Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 260827 AFDPHI AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE MOUNT HOLLY NJ 427 AM EDT TUE MAY 26 2015 .SYNOPSIS... A HIGH PRESSURE SYSTEM WILL BE ANCHORED OFF THE EAST COAST THROUGH THE END OF THIS WEEK. A COLD FRONT WILL APPROACH FROM THE WEST WEDNESDAY AND EVENTUALLY DISSIPATE AS IT TRIES TO MOVE THROUGH THE FORECAST AREA THURSDAY. ANOTHER COLD FRONT IS FORECAST TO APPROACH THE AREA ON SUNDAY AND STALL IN OUR VICINITY ON MONDAY. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... FOR THE THIRD STRAIGHT DAY, HIGH PRESSURE WILL REMAIN PARKED OFF THE MID-ATLANTIC COAST TODAY. ALTHOUGH THE HUMIDITY WILL INCREASE SLIGHTLY TODAY OWING TO THE CONTINUED SOUTHERLY RETURN FLOW AROUND THE HIGH, IT WON`T BE ON PAR WITH A QUINTESSENTIAL MUGGY SUMMER DAY IN THE MID ATLANTIC. OFFICIAL TEMP FORECAST FALLS ON THE WARMER ENVELOPE OF GUIDANCE AFTER TAKING INTO ACCOUNT THE RECENT COOL BIAS EXHIBITED BY THE MODELS. EXPECT HIGHS TODAY TO BE IN THE MID TO UPPER 80S WHICH IS GENERALLY ON PAR WITH YESTERDAY, IF NOT A DEGREE OR TWO WARMER. DID NOT QUITE GO HIGH ENOUGH TO FORECAST THE FIRST 90 DEGREE DAY OF THE SEASON FOR PHILADELPHIA, BUT THE POSSIBILITY CANNOT BE COMPLETELY RULED OUT. THE UPPER RIDGE AXIS IS FORECAST TO SHIFT JUST EAST OF THE EAST COAST TODAY. THE LARGE-SCALE SUBSIDENCE ASSOCIATED WITH THIS RIDGE WILL WEAKEN OVERHEAD A BIT, WHICH MAY RESULT IN SOME POP UP SHOWERS AND STORMS FORMING OVER THE HIGHER TERRAIN OF NE PA AND NW NJ. THE WINDOW FOR CONVECTION APPEARS TO BE MID TO LATE AFTERNOON WHEN DIURNAL INSTABILITY PEAKS AND A SUBTLE SHORTWAVE TROUGH THAT IS CURRENTLY OVER THE SOUTHERN APPALACHIANS MOVES AROUND THE RIDGE AND INTO EASTERN PA. MLCAPE PROFILES BELOW 1000 J/KG AND DEEP-LAYER BULK SHEAR BELOW 25 KT ARE NOT SUPPORTIVE OF INTENSE/ORGANIZED STORMS. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM WEDNESDAY/... A MILD NIGHT IS IN STORE TONIGHT WITH PERSISTENT SOUTHERLY FLOW AROUND THE OFFSHORE HIGH. HOURLY POPS FOR TONIGHT EXHIBIT A SEMI- DIURNAL TREND; DECREASING AFTER SUNSET DUE TO THE LOSS OF DAYTIME INSTABILITY. HOWEVER, ISOLATED SHOWERS MAY LAST INTO PART OF THE NIGHT OVER THE HIGHER TERRAIN AS A WEAK DISTURBANCE MOVES THROUGH THE AREA. AT THE VERY LEAST, THE AFOREMENTIONED DISTURBANCE WILL RESULT IN AN INCREASE IN MID AND HIGH CLOUDS OVER A MAJORITY OF THE FORECAST AREA. THE CLOUDS WILL LIMIT RADIATIONAL COOLING AND KEEP TEMPS WARMER THAN IN PREVIOUS NIGHTS. FORECAST LOWS ARE GENERALLY IN THE MID TO UPPER 60S. CLOUDS WILL LIMIT THE FOG POTENTIAL, BUT THERE MAY BE PATCHY FOG THAT DEVELOPS TOWARD DAWN AT INLAND LOCATIONS THAT RECEIVE ANY RAINFALL TODAY. && .LONG TERM /WEDNESDAY THROUGH MONDAY/... 5-WAVE MODEL GUIDANCE SHOWS EAST COAST RIDGING AS THE DOMINANT FORCE THROUGH MOST OF THE LONGTERM PERIOD, WITH A RIPPLE OR TWO. BY THE WEEKEND THE RIDGING BEGINS TO BREAK DOWN, MOVES OFF TOWARDS THE NORTHEAST, WHILE GREAT LAKES TROUGHING STARTS TO PUSH IN EARLY NEXT WEEK. OVERALL THE LONGTERM REMAINS IN TACT FROM THE PREVIOUS SHIFT WITH DAILY CHANCES FOR SHOWERS AND THUNDERSTORMS EACH AFTERNOON TIME PERIOD...BETTER WEST THAN EAST WITH LEE TROUGHING POSSIBLE AND DIFFERENTIAL HEATING ALONG THE RIDGE TOPS. WEDNESDAY...COLD FRONT MOVES CLOSER TO THE REGION FROM THE NORTHWEST. EXPECTING A THERMAL/LEE SIDE TROUGH TO DEVELOP TO OUR IMMEDIATE WEST DURING THE PEAK HEATING HOURS AND SLOWLY MOVE IT TO THE EAST THROUGH THE EVENING HOURS. BETTER HEATING ALONG THE RIDGE TOPS SHOULD BE THE PRIME FOCUS FOR CONVECTIVE INITIATION DURING THE AFTERNOON HOURS WHERE HIGHER SURFACE BASED CAPE VALUES SETUP AND SOMEWHAT BETTER 0-4KM SHEAR VALUES...AROUND 35 KNOTS. ONE ISSUE IS STRICTLY TIED TO THE TIMING OF THE TROUGHS PASSAGE AS THE SHEAR VALUES ARE EXPECTED TO DECREASE THROUGH THE AFTERNOON. NONE-THE-LESS POP-UP/AIRMASS THUNDERSTORMS ARE EXPECTED WITH BETTER ORGANIZED CONVECTION TO OUR SOUTHWEST AND NORTH. WHILE THUNDERSTORMS WILL BE STRONG, HIGH PWATS WILL ALSO PRODUCE LOCALLY HEAVY DOWNPOURS. TEMPERATURES STILL RUNNING WELL ABOVE NORMAL THOUGH COULD END UP A DEGREE OR TWO COOLER THAN TUESDAY WITH A BIT MORE CLOUD COVER IN PLACE. NOT EXPECTING TO HAVE ISSUES WITH HEAT INDEX VALUES AS WE SHOULD BE ABLE TO MIX OUT THE DEWPOINTS A BIT, THOUGH MUCH LESS THAN PREVIOUS DAYS, AND KEEP VALUES SLIGHTLY STICKIER THAN PREDICTED AFTERNOON MAXES. THURSDAY - FRIDAY...COLD FRONT LOOKS TO STALL AND EVENTUALLY DISSIPATE ACROSS THE REGION BY FRIDAY. THE FORCING WITH THE FRONT DECREASES WITH TIME BECOMING NIL BY FRIDAY. AGAIN, WITH A WARM AIRMASS IN PLACE, THOUGH SLIGHTLY COOLER THAN PREVIOUS DAYS, WE WILL HAVE PLENTY OF DEWPOINT POOLING OCCURRING TO KEEP THE CHANCE POPS BOTH DAYS...DIFFUSE FRONT WILL ACT AS A MOISTURE CONDUIT WITH POCKETS OF HEAVIER RAIN BEING THE BIGGER ISSUE COMPARED TO SEVERE STORMS. SATURDAY - SUNDAY...WE WILL KEY ON SUNDAY VS SATURDAY AS THE CONVECTIVELY MORE ACTIVE DAY GIVEN THE BETTER MODEL CONSENSUS AND OVERALL TENDENCY FOR A SLOWER TIMING. POPS HIT A RELATIVE LOW ON SATURDAY BEFORE RAMPING UP AGAIN ON SUNDAY. ALSO GIVEN THE SLOWER FRONTAL TIMING, MAX TEMPS ARE MUCH CLOSER TO WPC GUIDANCE AND EC MOS GUIDANCE THAN GFS MOS. MONDAY...INITIAL LOOK AT MONDAY HAS THE FRONT STALLED JUST SOUTH OF OUR CWA, THIS SHOULD BE RELATIVELY COOLER WITH STILL SOME SHOWERS AND THUNDERSTORMS AROUND. EASTERLY ONSHORE FLOW WILL HELP TO KEEP MOISTURE STREAMING INTO THE REGION WHICH MAY PRODUCE A MUCH NEEDED WIDESPREAD SOAKING RAIN WITH SUPPORT FROM THE UPPER LEVEL DYNAMICS...JET INTERACTION AND PASSING SHORTWAVES. && .AVIATION /08Z TUESDAY THROUGH SATURDAY/... THE FOLLOWING DISCUSSION IS FOR KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY AND SURROUNDING AREAS. PREDOMINATELY VFR TODAY AND THIS EVENING. SOUTHERLY WINDS 5-10 KT THIS MORNING WILL INCREASE THIS AFTERNOON WITH GUSTS UP TO 20 KT POSSIBLE. MAY SEE AN ISOLATED SHRA OR TSRA IN VC OF ABE/RDG BUT COVERAGE TOO LOW TO INCLUDE IN THE 06Z TAF ISSUANCE. MOST TAF SITES SHOULD REMAIN AT VFR OVERNIGHT BUT PATCHY FOG MAY DEVELOP AT THE USUAL FOG-PRONE TERMINALS. OUTLOOK... WEDNESDAY...POSSIBLE MORNING MVFR FOG. NUMEROUS SHOWERS AND THUNDERSTORMS EXPECTED DURING THE AFTERNOON/EARLY EVENING HOURS...BETTER CHANCES ACROSS RDG/ABE. THURSDAY-SATURDAY...EARLY MORNING FOG/STRATUS FOLLOWED BY SCATTERED AFTERNOON SHOWERS AND THUNDERSTORMS. && .MARINE... THE SCA THAT WAS IN EFFECT FOR PORTIONS OF THE COASTAL WATERS EARLY THIS MORNING WAS CANCELLED AS WINDS ARE BELOW 20 KT AND SEAS 4 FT OR LESS. DEBATED ON WHETHER OR NOT TO ISSUE A SCA FOR THIS AFTERNOON AND TONIGHT AS WINDS WILL BE GUSTY FOR A PERIOD LATE IN THE DAY TODAY AND WAVES WILL RESPOND ACCORDINGLY. OPTED TO HOLD OFF ON ANY MARINE HEADLINE SINCE THE WIND FIELD WILL BE SLIGHTLY WEAKER THIS AFTERNOON THAN YESTERDAY, SO NEAR-SHORE GUSTS SHOULD GENERALLY BE BELOW 25 KT. WINDS WILL BE STRONGER A FEW MILES OFFSHORE BUT MIXING WILL BE SIGNIFICANTLY WEAKER DUE TO THE COOLER WATERS. SEAS TONIGHT SHOULD GENERALLY BE AROUND 4 FT IN THE COASTAL ATLANTIC. OUTLOOK... WEDNESDAY...POSSIBLE SCA CONDITIONS WITH A STRONG SOUTHERLY FLOW INPLACE. GUSTS MAY REACH 25 KNOTS WITH SEAS REACHING 5 FEET ON THE OCEAN WATERS. STILL DO NOT HAVE THE CONFIDENCE TO ISSUE A SCA ON THIS SHIFT. WE WILL CONTINUE TO MONITOR. THURSDAY THROUGH SATURDAY...WINDS AND WAVES SHOULD BE BELOW SCA THRESHOLD AS A WEAKER PRESSURE PATTERN SHOULD PREVAIL. && .CLIMATE... A TOP 10 WARMEST MAY WILL OCCUR IN LARGE PORTIONS OF OUR FORECAST AREA. THE FIRST 25 DAYS OF MAY IN OUR FORECAST AREA WERE AVERAGING ABOUT 5 TO 6 DEGREES ABOVE NORMAL. RUNNING THE NUMBERS FROM THE FCST THROUGH THE END OF THE MONTH WE FIND THE FOLLOWING. FOR PHILADELPHIA...PERIOD OF RECORD (POR) BACK TO 1874. PROJECTING A MONTHLY AVERAGE TEMPERATURE OF 69 TO 70 RISING FROM THE CURRENT VALUE OF 67.8, AT LEAST 5 TO 6 DEGREES ABOVE NORMAL. HEADING FOR MOST LIKELY THE THIRD WARMEST MAY ON RECORD. WARMEST AVERAGE TEMPERATURE IN MAY ON RECORD WAS 70.8 IN 1991. THEN 69.2 2004 AND 68.2 IN 2012. FOR ALLENTOWN...PERIOD OF RECORD (POR) BACK TO 1922. THE MONTHLY AVERAGE TEMPERATURE IS CURRENTLY 63.9 DEGREES. IT IS LIKELY TO INCREASE BY A DEGREE OR TWO LATER THIS WEEK, LEADING TO A TOP 10 WARMEST MAY THIS YEAR AT ALLENTOWN. THE WARMEST AVERAGE TEMPERATURE IN MAY WAS 67.2 BACK IN 1991. FOR WILMINGTON.... THE MONTH OF MAY IS LIKELY TO END UP IN THE TOP 5- 10 WARMEST. THE CURRENT MONTHLY TEMPERATURE IS 65.8 DEGREES AND SHOULD RISE A DEGREE OR TWO BY MONTHS END. THE WARMEST MAY WAS IN 1896 WHERE THE MONTHLY TEMPERATURE WAS 70.1 DEGREES. IN 1991 THE MONTHLY TEMPERATURE WAS 69.1 DEGREES. FOR ATLANTIC CITY... THE MONTH IS LIKELY TO END UP IN THE TOP THREE WARMEST MAYS. CURRENTLY, THE AVERAGE TEMPERATURE FOR THE MONTH IS 64.5 DEGREES AND IS LIKELY TO RISE A DEGREE TOR TWO BY MONTHS END. THE WARMEST MAY WAS BACK IN 2004 WHERE THE AVERAGE TEMPERATURE REACHED 66.9 DEGREES. IN 1991 THE MONTHLY AVERAGE TEMPERATURE WAS 66 DEGREES. BOTH 2011 AND 2012 SAW A MONTHLY AVERAGE TEMPERATURE OF 65 DEGREES. NORMAL FIRST DATE OF 90 OR HIGHER. PHILADELPHIA MAY 29 ALLENTOWN MAY 30 WILMINGTON JUNE 4 ATLANTIC CITY JUNE 11. ************************************************************* HERE IS OUR ANNUAL LOOK AS TO HOW HOT, HOT, HOT THE SUMMER WILL BE BASED ON PAST PERFORMANCES. A FULL FLEDGED EL NINO IS UNDERWAY IN THE TROPICAL PACIFIC. ALL DYNAMICAL AND STATISTICAL MODELS KEEP IT GOING, THE DIFFERENCES THAT EXIST ARE TO ITS STRENGTH THROUGH THIS UPCOMING SUMMER. THE DYNAMICAL AND STATISTICAL MODELS ARE AVERAGING A MODERATE EL NINO FOR THE COMBINED SUMMER MONTHS. SOME MODELS, INCLUDING THE CFS2 AND ECMWF ARE PREDICTING A STRONG EL NINO. IN THE CPC MONITORING EL NINO ERA STARTING SINCE 1950, THE HIGHEST (WARMEST) SUMMER ANOMALY RECORDED WAS +1.5C (NEAR OR AT THE THRESHOLD FOR STRONG EL NINOS) DURING THE SUMMER OF 1997. THIS UPCOMING WEEK WILL REALLY SEAL THE DEAL THAT THIS MAY WILL BE UNSEASONABLY WARM IN PHILADELPHIA. IT "MAY" EVEN BE A TOP THREE WARMEST. THIS FOLLOWS ON THE HEELS OF AN UNSEASONABLY WARM APRIL. SINCE 1872, THE COMBINATION OF BOTH WARM (TOP THIRD OF ALL YEARS) APRILS AND MAYS HAS ONLY OCCURRED TWENTY-THREE TIMES. BUT IT HAS OCCURRED ELEVEN TIMES SINCE 1990 AND SIX OF THE LAST SEVEN SPRINGS. SO WE SCOURED THE CLIMATE RECORDS TO FIND BUDDING OR IN PROGRESS EL NINOS THAT COINCIDED WITH BOTH WARM APRILS AND MAYS LOCALLY IN THE CPC MONITORING ERA. WE FOUND THREE PAST SUMMERS THAT MET THE CRITERIA. THE ONE SIGN IS THAT ALL THREE ANALOG SUMMERS WERE WETTER THAN NORMAL, HOPEFULLY KEEPING DROUGHT CONDITIONS FROM INTENSIFYING FURTHER. THE NORTH AMERICAN MULTI MODEL ENSEMBLE PREDICTIONS FOR THIS UPCOMING SUMMER HAS WARMER AND DRIER THAN NORMAL WEATHER EXPECTED IN OUR AREA. THE INTERNATIONAL MULTI MODEL ENSEMBLE FORECASTS ARE SLIGHTLY MORE OPTIMISTIC PREDICTING NEAR NORMAL TEMPERATURES AND PRECIPITATION. THE ANALOGS FOR PHILADELPHIA ARE: YEAR JUNE AVG JULY AVG AUGUST AVG SUMMER AVG SUMMER PCPN 1969 73.4 75.1 75.2 74.6 18.30 1977 68.6 77.8 76.2 74.2 15.50 1991 75.7 79.0 79.0 77.9 12.01 AVG 72.6 77.3 76.8 75.6 15.27 1981- 2010 73.3 78.1 76.6 76.0 11.28 NORMAL THE OFFICIAL CPC OUTLOOK FOR OUR FORECAST AREA IS FOR EQUAL CHANCES OF ABOVE AND BELOW NORMAL TEMPERATURES AS WELL AS EQUAL CHANCES OF ABOVE AND BELOW NORMAL PRECIPITATION. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...NONE. NJ...NONE. DE...NONE. MD...NONE. MARINE...NONE. && $$ SYNOPSIS...GIGI/KLEIN NEAR TERM...KLEIN SHORT TERM...KLEIN LONG TERM...GIGI/HEAVENER AVIATION...HEAVENER/KLEIN MARINE...HEAVENER/KLEIN CLIMATE...

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