Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 132334 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 734 PM EDT Sun Aug 13 2017 .SYNOPSIS...
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High pressure will influence our weather into Monday. A weak disturbance in the upper atmosphere, approaching from the southwest, is forecast to pass over our region on Monday evening. Tropical Storm Gert is expected to pass well to our southeast and east on Tuesday and Tuesday night. A frontal boundary is anticipated to arrive from the north during the mid week period, then it should meander near our region into the weekend.
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&& .NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
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High pressure over the Great Lakes region will into the area through tonight. Meanwhile, the cold front that moved through the forecast area last night will stall to our south across the Carolinas and southeastern VA. Overnight low temperatures in the 50s are attainable in the typical sheltered valleys of northeastern PA and northwestern NJ where the boundary layer is fully expected to decouple. Farther south and east across the area, the floor for temperatures will be in the low to mid 60s based on current and projected dewpoints. However, a light northerly breeze may persist tonight with the center of the high passing to our north while some mid and high level clouds spread northward especially over the southern half of the region. This would limit radiational cooling a bit and potentially keep some spots in the urban I-95 corridor and coastal plain slightly warmer.
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&& .SHORT TERM /6 AM MONDAY MORNING THROUGH 6 PM MONDAY/... High pressure will move off the New England coast and weaken on Monday. Southerly return flow develops over the region around the backside of the surface high and ahead of an upstream shortwave trough over the Mid South/Ohio Valley. This will cause the stalled frontal boundary to our south to return back northward. However, with the steering flow rather weak, the northward movement of the frontal boundary should be gradual. Recent guidance has trended weaker/slower with the development of a wave of low pressure along the front. Models consensus has been to keep the warm front and more organized band of convection generally south and west of the area tomorrow. However, the 12Z NAM, 12Z ECMWF and about a quarter of both the 09Z SREF and 00Z NCAR CAM ensemble members still bring showers into our far southern (mid Delmarva) and western (southeast PA) late in the afternoon. The 12Z 3 km NAM Nest is even more aggressive with the development of afternoon showers across much of the CWA. This latter solution may be a bit overdone with the bulk of the synoptic lift expected to remain upstream of the region. Following model trends and consensus, PoPs were lowered near the coast but raised for western (especially southwestern) zones during the mid to late afternoon hours. PoPs were capped in the 20-30 percent range with the expectation that coverage of showers should be rather sparse. Forecast soundings from all available guidance reveal a relatively stable environment that is not supportive of thunderstorms (which makes sense with the region located in the cool sector north of the front and with increasing clouds helping to limit the degree of boundary layer destabilization). Trended cloudier than previous forecasts for Monday as model x- sections show a deepening saturated layer (wrt/ ice) in the 700-400 mb layer. This pattern would indicate a rapid increase in mid and hi clouds from SW to NE during the late morning and afternoon. Owing to the cloud cover, weighted temperatures more toward the cooler MET. Forecast highs are generally in the low to mid 80s (slightly cooler in the Poconos and along the coast). && .LONG TERM /MONDAY NIGHT THROUGH SUNDAY/...
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A mid level disturbance is forecast to be overhead on Monday evening before moving off the coast on Monday night. The models are trending wetter for Monday night, so we will include at least a slight chance for showers and thunderstorms at that time. However, the NAM continues to appear quite overdone with the rainfall amounts. Of course, you cannot go wrong forecasting rain for central and southern Delaware and the adjacent counties of eastern Maryland. It appears as though Tropical Storm Gert over the western North Atlantic will stay well enough to our southeast and east on Tuesday and Tuesday night as not to impact our weather. However, a mid level short wave trough is forecast to approach from the Great Lakes on Tuesday before passing overhead on Tuesday night. We will include a chance of showers and thunderstorms for Tuesday afternoon and evening, mainly in eastern Pennsylvania and northern New Jersey. The mid level short wave is anticipated to pull a frontal boundary into our region from the north on Tuesday night. The boundary is expected to remain over or near eastern Pennsylvania, northeastern Maryland, Delaware and New Jersey for the balance of the week into the weekend. The model guidance differs in handling weak waves of low pressure travailing along the front, so the timing of any precipitation in the forecast remains difficult. Regardless, it seems as though the period from Thursday into Saturday will be rather unsettled with a chance of showers and thunderstorms each day. High temperatures are forecast to favor the 80s during the period from Tuesday through Sunday with lows generally in the 60s and lower 70s.
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&& .AVIATION /00Z MONDAY THROUGH FRIDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Tonight...VFR overall. There is the potential for MVFR visibilities in the 07-12z timeframe at the more fog-prone terminals such as KMIV, KACY, KABE and KRDG. For now, included a TEMPO for light fog at KRDG and KMIV only. Northwesterly winds near 5 knots, becoming light and variable at most terminals. Monday...Other than some local MVFR visibilities (fog) possible early, a developing VFR ceiling mostly in the 8000-15000 feet range is expected. A few showers are possible mainly late in the afternoon south and west of KPHL, however the chance is rather low. Northeast to east winds less than 10 knots, becoming southeast in the afternoon. OUTLOOK... Monday night and Tuesday...Mainly VFR. Late night and early morning low clouds and visibility restrictions are possible. A chance or slight chance of showers and thunderstorms. Tuesday night through Wednesday night...Mainly VFR. Late night and early morning low clouds and visibility restrictions are possible. Thursday through Friday...Mainly VFR with a chance of showers and thunderstorms. Late night and early morning low clouds and visibility restrictions are possible.
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&& .MARINE...
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No marine hazards expected tonight and Monday. Winds off the NJ coast become southerly this evening, then light and variable winds overnight. A wind shift to E 5-10 kt during Monday morning and eventually SE during the afternoon. Seas in our coastal waters are generally around 3 ft and less than 2 ft in the Delaware Bay. Wave heights are not forecast to change much through Monday. RIP CURRENTS... A medium-period (6-8 seconds) south-southeast swell and 3-4 feet surf looks to continue into Monday. Wind speeds will be similar Monday compared to today. However, the wind direction changes to easterly during the morning and southeasterly by afternoon. Despite the onshore flow, it is forecast to be light enough and therefore we will go with a low risk for the development of dangerous rip currents for Monday. This will be re-evaluated early Monday morning. Tropical Storm Gert is forecast to pass well to our southeast and east on Tuesday and Tuesday night. Swells from the system will likely bring at least a moderate risk for the development of dangerous rip currents for Monday night and especially Tuesday and Tuesday night.
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&& .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Iovino Near Term...Klein Short Term...Klein Long Term...Iovino Aviation...Gorse/Iovino/Klein Marine...Gorse/Iovino/Klein

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