171  
FXUS61 KPHI 230828  
AFDPHI  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE MOUNT HOLLY NJ  
428 AM EDT SAT JUN 23 2018  
   
SYNOPSIS
 
 
A LOW PRESSURE SYSTEM IN THE GREAT LAKES REGION LEADS TO A WARM  
FRONTAL PASSAGE IN OUR AREA TODAY AND A COLD FRONTAL PASSAGE  
TOMORROW. HIGH PRESSURE OVER ONTARIO BUILDS INTO THE REGION ON  
MONDAY AND SLOWLY MOVES OFF THE EASTERN SEABOARD TO A POSITION NEAR  
BERMUDA BY THURSDAY. A TROUGH MAY MOVE THROUGH OUR REGION WEDNESDAY  
NIGHT INTO THURSDAY.  
 

 
   
NEAR TERM /UNTIL 6 PM THIS EVENING/
 
 
THROUGH 10 AM...  
 
MID-LEVEL SHORTWAVE ENERGY AND THE LF QUADRANT OF THE UPPER JET,  
AIDED BY OROGRAPHIC LIFT IN THE EASTERLY LOW-LEVEL FLOW, WITHIN A  
VERY MOIST AIRMASS (PW VALUES ~ 2 INCHES) WILL LEAD TO HEAVY SHOWERS  
ACROSS THE LEHIGH VALLEY AND SOUTHERN POCONOS. FURTHER SOUTHEAST, A  
WAVE OF LOW PRESSURE DEVELOPING ALONG THE WARM FRONT JUST TO THE  
SOUTH WILL LEAD TO ENHANCED ISENTROPIC ASCENT ACROSS PORTIONS OF  
SOUTHERN NJ AND DELAWARE, WHERE HEAVY SHOWERS ARE ALSO POSSIBLE.  
CANNOT RULE OUT A RUMBLE OF THUNDER, WITH THE GREATEST PROBABILITY  
OVER FAR SOUTHERN NJ AND DELMARVA. IN ADDITION, SOME OF THE HEAVIEST  
RAIN SHOWERS COULD LEAD TO LOCALIZED RAINFALL AMOUNTS UP TO 1"/HOUR  
BUT FAST MOVEMENT SHOULD PRECLUDE MORE THAN POOR DRAINAGE FLOODING.  
 
10 AM THRU 6 PM...  
 
THE WARM FRONT WILL BEGIN TO MAKE NORTHWARD PROGRESS, BUT THERE IS  
STILL UNCERTAINTY REGARDING ITS POSITION BY LATE AFTERNOON. THE GFS  
AND ECMWF ARE IN BETTER AGREEMENT WITH THE NAM FOR THE 00Z/23 MODEL  
SUITE, AND HAVE THE FRONT LOCATED INVOF I-276 IN PA NORTHEASTWARD TO  
I-95 IN NJ BY LATE AFTERNOON. ANY DEVIATION IN THIS POSITION NORTH  
OR SOUTH WILL HAVE A PROFOUND IMPACT ON SEVERE WEATHER POTENTIAL AND  
HIGH TEMP FORECAST.  
 
SEVERE WEATHER POTENTIAL...  
 
SPC HAS OUTLOOKED A SLIGHT RISK FOR EXTREME SOUTHEAST PA AND  
SOUTHERN NJ, WITH A MARGINAL RISK FROM PORTIONS OF THE LEHIGH VALLEY  
TO INCLUDE ALL OF OCEAN COUNTY NJ.  
 
ACROSS THE SLIGHT RISK AREA, FAVORABLE PARAMETERS CONTINUE, WITH ML  
CAPE ~1,500 J/KG AND BULK SHEAR AROUND 45 KT. GUSTY WINDS ARE  
POSSIBLE, ALTHOUGH MODEST 0-3 KM LAPSE RATES AND MEAGER D-CAPE  
SUGGEST DAMAGING WINDS WILL BE LIMITED IN COVERAGE. STILL VERY  
CONCERNING IS THE ENVIRONMENT CLOSER TO THE WARM FRONT, WHERE 0-1 KM  
SHEAR OVER 20 KTS AND LOW LCLS COULD FAVOR A BRIEF, WEAK TORNADO.  
HAIL PARAMETERS, INCLUDING LAPSE RATES IN THE HAIL GROWTH ZONE,  
REMAIN WEAK SO WOULD EXPECT MAINLY SMALL HAIL. THE EXCEPTION WOULD  
BE ANY ROTATING UPDRAFTS CLOSER TO THE WARM FRONT, WHICH WOULD BE  
MORE FAVORABLE FOR LARGE HAIL.  
 
WITH PRECIPITABLE WATER VALUES ~2 INCHES, HEAVY DOWNPOURS ARE  
LIKELY, ESPECIALLY IF THUNDERSTORMS TRAIN NEAR THE WARM FRONT,  
WHICH MAY RESULT IN URBAN AND POOR DRAINAGE FLOODING.  
 
HIGH TEMPERATURES...  
 
A WARM AND HUMID AIRMASS WILL ADVECT NORTHWARD TO THE AFOREMENTIONED  
AREA. SOUTH OF THE FRONT, SOUTHERLY FLOW, PARTLY CLOUDY SKIES, AND  
MIXING UP TO 850 HPA SUPPORT HIGHS IN THE MID TO UPPER 80S. NORTH OF  
THE FRONT, CONSIDERABLE CLOUDINESS AND AN EASTERLY FLOW WILL KEEP  
TEMPS IN THE 70S. ANY OSCILLATION IN THE FRONTAL POSITION COULD  
RAISE/LOWER TEMPERATURES 10-15 DEGREES AT A GIVEN LOCATION.  
 

 
   
SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SUNDAY/
 
 
THE SEVERE WEATHER THREAT SHOULD DIMINISH AROUND 03Z WITH THE LOSS  
OF INSTABILITY AND SHEAR. THE SHORTWAVE AXIS AND PRE-FRONTAL TROUGH  
ALSO MOVE OFFSHORE AROUND MIDNIGHT, WITH SUBSIDENCE TAKING HOLD  
THROUGH COLUMN AND DRYER AIR ADVECTING IN. THERE MAY BE SOME PARTIAL  
CLEARING, AND WITH THE MOIST GROUND, THIS WOULD BE CONDUCIVE FOR THE  
DEVELOPMENT OF FOG. BUT NEAR SURFACE WINDS OUT OF THE WEST ARE  
EXPECTED TO REMAIN ELEVATED MUCH OF THE NIGHT, SO CONFINED PATCHY  
FOG TO THE MORE SHELTERED LOCATIONS OF NORTHERN NJ AND SOUTHEAST PA.  
LOW TEMPERATURES ARE EXPECTED TO BE A FEW DEGREES ABOVE NORMAL.  
 

 
   
LONG TERM /SUNDAY THROUGH FRIDAY/
 
 
SUMMARY: THUNDERSTORMS ARE POSSIBLE ON SUNDAY AS A COLD FRONT  
ARRIVES IN THE REGION. HIGH PRESSURE WILL BRING TRANQUIL WEATHER  
MONDAY AND TUESDAY. CHANCES FOR SHOWERS AND THUNDERSTORMS AGAIN  
INCREASE WEDNESDAY THROUGH THURSDAY AS A TROUGH APPROACHES FROM  
THE WEST.  
 
DETAILS:  
SUNDAY...IN THE MID AND UPPER LEVELS, WINDS BECOME MORE WESTERLY  
EARLY IN THE DAY ON SUNDAY RESULTING IN DRY AIR ADVECTION. A MID  
AND UPPER LEVEL TROUGH DIGGING SOUTHEAST WILL ARRIVE JUST BEHIND  
THE SURFACE COLD FRONT LATE IN THE DAY. DESPITE STILL BEING IN  
THE WARM SECTOR THAT WILL BE RESPONSIBLE FOR THUNDERSTORMS  
TODAY, THE DRY AIR ADVECTION AND A MID LEVEL INVERSION SHOULD  
LIMIT INSTABILITY ON SUNDAY. THUNDERSTORMS ARE POSSIBLE, BUT  
STRONG OR SEVERE THUNDERSTORMS ARE NOT EXPECTED AT THIS TIME.  
 
MONDAY AND TUESDAY...CANADIAN HIGH PRESSURE BUILDS SOUTH. THE  
COLD FRONT WILL ONLY DROP TEMPERATURES TO NEAR NORMAL. FORECAST  
TEMPERATURES, ESPECIALLY ON TUESDAY ARE HIGHER THAN MOST  
GUIDANCE AS THE PATTERN FAVORS VERY LARGE DEW POINT DEPRESSIONS  
IN THE BOUNDARY LAYER. MODELS TYPICALLY UNDERESTIMATE THE DEPTH  
OF THE MIXING LAYER IN THESE CASES WHICH COULD RESULT IN HIGHER  
THAN FORECAST MAX TEMPERATURES AND LOWER THAN FORECAST DEW  
POINTS.  
 
WEDNESDAY AND THURSDAY...ANOTHER LARGE LOW PRESSURE SYSTEM,  
TAKING A TRACK FURTHER NORTH THAN THE ONE TODAY, WILL BRING A  
TROUGH INTO THE REGION EITHER WEDNESDAY NIGHT OR THURSDAY. AHEAD  
OF THIS TROUGH, SOUTHERLY LOW LEVEL RETURN FLOW DEVELOPS,  
RESULTING IN MOISTURE AND WARM AIR ADVECTION. THERE ARE STILL  
SOME TIMING DIFFERENCES BETWEEN THE MODELS ON THE TROUGH, WHICH  
HAS AN IMPACT BOTH ON RAIN CHANCES AND TEMPERATURES. HAVE GONE  
WITH A MIDDLE OF THE ROAD SOLUTION, BRINGING THE TROUGH THROUGH  
LATE WEDNESDAY NIGHT/THURSDAY MORNING, RESULTING IN MODEST  
WARMING TREND ON WEDNESDAY, BUT SIGNIFICANTLY WARMER ON  
THURSDAY. IN THIS SCENARIO, THE PRIME PERIOD FOR SHOWERS AND  
THUNDERSTORMS ACROSS OUR REGION WOULD BE WEDNESDAY NIGHT AND  
THURSDAY, THOUGH SOME MAY DEVELOP PRIMARILY WEST OF THE FALL  
LINE ON WEDNESDAY AFTERNOON THANKS TO ENHANCED OROGRAPHIC LIFT.  
 
FRIDAY...A HIGH SHOULD START BUILDING IN FROM THE SOUTHWEST. HOW  
QUICKLY IT DOES SO WILL DETERMINE IF WE SEE ANOTHER ROUND OF  
SHOWERS AND THUNDERSTORMS DURING THE DAY FRIDAY AND HOW QUICKLY  
REALLY MUGGY CONDITIONS RETURN TO THE AREA. GIVEN THE SOURCE  
REGION OF THIS AIR MASS, WOULD NOT BE SURPRISED IF HEAT INDEX  
VALUES ONCE AGAIN GET ABOVE 100, IF NOT ON FRIDAY, THEN LIKELY  
BY SATURDAY.  
 

 
   
AVIATION /07Z SATURDAY THROUGH WEDNESDAY/
 
 
THE FOLLOWING DISCUSSION IS FOR KPHL, KPNE, KTTN, KABE, KRDG,  
KILG, KMIV, KACY AND SURROUNDING AREAS.  
 
TODAY...  
 
WIDESPREAD IFR CIGS THIS MORNING WILL GRADUALLY IMPROVE LATE THIS  
MORNING WITH VFR AT MOST LOCATIONS BY LATE AFTERNOON. THE EXCEPTION  
WILL BE ABE AND RDG, WHERE CIGS WILL ONLY IMPROVE TO MFVR. MEDIUM  
CONFIDENCE.  
 
WINDS EASTERLY THIS MORNING, GRADUALLY SHIFTING TO SOUTH-SOUTHWEST  
LATE THIS MORNING INTO THE AFTERNOON, EXCEPT ABE AND RDG WHERE WINDS  
REMAIN EASTERLY. SOUTH-SOUTHWEST WINDS COULD GUST UP TO 25 MPH. LOW  
CONFIDENCE.  
 
DURING THE AFTERNOON INTO THE EVENING, SHRA AND TSRA ARE EXPECTED AT  
MOST TERMINALS, ACCOMPANIED BY BRIEF CIG/VSBY RESTRICTIONS AND GUSTY  
WINDS. HIGH CONFIDENCE.  
 
TONIGHT...  
 
SATURDAY NIGHT...TSRA POSSIBLE UNTIL AROUND 03Z. THEREAFTER, MVFR IN  
LOW CLOUDS AND PATCHY FOG, MAINLY ABE, RDG, MIV, AND ACY. WEST WINDS  
LESS THAN 10 KT. LOW CONFIDENCE.  
 
OUTLOOK...  
SUNDAY...MVFR CONDITIONS WILL BE POSSIBLE WITH ANY SHOWERS AND  
THUNDERSTORMS THAT DEVELOP. OTHERWISE, VFR CONDITIONS ARE  
EXPECTED. WINDS WILL BE SHIFTING TO NORTHWESTERLY BEHIND A COLD  
FRONT, BUT WIND SPEEDS SHOULD REMAIN NEAR OR BELOW 10 KT.  
MODERATE CONFIDENCE.  
 
MONDAY AND TUESDAY...VFR CONDITIONS ARE EXPECTED. NORTHWESTERLY  
WINDS ON MONDAY BECOME LIGHT AND VARIABLE MONDAY NIGHT INTO  
TUESDAY. HIGH CONFIDENCE.  
 
WEDNESDAY...MOSTLY VFR CONDITIONS ARE EXPECTED. THERE IS A  
CHANCE FOR SHOWERS AND THUNDERSTORMS TO MOVE IN THROUGH THE DAY,  
ESPECIALLY AT KABE AND KRDG. IF THIS HAPPENS, MVFR CONDITIONS  
WILL BE POSSIBLE. MODERATE CONFIDENCE.  
 

 
   
MARINE
 
 
SCA CONTINUES FOR THE ATLANTIC DE/NJ AND LOWER DE BAY WATERS TODAY  
AND TONIGHT. EAST-SOUTHEAST WINDS THIS MORNING 15-20 KT, SHIFTING TO  
SOUTH-SOUTHWEST THIS AFTERNOON 10-20 KT, BECOMING WEST-SOUTHWEST  
AROUND 10 KT TONIGHT. GUSTS AROUND 25 KT ARE POSSIBLE TODAY. SEAS  
REMAINING ELEVATED AROUND 5 FT TODAY AND TONIGHT. HIGHER WINDS AND  
SEAS POSSIBLE NEAR THUNDERSTORMS THIS AFTERNOON AND EVENING.  
 
OUTLOOK...  
SUNDAY THROUGH TUESDAY...WINDS AND SEAS SHOULD STAY BELOW SCA  
CRITERIA. THUNDERSTORMS ARE POSSIBLE ON SUNDAY, BUT THEN HIGH  
PRESSURE BUILDS IN LEADING TO DRY CONDITIONS.  
 
WEDNESDAY...SCA CONDITIONS, FOR BOTH WINDS AND SEAS, ARE  
POSSIBLE. SOUTHERLY WINDS INCREASE AHEAD OF THE NEXT LOW  
PRESSURE SYSTEM. SEAS SHOULD SUBSEQUENTLY INCREASE AS WELL.  
 
RIP CURRENTS...  
 
EASTERLY WINDS THIS MORNING INTO THE EARLY AFTERNOON SHIFTING TO THE  
SOUTH-SOUTHWEST. WINDS MAY GUST UP TO 25 MPH AT TIMES. SEAS IN THE  
SURF ZONE 3 TO 4 FT. A LOW RISK FOR THE FORMATION OF DANGEROUS TODAY  
FOR THE DE BEACHES. WITH MORE OF AN ONSHORE COMPONENT OF FLOW AND  
SWELL, THERE IS A MODERATE RISK FOR THE NJ BEACHES.  
 

 
   
PHI WATCHES/WARNINGS/ADVISORIES
 
 
PA...NONE.  
NJ...NONE.  
DE...NONE.  
MD...NONE.  
MARINE...SMALL CRAFT ADVISORY UNTIL 6 AM EDT SUNDAY FOR ANZ450>455.  
SMALL CRAFT ADVISORY UNTIL 6 AM EDT EARLY THIS MORNING FOR  
ANZ431.  
 

 
 

 
SYNOPSIS...JOHNSON  
NEAR TERM...LF  
SHORT TERM...LF  
LONG TERM...JOHNSON  
AVIATION...JOHNSON/LF  
MARINE...JOHNSON/LF  
 
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