548  
FXUS61 KPHI 251047  
AFDPHI  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE MOUNT HOLLY NJ  
647 AM EDT MON OCT 25 2021  
   
SYNOPSIS
 
 
A WARM FRONT WILL SURGE NORTHWARD THROUGH THE REGION THIS MORNING AS  
A SURFACE LOW MOVES EAST THROUGH THE OHIO VALLEY TODAY, WEAKENING AS  
IT APPROACHES THE APPALACHIANS AND MID-ATLANTIC TONIGHT. MEANWHILE,  
A SURFACE LOW WILL DEVELOP OFF THE SOUTHEAST COAST TODAY, LIFTING  
NORTHEASTWARD OFF THE MID-ATLANTIC COAST TONIGHT AND TUESDAY AS IT  
INTENSIFIES. THIS LOW IS EXPECTED TO STALL JUST SOUTH OF NEW ENGLAND  
TUESDAY AND TUESDAY NIGHT BEFORE PIVOTING EASTWARD ON WEDNESDAY.  
ANOTHER STRONG SURFACE LOW WILL MOVE FROM THE SOUTHERN PLAINS ON  
WEDNESDAY NIGHT TO THE NORTHEAST BY THE END OF THE WEEK. THIS LOW  
WILL LIKELY LIFT NORTHEASTWARD INTO THE CANADIAN MARITIMES DURING  
THE WEEKEND.  
 

 
   
NEAR TERM /THROUGH TUESDAY/
 
 
NO SIGNIFICANT CHANGES WITH THE 7 AM UPDATE. THIS REMAINS A LOW  
CONFIDENCE BUT POTENTIALLY HIGH IMPACT FORECAST. THE MAIN  
FEATURES TO WATCH OVER THE NEXT 24 TO 36 HOURS ARE THE CLOSED  
LOW CURRENTLY OVER THE MID MISSISSIPPI VALLEY, THE WARM FRONT  
CURRENTLY BISECTING OUR REGION, AND A MID LEVEL SHORT WAVE  
TROUGH WHICH WAS OVER AL/GA AS OF THE 00Z SOUNDINGS. THE  
INTERACTION OF THESE FEATURES IS WHAT IS EXPECTED TO RESULT IN  
CYCLOGENESIS LATER TODAY OFF THE GA/SC COAST WHICH WILL BECOME  
THE COASTAL STORM TO IMPACT OUR REGION TONIGHT CONTINUING  
THROUGH TUESDAY.  
 
AS MENTIONED ABOVE, THIS IS A LOW CONFIDENCE FORECAST. THE WARM  
FRONT IS MUCH SLOWER THAN ANY OF THE MODELS HAD DEPICTED EVEN WITH  
THE 00Z RUNS. I AM ALSO CONCERNED THAT AT LEAST SOME 00Z MODELS  
APPEAR TO HAVE INITIALIZED WITH THE CENTER OF THE MID LEVEL LOW  
TOO FAR SOUTH AND EAST COMPARED TO WHAT 00Z SOUNDINGS AND  
SATELLITE TRENDS WOULD SUGGEST. IF THERE IS IN FACT MORE  
SEPARATION (DISTANCE WISE) BETWEEN THE MID LEVEL SHORT WAVE  
TROUGH AND THE CLOSED LOW, THEN IT IS POSSIBLE THE COASTAL LOW  
WILL DEVELOP FURTHER OFF SHORE. CYCLOGENESIS OCCURING ENTIRELY  
OFF THE COAST IS ALREADY A NOTORIOUSLY HIGH UNCERTAINTY PATTERN,  
AND THE PREVIOUSLY MENTIONED TRENDS JUST ADD TO MY UNCERTAINTY  
WITH THIS FORECAST.  
 
THAT BEING SAID, THIS HAS THE POTENTIAL TO BE HIGH IMPACT FOR OUR  
REGION, SO HERE IS A SUMMARY OF THE POTENTIAL HAZARDS THROUGH  
TUESDAY:  
 
HEAVY RAIN: HEAVY RAIN AT THIS POINT IS MY BIGGEST CONCERN, AND THE  
AREA OF CONCERN HASN'T CHANGED MUCH, MOSTLY COASTAL AND NORTHERN  
NJ. THERE IS POTENTIAL THAT THE AXIS OF HEAVIEST RAIN COULD BE  
SLIGHTLY FURTHER SOUTH ALONG THE SOUTHERN NJ SHORE, SO CAPE MAY  
AND ATLANTIC COUNTIES WERE ADDED IN TO THE WATCH. ADDITIONALLY,  
BASED ON GUIDANCE TRENDS, INCREASED THE QPF SLIGHTLY.  
 
SEVERE THUNDERSTORMS: LIMITED INSTABILITY, BUT SIGNIFICANT SHEAR  
(BOTH SPEED AND DIRECTIONAL SHEAR) MEANS THAT THERE IS POTENTIAL IF  
ANY THUNDERSTORMS (OR EVEN JUST SHOWERS WITH A STRONGER UPDRAFT)  
DEVELOP, THAT STRONGER WINDS COULD MIX DOWN TO THE SURFACE.  
 
AS FAR AS TIMING, EXPECT MOST OF TODAY TO BE DRY, BUT SOME INITIAL  
SHOWERS COULD MOVE IN OR DEVELOP (ESPECIALLY IF THE WARM FRONT IS  
STILL IN THE AREA) BY LATE IN THE AFTERNOON. THE BULK OF THE RAIN  
SHOULD BE FROM LATE THIS EVENING THROUGH MUCH OF THE DAY ON  
TUESDAY.  
 

 
   
SHORT TERM /TUESDAY NIGHT THROUGH WEDNESDAY NIGHT/
 
 
MAJOR STORM JUST OFF THE COAST WILL BE IN THE PROCESS OF PIVOTING  
EASTWARD TUESDAY NIGHT AND WEDNESDAY AS A DIGGING VORT MAX REACHES  
THE MID-ATLANTIC. AS IT REACHES THE BASE OF THE LONGER-WAVE TROUGH  
AROUND 12Z WEDNESDAY, THE ORIENTATION OF THE VORTICITY MAXIMUM WILL  
ALLOW FOR THE INCREASE IN EASTWARD MOMENTUM OF THE MAIN UPPER LOW BY  
WEDNESDAY AFTERNOON. OF COURSE, A NUMBER OF QUESTIONS REMAIN ABOUT  
THIS PROCESS, SINCE THE INTERACTIONS AMONG THE VARIOUS PERTURBATIONS  
AND THE PREDECESSOR IMPACTS OF WIDESPREAD PRECIPITATION/CONVECTION  
IN TANDEM WITH THE RAPIDLY DEEPENING LOW ARE GENERALLY LOW-  
PREDICTABILITY PHENOMENA. NEVERTHELESS, WITH THE NOTABLE MODEL  
TRENDS ON KEEPING THE LOW CLOSER TO THE COAST AND FOR A LITTLE WHILE  
LONGER, SUSPECT STRONGER WINDS WILL LINGER NEAR THE COAST FOR A TIME  
TUESDAY NIGHT AND WEDNESDAY MORNING, WITH GUSTS EXCEEDING 35 TO 40  
MPH PROBABLE ALONG THE IMMEDIATE COAST. NOT OUT OF THE QUESTION A  
WIND ADVISORY WILL BE NEEDED FOR THESE AREAS. ADDITIONALLY, SHOWERS  
WILL LIKELY ROTATE SOUTH-SOUTHEASTWARD ON THE WEST SIDE OF THE LOW  
IN MUCH OF THE AREA TUESDAY EVENING, SLOWLY SHIFTING EASTWARD AS THE  
LOW BEGINS ITS INEVITABLE ACCELERATION OFFSHORE. BY WEDNESDAY, MOST  
IF NOT ALL OF THE PRECIPITATION SHOULD BE DONE FOR THE REGION, WITH  
BREEZY NORTH WINDS ALSO GRADUALLY DECLINING DURING THE DAY. HIGHS ON  
WEDNESDAY SHOULD RANGE FROM THE MID 50S IN THE POCONOS TO THE MID  
60S IN DELMARVA.  
 
SKY COVER MAY DIMINISH WEDNESDAY NIGHT AS NORTH TO NORTHEAST WINDS  
SLOWLY DECREASE, WHICH MAY BE FAVORABLE FOR SOME STRONGER COOLING  
THAN ON THE PREVIOUS NIGHT. STILL, THINK WINDS WILL NOT COMPLETELY  
DECOUPLE, SO FORECAST LOWS ARE MAINLY IN THE MID 40S TO AROUND  
50.  
 

 
   
LONG TERM /THURSDAY THROUGH SUNDAY/
 
 
TRANSIENT/AMPLIFIED RIDGING ON THURSDAY WILL QUICKLY GIVE WAY TO THE  
NEXT STRONG SYSTEM LIFTING NORTHEASTWARD FROM THE SOUTHERN U.S.  
THURSDAY NIGHT AND FRIDAY. A STRONG SURFACE LOW WILL RAPIDLY BECOME  
VERTICALLY STACKED WITH A LARGE UPPER LOW IN THE MISSISSIPPI VALLEY  
THURSDAY NIGHT. A FRONT WILL EXTEND WELL EASTWARD/SOUTHEASTWARD FROM  
THE PARENT LOW TO THE SOUTHEAST COAST. WIDESPREAD  
PRECIPITATION/CONVECTION SHOULD OCCUR IN ADVANCE (NORTH AND EAST) OF  
THE FRONT AS IT TRANSLATES RAPIDLY NORTHEASTWARD THURSDAY AND  
FRIDAY. TIMING OF IMPACTS FOR OUR AREA LOOKS MOST LIKELY FROM EARLY  
FRIDAY MORNING INTO FRIDAY EVENING, AS A POTENT SHORTWAVE TROUGH  
LIFTS NORTHEASTWARD ON THE EAST SIDE OF THE UPPER LOW NEAR THE  
COAST. NOTABLY, A STIFF EASTERLY WIND IS LIKELY AS THE SYSTEM MOVES  
THROUGH THE AREA, MAKING FOR A RATHER UNPLEASANT DAY.  
 
THEREAFTER, THE UPPER LOW WILL ONLY MAKE SLOW PROGRESS EAST-  
NORTHEAST INTO THE NORTHEAST FRIDAY NIGHT AND SATURDAY, AS  
DOWNSTREAM RIDGING PUTS THE BRAKES ON THE THE CLOSED LOW. SHOWERY  
WEATHER SHOULD CONTINUE DURING THIS PERIOD, WITH CONDITIONS SLOWLY  
DRYING OUT SATURDAY NIGHT AND SUNDAY, AIDED IN PART BY A NORTHERN-  
STREAM VORT MAX SHIFTING EASTWARD THROUGH SOUTHEAST CANADA ON SUNDAY.  
 
TEMPERATURES DURING THIS PERIOD WILL GENERALLY BE A FEW DEGREES  
ABOVE SEASONAL AVERAGES.  
 

 
   
AVIATION /12Z MONDAY THROUGH FRIDAY/
 
 
THE FOLLOWING DISCUSSION IS FOR KPHL, KPNE, KTTN, KABE, KRDG, KILG,  
KMIV, KACY AND SURROUNDING AREAS.  
 
TODAY...MOSTLY VFR CONDITIONS ARE EXPECTED THROUGH THE DAY. THERE IS  
A SMALL CHANCE THAT SHOWERS AND STORMS COULD MOVE INTO THE REGION  
LATE THIS AFTERNOON, BUT IT IS MORE LIKELY THAT WE WILL STAY DRY  
THROUGH THE DAY. AT KILG, KMIV, AND KACY, WINDS WILL BE SOUTHERLY 5  
TO 10 KT, WITH SOME GUSTS NEAR 15 KT POSSIBLE THROUGH THE DAY. FOR  
KPHL, KPNE, KRDG, KABE, AND KTTN, WINDS WILL START LIGHT AND  
VARIABLE, OR EVEN LIGHT EASTERLY, BUT SHOULD EVENTUALLY SETTLE OUT  
OF THE SOUTH AND SOUTHWEST BY MID DAY WITH SPEEDS 5 TO 10 KT WITH  
SOME GUSTS NEAR 15 KT POSSIBLE. HIGH CONFIDENCE ON MOST ASPECTS OF  
THE FORECAST, BUT MODERATE CONFIDENCE ON WHEN THE SHIFT TO SOUTHERLY  
WINDS WILL OCCUR FOR THE TAF SITES FROM KPHL NORTHWARD.  
 
TONIGHT...SHOWERS AND A FEW THUNDERSTORMS ARE EXPECTED TO SPREAD  
OVER THE REGION, ESPECIALLY AFTER 00Z. ALTHOUGH SOME VISIBILITY  
RESTRICTIONS WILL BE POSSIBLE IN HEAVY RAIN AND POSSIBLY EVEN FOG  
(ESPECIALLY AFTER 06Z), THE CONTROLLING ELEMENT FOR FLIGHT CATEGORY  
IS EXPECTED TO BE THE CEILINGS. MVFR CEILINGS ARE EXPECTED THIS  
EVENING AS THE RAIN FIRST MOVES IN, AND IFR CEILINGS ARE POSSIBLE  
AFTER 03Z. WINDS WILL START LIGHT SOUTHERLY, BUT ARE EXPECTED TO  
SHIFT TO NORTHEASTERLY AT 5 TO 15 KT DURING THE LATER HALF OF THE  
NIGHT. VARIABLE AND GUSTY WINDS WILL BE POSSIBLE IN THE VICINITY OF  
ANY THUNDERSTORMS AND EVEN SOME SHOWERS. HIGH CONFIDENCE THAT  
PREVAILING CONDITIONS WILL BE BELOW VFR FOR ALMOST ALL OF THE 00 TO  
12Z PERIOD, BUT LOW CONFIDENCE ON OTHER ASPECTS OF THE FORECAST,  
ESPECIALLY TIMING DETAILS OF FLIGHT CATEGORY CHANGES, AND THE LOWEST  
POSSIBLE CEILINGS.  
 
TUESDAY...PREVAILING MVFR OR IFR MOSTLY DUE TO CEILINGS, THOUGH SOME  
VISIBILITY RESTRICTIONS WILL BE POSSIBLE AS WELL DURING THE HEAVIEST  
RAIN. EVEN AREAS THAT MAY SEE BREAKS IN THE RAIN (MOST LIKELY TO  
OCCUR AT KRDG), LOW CLOUDS ARE EXPECTED TO PERSIST THROUGH THE DAY.  
WINDS STARTING LIGHT NORTHEASTERLY, BECOMING NORTHWESTERLY 10 TO 20  
KT, WITH HIGHER GUSTS POSSIBLE ESPECIALLY AFTER 18Z. NON-CONVECTIVE  
LOW LEVEL WIND SHEAR IS POSSIBLE NEAR THE COAST THROUGH THE MORNING,  
INCLUDING KACY. MODERATE CONFIDENCE ON THE OVERALL PATTERN, BUT LOW  
CONFIDENCE ON DETAILS AND EXACT IMPACTS AT TAF SITES.  
 
OUTLOOK...  
 
TUESDAY NIGHT...SUB-VFR CONDITIONS PROBABLE WITH SCATTERED TO  
NUMEROUS SHOWERS. NORTH/NORTHWEST WINDS 10 TO 20 KT WITH HIGHER  
GUSTS POSSIBLE (STRONGER NEAR THE COAST). MODERATE CONFIDENCE.  
 
WEDNESDAY...LINGERING SUB-VFR POSSIBLE, BUT CONDITIONS SHOULD  
GRADUALLY IMPROVE TO VFR, WITH SHOWERS EXITING THE REGION. NORTH  
WINDS 5 TO 15 KT WITH HIGHER GUSTS. MODERATE CONFIDENCE.  
 
WEDNESDAY NIGHT AND THURSDAY...MAINLY VFR WITH LIGHT NORTHEAST WINDS  
WEDNESDAY NIGHT BECOMING EAST 5 TO 15 KT ON THURSDAY. HIGH  
CONFIDENCE.  
 
THURSDAY NIGHT AND FRIDAY...RESTRICTIONS LIKELY WITH A GOOD CHANCE  
FOR SHOWERS (MAINLY ON FRIDAY). EAST WINDS 5 TO 15 KT INCREASING TO  
10 TO 20 KT ON FRIDAY (STRONGER NEAR THE COAST). MODERATE CONFIDENCE.  
 

 
   
MARINE
 
 
SCA CONDITIONS ARE UNLIKELY TO DEVELOP, SO HAVE CANCELED THE  
REMAINING SMALL CRAFT ADVISORY. WINDS AND SEAS SHOULD STAY BELOW  
SCA CRITERIA THROUGH THE REMAINDER OF TODAY AND THIS EVENING,  
THOUGH SEAS COULD GET CLOSE TO 5 FT ON THE NJ ATLANTIC COASTAL  
WATERS LATE TODAY.  
 
BY LATE MONDAY NIGHT/EARLY TUESDAY MORNING, NORTHEASTERLY WINDS AND  
SEAS SHOULD INCREASE IN RESPONSE TO A DEVELOPING COASTAL LOW. GALE  
FORCE WINDS ARE LIKELY TO DEVELOP BY LATE TUESDAY AFTERNOON, BUT  
GIVEN CONSIDERABLE UNCERTAINTY IN THE TIMING OF WHEN THIS WILL  
DEVELOP, HAVE ISSUED A GALE WATCH STARTING TUESDAY MORNING. IF  
CONFIDENCE INCREASES, THIS START TIME MAY BE REFINED FURTHER.  
 
OUTLOOK...  
 
TUESDAY NIGHT AND WEDNESDAY...GALES LIKELY (PRIMARILY TUESDAY NIGHT  
FOR DELAWARE BAY). A GALE WATCH IS IN EFFECT. CONDITIONS WILL BE  
SHOWERY, WITH POTENTIAL FOR SOME VISIBILITY RESTRICTIONS AS WELL.  
 
WEDNESDAY NIGHT...ADVISORY CONDITIONS FOR THE ATLANTIC WATERS, WITH  
NORTH TO NORTHEAST WINDS GUSTING TO 30 KT AND SEAS OF 6 TO 9 FEET.  
SUB-ADVISORY CONDITIONS MAY OCCUR ON DELAWARE BAY.  
 
THURSDAY...SUB-ADVISORY WINDS, BUT SEAS WILL LIKELY REMAIN ABOVE 5  
FEET ON THE ATLANTIC WATERS.  
 
THURSDAY NIGHT AND FRIDAY...STRONG NORTHEAST TO EAST WINDS WILL  
DEVELOP, WITH GALES POSSIBLE ON THE ATLANTIC WATERS ON FRIDAY. SEAS  
MAY BUILD TO 6 TO 10 FEET.  
 

 
   
TIDES/COASTAL FLOODING
 
 
AS A MAJOR COASTAL STORM DEVELOPS TONIGHT AND LINGERS OFF THE  
SOUTHERN NEW ENGLAND COAST THROUGH WEDNESDAY, STRONG NORTHEAST TO  
NORTH WINDS WILL DEVELOP ALONG THE COAST TUESDAY BEFORE BECOMING  
MORE NORTHWESTERLY ON TUESDAY NIGHT. THE HIGH TIDE IS ON LATE  
TUESDAY EVENING ON THE ATLANTIC COAST, AND THE EVOLUTION OF THE  
WINDS WILL BE CRITICAL IN DETERMINING THE DEGREE OF COASTAL FLOODING  
OBSERVED. FORTUNATELY, ASTRONOMICAL TIDES WILL BE LOW, SO DEPARTURES  
OF 2-3+ FEET ARE REQUIRED FOR MINOR FLOODING TO BE OBSERVED. GIVEN  
THE RAPIDLY STRENGTHENING SYSTEM OFF THE COAST ON TUESDAY, THESE  
DEPARTURES MAY BE REALIZED ON THE ATLANTIC COAST IF STRONG ONSHORE  
WINDS LAST FOR A CONSIDERABLE DURATION LEADING UP TO THE HIGH TIDE.  
A PERIOD OF NORTHWEST WINDS WILL OCCUR TUESDAY NIGHT INTO WEDNESDAY  
MORNING AS THE LOW STALLS, BUT IF THE LOW MOVES MORE SOUTHEASTWARD  
(AND MORE SLOWLY) BY WEDNESDAY AFTERNOON, WINDS MAY AGAIN BECOME  
MORE NORTHEASTERLY PRECEDING THE WEDNESDAY NIGHT HIGH TIDE. ANOTHER  
ROUND OF COASTAL FLOODING MAY OCCUR FOR THAT HIGH TIDE, AS A RESULT.  
 
AS THE ABOVE SUGGESTS, THE GREATEST THREAT FOR COASTAL FLOODING  
TUESDAY EVENING THROUGH WEDNESDAY NIGHT IS ON THE ATLANTIC COAST,  
WITH TIDAL FLOODING ALSO POSSIBLE ON DELAWARE BAY. FOR NOW, THINK  
THE THREAT IS RELATIVELY LOW ON THE TIDAL DELAWARE RIVER, WITH NO  
TIDAL FLOODING EXPECTED ON THE EASTERN SHORE OF CHESAPEAKE BAY  
THROUGH WEDNESDAY NIGHT.  
 

 
   
CLIMATE
 
 
OCTOBER 25TH MAXIMUM TEMPERATURE RECORDS AND FORECAST:  
 
.......................RECORD............FORECAST  
 
ALLENTOWN...................78/1963...........74  
ATLANTIC CITY...............79/2001...........78  
ATLANTIC CITY MARINA........79/2001...........74  
GEORGETOWN..................81/2001...........79  
MOUNT POCONO................75/1963...........65  
PHILADELPHIA................76/2010...........78  
READING.....................79/1963...........75  
TRENTON.....................77/1963...........76  
WILMINGTON..................79/2001...........78  
 

 
   
PHI WATCHES/WARNINGS/ADVISORIES
 
 
PA...NONE.  
NJ...FLASH FLOOD WATCH FROM THIS EVENING THROUGH TUESDAY AFTERNOON  
FOR NJZ001-007>010-012>015-019-020-022>027.  
DE...NONE.  
MD...NONE.  
MARINE...GALE WATCH FROM TUESDAY MORNING THROUGH WEDNESDAY AFTERNOON  
FOR ANZ450>455.  
GALE WATCH FROM TUESDAY AFTERNOON THROUGH LATE TUESDAY NIGHT  
FOR ANZ430-431.  
 

 
 

 
SYNOPSIS...CMS  
NEAR TERM...JOHNSON  
SHORT TERM...CMS  
LONG TERM...CMS  
AVIATION...CMS/JOHNSON  
MARINE...CMS/JOHNSON  
TIDES/COASTAL FLOODING...WFO PHI  
CLIMATE...WFO PHI  
 
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