE WILL REMAINS PARKED OFFSHORE WHILE A COLD FRONT APPROACHES FROM THE WEST TONIGHT AND MONDAY. THE FRONT IS EXPECTED TO GRADUALLY TRAVERSE THE REGION MONDAY NIGHT THROUGH TUESDAY NIGHT BEFORE MOVING OFFSHORE WEDNESDAY MORNING. HIGH PRESSURE BUILDS INTO THE AREA LATER WEDNESDAY THROUGH THURSDAY. THE HIGH WILL EVENTUALLY SHIFT OFF THE COAST BY THE BEGINNING OF NEXT WEEKEND. && .NEAR TERM /UNTIL 6 AM MONDAY MORNING/... THE SEVERE THUNDERSTORMS WATCH HAS BEEN EXTENDED FOR ANOTHER HOUR THROUGH MIDNIGHT AS A LINE OF STRONG TO SEVERE THUNDERSTORMS CONTINUES TO MOVE EASTWARD ACROSS THE AREA. THE STORMS SHOULD MOVE OUT OF THE WATCH AREA AROUND MIDNIGHT AND OUT OF THE AREA BY 2 AM. ADDITIONAL SHOWERS AND THUNDERSTORMS ARE POSSIBLE THROUGH THE REMAINDER OF THE NIGHT AS SEVERAL VORTICITY IMPULSES CONTINUE TO SLIDE ACROSS THE AREA. HOWEVER, THUNDERSTORMS OVERNIGHT ARE NOT EXPECTED TO BE AS WIDESPREAD AS THIS EVENING, ALTHOUGH ANY THUNDERSTORMS THAT DO OCCUR OVERNIGHT COULD CONTAIN GUSTY WINDS AND HEAVY RAINFALL. AFTER THESE T-STORMS MOVE THROUGH THE REST OF THE NIGHT SHOULD BE RATHER WARM AND MUGGY WITH AREAS OF FOG FORMING...ESPECIALLY WHERE RAIN HAS OCCURRED. && .SHORT TERM /6 AM MONDAY MORNING THROUGH 6 PM MONDAY/... LOW PRESSURE WILL MOVE ACROSS CANADA AND A COLD FRONT WILL APPROACH FROM THE N/W. SCT SHOWERS AND TSTMS WILL BECOME MORE NUMEROUS DURING THE AFTERNOON AND PERSIST INTO THE EVENING. THE SPC HAS OUTLINED OUR AREA FOR SVR POTENTIAL TOMORROW. DOWNPOURS WILL PROBABLY CREATE SOME LOCAL FLASH FLOODING CONCERNS LATE...BUT THE AREAS WITH THE LOWEST FF GUIDANCE (N/W) MAY NOT GET THE HIGHEST QPF UNTIL LATE MON OR MON NIGHT...SO WE WILL HOLD WITH THE FF WATCH...BUT STILL OFFER THE POSSIBILITY FOR A WATCH TO BE RAISED TONIGHT. HOT AND HUMID MONDAY WITH HIGHS IN THE UPPER 80S AND LOW 90S AND HEAT INDEX VALUES IN THE MID/UPPER 90S. && .LONG TERM /MONDAY NIGHT THROUGH SUNDAY/... STORMY WEATHER PATTERN EXPECTED MONDAY NIGHT THRU TUESDAY NIGHT AS A COLD FRONT MOVES SLOWLY THRU THE AREA. MULTIPLE ROUNDS OF SEVERE WEATHER AND HEAVY RAINFALL ARE POSSIBLE DURING THIS TIME. SEE HYDRO SECTION FOR MORE INFO ON THE FLASH FLOODING POTENTIAL. THE COLD FRONT WILL MOVE SOUTHEASTWARD FROM THE GREAT LAKES TOWARD THE OHIO VALLEY MONDAY NIGHT. MODELS SHOW THE DEVELOPMENT OF A LEE TROUGH IN CENTRAL/ERN PA AHEAD OF THE COLD FRONT BY THE EVENING. WITH DEWPOINTS IN THE LOWER 70S AND AN ANOMALOUSLY STRONG WIND FIELD, SHOWERS AND THUNDERSTORMS WILL LIKELY CONTINUE THRU THE EVENING AND POSSIBLY OVERNIGHT AS THE LATEST NCEP MODEL SOUNDINGS INDICATE MUCAPE AOA 1000 J/KG FOR MUCH OF THE NIGHT. EVEN IF CONVECTION BECOME ELEVATED, THERE SHOULD BE ENOUGH LIFT WITH THE PRE-FRONTAL TROUGH FOR PARCELS TO REACH THEIR LFC AND CONTINUE AFTER PEAK HEATING HRS. THE COLD FRONT IS THEN EXPECTED TO JUMP THE APPALACHIANS TOWARD THE LEADING SFC TROUGH OVER CENTRAL PA TUESDAY MORNING. MODELS HAVE COME INTO MUCH BETTER AGREEMENT WITH THE POSITION AND TIMING OF THE COLD FRONT ON TUESDAY IN RECENT RUNS. THE TREND HAS BEEN TO SLOW DOWN THE MOVEMENT OF THE FRONT ENOUGH TO KEEP THE FORECAST AREA IN THE WARM/ MOIST PRE-FRONTAL SECTOR ON TUESDAY AND EVEN INTO TUESDAY NIGHT. THIS SLOWER SCENARIO WOULD MAKE SENSE GIVEN THE TRACK OF THE UPPER LOW CUTTING WELL TO OUR NORTHWEST AND THE AMPLIFYING S-SW STEERING FLOW AHEAD OF THE TROUGH BECOMING INCREASINGLY PARALLEL TO THE FRONTAL BOUNDARY. WHILE THE BEST HEIGHT FALLS AND FORCING FOR ASCENT FROM THE UPPER LOW WILL BE RESIDE OVER THE EASTERN GREAT LAKES, SYNOPTIC LIFT WILL INCREASE OVER OUR REGION IN THE RIGHT ENTRANCE-REGION OF A 130+ KT ULVL JET STREAK AND ALSO WITH THE COLD FRONT ITSELF. FCST POPS OF 60-70 PERCENT FOR TUESDAY-TUESDAY NIGHT ARE RELATIVELY HIGH FOR CONVECTIVE SETUPS BUT IT SEEMS WARRANTED GIVEN THE PATTERN. THE RISK FOR FLASH FLOODING IS ENHANCED IN THIS SETUP. ONE OF THE BIG FORECAST UNCERTAINTIES RELATES TO CLOUD COVER, WHICH WILL PLAY A BIG ROLE IN DETERMINING THE POTENTIAL FOR SEVERE THUNDERSTORMS TUESDAY AFTERNOON-EVENING. THERE WILL LIKELY BE CLOUD COVER FROM OVERNIGHT CONVECTION OVER OUR AREA TUESDAY MORNING, BUT A CONDITIONAL RISK FOR SEVERE STORMS EXISTS IF APPRECIABLE BREAKS OCCUR THAT WOULD ALLOW FOR POCKET OF HIGHER INSTABILITY TO DEVELOP DURING THE PEAK HEATING HOURS. THE SETUP WOULD FAVOR A LINEAR MODE OF CONVECTION WITH THE FRONT AND THE BIGGEST THREAT WOULD BE DAMAGING WINDS. UNCERTAINTY REMAINS TOO HIGH ATTM TO ENHANCE WORDING IN THE HWO FOR A SIGNIFICANT SEVERE WEATHER EVENT, BUT THE POSSIBILITY OF IT CANNOT BE RULED OUT IF FULL DESTABILIZATION IS REALIZED GIVEN THE UNSEASONABLY STRONG 30-50 KT DEEP-LAYER SHEAR THAT IS FORECAST TO OVERSPREAD THE AREA BY 00Z WED. ATTM, THE SOUTHEASTERN HALF OF THE FORECAST AREA INCLUDED BY SPC IN A 5 PERCENT CHANCE FOR SEVERE STORMS ON TUESDAY. THE MAJORITY OF 12Z MODEL GUIDANCE HAS THE COLD FRONT MOVING OFF THE NJ AND DE COAST AROUND DAYBREAK WEDNESDAY MORNING. THE NW WINDS BEHIND THE FRONT ARE FORECAST TO BE LIGHT IN THE BOUNDARY LAYER, SO DRIER AND COOLER AIR AT THE SFC MAY BE SLOW TO ARRIVE DURING THE DAY, ESPECIALLY ALONG AND EAST OF I-95 IF THE FRONT SLOWS DOWN FURTHER OR STALLS JUST OFF THE COAST. IN ANY EVENT, ENOUGH SUBSIDENCE ALOFT SHOULD ALLOW FOR SKIES TO CLEAR FROM WEST TO EAST, WITH SOME AREAS BECOMING MOSTLY SUNNY DURING THE AFTERNOON. HIGH PRESSURE BUILDS IN FROM THE MIDWEST LATE WEDNESDAY INTO THURSDAY. MAX TEMPS BOTH WEDNESDAY AND THURSDAY WILL BE AROUND 80F ACROSS MOST OF THE AREA (EXCEPT COOLER IN THE POCONOS AND ALONG THE COAST). THE BIGGEST DIFFERENCE BETWEEN BOTH DAY FOR LOCATIONS ALONG AND EAST OF I-95 WILL LIKELY BE THE HUMIDITY. WHILE IT WON`T FEEL MUGGY ON WEDNESDAY (ESPECIALLY FOR JULY STANDARDS) WITH DEWPOINTS IN THE 60S ON WEDNESDAY, THURSDAY WILL BE MUCH DRIER WITH DEWPOINTS IN THE 50S. THE HIGH STARTS TO MOVE OFFSHORE LATE IN THE WEEK. SOUTHERLY RETURN FLOW DEVELOPING ON THE BACKSIDE OF THE HIGH WILL BE ACCOMPANIED BY AN INCREASE IN THE HEAT AND HUMIDITY. DEPENDING ON HOW QUICKLY THE HIGH MOVES OFFSHORE, SHOWER/THUNDERSTORM CHANCES COULD RETURN AS EARLY AS FRIDAY. BOTH THE 12Z GFS/ECMWF AND SEVERAL OF THE ENSEMBLE MEMBERS INDICATE A WARM FRONT LIFTING NORTHWARD TOWARD THE REGION AND WAVES OF LOPRES DEVELOPING ALONG THE BOUNDARY BY THE BEGINNING OF NEXT WEEKEND. PRECIP CHANCES INCREASE SATURDAY ACCORDINGLY. && .AVIATION /03Z MONDAY THROUGH FRIDAY/... THE FOLLOWING DISCUSSION IS FOR KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY AND SURROUNDING AREAS. A LINE OF SHOWERS AND T-STORMS IS MOVING ACROSS THE AREA AT THIS TIME. SOME OF THESE STORMS WILL LIKELY PRODUCE TEMPORARY IFR CONDS AS WELL AS GUSTY WINDS TO 30 OR 40 KT. WHEN THE SHOWERS/STORMS MOVE OUT, CONDITIONS SHOULD RETURN TO VFR. HOWEVER, WITH THE RAIN, THERE IS THE POTENTIAL FOR FOG AND LOWER VSBYS TO FORM OVERNIGHT. OUTLOOK... MONDAY NIGHT THROUGH TUESDAY NIGHT...MULTIPLE ROUNDS OF SHRA/TSRA DURING THIS PERIOD WILL LEAD TO PERIODS OF SUB-VFR CONDITIONS. SWLY WINDS AROUND 10-15 KT AHEAD OF A COLD FRONT. COLD FRONT SLOWLY MOVES THRU TUESDAY NIGHT. FROPA WILL BE MARKED BY A WIND SHIFT OUT OF THE NW BUT SPEEDS WILL BE LIGHT. WEDNESDAY AND THURSDAY...VFR WITH HIPRES MOVING OVERHEAD. ONLY EXCEPTION MAY BE A BRIEF PERIOD OF LOW CLOUDS NEAR MIV/ACY WEDNESDAY MORNING AS THE COLD FRONT IS SLOWER TO MOVE THRU. DRIER AIR MOVES INTO THE REGION, SO DO NOT EXPECT MUCH IN THE WAY OF FOG AT NIGHT. FRIDAY...HIPRES STARTS TO SHIFT OFFSHORE LATE. MAYBE AN ISO TSRA IN VC OF THE FAR SRN AND WRN TERMINALS LATE IN THE DAY. && .MARINE... WE WILL CONTINUE WITH THE SCA FLAG FOR THE OCEAN WATERS INTO THE EVENING AND LEAVE THE EXPIRATION TIME AS IS FOR NOW. OCNL GUSTS TO 25 KTS AND A SLOWLY BUILDING SEA ACROSS THE WATERS (MOSTLY NORTH) AS THE PRESSURE GRADIENT REMAINS ENHANCED WITH THE APPROACHING LOW/FRONT TO THE WEST. SCT TSTMS THIS EVENING AND AGAIN MON AFTERNOON WILL HAVE LOCAL HIGHER WINDS AND SEAS. OUTLOOK... MONDAY NIGHT THROUGH TUESDAY... GUSTY S-SW WINDS EXPECTED AHEAD OF A COLD FRONT MONDAY NIGHT THRU TUESDAY. SMALL CRAFT ADVISORIES MAY BE NEEDED DURING THIS TIME AS GUSTS TO 25 KT AND SEAS OF 4-5 ARE FORECAST. WINDS SUBSIDE TUESDAY NIGHT WITH THE PRESSURE GRADIENT WEAKENING OVER THE WATERS AS THE COLD FRONT APPROACHES FROM THE WEST. WINDS SHIFT OUT OF THE NW WEDNESDAY AND WEST THURSDAY BUT REMAIN LIGHT. && .HYDROLOGY... A MOISTURE-LADEN AIR MASS IS FORECAST TO BE IN PLACE TONIGHT THROUGH TUESDAY NIGHT AHEAD OF A SLOW-MOVING COLD FRONT. THIS IS CHARACTERIZED BY PW VALUES AROUND 2.00 INCHES. THE PRESENCE OF A LEE SIDE TROUGH MONDAY MAY HELP FOCUS HEAVY RAIN PRODUCING CONVECTION ALONG AND NORTHWEST OF INTERSTATE 95 CORRIDOR, POTENTIALLY RESULTING IN AN INCREASED FLASH FLOOD THREAT. THE FLOW IS FORECAST TO INCREASE THEREFORE THE STORM MOTIONS SHOULD BE DECENT, HOWEVER CONVECTION SHOULD BE EFFICIENT RAINFALL PRODUCERS. THERE ARE AREAS PARTICULARLY IN PORTIONS OF SOUTHEASTERN AND NORTHEASTERN PENNSYLVANIA AND ALSO NORTHERN NEW JERSEY WHERE THE 1-HOUR FLASH FLOOD GUIDANCE IS 1.50 INCHES OR LESS. THE HIGHLY URBAN AREAS ALONG THE INTERSTATE 95 CORRIDOR ARE ALSO VULNERABLE TO QUICK RUNOFF DURING TORRENTIAL DOWNPOURS. THE SETUP FOR HEAVY RAINFALL INCREASES THROUGHOUT THE PERIOD WITH POTENTIALLY THE HIGHEST RISK FOR FLASH FLOODING ON TUESDAY AFTERNOON AND EVENING WHEN THE COLD FRONT EVER-SO-SLOWLY MOVES THRU. THE UNCERTAINTY REVOLVES AROUND WHERE THE AXIS OF HEAVIER RAIN OCCURS AS THERE IS SOME MODEL SPREAD, AND ALSO THE ORGANIZATION OF THE CONVECTION. THE LATEST QPF FROM WPC GIVES US 2-4 INCHES OF RAINFALL OVER THE NEXT THREE DAYS. WHILE SOME LOCATIONS MAY SEE LESS THAN THIS, THERE IS A CHANCE THAT OTHER LOCATIONS SEE THIS AMOUNT OF RAINFALL IN ONLY A FEW HOURS IN HEAVIER/SLOWER MOVING STORMS. NO FLASH FLOOD WATCH ATTM FOR MONDAY BUT WILL MAINTAIN A MENTION IN THE HAZARDOUS WEATHER OUTLOOK. THIS WILL BE REVISITED IN LATER SHIFTS. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...NONE. NJ...NONE. DE...NONE. MD...NONE. MARINE...SMALL CRAFT ADVISORY UNTIL 6 AM EDT MONDAY FOR ANZ450>455. && $$ SYNOPSIS... NEAR TERM...AMC/O`HARA SHORT TERM...O`HARA LONG TERM...KLEIN AVIATION...AMC/KLEIN/O`HARA MARINE...KLEIN/O`HARA HYDROLOGY...

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