Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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353 FXUS61 KPHI 252047 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 347 PM EST Sat Feb 25 2017 .SYNOPSIS... In the wake of the strong cold front moving through our region Saturday afternoon, cooler air will settle in over the region. High pressure then builds south of the region on Sunday before moving offshore on Monday. Low pressure approaching from the west will lift a warm front through the region on Tuesday. The aforementioned low will propagate from the Great Lakes region into eastern Canada. The associated cold front will move through Wednesday night or Thursday. A quick moving low pressure system will slide over the Mid Atlantic at the end of the week. && .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
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345 pm update: Severe thunderstorm watch 39 expanded to include Sussex County, DE, and the lower portion of Delaware Bay. Previous discussion below... Negatively-tilted trough is approaching the Northeast this afternoon with considerable large-scale ascent present from the Carolinas northward to New England, thanks in large part to strong differential cyclonic vorticity advection downstream of the trough axis and enhanced upper-level divergence via jet coupling. The warm sector has destabilized more than forecast this afternoon, with SBCAPE exceeding 1000 J/kg in much of the CWA south of I-76. With very strong shear profiles present (effective bulk shear 50+ knots; 0-1 km SRH ~100-150 J/kg), environment is quite favorable for severe storms generally along and west of the Delaware River this afternoon pushing toward the coast this evening. The orientation of the deep-layer shear vectors is generally parallel to the cold front approaching the East Coast this afternoon. However, the strength of the low- level shear and the increased CAPE from afternoon surface heating has enabled more of a mixture of line segments and discrete cells as the storms have progressed eastward. This has increased the hail and tornado threat with the storms somewhat, particularly south of the I-76 corridor. As storm interactions continue and large- scale ascent continues to increase, cold pool interactions should become more pronounced, and convection is expected to congeal into a larger, more contiguous line, which should make damaging winds the primary threat as the event wears on. This process already appears to be occurring with storms along the PA/MD border, for example. Severe threat is generally lower for the counties immediately adjacent to the Atlantic, as trajectories from the cooler ocean waters have limited surface heating considerably. High- resolution models continue to show a rapid diminishing trend in convection as storms move east of the Delaware River this evening. Cold front will move through the CWA this evening, with storms exiting the area by 03Z or shortly thereafter. Winds will quickly veer to westerly and become gusty after frontal passage. Temperatures will plummet behind the front, with temperatures expected to bottom out in the middle 20s in the southern Poconos to the middle 30s along the coast.
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&& .SHORT TERM /6 AM SUNDAY MORNING THROUGH 6 PM SUNDAY/... Surface high will move through the Southeast on Sunday, but the pressure gradient on the northeast side of the high will permit a rather breezy day across the area, especially during the morning hours. Cold air advection on the upstream side of the potent vort max moving through the area today will make for a sharply colder day. High temperatures are forecast to be near seasonal averages, or about 30 degrees colder than what we saw today. && .LONG TERM /SUNDAY NIGHT THROUGH SATURDAY/... Few changes made in the long term so as to focus on current hazards this evening. Unsettled weather pattern is on tap through the week. Monday and Tuesday are very low confidence especially with respect to precip chances. As long as the high isn`t more progressive in shifting off shore, during the day Monday should stay dry. Monday night into Tuesday there is low confidence as to the extent of rain ahead of the arrival of the warm front and with the timing of the warm front. Models have continued their run-to-run jumpiness with the timing of this front. Wednesday and Wednesday night looks to be the highest chance of precip with the region solidly in the warm sector. Thunderstorms may be possible as well if we can warm up enough to realize some modest surface based instability. Cold front still on track to move through late Wedensday night or Thursday. Once it moves through, we should see a lull in the precip, albeit a brief one. The next question will be the timing and track of a quick moving low which could move across the region Friday or Friday night. Moisture may be limited as there will be strong dry air advection behind the cold front, but there is some chance for precip around the area. && .AVIATION /21Z SATURDAY THROUGH THURSDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. 21Z amendments: Tweaked the timing of storm passage and increased wind gusts expected with the storms given recent obs. Isolated severe wind gusts (50+ kts) possible. For the 18Z TAFs...Mostly VFR before storms move in this afternoon. Timing looks to be 19-23Z for KRDG/KABE, 20-00Z for KPHL/KPNE/KTTN/KILG, and 22-02Z for KMIV and KACY. Sub-VFR VSBYs/CIGs expected as the precipitation moves through, with gusty/erratic winds 30-45 kts possible in proximity to the stronger storms. Winds will quickly veer to westerly or northwesterly after shower/storm passage late this afternoon/evening, with winds gusting 20-30 kts through tomorrow afternoon before diminishing. Outlook... Sunday night and Monday...VFR conditions expected. Monday night through Thursday...periods of rain and showers are possible. When/if this does occur, MVFR ceilings or lower are possible. Highest chance is Wednesday and Wednesday night. There is considerable forecast uncertainty in this period.
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&& .MARINE... Solid small craft advisory conditions expected through Sunday afternoon, with potential for gale force gusts both with the passing line of storms this evening and for about a six-hour period after the cold front moves through the area this evening. Threat continues to look too brief/marginal to upgrade to a gale warning, but will continue to monitor. Current obs continue to indicate patchy fog along the New Jersey coastal waters, but this should rapidly diminish as storms approach the area by 6 pm. Currently expect the dense fog advisory to be allowed to expire as scheduled. Outlook... Sunday night and Monday...wind gusts to 25 kt may linger for a few hours on Sunday evening, but will be below SCA criteria for the remainder of this period. Tuesday through Wednesday...winds and seas should generally stay below SCA criteria, though wind gusts above 20kt will be possible on Wednesday. Periods of rain/showers possible, especially on Wednesday. Wednesday night and Thursday...Northwesterly wind gusts around 25 kt are possible && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Dense Fog Advisory until 6 PM EST this evening for ANZ450>453. Small Craft Advisory until 6 PM EST Sunday for ANZ450>455. Small Craft Advisory until 6 PM EST Sunday for ANZ430-431. && $$ Synopsis...Johnson Near Term...CMS Short Term...CMS Long Term...Johnson Aviation...CMS/Johnson Marine...CMS/Johnson

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