Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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743 FXUS61 KPHI 221342 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 942 AM EDT Sun Apr 22 2018 .SYNOPSIS... High pressure over eastern Canada and the Northeast United States will slowly build east and move off the New England coast on Monday. Meanwhile, low pressure over the western Gulf Coast states will track to the north and east and impact the Mid-Atlantic and Northeast for the middle of the week. Another low may affect the area at the end of the week, and then a cold front may pass through the region next weekend. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... For the 930 AM update, made some adjustments to the temperatures as they are quickly rising so far with ample boundary layer heating within a rather dry airmass. The surface dew points vary quite a bit this morning due to not much mixing so far and earlier radiational cooling. Rather dry low-level air looks to continue and therefore the dew points were lowered especially near and north/west of the fall line through this afternoon. To the south and east of there, dew points should increase some by late afternoon as a sea breeze makes some inland progress. This of course will have an impact on temperatures as well, and the going forecast looks to have a good depiction of these anticipated trends. The high level clouds have thinned quite a bit across the southern areas this morning, however some additional cloudiness is to the west and heading east-southeastward. As a result, kept the most clouds across the far southern areas although may need to expand this farther north some if the incoming thicker cirrus holds together. There is a stronger shortwave within a trough axis aloft back to our west that swings east through the afternoon, therefore this could provide for some more high cloudiness farther north. Otherwise, the development and progress inland of a sea breeze will be watched. According to high-resolution simulations, sea/bay breezes will likely progress well into New Jersey by late afternoon, and may reach Philadelphia by late in the day. With the sea/bay breeze passage, may see a burst of wind and a slight drop in temperatures. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM MONDAY/... Main forecast concern tonight is the development of fog near the coast as remnant (light) onshore flow from the sea breeze combines with dropping temperatures to allow for low-level saturation. Spread the mention of fog a little bit further inland in southern/eastern NJ and coastal Delaware, though not expecting widespread/dense fog at this point. With the surface ridge in place, winds will likely decouple fast, and with clear skies thanks to being upstream of passing northwest- flow midlevel perturbations, temperatures will fall readily. I went a little below guidance for lows tonight, and would not be surprised to see the low-lying spots with a little frost late. However, suspect this will be quite patchy at best, so made no mention of this in the grids at this point. Forecast lows are similar to tonight`s values. && .LONG TERM /MONDAY THROUGH SATURDAY/... Unsettled weather on tap for much of the new week as low pressure lifts along the coast. High pressure over the Northeast U.S. moves off the New England coast on Monday, and then slowly tracks out to sea Monday night and Tuesday. Onshore flow develops, and although temperatures climb into the 60s for most inland areas, temperatures will be colder along the coast with highs staying in the 50s. Low pressure tracking along the Gulf Coast states will be over the Southeast U.S. Monday evening, and then the low lifts along the coast Monday night and Tuesday. Onshore flow will usher in an increasingly moist airmass into the region, and PWATs will increase to around 1.25" Tuesday night and Wednesday morning. This may allow for a period of moderate to locally heavy rain during this time. The low will be off the Mid-Atlantic coast Wednesday morning, and then the low tracks north during the day Wednesday and will be north of the region by Wednesday night. From there, conditions generally dry out Wednesday night and Thursday, but there is the potential for unsettled weather to continue into Friday as several weak upper level disturbances pass through the region. There is the chance for another coastal low on Friday, as the CMC-GDPS indicates this low, but the GFS and the ECMWF do not. The GFS has a quick moving cold front that the ECMWF holds off until Saturday. For now, will carry chance PoPs Friday through Saturday due to low confidence. Temperatures during this time will generally be near or below normal levels for most of the week. && .AVIATION /14Z SUNDAY THROUGH THURSDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Today...VFR with some high clouds. Light and variable winds, locally becoming west or northwest mainly below 10 knots. A sea/bay breeze though should result in a quick wind shift to south or southeast around 10 knots this afternoon at ACY, MIV, and ILG (maybe even PHL by late afternoon). Tonight...VFR. Winds quickly becoming light and variable to locally calm. Monday...VFR. There is a small chance of MVFR visibility due to light fog at MIV and ACY early in the morning. Light and variable to calms winds, becoming south-southeast 5-10 knots in the afternoon. Outlook... Monday night...VFR. Winds mainly light and variable. Tuesday through Thursday...MVFR/IFR conditions expected in rain and fog, especially Tuesday night through Wednesday afternoon. Easterly wind gusts up to 20 kt are possible Tuesday night into Wednesday. Otherwise, generally light winds (less than 10 kt) are expected. Medium confidence on flight categories, but low confidence on the timing of any flight category changes. && .MARINE... Winds/seas will remain below advisory levels through tonight. Winds will become southerly during the afternoon, with speeds 10 to 15 kts, before becoming light and variable overnight. Seas will likely remain below 3 feet through the period. There is potential for at least patchy fog overnight, with visibility restrictions possible. Chances are particularly high in Delaware Bay and adjacent portions of the southern NJ/DE Atlantic coastal waters, where light easterly/southeasterly surface flow may remain for most of the night. Outlook... Monday night...Winds and seas are expected to remain below SCA criteria. Tuesday and Wednesday...East winds increase to 15-20 kt with gusts up to 25 kt, and seas build to 8-10 feet on the ocean. Wind gusts to 25 kt also expected on DE Bay. Wednesday night and Thursday...Winds shift to the northwest and subsequently decrease. The main question will be how long seas remain above 5 feet on the coastal waters. Current forecast is for elevated seas into late Thursday. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...MPS Near Term...CMS/Gorse Short Term...CMS Long Term...MPS Aviation...CMS/Gorse/MPS Marine...CMS/MPS is the U.S. government's official web portal to all federal, state and local government web resources and services.