403  
FXUS61 KPHI 290003  
AFDPHI  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE MOUNT HOLLY NJ  
803 PM EDT WED JUN 28 2017  
   
SYNOPSIS
 
 
A BROAD AREA OF HIGH PRESSURE WILL SLOWLY DRIFT EASTWARD INTO THE  
WESTERN ATLANTIC THROUGH THE END OF THE WEEK. A COLD FRONT WILL  
GRADUALLY SAG SOUTHEASTWARD FROM THE GREAT LAKES AND SOUTHEAST  
CANADA THIS WEEKEND TO THE OHIO VALLEY AND SOUTHERN NEW ENGLAND  
EARLY NEXT WEEK.  
 

 
   
NEAR TERM /UNTIL 6 AM THURSDAY MORNING/
 
 
NO BIG CHANGES NEEDED. THE DAYTIME CU HAS DISSIPATED AND THE  
ONLY CLOUD COVER TONIGHT WILL BE THE CI/CS SHIELD OVERHEAD.  
 
IT LOOKS LIKE RADIATIONAL COOLING WILL BE RATHER EFFICIENT OVER  
NIGHT. THEREFORE, STAYED CLOSE TO THE PREVIOUS FORECAST WHICH  
IS SLIGHTLY BELOW THE NAM AND GFS MOS GUIDANCE ESPECIALLY NORTH  
AND WEST OF THE FALL LINE AND OVER THE PINE BARRENS.  
 
A FEW ADJUSTMENTS TO HOURLY TEMPS IN THE NEXT FEW HOURS (MOSTLY  
UPWARD).  
 

 
   
SHORT TERM /6 AM THURSDAY MORNING THROUGH 6 PM THURSDAY/
 
 
WITH THE SURFACE HIGH TO OUR EAST, AND INCREASING PRESSURE GRADIENT  
IN RESPONSE TO THE SURFACE LOW OVER THE GREAT LAKES REGION MOVING  
FURTHER EAST, EXPECT LOW LEVEL SOUTHWESTERLY FLOW TO DEVELOP AND  
INCREASE. IN RESPONSE, WE SHOULD SEE A CONSIDERABLE WARMING TREND  
TOMORROW, WITH HIGHS ABOUT 5 TO 10 DEGREES HIGHER THAN TODAY.  
 
ALSO, LATE IN THE DAY AS THE LOW CONTINUES TO SLOWLY PROPAGATE  
EASTWARD, WE COULD SEE ISOLATED SHOWERS AND THUNDERSTORMS GENERALLY  
FROM THE LEHIGH VALLEY TO NW NJ AND FURTHER NORTHWEST. I HAVE KEPT  
IT AT A SLIGHT CHANCE THOUGH AS THE LOW SHOULD STAY WELL TO OUR WEST  
THROUGH THIS TIME AND THE UPPER LEVEL JET WILL BE TOO FAR NORTH TO  
PROVIDE MUCH IN THE WAY OF LIFT ACROSS OUR REGION.  
 

 
   
LONG TERM /THURSDAY NIGHT THROUGH WEDNESDAY/
 
 
A LONG DURATION OF HOT AND HUMID WEATHER IS IN STORE FOR THE  
REGION, WITH CHANCES OF PRECIPITATION INCREASING THIS WEEKEND  
WITH OCCASIONAL CHANCES CONTINUING NEXT WEEK.  
 
OPERATIONAL MODELS ARE IN REASONABLY GOOD AGREEMENT WITH THE  
SYNOPTIC-SCALE DETAILS THROUGH THE LONG-TERM PERIOD. SMALLER-  
SCALE VORTICITY MAXIMA WITHIN THE SOUTHWEST UPPER-LEVEL FLOW  
DEVELOPING THIS WEEKEND AND LARGE-SCALE TROUGH PASSAGE NEXT WEEK  
HAVE TYPICAL STRENGTH/TIMING DIFFERENCES AMONG THE MODEL SUITE,  
WITH NO OBVIOUS MODEL BIASES NOTED. AS SUCH, PRETTY MUCH LOADED  
THE GRIDS WITH A MODEL BLEND THROUGH THE LONG TERM WITH MINOR  
MODIFICATIONS FOR BETTER OFFICE COLLABORATION, SOMEWHAT LOWER  
WEIGHTING FOR THE 12Z CMC LATE IN THE PERIOD OWING TO ITS  
INCREASED DISCREPANCIES FROM CONSENSUS, AND DAMPENED POPS MONDAY  
ONWARD OWING TO LARGER UNCERTAINTIES ASSOCIATED WITH VORT MAX  
TIMING.  
 
AT 00Z FRIDAY, A POTENT VORT MAX WILL BE MOVING THROUGH SOUTHERN  
ONTARIO AND THE ADJACENT GREAT LAKES WITH DOWNSTREAM RIDGE  
AMPLIFICATION FULLY COMMENCED IN THE NORTHEAST. AFTER A RAPID  
WARMUP ON THURSDAY, THIS WILL BE FOLLOWED BY A RAPID INCREASE IN  
SURFACE DEW POINTS ON FRIDAY AND SATURDAY. WITH HIGHS NEAR OR  
ABOVE 90 IN THE URBAN CORRIDOR, THIS LEADS TO A RATHER  
UNCOMFORTABLE PERIOD FOR THE AREA. I WAS A LITTLE HESITANT TO  
INCREASE DEW POINTS TO THE VALUES OF MAV/MEX MOS, WHICH HAVE  
BEEN A LITTLE ON THE HIGH SIDE IN SIMILAR PATTERNS SO FAR THIS  
SPRING/EARLY SUMMER. AS SUCH, I DROPPED HEAT INDICES SOMEWHAT  
BOTH FRIDAY AND SATURDAY, ESPECIALLY SINCE MODEL SOUNDINGS SHOW  
WELL-MIXED SOUNDINGS DURING THE AFTERNOONS. WITH A RATHER DEEP  
MIXED LAYER, I STRONGLY SUSPECT MOS IS OVERDOING SURFACE  
MOISTURE, ESPECIALLY ON FRIDAY. MUCH STRONGER WEIGHTING WAS  
GIVEN TO MET GUIDANCE FRIDAY AND MANUAL ADJUSTMENT SATURDAY  
GIVEN NAM BUFKIT SOUNDINGS.  
 
REGARDING THE AFOREMENTIONED VORT MAX, AS IT SLIDES EASTWARD  
INTO FAR NORTHERN NEW ENGLAND, AN UPSTREAM VORT MAX WILL MOVE  
INTO THE UPPER MIDWEST BY FRIDAY AFTERNOON. THIS WILL BE THE  
START OF PROLONGED SOUTHWESTERLY MIDLEVEL FLOW IN THE EASTERN  
U.S. SEVERAL PERTURBATIONS IN THE FASTER FLOW WILL PROGRESS  
NORTHEASTWARD FROM THE CENTRAL U.S. TO THE NORTHEAST FOR THE  
FOLLOWING SEVERAL DAYS. WITH A STRONG BERMUDA HIGH REMAINING  
INFLUENTIAL INTO THE SOUTHEASTERN U.S., THIS WILL FORCE THE  
ASSOCIATED VORTICITY MAXIMA TO ELONGATE IN A POSITIVELY-TILTED  
ORIENTATION AS THEY PROGRESS NORTHEASTWARD. UPSTREAM  
PERTURBATIONS WILL THEN AMPLIFY THE VORTICITY WITH DECREASING  
LATITUDE (FARTHER SOUTHWARD) WITH TIME.  
 
THE RESULT OF THIS ABOVE-SURFACE PATTERN IS A FRONTAL BOUNDARY  
MOVING AT A SNAIL'S PACE SOUTHEASTWARD AS MIDLEVEL FLOW BECOMES  
PARALLEL TO ITS ORIENTATION, BROADLY FROM NEW ENGLAND TO THE  
GREAT LAKES AND SOUTHWESTWARD THIS WEEKEND. SEVERAL EMBEDDED  
SURFACE WAVES OF LOWER PRESSURE WILL PROGRESS NORTHEASTWARD  
ALONG THIS BOUNDARY, LIKELY ENHANCING CONVECTION IN SURROUNDING  
REGIONS. PRECIPITATION CHANCES INCREASE GRADUALLY FRIDAY THROUGH  
SUNDAY IN THIS REGIME FROM NORTHWEST TO SOUTHEAST. MODELS ARE  
IN REASONABLY GOOD CONSENSUS SHOWING A PARTICULARLY STRONG WAVE  
MOVING THROUGH THE MID-ATLANTIC REGION SATURDAY AFTERNOON  
THROUGH SUNDAY MORNING.  
 
THIS BRINGS ABOUT THE MOST CHALLENGING ASPECTS OF THE WEEKEND  
FORECAST. THE FIRST IS HIGHS ON SATURDAY, WITH  
MODELS/STATISTICAL GUIDANCE TRENDING NOTICEABLY DOWNWARD WITH  
TEMPERATURES ON SATURDAY AS POTENTIAL SKY COVER/PRECIPITATION  
INFLUENCES MAY INHIBIT MAXIMUM WARMING. THE SECOND IS FIGURING  
OUT TIMING/LOCATION OF HIGHEST POPS. DECIDED TO KEEP A  
CONSERVATIVE APPROACH FOR THIS FORECAST, BOTH BY LOWERING MAX  
TEMPERATURES SLIGHTLY ON SATURDAY AND BY CAPPING POPS TO CHANCE  
SATURDAY THROUGH SUNDAY. SUNDAY MAY END UP BEING FAIRLY DRY IF  
THE VORT MAX PROJECTED TO MOVE THROUGH SATURDAY NIGHT IS  
FOLLOWED BY FAIRLY STRONG TRANSIENT RIDGING (AS DEPICTED TO  
SOME DEGREE BY THE GFS AND ECMWF). THE 12Z CMC, MEANWHILE,  
BRINGS A QUICK-TO-FOLLOW PERTURBATION THROUGH THE AREA ON  
SUNDAY, GENERATING MORE CONVECTION IN MUCH OF THE REGION. WITH  
SO MUCH UNCERTAINTY WITH THESE SMALL-SCALE DETAILS, IT IS  
FORMIDABLY CHALLENGING TO GET TOO SPECIFIC WITH POPS AT THIS  
POINT. THERE ARE DECENT INDICATIONS OF A PUSH OF DRIER SURFACE  
AIR MOVING INTO THE AREA BY SUNDAY NIGHT AS THE COLD FRONT MAKES  
ITS STRONGEST PROGRESS INTO THE REGION.  
 
FOR EARLY NEXT WEEK, MODELS ARE SHOWING A SOMEWHAT DRIER DAY  
MONDAY UPSTREAM OF THE PERTURBATIONS MOVING THROUGH DURING THE  
WEEKEND FOLLOWED BY A POTENT TROUGH MOVING THROUGH THE NORTHEAST  
TUESDAY AND WEDNESDAY. KEPT SLIGHT CHANCE TO CHANCE POPS FOR  
MUCH OF THE AREA (HIGHEST NORTH) TUESDAY AND WEDNESDAY, BUT KEPT  
MONDAY MOSTLY DRY GIVEN THE REASONABLY STRONG CONSENSUS AT THIS  
POINT. HOWEVER, WITH THE FRONT FAILING TO MAKE PROGRESS  
SOUTHEASTWARD THROUGH THE REGION, TEMPERATURES WILL REMAIN WARM,  
AND HUMIDITY WILL REMAIN HIGH.  
 

 
   
AVIATION /00Z THURSDAY THROUGH MONDAY/
 
 
THE FOLLOWING DISCUSSION IS FOR KPHL, KPNE, KTTN, KABE, KRDG, KILG,  
KMIV, KACY AND SURROUNDING AREAS.  
 
MOSTLY VFR CONDITIONS ARE EXPECTED THROUGH THE TAF PERIOD. AFTER 18Z  
THURSDAY, THERE IS A SLIGHT CHANCE OF SHOWERS AND THUNDERSTORMS  
ACROSS PORTIONS OF SOUTHEASTERN PA INCLUDING KRDG AND KABE. HOWEVER,  
COVERAGE IS TOO LIMITED TO INCLUDE IN THE TAFS AT THIS TIME.  
 
WINDS WILL BECOME LIGHT FROM THE SOUTHWEST TONIGHT. BY 12Z TOMORROW  
THOUGH, WE SHOULD SEE WINDS ACROSS THE REGION SHIFT TO SOUTH-SOUTHWESTERLY  
AND INCREASE TO 10 TO 15 KT WITH GUSTS ABOVE 20KT.  
 
OUTLOOK...  
THURSDAY NIGHT AND FRIDAY...VFR CONDITIONS EXPECTED. SLIGHT  
CHANCE OF STORMS NORTHWEST OF KPHL, WHICH MAY LOCALLY LOWER  
CIGS/VSBYS. SOUTHWEST WINDS 5 TO 15 KTS (STRONGER DURING THE  
DAY), WITH GUSTS TO 20 KTS POSSIBLE.  
 
FRIDAY NIGHT THROUGH SUNDAY...MOSTLY VFR, BUT CHANCES OF STORMS  
THROUGH THE PERIOD, ESPECIALLY SATURDAY AFTERNOON THROUGH  
SUNDAY. LOCALLY LOWER VISIBILITY AND CEILINGS LIKELY. WEST TO  
SOUTHWEST WINDS 5 TO 15 KTS, WITH GUSTS TO 20 KTS POSSIBLE  
DURING THE DAY (MORE SO SATURDAY).  
 
SUNDAY NIGHT AND MONDAY...A POTENTIAL DRY PERIOD, WITH MOSTLY  
VFR CONDITIONS. LIGHT WINDS, PRIMARILY WESTERLY.  
 

 
   
MARINE
 
 
WINDS AND SEAS WILL BEGIN TO BUILD MID DAY THURSDAY. SMALL CRAFT  
ADVISORY CONDITIONS ARE EXPECTED TO DEVELOP BY MID AFTERNOON ALONG  
THE ATLANTIC COASTAL WATERS.  
 
ON THE DELAWARE BAY, IT IS UNLIKELY THAT WINDS WILL GET ABOVE 25 KT  
THROUGH THE DAY TIME HOURS. HOWEVER, SOUTHERLY WIND GUSTS ABOVE 20KT  
ARE POSSIBLE ON THE LOWER BAY.  
 
OUTLOOK...  
THURSDAY NIGHT AND FRIDAY...ADVISORY CONDITIONS LIKELY, WITH  
GUSTS NEAR GALE FORCE POSSIBLE THURSDAY EVENING.  
 
FRIDAY NIGHT THROUGH SUNDAY...ADVISORY CONDITIONS MAY LINGER  
FRIDAY NIGHT, WITH GENERALLY SUB-ADVISORY CONDITIONS THEREAFTER.  
HOWEVER, A PERIOD OF STRONGER WINDS MAY OCCUR SATURDAY NIGHT AS  
WELL. FREQUENT CHANCES OF STORMS THROUGH THE PERIOD.  
 
SUNDAY NIGHT AND MONDAY...A DRIER PERIOD MAY OCCUR. SUB-ADVISORY  
CONDITIONS ARE EXPECTED.  
 
RIP CURRENTS...  
 
THE OUTLOOK FOR TOMORROW IS THAT THE LOW RISK FOR THE FORMATION OF  
DANGEROUS RIP CURRENTS WILL CONTINUE INTO AT LEAST TOMORROW MORNING.  
DEPENDING ON HOW QUICKLY THE WINDS INCREASE TOMORROW AFTERNOON, THE  
RISK MAY INCREASE TO MODERATE BY LATE IN THE DAY, BUT THAT REMAINS  
UNCERTAIN AT THIS TIME.  
 
LOW RISK DOES NOT MEAN NO RISK AND IT IS ALWAYS ADVISED FOR ULTIMATE  
SAFETY, SWIM IN THE PRESENCE OF THE LIFEGUARDS. THEY HAVE THE  
FLOTATION DEVICES THAT CAN MORE EASILY SAVE A LIFE.  
 
WHERE THE SURF ZONE WATERS ARE STEEPER, THERE MAY BE FEWER RIP  
CURRENTS, BUT THERE, THE DANGER OF WAVE RELATED INJURY INCREASES.  
 
AN ONGOING DE 6+ YEAR STUDY SHOWS THE 10-20 YEAR AND 40-60 YEAR  
OLD AGE GROUPS MOST VULNERABLE. THE 10-20 YEAR OLD SURF ZONE  
INJURIES ARE ASSOCIATED WITH BODY BOARDING AND BODY SURFING,  
WHILE THE 40-60 YEAR OLD WADERS ARE KNOCKED DOWN BY WAVE ACTION,  
ESPECIALLY WITH BACK TURNED TO THE OCEAN.  
 
MALES ARE STATISTICALLY FAR MORE LIKELY TO BE INJURED OR LOSE  
THEIR LIFE IN THE SURF ZONE BUT AS JUNE 15 REMINDS US...ANYONE  
IS VULNERABLE.  
 

 
   
CLIMATE
 
 
MONTHLY AVG TEMP FOR JUNE  
 
PHL PROJECTING 74.5 OR 1.2 DEGREES ABOVE THE AVERAGE OF 73.3  
 
ABE PROJECTING 70.8 OR 1.7 DEGREES ABOVE THE AVERAGE OF 69.1  
 
SEVEN OF OUR EIGHT LONG TERM CLIMATE SITES WILL AVERAGE ABOVE  
NORMAL PROBABLY FROM ABOUT 8 TENTHS OF A DEGREE FOR TTN/ILG TO  
AS MUCH AS 2 DEGREES FOR GED.  
 
MOUNT POCONO IS OUR ONLY BELOW NORMAL AVERAGE AND PROJECTING  
1-1.5 BELOW).  
 
THESE PROJECTIONS ARE BASED ON OUR 330 AM FORECAST.  
 

 
   
PHI WATCHES/WARNINGS/ADVISORIES
 
 
PA...NONE.  
NJ...NONE.  
DE...NONE.  
MD...NONE.  
MARINE...SMALL CRAFT ADVISORY FROM 2 PM THURSDAY TO 6 AM EDT FRIDAY FOR  
ANZ450>455.  
SMALL CRAFT ADVISORY FROM 6 PM THURSDAY TO MIDNIGHT EDT  
THURSDAY NIGHT FOR ANZ431.  
 

 
 

 
SYNOPSIS...CMS/99  
NEAR TERM...JOHNSON/O'HARA  
SHORT TERM...JOHNSON  
LONG TERM...CMS  
AVIATION...CMS/JOHNSON/O'HARA  
MARINE...CMS/JOHNSON  
CLIMATE...  
 
CLICK HERE TO GO TO PREVIOUS BULLETINS.

The Nexlab PA Page
The Nexlab NJ Page
The Nexlab DE Page
The Nexlab MD Page Main Text Page