Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 242239 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 639 PM EDT Mon Apr 24 2017 .SYNOPSIS...
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Low pressure over the Southeast States will slowly lift north along the Atlantic Seaboard tonight through Tuesday night, then track east on Wednesday. High pressure builds along the coast on Thursday. A cold front approaches on Friday, but may become stationary north of our area into the weekend as high pressure becomes entrenched over the western Atlantic Ocean. A warm front lifts north through the area for early next week.
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&& .NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...
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For the 630 PM update, made some adjustments to PoPs to reflect some lower values initially this evening as showers are in waves but are mainly light. Dry low-level air hanging on across the far northern areas with dew points much lower compared to southern Delaware and far southeastern NJ. This may result in some sprinkles for awhile before actual showers settle in. It looks more showery tonight overall. The hourly temperature and dew points were adjusted based on the latest observations, which included showing initially warmer values up north and cooler across the south and coast. Low temperatures were lowered some mainly across the northern areas where dew points are currently near freezing. Otherwise, surface ridge axis will be situated across the area overnight, with a cold front to the north and low pressure to the south. This will maintain an overunning regime with rain showers across much of the area. Precip will have a tougher time making it north of I-78, with drier air in place. Cloud cover and an easterly flow will not allow temperatures to drop off much. Generally upper 40s to mid-50s.
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&& .SHORT TERM /6 AM TUESDAY MORNING THROUGH 6 PM TUESDAY/... Closed mid-level low lifting northward along the eastern seaboard will entrain some tropical moisture as it does so. There is model disagreement between the NAM/GFS, with most of the international models supporting the wetter NAM solution. This brings a strong easterly 850 hPa jet of 50 to 60 knots across the region, which will lead to heavy downpours. Stability indices also decrease, as a plume of steep mid-level lapse rates promotes elevated instability. As a result, we have inserted thunder into the forecast across much of the area. Precip amounts of 1-2 inches are expected, especially southeast of the NJ Turnpike/I-95, with resulting urban and poor drainage flooding the biggest concern. High temperatures will be in the 50s over PA and NJ, with low to mid 60s in Delmarva. && .LONG TERM /TUESDAY NIGHT THROUGH MONDAY/... By Tuesday evening, low pressure should be centered just over the Delmarva Peninsula. Swaths of moderate to locally heavy rain will continue to track northeast along the coast through Tuesday evening, gradually tapering off from southwest to northeast as the low slowly tracks along the NJ coast. Wednesday afternoon, the surface low will be just east of the region, moving into NYC and western Long Island. Although the bulk of the rainfall will be tapering off, the upper level low will be just to the west of the surface low. This will keep at least some showers in the region through Wednesday, but for now, will keep PoPs capped at chance through the morning, diminishing to slight chance as the afternoon progresses. Upper level ridging will build into western NY/PA Wednesday night as weak high pressure builds towards the East Coast. With an abundance of moisture over the area due to the rainfall, patchy fog is possible Wednesday night as skies clear out and winds become nearly calm. That ridge builds east on Thursday, and although there will be an onshore component to the winds, temperatures will rise into the 70s for much of the area, and possibly into the lower 80s over the Delmarva. Right along the coasts, however, highs should stay in the 60s. For the end of this week and into the start of the new week, Bermuda high pressure looks to become entrenched over the western Atlantic waters. There is a cold front that will approach on Friday, but the strength of the offshore high should be enough to keep the front from making too much eastern headway. Southwest flow develops ahead of this front, and this will allow a very warm and humid airmass to spread into the Mid- Atlantic and Northeast. With H8 temps between 14C-16C, highs on Friday will climb into the low to mid 80s for most of the area, although the Poconos will top off in the mid 70s, and coastal NJ/DE will remain in the 70s due to the sea breeze. That frontal boundary will make a slow eastward progression over the weekend as the high remains over the western Atlantic waters. Conditions remain warm and humid into the weekend with highs generally in the 70s and 80s. Although the main system will remain to the west, several weak upper level shortwaves will spin off ahead of the front, and with a warm and humid airmass in place, there should be enough instability to touch off some showers and thunderstorms over the weekend and into the new week. For now, will keep PoPs capped at slight chance/chance. && .AVIATION /00Z TUESDAY THROUGH SATURDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Tonight...VFR conditions lower to MVFR/IFR this evening and especially overnight. Rounds of showers will move through, light initially then some heavier downpours are possible overnight. Northeasterly winds in the 5-15 knot range overall, with local gusts at times to around 20 knots especially closer to the coast. Tuesday...IFR conditions, with occasional LIFR, due to low clouds and periods of rain. Some of the rain will be heavy at times, allowing for times of even lower visibility. Northeast to east winds around 15 knots with gusts to around 25 knots. The highest winds are expected to from about PHL south and east. OUTLOOK... Tuesday night...IFR and lower conditions in rain. E-NE winds 10-15 KT with gusts up to 25 KT, diminishing towards daybreak. Wednesday...Lingering showers through Wednesday afternoon with MVFR/IFR conditions. Improvement expected late. NE winds become N at 5-10 KT. Wednesday night through Thursday morning...IFR and lower conditions possible in fog/stratus. Thursday afternoon...VFR. SE winds 5-10 KT. Friday through Saturday...VFR. SW winds 5-10 KT.
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&& .MARINE...
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We have posted a Gale Warning for all the coastal waters expect upper Delaware Bay where a SCA remains in effect. Gales run from 10Z to 22Z Tuesday, and the SCA through 10Z Wednesday. The strongest east winds are expected from late morning into the afternoon on Tuesday. Seas will build and range from 2 to 4 feet on Delaware Bay and 7 to 10 feet on the ocean. OUTLOOK... Tuesday night through Wednesday...Gale force winds will diminish to 25-30 KT Tuesday evening, and then winds diminish to less than 20 KT by daybreak Wednesday. However, seas on the ocean will remain elevated at 7-10 KT, gradually diminishing to 4-6 FT by late Wednesday. Wednesday night through Friday...Lingering 3-5 ft seas on the ocean. Saturday...Sub-SCA conditions.
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&& .TIDES/COASTAL FLOODING... The cumulative effects of the onshore flow into Tuesday night and the approaching new moon will likely result in minor coastal. The minor flooding is anticipated along the New Jersey shore, at the Delaware beaches and along Delaware bay and on the far lower part of the Delaware River around Tuesday evening` high tide. The minor flooding should affect the back bays and the estuaries, as well. We will take a look at the trends around this evening`s high tide. We will likely issue a Coastal Flood Advisory for Tuesday evening with our forecast update late this evening. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Gale Warning from 6 AM to 6 PM EDT Tuesday for ANZ431-450>455. Small Craft Advisory until 6 AM EDT Wednesday for ANZ430. && $$ Synopsis...MPS Near Term...Franck/Gorse Short Term...Franck Long Term...MPS Aviation...Franck/Gorse/MPS Marine...Franck/MPS Tides/Coastal Flooding...Iovino

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