514  
FXUS61 KPHI 260426  
AFDPHI  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE MOUNT HOLLY NJ  
1226 AM EDT THU APR 26 2018  
   
SYNOPSIS
 
 
AN AREA OF LOW PRESSURE OVER THE AREA THIS EVENING WILL MOVE  
INTO NEW ENGLAND OVERNIGHT. WEAK HIGH PRESSURE TRIES TO BUILD  
INTO THE REGION LATE THURSDAY, BEFORE ANOTHER LOW PRESSURE MOVES  
NORTHWARD ACROSS THE MID ATLANTIC REGION FRIDAY. THIS LOW WILL  
COMBINE WITH ANOTHER LOW AS IT MOVES ACROSS NEW ENGLAND SATURDAY  
BEFORE MOVING INTO EASTERN CANADA. A COLD FRONT IS EXPECTED  
SATURDAY, WITH ADDITIONAL SURFACE TROUGHS SUNDAY INTO MONDAY.  
HIGH PRESSURE ACROSS THE GREAT LAKES AND OHIO VALLEY SUNDAY WILL  
BUILD TO OUR SOUTH MONDAY, AND REMAIN TO OUR SOUTH THROUGH  
WEDNESDAY.  
 

 
   
NEAR TERM /UNTIL 6 AM THIS MORNING/
 
 
930 PM UPDATE...SURFACE LOW PRESSURE WAS POSITIONED OVER THE  
FORECAST AREA. LOW-LEVEL CONVERGENCE COMBINED WITH DEEPER LIFT  
AHEAD OF A MID-LEVEL SHORTWAVE TROUGH AND SOME DAYTIME HEATING  
HAD BEEN RESPONSIBLE FOR THE DEVELOPMENT OF SHOWERS AND STORMS  
ACROSS DELMARVA AND SOUTHERN NJ LATE THIS AFTERNOON AND SO FAR  
THIS EVENING. THIS CONVECTION WAS NOT ALL THAT ROBUST/ORGANIZED  
BUT THEY WERE SLOW MOVERS AND EFFICIENT RAINFALL PRODUCERS. TWO  
FLOOD ADVISORIES WERE ISSUED FOR THESE AREAS WITH LOCALIZED  
RAINFALL OF 2-3". THE THREAT OF ISOLATED HEAVY RAIN AND EVEN  
THUNDER WILL QUICKLY WANE OVER THE NEXT FEW HOURS AS CONVECTION  
WAS IN PART DIURNALLY DRIVEN. THE BACK EDGE OF THE PRECIP THAT  
IS CURRENTLY WEST OF I-81 IN NORTHEASTERN PA SHOULD EXIT THE  
REGION (MAINLY AFFECTING NE PA AND N NJ) BY AROUND 2 OR 3 AM.  
THE DENSE FOG ADVISORY REMAINS IN EFFECT FOR COASTAL NJ THRU  
MIDNIGHT ALTHOUGH SHOWERS APPROACHING THE COAST MAY HELP MIX OUT  
THE DENSE FOG LATE THIS EVENING. THE OPPORTUNITY FOR FOG TO  
BECOME DENSE AGAIN BEHIND THESE SHOWERS WILL BE FLEETING AS W-NW  
WINDS BEHIND A COLD FRONT ADVECT DRIER AIR INTO THE REGION.  
 
6 PM UPDATE...BASED ON CURRENT WEB CAMS AND SATELLITE, WE HAVE  
EXTENDED THE DENSE FOG ADVISORY ALONG COASTAL NJ UNTIL MIDNIGHT  
AFTER WHICH TIME CONDITIONS SHOULD IMPROVE AS A COLD FRONT MOVES  
THROUGH. OTHERWISE, JUST SOME MOSTLY MINOR ADJUSTMENTS TO POP,  
T/TD, AND SKY GRIDS BASED ON CURRENT CONDITIONS AND TRENDS.  
 
PREVIOUS DISCUSSION...LOW PRESSURE IS CURRENTLY LOCATED OVER  
DELMARVA AND WILL CONTINUE TO MOVE TO THE NORTH AND EAST THROUGH  
OUR AREA TONIGHT. THE ASSOCIATED UPPER TROUGH WILL ROTATE  
THROUGH THE AREA THROUGH THE OVERNIGHT HOURS. A GOOD AMOUNT OF  
ENERGY ASSOCIATED WITH THE UPPER TROUGH WILL ALLOW FOR THE  
CONTINUATION OF SHOWERS LATE TONIGHT WITH MAJORITY OF THE RAIN  
ENDING BETWEEN MIDNIGHT AND 2 AM. EVEN THOUGH THE SYSTEM WILL  
MOVE OFF TO THE NORTH OF THE AREA OVERNIGHT, LOW CLOUDS AND FOG  
WILL BE SLOWER TO CLEAR AS PLENTY ARE LIKELY TO REMAIN THROUGH  
THE EARLY MORNING HOURS, ESPECIALLY ACROSS THE NORTHERN PARTS OF  
OUR AREA.  
 
PRECIPITATION IS SPOTTY THIS AFTERNOON. SOME ISOLATED HEAVIER  
SHOWERS, WITH EMBEDDED THUNDERSTORMS, REMAIN POSSIBLE ACROSS  
SOUTHERN DELAWARE AND INTO SOUTHERN NEW JERSEY AS THE SUN HAS  
BROKEN OUT ACROSS THOSE AREAS THIS AFTERNOON. WE SHOULD LOSE THE  
CONVECTIVE POTENTIAL AS WE CONTINUE TO HEAD TOWARDS SUNSET.  
SOME LIGHT RAIN WAS MOVING NORTHWARD ACROSS NORTHERN NEW JERSEY  
WITH JUST SOME SPOTTY SHOWERS OR DRIZZLE REMAINING. AREAS TO THE  
NORTH AND WEST OF PHILADELPHIA CONTINUE TO SEE AREAS OF FOG  
WITH VARYING VISIBILITIES.  
 
WE HAVE TAKEN DOWN THE DENSE FOG ADVISORY FOR THE DELAWARE  
COAST AS THE WEB CAMS SHOW THAT THINGS HAVE CLEARED UP NICELY  
THERE THIS AFTERNOON. DENSE FOG ADVISORY REMAINS IN PLACE ALONG  
THE NEW JERSEY COAST THROUGH 7PM. AREAS ALONG THE NEW JERSEY  
COAST SHOW CONSIDERABLE FOG THIS AFTERNOON ON AREA WEB CAMS.  
 
OVERNIGHT LOWS WILL BE MILD AND IN THE 40S TO AROUND 50 ACROSS  
THE REGION.  
 

 
   
SHORT TERM /6 AM THIS MORNING THROUGH 6 PM FRIDAY/
 
 
THURSDAY BRINGS A BRIEF RETURN TO HIGH PRESSURE AND FAIR  
WEATHER ACROSS OUR AREA.  
 
THE DAY MAY START OFF A LITTLE CLOUDY, ESPECIALLY ACROSS  
NORTHERN AREAS, BUT WE SHOULD SEE PLENTY OF SUNSHINE THROUGH THE  
DAY. WINDS WILL BE OUT OF THE WEST TO NORTHWEST AROUND 10 TO 15  
MPH.  
 
TEMPERATURES ARE EXPECTED TO RISE INTO THE MID TO UPPER 60S  
ACROSS THE MAJORITY OF THE AREA WITH UPPER 50S ACROSS THE HIGHER  
TERRAIN OF THE POCONOS AND LOWER 70S ACROSS SOUTHERN NEW JERSEY  
AND SOUTHERN DELAWARE.  
 

 
   
LONG TERM /FRIDAY NIGHT THROUGH THURSDAY/
 
 
AFTER A BRIEF PERIOD OF POSSIBLE UNSETTLED WEATHER AT THE  
BEGINNING OF THE PERIOD, DRY WEATHER TAKES HOLD OF THE FORECAST  
FOR THE REST OF THE LONG TERM FORECAST.  
 
ON THURSDAY NIGHT, AN AREA OF LOW PRESSURE WILL BE APPROACHING  
THE AREA FROM THE SOUTHWEST, BEFORE MOVING ACROSS THE AREA  
DURING THE DAY FRIDAY. THERE REMAINS SOME TIMING AND LOCATIONS  
DIFFERENCES WITH THE LOW BETWEEN THE GUIDANCE, HOWEVER, THERE IS  
BETTER AGREEMENT THAN THERE WAS YESTERDAY. THEY ALL INDICATE AN  
INCREASE IN MOISTURE WHILE SEVERAL SHORT WAVE/VORTICITY  
IMPULSES SLIDE ACROSS THE AREA AS WELL, ESPECIALLY DURING THE  
DAY FRIDAY, WHICH HAS THE BEST CHANCE OF RAINFALL.  
 
ONCE THIS LOW LIFTS TO OUR NORTHEAST, IT WILL COMBINE WITH  
ANOTHER LOW PRESSURE THAT WILL BE MOVING OUT OF THE GREAT LAKES  
REGION AND ACROSS EASTERN CANADA AND NORTHERN NEW ENGLAND. THIS  
WILL PULL A COLD FRONT ACROSS THE AREA DURING THE DAY SATURDAY.  
LOW-MID LEVEL LAPSE RATES INCREASE AHEAD OF THE FRONT WHILE  
THERE WILL BE AN INCREASE IN LOW-MID LEVEL MOISTURE. THIS COULD  
BRING ANOTHER CHANCE OF SHOWERS DURING THE DAY AS THE FRONT  
MOVES ACROSS THE AREA. NOT MUCH IN THE WAY OF INSTABILITY IS  
FORECAST, SO WE ARE NOT INCLUDING THUNDERSTORMS AT THIS TIME.  
 
BEHIND THIS FRONT, DRIER CONDITIONS RETURN TO THE FORECAST.  
 
THE LOW PRESSURE ACROSS EASTERN CANADA WILL STAY NEAR THE  
CANADIAN MARITIMES THROUGH MONDAY, WHILE HIGH PRESSURE ACROSS  
THE GREAT LAKES AND OHIO VALLEY SUNDAY WILL BUILD TO OUR SOUTH  
MONDAY. A COUPLE OF SURFACE TROUGHS WILL CROSS THE AREA DURING  
THIS TIME PERIOD, BUT ARE EXPECTED TO REMAIN DRY. FOR SUNDAY AND  
MONDAY, THERE WILL BE A PRESSURE GRADIENT IN PLACE BETWEEN THE  
HIGH TO THE SOUTH AND LOW TO THE NORTH, SO A STEADY BREEZE WITH  
GUSTS LIKELY IN THE 20S IS EXPECTED, WITH SUNDAY HAVING THE  
STRONGEST WINDS.  
 
THE HIGH WILL REMAIN TO OUR SOUTH TUESDAY INTO WEDNESDAY,  
KEEPING DRY CONDITIONS IN THE FORECAST. WINDS WILL REMAIN OUT OF  
THE WEST, AND ALTHOUGH A STEADY BREEZE IS EXPECTED AND SLIGHTLY  
GUSTY AT TIMES, WINDS ARE NOT FORECAST TO BE AS GUSTY AS SUNDAY  
OR MONDAY.  
 

 
   
AVIATION /06Z THURSDAY THROUGH MONDAY/
 
 
THE FOLLOWING DISCUSSION IS FOR KPHL, KPNE, KTTN, KABE, KRDG,  
KILG, KMIV, KACY AND SURROUNDING AREAS.  
 
TONIGHT...AT 0130Z, CONDITIONS WERE HIGHLY VARIABLE (PRIMARILY  
CIGS) FROM TERMINAL TO TERMINAL, RANGING FROM VFR AT PHL/PNE TO  
LIFR AT TTN AND MVFR ELSEWHERE. WHILE A BRIEF DROP IN CIGS AND  
EVEN VISIBILITY CANNOT BE RULED OUT LATE THIS EVENING, IT WILL  
ONLY BE SHORT LIVED (AND ISOLATED IN COVERAGE) AS WINDS SHIFT  
OUT OF THE W-NW BEHIND A COLD FRONT. CONDITIONS IMPROVE TO VFR  
GENERALLY FROM W TO E DURING THE 04-07Z TIME FRAME.  
 
THURSDAY...MAINLY VFR CONDITIONS EXPECTED WITH CLEARING SKIES.  
WEST TO NORTHWEST WINDS PICK UP AROUND 10-14Z. WINDS WILL BE  
AROUND 10 TO 12 KNOTS WITH A FEW HIGHER GUSTS THROUGH THE  
AFTERNOON.  
 
OUTLOOK...  
 
THURSDAY NIGHT...GENERALLY VFR, ALTHOUGH MVFR CONDITIONS MAY  
BEGIN LATE IN THE NIGHT ACROSS THE SOUTHERN AREAS AS RAIN  
APPROACHES.  
 
FRIDAY...MVFR, POSSIBLE IFR, WITH A CHANCE OF SCATTERED  
SHOWERS.  
 
FRIDAY NIGHT...MVFR CONDITIONS POSSIBLE EARLY WITH SCATTERED  
SHOWERS, BECOMING VFR DURING THE EVENING AND OVERNIGHT.  
 
SATURDAY...MOSTLY VFR. A CHANCE OF SHOWERS WHICH MAY  
TEMPORARILY LOWER CONDITIONS.  
 
SATURDAY NIGHT-MONDAY...VFR. GUSTY WEST TO NORTHWEST WINDS  
20-25 KNOTS SUNDAY AND MONDAY.  
 

 
   
MARINE
 
 
THE DENSE FOG ADVISORY WAS LET GO AFTER MIDNIGHT. THE ARRIVAL OF  
THE SHOWERS HAS STIRRED THE ATMOSPHERE UP ENOUGH TO DISSIPATE  
SOME OF THE THICKER FOG. THE FRONT WILL CROSS THE AREA SHORTLY  
AND THE THREAT FOR THE DENSE FOG HAS PASSED.  
 
OTHERWISE, THE DELAWARE BAY LOOKS FAIRLY QUIET THROUGH  
THURSDAY. WINDS MAY NEAR 25 KNOTS THURSDAY MORNING BUT ARE  
CURRENTLY FORECAST TO REMAIN BELOW SCA CRITERIA.  
 
THE SCA ON THE OCEAN WILL REMAIN IN PLACE, MAINLY FOR SEAS,  
THROUGH THURSDAY. WINDS MAY GUST TO AROUND 25 KNOTS EARLY ON  
THURSDAY.  
 
OUTLOOK...  
 
THURSDAY NIGHT...SMALL CRAFT ADVISORY CONTINUES DUE TO ELEVATED  
SEAS.  
 
FRIDAY-FRIDAY NIGHT...ELEVATED SEAS MAY LINGER INTO FRIDAY AND  
FRIDAY NIGHT.  
 
SATURDAY NIGHT...CONDITIONS EXPECTED TO REMAIN BELOW ADVISORY  
LEVELS.  
 
SUNDAY-MONDAY...WINDS MAY APPROACH SMALL CRAFT ADVISORY LEVELS  
ON SUNDAY.  
 

 
   
PHI WATCHES/WARNINGS/ADVISORIES
 
 
PA...NONE.  
NJ...NONE.  
DE...NONE.  
MD...NONE.  
MARINE...SMALL CRAFT ADVISORY UNTIL 6 AM EDT FRIDAY FOR ANZ450>455.  
 

 
 

 
SYNOPSIS...ROBERTSON  
NEAR TERM...FITZSIMMONS/KLEIN/MEOLA  
SHORT TERM...MEOLA  
LONG TERM...ROBERTSON  
AVIATION...FITZSIMMONS/KLEIN/ROBERTSON/MEOLA  
MARINE...FITZSIMMONS/KLEIN/ROBERTSON/MEOLA/PO  
 
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