Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 241320 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 920 AM EDT Sat Aug 24 2019 .SYNOPSIS... High pressure across the upper Great Lakes region this morning will continue to move east across southern Canada today and tonight. The high will try to spread down the east coast, but not fully make it into the area. The high will retreat north and east next Tuesday and Wednesday. This will allow an area of low pressure to lift northward off the eastern seaboard during the mid week period and a cold front to track through the area later next week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 930 AM update: An area of clouds with a ceiling deck of 5000 feet or more is moving through southeastern PA and central NJ at this time. Satellite pictures indicate a back edge to this area with mainly clear skies in the Lehigh Valley and northern NJ. Skies will become mainly clear everywhere as this area continues to move southwest as the morning progresses. Otherwise the early morning forecast remains on track. North to northeast winds gusting to around 10 MPH on the land and around 20 KT over the coastal waters will continue through the day, becoming more northeast by afternoon. With surface high pressure building in from the north, a dry day is expected. Previous discussion... A trough of low pressure will remain aloft while a surface high pressure area moves across southern Canada today. This will bring more cooler and drier air from the north into our region. We`ll see more sunshine than Friday, so highs will be 4 to 6 degrees warmer that yesterday, but highs will be below normal for mid-late August. Readings will top out in the low/mid 70s N/W and close to 80 across SE PA, south NJ and Delmarva. Winds will be mostly N to NE at 5 to 10 mph this morning and the NE to E at 10 to 15 mph this afternoon. Some mid/high level clouds will move across the srn areas this morning and into the afternoon, but overall a nice day expected. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SUNDAY/... High pressure will continue to strengthen while it moves to ME/srn Quebec. This will bring an increasing NE to E flow across our area and could result in some increasing low clouds and perhaps some light rains near the coast towards daybreak Sunday. We have kept the slight chc pops and increased the clouds for these areas. Low will be cool with readings dropping into the mid/upper 50s N/W and over the Pine Barrens of NJ. Elsewhere, readings will be in the 60s with mid/upper 60s near the shore with the nearby warmer waters. Winds will be NE to E at 5 to 10 mph. The winds will be higher near the shore with 10 to 15 mph more common there. && .LONG TERM /SUNDAY THROUGH FRIDAY/... Main concerns for the medium-range forecast include the effects of prolonged onshore flow Sunday through Tuesday and a cold- frontal passage during the middle of next week. A fairly quiet period is in store for the area, but as usual, some subtle details are uncertain and may end up making a big difference in sensible weather "quality" the next few days. Perhaps the biggest question is the ultimate result of a lengthy period of east to northeast winds across the area late this weekend into early next week. Operational models have been hinting for a while at the development and inland penetration of lower clouds and possibly some light precipitation as the surface high to our north/northeast lingers the next few days. It appears the primary window for this potential is Sunday and Monday, as an upper low lingers off the coast on Sunday before drifting off to the northeast Monday. This will promote some ageostrophic acceleration southward on the east side of the Appalachians, allowing the surface high to build southward a bit through this period. Eventually, the flow should become somewhat more northeasterly or even north-northeasterly Sunday night as the acceleration peaks, and this will aid in shunting the higher low-level moisture content southward. By Monday, the effects may primarily be in lower Delmarva and perhaps the far southeastern portions of New Jersey with improvement in conditions to the north. Far western portions of the area may be much sunnier during this entire period. Confidence is low on just how far inland the thicker clouds and even light precipitation make it, but decided to go a little bit more pessimistic than consensus (which is close to the previous forecast). Kept temperatures close to continuity and a statistical blend during this period as well, but there may be a more pronounced gradient between the coast and the western CWA depending on how far inland the clouds make it (and how long they last). By Tuesday, the effects of a tropically-induced low off the coast will generally become most pronounced. The surface high to the north is expected to retreat northeastward, permitting a return to more easterly surface flow. Anticipating an increase in clouds and low-grade chances of showers. Think most of the precipitation will be to the south of the area, but given uncertainty with the track of the low, felt some low PoPs remain warranted. Additionally, should see some warming temperatures as low-level flow begins to tap into warmer/moister air in association with the low. The good news is a well-timed trough will deepen and progress eastward into the Great Lakes on Wednesday, which will force the low off the coast well away from the area. There will be an attendant cold front associated with the trough, with timing Wednesday night (plus or minus) for our area. The ECMWF is more aggressive with generating some storms along the front. The GFS is drier, which appears to be tied to how close the low off the coast tracks toward the area and the residual influence of the retreating surface high. The GFS is closer with the low track and stronger with the residual effects of the surface high, which precludes convection from maintaining itself as it reaches the area. The ECMWF is farther offshore with both, allowing more substantial large-scale lift to reach the area. Suspect the ECMWF is a little too aggressive, particularly with the retreat of the high to the north. For now, maintained chance PoPs Wednesday and Wednesday night given the uncertainty (only somewhat higher than climatology). Should see improving conditions late next week as high pressure builds in from the west. && .AVIATION /13Z SATURDAY THROUGH WEDNESDAY/... Today...VFR expected. N winds 5 to 10 knots early, then NE 8 to 12 knots late morning and into the afternoon. Some gusts close to 20 knots near KACY late morning and afternoon. Sct-Bkn 050-060 KMIV/KACY this afternoon. Tonight..VFR continues. Low clouds possible late near the shore towards dawn Sunday. Confid is not high enough to include in TAFS attm. E winds 5 to 10 knots. OUTLOOK... Sunday...Mainly VFR, though sub-VFR conditions may occur near the coast (including MIV/ACY) as low ceilings move in with the persistence of onshore flow. Cannot rule out some drizzle or light rain/showers near the coast as well. East to northeast winds 7 to 15 kts with stronger gusts possible near the coast. Low confidence. Sunday night through Tuesday night...Conditions possibly varying from VFR to MVFR as lower ceilings move farther inland and retreat at times during the period. May see some drizzle/fog as well near the coast. Winds mainly east or northeast 5 to 15 kts, with higher gusts possible near the coast. Directions may become more southeasterly by Tuesday afternoon. Very low confidence. Wednesday...Mainly VFR as winds become more southerly/southwesterly (up to around 10 kts). A chance of storms, especially north/west of PHL. Low confidence. && .MARINE... High pressure moving north of the waters today and tonight will cause winds to be mostly N or NE today and then NE to E tonight. Winds and seas will slowly increase to near SCA conditions for awhile both this afternoon and late tonight. Not really expecting the SCA conditions to persist too long with only perhaps a gust or two and a few seas nearing 5 ft. We`ll hold on the SCA flag for now, although a short term SCA is possible later. Fair weather today and a slight chc for a shower late tonight. OUTLOOK... Sunday through Wednesday...Advisory conditions possible at times through the period as steady east to northeast winds persist, with seas building to around or slightly above 5 feet. Some drizzle or light precipitation is possible at times as well. RIP CURRENTS... Increasing northeasterly winds and seas building to around 4 ft by the afternoon will result in a moderate risk of rip currents today. This elevated rip current risk (either moderate or at times high) is expected to persist until at least Tuesday as a prolonged period of onshore flow and elevated seas looks likely. && .TIDES/COASTAL FLOODING... A prolonged northeast to east flow is expected today through at least Tuesday. This persistent onshore fetch, combined with increasing astronomical tides due to an approaching new moon on Thursday, will likely lead to increasing coastal water levels. Guidance is already indicating that minor coastal flooding is becoming more likely Monday and Tuesday over Delaware Bay and portions of the NJ shore. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Kruzdlo/O`Hara Near Term...Miketta Short Term...O`Hara Long Term...CMS Aviation...CMS/O`Hara Marine...CMS/Miketta/O`Hara Tides/Coastal Flooding... is the U.S. government's official web portal to all federal, state and local government web resources and services.