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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