Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 200014 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 814 PM EDT Thu Oct 19 2017 .SYNOPSIS... High pressure located west of the region will gradually build eastward through the end of the week. A cold front with several waves of low pressure will approach the region on Tuesday and slowly move east through the middle of next week. && .NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
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A cold front is making progress into western NY/PA as of 23Z, while pressure falls in advance of this boundary are maintaining a pre-frontal trough to the lee of the Appalachians. This front will continue moving east and through our region overnight into early Friday morning. A dry frontal passage is expected with some high clouds associated with the 250 hPa jet lifting to our north. While some low-level mixing will occur in association with the frontal passage, it should we weak enough for some of the more sheltered locations to decouple. Therefore, we have maintained patchy fog in the normally more prone locals. In fact, the boundary layer has decoupled more quickly this evening, with temperatures falling faster than expected, especially in the Pine Barrens. We have adjusted hourly temperatures accordingly, but expect them to level out, so the low temperature forecast remains in tact.
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&& .SHORT TERM /6 AM FRIDAY MORNING THROUGH 6 PM FRIDAY/... The cold front will move off the mid-Atlantic coast during the morning. The center of the surface high will shift northward along the Appalachian spine during the day. Minimal cold-air advection behind the front will be offset by warming from downsloping winds off the mountains and strong solar heating. The net result will be high temperatures that are similar to today- upper 60s across the higher terrain and low to mid 70s elsewhere. The pressure gradient between the departing cold front and high pressure to our west will produce a temperate northwest breeze around 10 mph with gusts 15-20 mph. There are some fire weather concerns for tomorrow with drier air arriving in wake of fropa. See fire weather section below for details. && .LONG TERM /FRIDAY NIGHT THROUGH THURSDAY/... Friday night through Monday: High pressure will gradually build eastward into the region later this week then move offshore by the early part of next week. This will continue the already dry and warm pattern in place. The blend of MET/MAV along with ensemble mean guidance will likely be too low with afternoon high temperatures each of these days. Highs have a decent shot to reach well into the 70`s with plenty of sunshine each day. Overnight lows will generally bottom out in the 40`s and 50`s. Winds will also be light till the high pressure system shifts offshore on Monday, when winds become more southerly and increase a bit. Monday night through Thursday: A cold front with several waves of low pressure will slowly move eastward across the Ohio Valley toward and then through our region throughout most of next week. A fairly strong ridge in the mid- levels is expected to be in place across the North Atlantic which will have the potential to really slow the eastward progression of the front. Ensemble spread on timing is fairly high in this time period. However, the general idea is for a strong LLJ and southerly advection to usher in moisture and increase winds even further ahead of the front. Gusts have the potential to reach 30 mph as well. Much needed rain will then occur ahead of the frontal passage. If this front checks up or stalls, additional areas of low pressure could keep rain going through the end of the forecast period. Right now, the GFS is likely falling into a typical progressive bias while the ECMWF/CMC may still be slow with the frontal passage. Rain shower chances extend through Tuesday and Wednesday given the uncertainty. Temperatures are also uncertain as well. Current thinking is to be slightly slower than the ensemble mean with the frontal timing, we trended a little warmer through the middle of week. High temperatures in the 60`s and 70`s are still expected with warmer lows in the 50`s and 60`s. Winds will likely become westerly and decrease after the frontal passage. && .AVIATION /00Z FRIDAY THROUGH TUESDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. VFR through Friday. A wind shift from SW to NW is expected to occur late tonight as a cold front moves through, with speeds around 5 knots or less. Winds increase again Friday morning to around 10 kt with some gusts of 15-20 kt. OUTLOOK... Friday night through Monday night: VFR. Winds variable and under 10 knots till Monday then southerly with gusts reaching 15 knots on Monday. Tuesday and Tuesday night: Sub-VFR with shower chances increasing throughout the day. Southerly winds 10-20 knots with gusts up to 30 knots. A windy period is likely but uncertainty exists with the timing.
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&& .MARINE...
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A wind shift from SW to NW is expected late tonight/early Friday morning and will accompany a fropa. Wind speeds 10-15 kt with gusts to 20 kt are expected, gradually decreasing during Friday afternoon. Seas of 3-4 ft can be expected in the coastal waters and 2 ft or less in the Delaware Bay with a longer period (9- 10 sec) SEly swell. OUTLOOK... Friday night through Monday night: Seas and winds will stay well under SCA criteria with seas of only a few feet. Winds will generally be under 10 knots till Monday then increase from the south. Tuesday and Tuesday night: Southerly SCA gusts with seas building over five feet by Tuesday night. Current forecast may be a little aggressive in building seas given southerly flow, but higher than normal timing uncertainty.
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&& .FIRE WEATHER... Northwest flow will advect dry air into the region behind a cold front on Friday. These winds will be somewhat breezy (10-15 mph with peak gusts to 20 mph), especially across NE PA and N NJ. RH values may bottom out in the lower to middle 30s by the afternoon. Additionally, fine fuels are dry (below 10 percent) across the region. These conditions could potentially support an enhanced risk for the spread of wildfires by the late morning or afternoon. A Special Weather Statement may need to be issued on the overnight shift for locations north of the Mason-Dixon line. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Gaines Near Term...Franck/Klein Short Term...Klein Long Term...Gaines Aviation...Franck/Gaines/Klein Marine...Franck/Gaines/Klein Fire Weather...

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