123  
FXUS61 KPHI 210146  
AFDPHI  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE MOUNT HOLLY NJ  
946 PM EDT SUN AUG 20 2017  
   
SYNOPSIS
 
 
HIGH PRESSURE OVER THE AREA MOVES OFFSHORE TONIGHT, THEN SLOWLY  
DRIFTS OUT TO SEA THROUGH TUESDAY. A COLD FRONT APPROACHES FROM  
THE WEST ON TUESDAY, THEN WORKS ITS WAY ACROSS THE REGION ON  
WEDNESDAY. CANADIAN HIGH PRESSURE BUILDS INTO THE REGION LATE IN  
THE WEEK AND INTO NEXT WEEKEND.  
 

 
   
NEAR TERM /UNTIL 6 AM MONDAY MORNING/
 
 
RELATIVELY TRANQUIL WEATHER CONDITIONS WILL PERSIST THROUGH  
TONIGHT AS HIGH PRESSURE REMAINS OVER THE AREA. SCANT CLOUD  
COVER AND LIGHT AND VARIABLE WINDS WILL LEAD TO FAVORABLE  
RADIATIONAL COOLING TONIGHT, WITH TEMPERATURES FALLING TO NEAR  
OR WITHIN A FEW DEGREES OF CLIMATOLOGICAL NORMALS.  
 
DEW POINTS HAVE INCREASED A BIT MORE THAN PREVIOUSLY FORECAST,  
SO EXPANDED THE MENTION OF PATCHY FOG, THOUGH AGREE WITH THE  
PREVIOUS SHIFT THAT ANY FOG WILL BE SHALLOW RADIATION FOG AND  
WILL LIKELY BE SHORT LIVED.  
 

 
   
SHORT TERM /6 AM MONDAY MORNING THROUGH 6 PM MONDAY/
 
 
AS HIGH PRESSURE SHIFTS OFFSHORE, FLOW WILL RETURN TO A MORE  
SOUTHERLY DIRECTION ALLOWING WARM/MOIST ADVECTION THROUGH THE  
AFTERNOON. TEMPERATURES WILL BE A FEW DEGREES HIGHER THAN  
SUNDAY, WHILE HUMIDITY WILL MARKEDLY INCREASE TOWARDS THE  
EVENING. THERE MAY BE A CHANCE FOR SOME SHOWERS AND  
THUNDERSTORMS AS A SHORT WAVE TROUGH OVER THE MIDWEST MOVES  
EASTWARD, REACHING THE AREA BY AFTERNOON/EVENING. ONE LIMITING  
FACTOR WILL BE RATHER DIFFUSE SURFACE BOUNDARIES - WITH LITTLE  
TO NO FORECAST CONVERGENCE EXPECTED TO FOCUS OR ENHANCE  
ACTIVITY. AS SUCH, THE BEST CHANCE FOR ANY DEVELOPMENT WILL BE  
ALONG HIGHER TERRAIN, MAINLY TO THE WEST OF PHILADELPHIA.  
 

 
   
LONG TERM /MONDAY NIGHT THROUGH SUNDAY/
 
 
SOME MID-LEVEL SHORTWAVE ENERGY WILL PASS THROUGH THE REGION  
MONDAY NIGHT. ONSHORE FLOW WILL USHER A WARM AND HUMID AIRMASS  
INTO THE REGION, AS SURFACE DEWPOINTS WILL CLIMB INTO THE UPPER  
60S TO LOW 70S, AND INTO THE MID 70S ACROSS THE DELMARVA AND  
EXTREME SOUTHERN NJ. THIS WILL RESULT IN ISOLATED TO SCATTERED  
SHOWERS AND THUNDERSTORMS IN THE EVENING, GRADUALLY TAPERING OFF  
AFTER MIDNIGHT. PATCHY FOG MAY FORM IN THE PRE-DAWN HOURS GOING  
INTO TUESDAY MORNING.  
 
ON TUESDAY, LOW PRESSURE OVER THE GREAT LAKES WILL LIFT TO THE  
NORTHEAST TOWARDS EASTERN CANADA. THIS WILL DRAG A COLD FRONT  
THROUGH THE GREAT LAKES AND INTO THE OHIO VALLEY LATE TUESDAY.  
PRE-FRONTAL TROUGH SETS UP AHEAD OF THE FRONT, AND THAT LOOKS  
TO TRIGGER AFTERNOON AND EVENING SHOWERS AND THUNDERSTORMS.  
ABUNDANT LOW LEVEL MOISTURE WILL BE IN PLACE WITH DEWPOINTS IN  
THE LOW TO MID 70S, AND WITH SEVERAL STRONG SHORTWAVES OUT AHEAD  
OF THE UPPER TROUGH/SURFACE COLD FRONT, CAN EXPECT SOME FAIRLY  
STRONG THUNDERSTORMS DURING THE OVERNIGHT HOURS TUESDAY NIGHT  
INTO WEDNESDAY MORNING. COLD FRONT WORKS ITS WAY TO THE SOUTH  
AND EAST WEDNESDAY THROUGH WEDNESDAY EVENING, THEN BECOMES  
NEARLY STATIONARY OVER THE MID-ATLANTIC AND DELMARVA FOR THE END  
OF THE WEEK. QUITE WARM AND HUMID FOR THE FIRST HALF OF THE  
LONG TERM PERIOD, WITH HIGHS IN THE UPPER 80S TO LOW 90S ON  
TUESDAY, THEN IN THE MID TO UPPER 80S ON WEDNESDAY. MAX HEAT  
INDEX VALUES ON TUESDAY LOOK TO BE IN THE UPPER 90S FOR AREAS  
ALONG AND SOUTH OF THE I-95 CORRIDOR, AND AROUND 100 FOR THE  
DELMARVA.  
 
SEVERAL WAVES OF LOW PRESSURE MAY DEVELOP ON THIS BOUNDARY, BUT  
LATEST MODEL GUIDANCE KEEPS THOSE SYSTEMS WELL OFFSHORE.  
 
CANADIAN HIGH PRESSURE THEN BUILDS IN FROM THE NORTH AND WEST  
TO CLOSE OUT THE WORK WEEK, WITH THE CENTER OF THE HIGH  
GRADUALLY PASSING NORTH OF THE REGION ON SATURDAY BEFORE MOVING  
OFFSHORE ON SUNDAY. DRY CONDITIONS EXPECTED FOR THE END OF THE  
WEEK AND INTO THE WEEKEND WITH TEMPERATURES RUNNING SEVERAL  
DEGREES BELOW NORMAL. HIGHS WILL TOP OFF IN THE UPPER 70S TO  
AROUND 80 FROM THURSDAY THROUGH SUNDAY, AND LOWS WILL DROP INTO  
THE UPPER 50S TO LOW 60S, EXCEPT COOLER UP NORTH.  
 

 
   
AVIATION /02Z MONDAY THROUGH FRIDAY/
 
 
THE FOLLOWING DISCUSSION IS FOR KPHL, KPNE, KTTN, KABE, KRDG,  
KILG, KMIV, KACY AND SURROUNDING AREAS.  
 
VFR CONDITIONS. THERE IS A LOW CHANCE OF PATCHY FOG DEVELOPMENT  
TOWARDS DAYBREAK TONIGHT BUT WITH DRIER CONDITIONS EXPECT ANY  
DEVELOPMENT TO BE LIMITED. WESTERLY WINDS THROUGH THIS AFTERNOON  
WILL GRADUALLY BECOME LIGHT SOUTHWESTERLY THEN LIGHT AND  
VARIABLE AS HIGH PRESSURE SHIFTS ACROSS THE AREA.  
 
BY MONDAY, HIGH PRESSURE WILL SHIFT OFFSHORE ALLOWING SOUTHERLY  
FLOW TO STRENGTHEN TO AROUND 5-10 KT THROUGH THE AFTERNOON.  
THERE IS AN INCREASING CHANCE FOR SHOWERS AND THUNDERSTORMS  
AFTER 18Z, GENERALLY 20 TO 30 PERCENT FOR ALL TAF SITES.  
HOWEVER, THE CHANCE IS TOO LOW TO INCLUDE IN THE TAFS AT THIS  
TIME.  
 
OUTLOOK...  
 
MONDAY NIGHT...SUB-VFR CONDITIONS POSSIBLE IN SCATTERED  
SHRA/TSRA. FOG/STRATUS ALSO POSSIBLE WITH SUB-VFR CONDITIONS.  
 
TUESDAY...MORNING FOG/STRATUS BURNS OFF, THEN PREDOMINANTLY  
VFR. SCATTERED SHRA/TSRA DEVELOP LATE IN THE DAY AND AT NIGHT  
WITH SUB-VFR CONDITIONS.  
 
WEDNESDAY...SUB-VFR CONDITIONS IN SHOWERS/THUNDERSTORMS, WHICH  
TAPER OFF LATE IN THE DAY. A WIND SHIFT FROM S TO NW LIKELY LATE  
IN THE DAY WITH THE PASSAGE OF A COLD FRONT.  
 
THURSDAY THROUGH FRIDAY...VFR AND LIGHT WINDS.  
 

 
   
MARINE
 
 
WINDS AND SEAS REMAIN TRANQUIL ACROSS THE WATERS THROUGH MONDAY  
AS HIGH PRESSURE GRADUALLY SHIFTS OFFSHORE. SEAS WILL REMAIN  
TWO FEET OR LESS THROUGH MONDAY AFTERNOON, WITH A FEW 3 FOOT  
SEAS POSSIBLE OFF THE NORTHERN NJ COAST BY EVENING AS SOUTHERLY  
WINDS BEGIN TO STRENGTHEN.  
 
OUTLOOK...  
 
MONDAY NIGHT...WINDS AND SEAS BELOW SCA THRESHOLDS.  
 
TUESDAY...SCA POSSIBLE LATE AFTERNOON AND NIGHT WITH S WINDS  
INCREASING TO AROUND 25 KT AHEAD OF COLD FRONT. BUILDING SEAS  
TO 4-6 FT ARE FORECAST FOR THE ATLANTIC COASTAL WATERS.  
 
WEDNESDAY...WINDS SHOULD DECREASE TO BELOW 25 KT BUT SCA  
CONDITIONS MAY CONTINUE INTO THE MORNING AS IT MAY TAKE LONGER  
FOR SEAS IN THE COASTAL WATERS TO DROP BELOW 5 FT.  
 
THURSDAY THROUGH FRIDAY...NO MARINE HAZARDS EXPECTED.  
 
RIP CURRENTS...  
AT THIS POINT WE EXPECT THE RISK FOR THE FORMATION OF DANGEROUS  
RIP CURRENTS ON MONDAY TO BEGIN LOW. HOWEVER, WITH WINDS  
INCREASING OUT OF THE SOUTH AND THE DOMINANT PERIOD TRENDING  
LONGER, THE RISK COULD INCREASE AS WE GO THROUGH THE DAY  
DEPENDING ON HOW QUICKLY THE WIND SHIFT OCCURS. FOR NOW WE WILL  
MENTION A LOW RISK IN THE SURF FORECAST BUT CONTINUE TO MONITOR  
CLOSELY ESPECIALLY AS WE GO THROUGH THE DAY ON MONDAY.  
 

 
   
TIDES/COASTAL FLOODING
 
 
MINOR COASTAL FLOODING IS POSSIBLE AT THE HIGH TIDE MONDAY  
EVENING. WITH HIGH PRESSURE OFFSHORE, ONSHORE WILL WINDS  
DEVELOP, AND THIS WILL COINCIDE WITH RELATIVELY HIGH  
ASTRONOMICAL TIDES DUE TO A NEW MOON. LATEST GUIDANCE IS SHOWING  
WATER LEVELS JUST TOUCHING MINOR COASTAL FLOODING THRESHOLDS.  
HOWEVER, WATER LEVELS WITH THE PAST THREE HIGH TIDE CYCLES HAVE  
BEEN HIGHER THAN MOST GUIDANCE, THUS WILL INCLUDE A MENTION IN  
THE HAZARDOUS WEATHER OUTLOOK FOR THE POTENTIAL OF MINOR TIDAL  
FLOODING. WILL WAIT TO MAKE A DECISION ON AN ADVISORY UNTIL WE  
GET THE NEXT RUN OF GUIDANCE IN AND HAVE A CHANCE TO RUN LOCAL  
REGRESSION EQUATIONS BASED ON THE TIDAL DEPARTURES WE SAW THIS  
EVENING.  
 

 
   
PHI WATCHES/WARNINGS/ADVISORIES
 
 
PA...NONE.  
NJ...NONE.  
DE...NONE.  
MD...NONE.  
MARINE...NONE.  
 

 
 

 
SYNOPSIS...MPS  
NEAR TERM...JOHNSON/99  
SHORT TERM...99  
LONG TERM...MPS  
AVIATION...JOHNSON/MPS/99  
MARINE...JOHNSON/MPS/99  
TIDES/COASTAL FLOODING...  
 
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