436  
FXUS61 KPHI 141945  
AFDPHI  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE MOUNT HOLLY NJ  
345 PM EDT TUE AUG 14 2018  
   
SYNOPSIS
 
 
THE LARGE LOW PRESSURE SYSTEM THAT HAS BEEN RESPONSIBLE FOR THE  
HEAVY RAIN IN THE AREA THE LAST FEW DAYS WILL CONTINUE TO SLOWLY  
LIFT NORTHEAST OVERNIGHT AWAY FROM THE REGION. THIS WILL BE FOLLOWED  
BY WEAK HIGH PRESSURE TOMORROW. BY FRIDAY, ANOTHER LOW CROSSING FROM  
THE GREAT LAKES REGION INTO NEW ENGLAND WILL BRING A COLD FRONT INTO  
OUR REGION. THIS FRONT IS EXPECTED TO STALL OVER OUR REGION FOR MUCH  
OF THE WEEKEND BEFORE SHIFTING SOUTHEAST SUNDAY NIGHT OR MONDAY.  
 

 
   
NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/
 
 
A CLEAR SPIN IN THE ATMOSPHERE WAS EVIDENT THIS AFTERNOON ON  
REGIONAL MOSAIC RADAR AND GOES-16 SATELLITE LOOPS IN ASSOCIATION  
WITH A CLOSED UPPER LOW OVER THE CATSKILLS REGION IN UPSTATE NY.  
THIS PLACES OUR REGION UNDER THE DRY SLOT AND IN WESTERLY FLOW TO  
THE SOUTH OF THE CYCLONE. HOWEVER, THIS DRY SLOT IS NOT VERY DRY AS  
THE PLUME OF TROPICAL MOISTURE THAT WAS DRAWN POLEWARD AHEAD OF THIS  
SYSTEM HAS WRAPPED AROUND THE LOW, RESULTING IN PWATS THAT ARE NEAR  
OR SLIGHTLY ABOVE CLIMO (1.5-1.75").  
 
SUFFICIENT MOISTURE AND DIURNAL INSTABILITY COMBINED WITH SEVERAL  
SOURCES OF LIFT, HAS LEAD TO ANOTHER EPISODE OF SHOWERS AND STORMS  
THIS AFTERNOON. CONVECTION WAS LOOSELY ORGANIZED EARLIER THIS  
AFTERNOON BUT HAS SINCE ORGANIZED A BIT INTO AT LEAST THREE BROKEN  
LINE SEGMENTS: (1) ALONG A PRE-FRONTAL SURFACE TROUGH THAT WAS OVER  
THE I-95 CORRIDOR AT THE MOMENT AND (2)/(3) A TRAILING SHORTWAVE  
TROUGHS LOCATED OVER NORTH-CENTRAL AND EASTERN PA. THIS MEANS  
SEVERAL ROUNDS OF SHOWERS AND STORMS WILL AFFECT EASTERN PA AND NJ  
THROUGH THE REMAINDER OF THE AFTERNOON AND INTO THE EVENING. THE  
BULK OF THIS ACTIVITY THIS AFTERNOON MAY PASS TO THE NORTH OF  
DELMARVA BUT THE TRAILING CONVECTION COULD ADVANCE FARTHER SOUTH  
INTO THESE SOUTHERN ZONES THIS EVENING.  
 
HEAVY DOWNPOURS WITH THESE SHOWERS AND STORMS ARE PRODUCING A QUICK  
ONE- TO THREE-QUARTERS OF AN INCH BUT A STEADY STORM MOTION (EAST AT  
10-20 MPH) HAS LIMITED THE DURATION OF THE HEAVY RAIN AND THE THREAT  
OF FLASH FLOODING. NONETHELESS, POOR DRAINAGE FLOODING IS STILL A  
POSSIBILITY FOR LOCATIONS THAT SEE MULTIPLE ROUNDS OF BRIEF HEAVY  
DOWNPOURS AND ARE PARTICULAR FLOOD PRONE DUE TO THE RECENT HEAVY  
RAINFALL. UNDER THE ASSUMPTION THAT STORMS DON'T ORGANIZE ALONG A  
MORE WEST-EAST LINE (PARALLEL TO STORM MOTION AND FAVORABLE TO  
TRAINING), THE RISK FOR FLASH FLOODING WILL BE LOW.  
 
SHOWERS AND STORMS WILL DIMINISH IN COVERAGE AND INTENSITY AFTER  
SUNSET, BUT AN ISOLATED SHOWER IS STILL POSSIBLE EARLY INTO THE  
OVERNIGHT. A LIGHT WEST WIND AND CLOUD COVER SHOULD LIMIT THE  
DEVELOPMENT OF FOG (IN TERMS OF COVERAGE AND DENSITY). HOWEVER,  
PATCHY FOG CANNOT BE RULED OUT IN THE RURAL VALLEYS IF CLOUD BREAKS  
DO DEVELOP.  
 

 
   
SHORT TERM /6 AM WEDNESDAY MORNING THROUGH 6 PM WEDNESDAY/
 
 
THE UPPER TROUGH WILL MOVE OFF THE NEW ENGLAND COAST WHILE THE  
ATTENDANT COLD FRONT PROGRESSES FARTHER OFF THE MID-ATLANTIC COAST.  
AN UPPER SHORTWAVE RIDGE INITIALLY POSITIONED WEST OF THE  
APPALACHIAN SPINE IN THE MORNING WILL BUILD EASTWARD TOWARD THE AREA  
THROUGH THE DAY.  
 
THE AFORMENTIONED RIDGE MAY BE FAR ENOUGH UPSTREAM OF OUR AREA TO  
GUARANTEE ONE OF THOSE FLEETING DAYS WITHOUT ANY SHOWERS OR STORMS.  
THE POSSIBILITY FOR A WEAK SHORTWAVE DISTURBANCE RIDING ATOP OF THE  
RIDGE, DEVELOPMENT OF A PRESSURE TROUGH AND TERRAIN CIRCULATIONS  
COULD PROVIDE A SOURCE(S) OF LIFT FOR CONVECTIVE INITIATION DURING  
PEAK HEATING HOURS, MAINLY ACROSS THE HIGHER TERRAIN AND NORTHERN  
TIER OF THE CWA (I-78 NORTH). COVERAGE (POPS) ARE FORECAST TO BE  
HIGHEST LATE IN THE AFTERNOON BUT LIMITED TO LOW CHANCE (30 PERCENT)  
ACROSS THE SOUTHERN POCONOS AND ISOLATED (20 PERCENT) ELSEWHERE N/W  
OF THE FALL LINE AND MASON-DIXON LINE.  
 
WAA IN WAKE OF THE EXITING TROUGH IN ADDITION TO STRONG DAYTIME  
HEATING AND DOWNSLOPING FLOW WILL ALL CONTRIBUTE TO A WARMING TREND  
COMPARED TO RECENT DAYS. HIGHS IN THE LOWER 90S ARE FORECAST FOR THE  
URBAN I-95 CORRIDOR AND COASTAL PLAIN WITH UPPER 80S MORE COMMON  
FARTHER INLAND IN THE HIGHER TERRAIN.  
 

 
   
LONG TERM /WEDNESDAY NIGHT THROUGH TUESDAY/
 
 
SUMMARY...RELATIVE CALM WEDNESDAY NIGHT AND THURSDAY WILL BE SHORT  
LIVED AS ANOTHER LOW PRESSURE SYSTEM WILL BRING MORE CHANCES FOR  
RAIN FRIDAY THROUGH THE WEEKEND. WE MAY HAVE ANOTHER PERIOD OF  
TRANQUIL WEATHER ON MONDAY, BEFORE A STRONGER COLD FRONT COULD BRING  
ANOTHER ROUND OF STORMS TUESDAY OR LATER.  
 
DETAILS:  
 
WEDNESDAY NIGHT AND THURSDAY...A MID LEVEL RIDGE IS EXPECTED TO  
PROGRESS EAST THROUGH THIS PERIOD, ALTHOUGH IT WILL BE WEAKENING AS  
IT DOES SO. THEREFORE, THE PERIOD SHOULD BE STARTING OUT MOSTLY DRY,  
THOUGH SOME SHOWERS AND STORMS COULD ENCROACH ON REGION FROM THE WEST  
ON THURSDAY AS THE NEXT LOW BEGINS TO DEVELOP. WITH THURSDAY  
STARTING OFF MOSTLY SUNNY, AND DEW POINTS EXPECTED TO REMAIN IN THE  
LOWER 70S, WE WILL SEE HOT AND MUGGY CONDITIONS DURING THE DAY  
THURSDAY. CURRENT FORECAST HAS CONDITIONS BELOW HEAT ADVISORY  
CRITERIA, BUT WILL CONTINUE TO MONITOR THIS AS WE GET CLOSER.  
 
FRIDAY THROUGH SUNDAY...DEVELOPING LOW OVER THE GREAT LAKES REGION  
WILL PROGRESS TOWARDS NEW ENGLAND THROUGH THE WEEKEND. THIS LOW WILL  
BRING A COLD FRONT INTO OUR REGION ON FRIDAY. HOWEVER, IT STILL  
LOOKS LIKELY THAT THIS FRONT WILL STALL OVER OUR REGION FOR MUCH OF  
THE REST OF THE WEEKEND. CONSEQUENTLY, EXPECT CONTINUED CHANCES FOR  
SHOWERS AND STORMS THROUGH THE WEEKEND, ESPECIALLY ON FRIDAY AND  
FRIDAY NIGHT. HOWEVER, THANKFULLY, IT LOOKS LIKE WE SHOULD HAVE  
FASTER STORM MOTIONS LIMITING THE FLOODING THREAT, ALTHOUGH  
PRECIPITABLE WATER VALUES REMAIN QUITE HIGH. TEMPERATURES THROUGH  
THIS PERIOD WILL BE VERY DEPENDENT ON WHERE THE FRONT EVENTUALLY  
STALLS. ESPECIALLY ON FRIDAY, WE COULD SEE A LARGE TEMPERATURE  
GRADIENT ACROSS OUR REGION WITH THE COASTAL PLAINS ONCE AGAIN IN THE  
LOWER 90S (HEAT INDEX VALUES NEAR 100), WHILE THE SOUTHERN POCONOS  
MAY STRUGGLE TO REACH 80.  
 
MONDAY...MID AND UPPER LEVEL SHORT WAVE RIDGE IS EXPECTED TO QUICKLY  
PROPAGATE EAST OVER OUR REGION RESULTING IN A MOSTLY DRY DAY.  
 
TUESDAY AND BEYOND...THIS WILL BE A PERIOD WORTH WATCHING AS ALMOST  
ALL OF THE OPERATIONAL MODELS ARE DEPICTING A STRONGER COLD FRONT  
APPROACHING OUR REGION FROM THE WEST. THERE REMAIN SOME MAJOR TIMING  
DIFFERENCES (THE GFS DOESN'T DEPICT THIS FRONT COMING THROUGH AT ALL  
THROUGH THE FIRST HALF OF THE WEEK, WHILE THE ECMWF HAS IT THROUGH  
THE REGION TUESDAY NIGHT), BUT IF THIS PATTERN CONTINUES, THIS COULD  
FINALLY BRING IN MUCH DRIER AIR (AFTER A PERIOD OF SHOWERS AND  
STORMS ALONG AND JUST AHEAD OF THE FRONT).  
 

 
   
AVIATION /20Z TUESDAY THROUGH SUNDAY/
 
 
THE FOLLOWING DISCUSSION IS FOR KPHL, KPNE, KTTN, KABE, KRDG, KILG,  
KMIV, KACY AND SURROUNDING AREAS.  
 
THROUGH THIS EVENING...SEVERAL BROKEN LINES OF SHOWERS AND STORMS  
WILL MOVE EASTWARD THROUGH THE REGION. THIS ACTIVITY DOES NOT LOOK  
TO AFFECT A TERMINAL FOR MORE THAN 15-20 MINUTES, RESULTING IN ONLY  
BRIEF MVFR OR IFR RESTRICTIONS. TIMING EACH ROUND HAS BEEN RATHER  
CHALLENGING WITH NEAR TERM TAF AMENDMENTS BASED ON UPSTREAM RADAR  
TRENDS SEEMS TO BE THE MOST PRACTICAL APPROACH. THIS ACTIVITY SHOULD  
DIMINISH LATE THIS EVENING.  
 
OVERNIGHT...A BROKEN MID-LEVEL DECK AND W-NW FLOW 4-8 KT SHOULD  
LIMIT THE POTENTIAL FOR FOG DEVELOPMENT. THE CURRENT FORECAST IS FOR  
VFR OVERNIGHT.  
 
WEDNESDAY...PREDOMINATELY VFR. AN ISOLATED SHOWER OR STORM IS  
POSSIBLE, MAINLY IN WESTERN TERMINALS (ABE/RDG) AND LATE IN THE  
AFTERNOON. ANY RESTRICTIONS WOULD BE BRIEF AND LOCALIZED. W WIND 5-  
10 KT WITH GUSTS OF 15-20 KT EXPECTED.  
 
OUTLOOK...  
 
WEDNESDAY NIGHT...VFR CONDITIONS EXPECTED WITH LIGHT WESTERLY WINDS.  
PATCHY FOG MAY DEVELOP NEAR MORE RURAL TAF SITES, BUT CHANCE FOR  
THIS OCCURING IS LOW. HIGH CONFIDENCE ON THE REST OF THE FORECAST  
ELEMENTS.  
 
THURSDAY THROUGH SUNDAY...PREDOMINANTLY VFR WITH TEMPORARY  
VISIBILITY AND CEILING RESTRICTIONS IF ANY SHOWERS AND THUNDERSTORMS  
AFFECT A TAF SITE. THE HIGHEST RISK FOR THIS OCCURING IS ON FRIDAY  
AND FRIDAY NIGHT. SOUTHWESTERLY WIND WILL BE SHIFTING TO NORTHERLY  
AND EVENTUALLY NORTHEASTERLY SATURDAY NIGHT INTO SUNDAY AS A COLD  
FRONT SINKS SOUTH.  
 

 
   
MARINE
 
 
W-SW WINDS 10-15 KT WITH GUSTS TO 20 KT ARE OCCURRING THIS AFTERNOON  
IN THE DE BAY AND COASTAL ATLANTIC WATERS. MEANWHILE SEAS AROUND  
AROUND 4 FT IN THE COASTAL WATERS. EXPECT THESE CONDITIONS TO  
CONTINUE INTO THIS EVENING EXCEPT FOR A WIND SHIFT TO W OR EVEN W-NW  
WILL OCCUR BEHIND A FRONT AROUND OR SHORTLY AFTER SUNSET. A PRESSURE  
SURGE BEHIND THE FRONT COULD ALSO YIELD GUSTS NEAR 25 KT (SCA  
CRITERIA) BUT IT SHOULD ONLY LAST FOR AN HOUR OR TWO PER MODEL  
FORECAST SOUNDINGS. ACCORDINGLY A SCA WAS NOT ISSUED. ISOLATED TO  
SCATTERED STORMS OVER LAND SHOULD REACH THE NJ WATERS LATE THIS  
AFTERNOON. ISOLATED GUSTS AT OR ABOVE 30 KT ARE POSSIBLE AND COULD  
NECESSITATE SPECIAL MARINE WARNINGS. ANOTHER BROKEN LINE OF STORMS  
EVEN FARTHER INLAND MAY REACH THE WATERS CLOSER TO SUNSET BUT IT IS  
UNCERTAIN IF THE INTENSITY WILL HOLD TOGETHER BY THE TIME IT REACHES  
THE COASTAL WATERS.  
 
W WINDS 10-15 WITH GUSTS TO 20 KT ARE EXPECTED ON WEDNESDAY.  
 
OUTLOOK...  
 
THURSDAY...WINDS AND SEAS SHOULD STAY BELOW SCA CRITERIA.  
 
FRIDAY...SOUTHERLY WINDS ON THE ATLANTIC WATERS WILL BE INCREASING  
AND COULD GUST ABOVE 20 KT AT TIMES. SEAS WILL SUBSEQUENTLY INCREASE  
AS WELL. HOWEVER, AT THIS TIME IT LOOKS LIKELY THAT BOTH WINDS AND  
SEAS WILL STAY JUST BELOW SCA CRITERIA.  
 
SATURDAY AND SUNDAY...WINDS WILL GRADUALLY SHIFT TO WESTERLY AND  
THEN ABRUPTLY SHIFT TO NORTHERLY AND NORTHEASTERLY SATURDAY NIGHT  
INTO SUNDAY. BEHIND THIS COLD FRONT, WIND GUSTS TO 25 KT AND SEAS  
APPROACHING 5 FEET ARE POSSIBLE.  
 
RIP CURRENTS...  
 
A LOW RISK FOR THE DEVELOPMENT OF DANGEROUS RIP CURRENTS IS EXPECTED  
FOR THE REMAINDER OF THIS AFTERNOON AND WEDNESDAY AS THE FLOW  
REMAINS OFFSHORE.  
 

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

 
 

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