272  
FXUS61 KPHI 271714  
AFDPHI  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE MOUNT HOLLY NJ  
114 PM EDT WED MAY 27 2015  
   
SYNOPSIS
 
 
THE EAST COAST REMAINS ON THE WESTERN PERIPHERY OF AN ATLANTIC  
HIGH PRESSURE SYSTEM TODAY. A COLD FRONT WILL MOVE ACROSS THE AREA  
THURSDAY INTO THURSDAY NIGHT. HIGH PRESSURE REESTABLISHES ITSELF  
OFF THE EAST COAST FRIDAY INTO SATURDAY. A COLD FRONT IS FORECAST  
TO MOVE ACROSS THE AREA SATURDAY NIGHT INTO SUNDAY. HIGH PRESSURE  
WILL RETURN TO THE NORTHEAST EARLY NEXT WEEK WHILE THE FRONTAL  
BOUNDARY STALLS TO OUR SOUTH.  
 

 
   
NEAR TERM /UNTIL 6 PM THIS EVENING/
 
 
A SEVERE THUNDERSTORM WATCH IS IN EFFECT FOR THE AREA UNTIL 700  
PM.  
 
WITH THE UPDATE, HOURLY TEMPERATURES AND DEWPOINTS WERE ADJUSTED  
SOMEWHAT USING THE LAV/LAMP GUIDANCE TO MORE CLOSELY MATCH THE  
LATEST OBS AND TRENDS, BUT MAX TEMPERATURES LOOK ON TRACK BASED ON  
THIS MORNINGS 12Z REGIONAL SOUNDINGS. WE CONTINUE TO MONITOR SOME  
LINGERING STRATUS ACROSS THE REGION AS THIS WILL PLAY A ROLE IN  
WHEN/HOW MUCH THE ATMOSPHERE DESTABILIZES LATER TODAY. THE HRRR  
AND OTHER GUIDANCE CONTINUES TO REFLECT CONVECTION MOVING INTO OUR  
AREA FOR MID-AFTERNOON TO EVENING, AND WE KEPT HIGHER POPS IN  
PLACE FOR THIS PERIOD.  
 
OTHERWISE, SURFACE HIGH PRESSURE AND ASSOCIATED UPPER RIDGE WILL  
CONTINUE TO PARK OFF THE EAST COAST TODAY. THE MID-ATLANTIC REGION  
WILL BE POSITIONED IN BETWEEN THIS DOWNSTREAM RIDGE AND AN  
APPROACHING UPPER TROUGH OVER THE MIDWEST/GREAT LAKES. SOUTH-  
SOUTHWEST WINDS THROUGHOUT THE TROPOSPHERE WILL BACK AND  
STRENGTHEN SLIGHTLY AS THE PRESSURE/GEOPOTENTIAL HEIGHT GRADIENT  
BETWEEN THESE TWO SYSTEMS TIGHTENS OVER THE EASTERN SEABOARD.  
 
A NOTICEABLE CHANGE IN THE SENSIBLE WX TODAY COMPARED TO RECENT  
DAYS WILL BE THE INCREASE IN HUMIDITY WITH DEWPOINTS RISING INTO  
THE MID 60S TO NEAR 70F THIS AFTERNOON.  
 
THE ATMOSPHERE IS FORECAST TO BECOME MODERATELY UNSTABLE THIS  
AFTERNOON AS A DIRECT RESULT OF THE THE INCREASING MOISTURE AND  
STRONG DIABATIC HEATING. MEAN MLCAPE VALUES FROM BOTH THE 00Z  
NCAR/MMM HI-RES ENSEMBLES AND 03Z SREF ARE AROUND 1500 J/KG FOR  
OUR INLAND AREAS. WHILE FORECAST 0-6 KM BULK SHEAR OF 20-25 KT  
WILL BE MODEST, A 35-40 KT JET AT 500 MB WILL ENHANCE SHEAR  
PROFILES IN THE 0-5 KM LAYER LATE IN THE DAY. THE MAIN QUESTION  
LEADING UP TO TODAY WAS WHETHER THERE WAS GOING TO BE A LIFTING  
MECHANISM TO INITIATE CONVECTION. IT APPEARS THAT THERE WILL BE  
SOME FORCING. THE HIRES GUIDANCE HAS COME INTO BETTER AGREEMENT,  
FOCUSING THE INITIAL CONVECTIVE DEVELOPMENT ALONG A LEE SURFACE  
TROUGH TO OUR WEST DURING THE EARLY AFTERNOON. A BROKEN LINE OF  
STORMS SHOULD PROGRESS EASTWARD INTO EASTERN PA DURING THE MID TO  
LATE AFTERNOON WITH ADDED UPPER FORCING BROUGHT TO YOU BY AN  
APPROACHING MID-LEVEL SHORTWAVE TROUGH. POPS ARE HIGHEST (LIKELY)  
WEST OF THE PHILLY METRO WHERE THE ARRIVAL OF THE LINE SHOULD  
COINCIDE WITH PEAK HEATING. OVERALL, THE FORECAST ENVIRONMENT  
SUPPORTS A POTENTIAL FOR LOCALLY DAMAGING STRAIGHT-LINE WIND GUSTS.  
THE LATEST D1 CONVECTIVE OUTLOOK FROM SPC HAS PLACED AREAS ALONG  
AND WEST OF I-95 IN A SLIGHT RISK THIS AFTERNOON.  
 
ALTHOUGH FORECAST 925/850 MB TEMPS ARE AROUND 1C HIGHER TODAY THAN  
WHAT WAS OBSERVED ON MONDAY AND TUESDAY, CLOUD COVER IS EXPECTED  
TO ALSO BE HIGHER TODAY (ESPECIALLY INLAND). THE MOST LIKELY  
OUTCOME IS MAX TEMPS TODAY THAT ARE VERY SIMILAR TO THE PREVIOUS  
TWO DAYS. OVERALL, THE OFFICIAL FORECAST TEMPS ARE ON THE HIGHER  
SIDE OF GUIDANCE AFTER ACCOUNTING FOR A PERSISTENT COOL BIAS SEEN  
IN THE MODELS.  
 

 
   
SHORT TERM /6 PM THIS EVENING THROUGH 6 AM THURSDAY/
 
 
LATEST HI-RES MODEL SIMULATIONS SHOW A BROKEN LINE OF SHOWERS AND  
STORMS OVER EASTERN PA THAT CROSS THE I-95 CORRIDOR AROUND SUNSET  
AND DISSIPATE BEFORE IT REACHES THE COAST. CONVECTION IS EXPECTED  
TO BECOME SEPARATED FROM THE CONVERGENCE/SOURCE OF LIFT NEAR THE  
LEE TROUGH AND INSTABILITY WILL WANE WITH THE LOSS OF DAYTIME  
HEATING, RESULTING IN A GENERALLY WEAKENING TREND AS IT MOVES  
EASTWARD. HOWEVER, THERE COULD STILL BE A FEW STRONG TO SEVERE  
STORMS EARLY. POPS DECREASE SHARPLY FROM WEST TO EAST ACCORDINGLY.  
 
CLOUD COVER AND MUGGY CONDITIONS WILL KEEP TEMPS MILD TONIGHT.  
FORECAST MIN TEMPS ARE GENERALLY IN THE MID TO UPPER 60S. IF WE  
ARE ABLE TO CLEAR OUT OVERNIGHT, WE MAY SEE PATCHY FOG DEVELOP.  
 

 
   
LONG TERM /THURSDAY THROUGH TUESDAY/
 
 
ON THURSDAY, A COLD FRONT IS FORECAST TO APPROACH THE AREA AND MOVE  
ACROSS THE AREA LATE IN THE DAY INTO THE EVENING HOURS. A SHORT WAVE  
ALOFT IS FORECAST TO MOVE ACROSS THE AREA ALONG WITH THE FRONTAL  
BOUNDARY DURING THE AFTERNOON AND INTO THE EVENING AS WELL. WITH AN  
INCREASE IN INSTABILITY DURING THE DAY, SCATTERED SHOWERS AND  
THUNDERSTORMS SHOULD BE ABLE TO DEVELOP ACROSS THE AREA. WHILE QPF  
AMOUNTS ARE NOT HIGH, 0.10-0.25 INCHES, PW VALUES ARE NEAR 1.5  
INCHES, SO IT IS POSSIBLE FOR SOME HEAVY DOWNPOURS. WITH A LACK OF  
STRONG SHEAR, THE POTENTIAL FOR SEVERE WEATHER IS LOW, ALTHOUGH ANY  
THUNDERSTORMS THAT DO DEVELOP COULD PRODUCE STRONG GUSTY WINDS AND  
HEAVY RAIN.  
 
ONCE THE FRONT MOVES THROUGH, HIGH PRESSURE IS FORECAST TO  
REESTABLISH ITSELF ACROSS THE ATLANTIC, WITH OUR AREA ON THE WESTERN  
EDGE. SLIGHT RIDGING ALOFT BUILDS ACROSS THE AREA AS WELL, WHICH  
SHOULD HELP KEEP THE WEATHER MOSTLY DRY INTO FRIDAY. WITH RETURN  
FLOW ACROSS THE AREA, INSTABILITY IS FORECAST TO DEVELOP ACROSS THE  
WESTERN AREAS. THIS DAYTIME INSTABILITY COULD LEAD TO A FEW ISOLATED  
SHOWERS/THUNDERSTORMS ACROSS OUR WESTERN AREAS.  
 
THE RIDGING ALOFT PUSHES EASTWARD SLIGHTLY SATURDAY, WITH SOUTHWEST  
FLOW ALOFT DEVELOPING, ALLOWING FOR A COUPLE OF SHORT WAVE/VORTICITY  
IMPULSES TO MOVE ACROSS THE AREA LATE SATURDAY. WITH DAYTIME  
INSTABILITY DEVELOPING ACROSS THE AREA, AN INCREASE IN  
SHOWER/THUNDERSTORM ACTIVITY IS POSSIBLE, ESPECIALLY FOR THE WESTERN  
HALF OF THE AREA.  
 
A COLD FRONT IS THEN FORECAST TO MOVE ACROSS THE AREA SATURDAY  
NIGHT INTO SUNDAY SUNDAY NIGHT. HOWEVER, THERE REMAINS TIMING  
DIFFERENCES BETWEEN THE GFS AND ECMWF REGARDING THIS FRONT. THE  
GFS HAS THE FRONT AND ITS PRECIPITATION MOVING THROUGH SATURDAY  
NIGHT INTO SUNDAY. THE ECMWF HAS THE FRONT MOVING THROUGH SUNDAY  
INTO SUNDAY NIGHT. WE'LL CONTINUE WITH ENHANCED POPS DURING THE  
TIME PERIOD, GENERALLY CENTERED AROUND SUNDAY AND WILL BE ABLE TO  
BETTER FINE TUNE TIMING AS MODELS COME TO AN AGREEMENT. PW VALUES  
INCREASE TO NEAR 2 INCHES, SO THERE COULD BE A PERIOD OF HEAVY  
RAINFALL WITH ANY SHOWERS THAT DEVELOP DURING THE TIME PERIOD.  
 
HIGH PRESSURE RETURNS TO THE NORTHEAST EARLY NEXT WEEK AS THE  
FRONTAL BOUNDARY PUSHES SOUTH AND LIKELY STALLS JUST SOUTH OF OUR  
AREA. AGAIN, THERE ARE DIFFERENCES BETWEEN THE GFS AND ECMWF. THE  
GFS KEEPS HIGH PRESSURE ACROSS THE AREA, KEEPING MOSTLY DRY  
WEATHER INTO THE EARLY PART OF THE WEEK. THE ECMWF STARTS TO LIFT  
THE FRONT BACK NORTH TOWARD THE AREA, BRINGING AN INCREASE IN  
SHOWER CHANCES. WE'LL KEEP A SMALL CHANCE OF SHOWERS INTO TUESDAY,  
BECAUSE THE GFS HAS DOES HAVE A FEW SHORT WAVE/VORTICITY IMPULSES  
FORECAST TO CROSS THE AREA. SO THIS COULD STILL PRODUCE SOME  
SCATTERED SHOWERS/THUNDERSTORMS WITH DAYTIME INSTABILITY.  
 

 
   
AVIATION /17Z WEDNESDAY THROUGH SUNDAY/
 
 
THE FOLLOWING DISCUSSION IS FOR KPHL, KPNE, KTTN, KABE, KRDG, KILG,  
KMIV, KACY AND SURROUNDING AREAS.  
 
SOME STRATUS WITH CIGS AROUND 1500 FT WILL CONTINUE TO LINGER  
ACROSS MUCH OF THE REGION THROUGH MID-MORNING, BEFORE LIFTING AND  
DISSIPATING BY LATE MORNING.  
 
EXPECT SHRA AND TSRA TO DEVELOP OVER THE HIGHER ELEVATIONS OF  
CENTRAL/EASTERN PA THIS AFTERNOON. KEPT A TSRA PROB30 GROUP IN THE  
06Z FOR RDG AND ABE AND ADDED IT TO THE 09Z PHL TAF FOR THE MID  
TO LATE AFTERNOON. FARTHER EAST, THERE IS HIGHER UNCERTAINTY IF  
STORMS WILL BE MAINTAINED BY THE TIME IT REACHES THE COASTAL PLAIN SO  
NO MENTION OF TSRA AT THIS TIME. STORMS UNLIKELY TO REACH ACY THIS  
EVENING.  
 
S-SW WINDS 5-10 KT THIS MORNING INCREASE TO 10-15 KT THIS AFTERNOON  
WITH GUSTS NEAR 20 KT.  
 
OUTLOOK...  
 
THURSDAY-THURSDAY NIGHT...GENERALLY VFR. SCATTERED  
SHOWERS/THUNDERSTORMS POSSIBLE DURING THE DAY WHICH MAY LOCALLY  
REDUCE CIGS/VSBYS.  
 
FRIDAY...GENERALLY VFR. ISOLATED SHOWERS/THUNDERSTORMS POSSIBLE FOR  
WESTERN TAFS.  
 
FRIDAY NIGHT...VFR EARLY, THEN LOWERING CIGS/VSBYS POSSIBLE  
OVERNIGHT.  
 
SATURDAY...LOWER CONDITIONS POSSIBLE EARLY, THEN BECOMING VFR DURING  
THE DAY. SCATTERED SHOWERS AND THUNDERSTORMS POSSIBLE WHICH MAY  
LOCALLY REDUCE CIGS/VSBYS. GUSTY SOUTH-SOUTHWEST WINDS AROUND 20  
KNOTS.  
 
SATURDAY NIGHT-SUNDAY...SUB-VFR CONDITIONS POSSIBLE, ESPECIALLY WITH  
SHOWERS/THUNDERSTORMS.  
 

 
   
MARINE
 
 
SCA WAS CANCELLED FOR THIS MORNING AS WINDS AND SEAS HAVE FALLEN TO  
BELOW SCA LEVELS. THE WINDS WILL INCREASE THIS AFTERNOON AND EARLY  
EVENING. EXPECT GUSTS TO 25 KT, ESPECIALLY NEAR THE COAST. SEAS WILL  
RESPOND TO THE STRENGTHENING WINDS WITH WAVE HEIGHTS FORECAST TO  
INCREASE TO NEAR 5 FT. ACCORDINGLY, THE SCA REMAINS IN EFFECT FOR  
THIS AFTERNOON AND TONIGHT.  
 
OUTLOOK...  
 
THURSDAY-SATURDAY...SUB-SMALL CRAFT ADVISORY CONDITIONS CURRENTLY  
EXPECTED, ALTHOUGH WINDS MAY GUST AROUND 20 KNOTS AT TIMES.  
 
SATURDAY NIGHT...SEAS MAY RETURN TO 5 FEET OR GREATER OVERNIGHT  
AHEAD OF AN APPROACHING COLD FRONT.  
 

 
   
RIP CURRENTS
 
 
BASED ON IN HOUSE REGRESSION GUIDANCE, HAVE INCLUDED A MODERATE  
RISK OF RIP CURRENTS TODAY FOR THE NJ BEACHES UNDER A BREEZY  
SOUTH WIND.  
 

 
   
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...SMALL CRAFT ADVISORY UNTIL 6 AM EDT THURSDAY FOR ANZ450>455.  
 

 
 

 
SYNOPSIS...ROBERTSON  
NEAR TERM...KLEIN/KLINE  
SHORT TERM...KLEIN  
LONG TERM...ROBERTSON  
AVIATION...KLEIN/KLINE/ROBERTSON  
MARINE...KLEIN/ROBERTSON  
RIP CURRENTS...  
CLIMATE...  
 
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