Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 252022 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 422 PM EDT Thu May 25 2017 .SYNOPSIS... An area of low pressure will move across the area tonight, then move away to our northeast through Friday. A weak surface trough is expected to cross the area during the day Friday. Weak high pressure may briefly move across the area Friday night into early Saturday. A weak low is forecast to move along a developing stationary front to our south Saturday into Saturday night. Another area of low pressure is forecast to weaken as it moves into the eastern Great Lakes Sunday into Sunday night, then an occluded frontal system is expected to affect the area Sunday night into early Monday. A series of cold fronts are forecast to move across the area Monday through Wednesday. && .NEAR TERM /UNTIL 6 AM FRIDAY MORNING/... Warm front is now draped around the Mason-Dixon Line, with rapid scattering of the clouds and warming temperatures to the south (temps near 80 in southern Delmarva) and a low overcast and chilly conditions to the north. As a result, temperature forecast is a real pain today and will continue to be so through the evening hours, with low confidence on the northward progress the front will make. Main story, however, is the potential for convection this evening. Storms have already developed in the lee of the Appalachians in Maryland/Virginia just downstream of an upper low near the intersection of OH/PA/WV. A potent vort max on the equatorward side of the low will surge northeastward tonight, with attendant large-scale lift downstream in much of the Mid- Atlantic. Associated cool air aloft will enable sufficient (though marginal) instability to develop to allow convection to continue northeastward into the CWA this evening. The shear profiles, particularly in the low levels, are not particularly strong, and an unfavorable veer-back-veer directional profile will likely develop this evening. Nevertheless, the strong dynamics of the approaching system and the steep low-level lapse rates may allow for some "loose" organization, the latter of which via the development/merging of strong cold pools. Convection-allowing model guidance is fairly aggressive in developing a fairly broad region of strong convection to the southwest, lifting it through the area this evening (generally between 6 pm and midnight). The main threats with these storms should be small to marginally severe hail, especially if convection can remain somewhat discrete, gusty winds given the dry midlevels and steep low- level lapse rates, and heavy rainfall. The main limiting factors at this time for a more robust threat include the marginal nature of CAPE and the aforementioned limitations of the vertical wind profile. Fast storm motions and dry midlevel air will likely preclude a widespread flash flood threat, but there is some concern that convective cells may train from south to north should more widespread development occur. Though the recent HRRR output looks a bit on the extreme side, the suggestion of small swaths of heavy rains (1-2+ inches) demands increased attention to this threat this evening. Once the main convection exits northeast late this evening, the pivoting nature of the approaching upper low suggests wraparound showers remain a possibility for the overnight hours, especially north of the Mason-Dixon Line. Kept higher PoPs in this area through 6 am, though kept thunder chances to slight after 2 am, when the main show should be done. Low-confidence temp/wind forecast, with several updates likely overnight. Winds should transition to westerly by late tonight. Lows generally in the 50s, maybe near or slightly above 60 near the coast and in Delmarva. && .SHORT TERM /6 AM FRIDAY MORNING THROUGH 6 PM FRIDAY/... The main upper low shifts into New England on Friday, with westerly flow prevailing across the area during the day. Several perturbations in the flow are likely to move through the area during the day, especially in the morning. This should keep skies partially to mostly cloudy most of the day, and may allow for a sprinkle or light shower to occur, especially north of the Mason-Dixon Line. The 18Z NAM has a few showers farther to the south as a vort max rotates southeastward through the Mid- Atlantic around midday, but there is not a lot of support from higher-resolution guidance at the moment. As such, kept PoPs low or non-mentionable for the most part south of the I-78/I-195 corridor. Main sensible weather impact tomorrow may be the winds, with some stronger mixing likely to occur during the afternoon hours. Winds above the boundary layer are not particularly strong (20-30 kts), but gusts of 20-30 mph are likely by afternoon across most of the area. Forecast max temperatures range from the mid 60s in the Poconos to the mid 70s in the urban corridor to around 80 in southern Delmarva. && .LONG TERM /FRIDAY NIGHT THROUGH THURSDAY/... An unsettled weather pattern continues for much of the extended forecast with several periods of rain possible. Friday night through the early part of Saturday is expected to be mostly dry as weak high pressure quickly moves across the area. However, later in the day, a weak area of low pressure will be approaching from the west but passing to our south along a developing frontal boundary to our south. Also, a short wave/vorticity impulse is expected to move across the area during the day, so there will be a chance of an isolated shower during the day Saturday. On Saturday night, the frontal boundary is expected to remain to our south, but should begin lifting northward toward the area Sunday. It will likely not make its way into the area until Sunday night. There will be a chance of showers overnight Saturday into Sunday, but the best chance of showers is expected to be Sunday night as the occluded front moves into the area. A series of cold fronts are forecast to move across the area Monday through Wednesday of next week as low pressure remain nearly stationary across southern Canada just north of the Great Lakes region.As any short wave/vorticity impulses slide across the area, they could lead to additional showers or thunderstorms early next week. High pressure tries to build back into the area for Thursday, which would provide a dry forecast. && .AVIATION /20Z THURSDAY THROUGH TUESDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Clearing line/associated warm front now just south of KPHL, with CIGs potentially rising and scattering within the next hour or so. Expect IFR CIGs and transient sub-VFR VSBYs to continue through late this afternoon north of KPHL. Scattered storms likely in much of the area this evening, generally from 22Z-04Z, with lingering showers possible through 09Z, especially in NJ. Slow improvement in CIGs this evening/overnight, with generally VFR conditions by daybreak Friday. Winds 10 kts or less through tonight, but becoming west/gusty tomorrow. Confidence in TAFs very low overall, especially for the first 12 hours. OUTLOOK... Friday night...Generally VFR. Saturday...Generally VFR. Showers moving into the area later in the day, which could lead to lowering ceilings. Saturday night-Sunday...MVFR CIGS possible. Chance of scattered showers. Sunday night...MVFR to IFR conditions possible with periods of low clouds and rain. Monday...Generally improving to VFR during the morning. Scattered showers and thunderstorms possible during the day. Monday night-Tuesday...Generally VFR. Chance of showers or thunderstorms on Tuesday. && .MARINE... Small craft advisory continues, mainly for high seas, through tomorrow afternoon. Buoys are generally hovering around 4-5 feet this afternoon, which is below criteria, but with easterly flow continuing through this evening - felt it was wise to keep the advisory going for now, as seas will likely remain near the threshold of 5 feet for a while. Winds, however, have generally been below criteria today, and I do not expect persistent advisory-level gusts through Friday. Thus, if seas do not climb above 5 feet again later this evening, the advisory will likely be cancelled early. A good chance of storms exists tonight. Gusty winds, small hail, and frequent cloud-to-ground lightning are all threats from these storms. Locally strong winds/rough seas are likely in vicinity of these storms. Winds become westerly tomorrow, but should remain below 20 kts in general. A light shower or two may occur, especially off the New Jersey coast. RIP CURRENTS... A moderate risk of rip currents continues through this evening given the easterly-to-southerly flow in place. However, as winds become westerly tonight, conditions will become less favorable for the development of dangerous rip currents. For now, think chances remain in the low category tomorrow. OUTLOOK... Friday night-Tuesday...Conditions expected to remain below advisory levels, although winds may gust around 20 knots at times. && .TIDES/COASTAL FLOODING... A Coastal Flood Advisory remains for this evening for coastal counties of Delaware and New Jersey, as well as counties along the Chesapeake, Delaware, and Raritan Bays. The astronomical tides with the new moon are particularly high with the evening high tide cycle, and the persistent onshore flow has lead to a 1-2 foot tidal departure. It is possible that additional spotty flooding may occur with the evening high tides Friday and Saturday. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...Coastal Flood Advisory from 7 PM this evening to 1 AM EDT Friday for NJZ012>014-020>027. DE...Coastal Flood Advisory from 7 PM this evening to 1 AM EDT Friday for DEZ002>004. MD...Coastal Flood Advisory until 8 PM EDT this evening for MDZ015- 019-020. MARINE...Small Craft Advisory until 6 PM EDT Friday for ANZ450>455. && $$ Synopsis...Robertson Near Term...CMS Short Term...CMS Long Term...Robertson/Thaler Aviation...CMS/Robertson Marine...CMS/Robertson Tides/Coastal Flooding...Iovino/Robertson is the U.S. government's official web portal to all federal, state and local government web resources and services.