Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 181855 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 255 PM EDT Wed Oct 18 2017 .SYNOPSIS...
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High pressure remains in control of the weather for much of the rest of the week into the weekend. A dry frontal boundary may move into the area Thursday into Friday. A front and low pressure moving along it will bring unsettled conditions for later Monday into Wednesday.
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&& .NEAR TERM /THROUGH TONIGHT/...
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High pressure remains over the mid-Atlantic region. However, this high will undergo gradual weakening tonight as an upstream shortwave trough approaches from the Ohio Valley. This upper shortwave trough will be associated with increasing high clouds responsible prior to its arrival. Do not anticipate these clouds to have a significant impact on radiational cooling given the high base of the cirrus. Accordingly, temperatures should drop rather quickly this evening once the boundary layer decouples and winds become calm. Compared to recent nights, higher dewpoints tonight will (1) raise the floor for how low temperatures can bottom out, (2) inhibit frost formation and (3) increase the risk for radiational fog. At this point, am skeptical to think fog will be much more than patchy in coverage and shallow in depth (mainly ground fog). Forecast lows range from the upper 30s/lower 40s in the sheltered valleys of NE PA/NW NJ and the NJ Pine Barrens to the lower 50s for the urban I-95 cities and along the coast.
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&& .SHORT TERM /THURSDAY/...
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High pressure sinks southward toward the Southeast U.S. on Thursday. Modest pressure (height) falls at the surface (aloft) occurs across the northern mid-Atlantic region as a weak upper shortwave trough and surface cold front progress eastward. The aforementioned cold front will trail the upper trough enough to remain upstream of the forecast area through the period. The setup will allow us to remain situated in the pre-frontal warm sector and for this warming trend to continue into tomorrow. Sided closer to the warmer MAV guidance for temperatures, yielding forecast highs in the lower to middle 70s (except cooler in the higher elevations of NE PA-NW NJ). Filtered sunshine with high clouds during the morning will give way to sunny skies by the afternoon. Meanwhile, SW winds 5-10 mph in the morning are forecast to increase to 10-15 mph by afternoon.
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&& .LONG TERM /THURSDAY NIGHT THROUGH WEDNESDAY/...
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Thursday night thru Monday...High pressure at the surface and aloft will be in control of the weather with fair weather expected. Temperatures generally 5 to 10 degrees above normal. Monday night thru Wednesday...A pattern change with a sharp h5 trough moving into the ern part of the country. Low pressure and a slow moving front associated with the upper trough will affect the area with occasional rains. Locally heavy rains are possible with an upper trough beginning to take a negative tilt as it crosses the area. The 12Z NA models are showing QPF totals generally in the 1 to 3 inch range across the area for the period Mon_Wed. The higher totals are across the wrn counties. Much colder temperatures will be ushered in behind the system with below normal temperatures for the middle part of next week.
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&& .AVIATION /19Z WEDNESDAY THROUGH MONDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Generally VFR Thursday. However, patchy ground fog may develop at the fog-prone, rural TAF sites between 05Z-11Z Thursday which could result in a brief period of MVFR visibility. Confidence in fog development was too low to mention in 18Z TAFs except at MIV. SW winds 5-8 kt this afternoon will diminish to 5 KT or less after sunset this evening. SW winds continue on Thursday, gradually increasing to from 5-8 kt in the morning to 8-15 kt in the afternoon. OUTLOOK... Thu night thru Monday...VFR expected thru the period.
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&& .MARINE...
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Winds this afternoon are generally from the W-SW between 5 and 10 kt. However, winds are backed locally out of the S in the nearshore NJ waters owing to the formation of a sea breeze front. Winds may briefly increase to 10-15 kt late this afternoon and early evening a few miles farther off the NJ coast in association with an Ambrose jet. SW winds 5-10 kt will continue into tonight and Thursday morning, then increase 10-15 kt across the coastal Atlantic waters during the afternoon. Wind field may become locally enhanced to 15-20 kt off the coast of Ocean and Monmouth Counties via Ambrose jet late in the day. Held off on a SCA at this juncture since poor mixing profiles should inhibit higher winds aloft to reach the surface. Accordingly, capped gusts to below 25 kt. Seas will generally be around 3 ft in the coastal waters and 2 ft or less in the Delaware Bay. OUTLOOK... Thu night...Near SCA winds across the nrn NJ coastal waters Thu evening, but not confid enough for a flag attm. Fair. Friday thru Sunday...Sub- SCA conditions thru the period with fair weather.
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&& .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...O`Hara Near Term...Klein Short Term...Klein Long Term...O`Hara Aviation...Klein/O`Hara Marine...Klein/O`Hara

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