Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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482 FXUS61 KPHI 180130 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 930 PM EDT Thu Aug 17 2017 .SYNOPSIS... A warm front will lift north of our region later tonight. As low pressure tracks well to our north Friday, a cold front moves through Friday night into early Saturday. High pressure then builds in later Saturday and remains in place into Monday before shifting offshore. The next cold front is scheduled to arrive during Wednesday. && .NEAR TERM /UNTIL 6 AM FRIDAY MORNING/... 930 PM ESTF: Based on downstream trends, PoPs were trimmed back until 03Z, then expanded from west east across the region. As a warm front lifts north across the area overnight, we have likely PoPs toward dawn in the northwest with chance PoPs elsewhere. Given the moist and unstable air mass in place, isolated thunder possible with localized heavy downpours. Previous Discussion... High pressure centered over New England continues to drift north and east tonight. Meanwhile, low pressure over the western Great Lakes will also continue to lift to the north and east tonight, and this will drag a warm front into the region tonight. This front should make it through the Delmarva and Delaware Valley late tonight, but then gets hung up. Warm fronts are notorious for having a hard time lifting too far north during the overnight hours, so do not think it will make it too much past a line from around KABE to KTTN to KMJX. There are several aspects to watch for this evening and tonight. For the Delmarva, low level moisture continues to pool around a weak stationary boundary that runs through northeast MD and DE. Some weak shortwave energy is sliding along that boundary, and touching off isolated to scattered showers and perhaps a few thunderstorms during the early evening hours. The second aspect to watch is the approaching warm front. Scattered showers and thunderstorms will develop out ahead of this front, and look to make it into far western zones this evening. However, looks as if more widespread showers and thunderstorms will develop in the pre-dawn hours on Friday as a strong H5 shortwave moves into the Lehigh Valley and northwest NJ at that time. Will carry the highest PoPs (likely) in association with that shortwave to the north, and not until well after midnight. A warm and muggy night on tap. Lows will be in the low to mid 70s, and surface dewpoints will creep up into the lower 70s. Given the widespread cloud cover that will spread into the region associated with that warm front, not expecting much fog to develop, but patchy fog is possible in some areas. && .SHORT TERM /6 AM FRIDAY MORNING THROUGH 6 PM FRIDAY/... Friday looks to be an active weather day. Warm front continue to lift to the northeast Friday morning, and showers and thunderstorms will continue to be associated with it. Most activity will taper off from south to north in the morning, and it looks as if there should be a period of dry weather from late morning through early afternoon. With the warm front north of the region, southerly flow users an unusually warm and humid airmass into the region. Surface dewpoints will soar into the mid and upper 70s, along with highs in the upper 80s to around 90 in the Delmarva, and in the mid to upper 80s for most of NJ and southeast PA, and in the low 80s along the coasts and in the mountains. Going through the afternoon and into the evening, surface low pressure over the western Great Lakes lifts north and east towards Ontario. This drags a surface cold front and H5 trough with several strong shortwaves east. 12Z NAM indicating low pressure forming over the Delmarva early in the evening. For the forecast area, 12Z NAM showing SB CAPE of over 3500 J/kg and MU CAPE almost 5000 J/kg. This is in addition to a Lifted Index as low as -8C, and 0-6 km Bulk Shear of 30-40 KT. PWATs also range from 2-2.5 inches. All of this lines up for a period of strong to severe thunderstorms. SPS has the area in a Marginal Risk for severe weather, and WPC has the area in a Slight Risk for excessive rain. There is a threat for hail and damaging winds, but with all of this moisture, and based on model guidance, looks as if heavy rain and flooding will be of primary concern. NAM showing a bullseye right over the Delmarva and southern NJ, while the GFS is a bit farther north with that bullseye. It all depends on how training of thunderstorms develops, but it is looking as if somebody will pick up a significant amount of rain Friday afternoon/evening. && .LONG TERM /FRIDAY NIGHT THROUGH THURSDAY/... Summary...Convection ends Friday night followed by some lowering of the dew points Saturday and Sunday. Some convection returns next week especially Wednesday with the next cold front, then more noticeable cooling should take place in the wake of that front. Synoptic Overview...An upper-level trough swings across the Great Lakes Friday into Saturday, then into the Northeast later Saturday. The flow aloft should then turn more zonal as we start next week, however another trough is forecast to amplify across eastern Canada and eventually into the Great Lakes to the Northeast. We used a blend of continuity/guidance for Friday night through Saturday night, then blended in the 12z WPC Guidance. For Friday night...A convectively active evening should be in store as a cold front arrives along with an attendant short wave. The guidance indicates either mixed modes or a line of convection moving offshore toward later in the evening, with perhaps some leftover convection lingering into the overnight as the cold front arrives. There is the potential for some severe thunderstorms in the evening prior to the boundary layer cooling and/or the airmass becoming worked over. Given the potential for at least some convective organization within a rather moist airmass, any robust convection could be capable of producing locally strong winds (wet microbursts). This is especially if cold pool development becomes efficient. Otherwise, an axis of precipitable water in the 2.0-2.5 inch range will lead to torrential downpours. While the convection should be moving, efficient rainfall rates could lead to a localized flash flood risk especially in urban areas. The convection should be weakening as it reaches the coast and beyond overnight. If some thinning occurs in the clouds with light winds, some fog may develop. For Saturday and Sunday...Low pressure is forecast to lift well north of our area with an associated cold front shifting off our coast early in the morning. The main upper-level trough however lags behind, with it arriving across the Northeast and Mid Atlantic late in the day Saturday. Given the passage of the cold front, the precipitable water decreases through the day. As a result the dew points should lower at least some during the course of Saturday into Sunday, although the dry air push looks to be most robust to our north through Sunday despite high pressure building in. As the main trough axis aloft arrives later Saturday, some guidance indicates enough lift and instability to develop some convection to our west. This may weaken by the time it reaches our area, plus the column should be drying some. As a result, we kept Saturday post-frontal dry for now. Sunday is looking warm with some humidity along with plenty of sunshine anticipated. For Monday and Tuesday...The flow looks to be more zonal aloft during this timeframe. This drives high pressure eventually offshore as it weakens some later Monday. The next upper-level trough amplifies eastward from the Midwest and Upper Great Lakes Tuesday. This will start to back the flow more southwesterly, and there may be a lee-side trough present on Tuesday. As the moisture begins to increase once again combined with the possibility for some convergence and enough instability, some convective development is possible Tuesday afternoon. We will continue with slight chance to low chance PoPs as a result for Tuesday. Any convection should tend to diminish during the night. For Wednesday and Thursday...The prospects for convection will depend on the timing of an incoming cold front. This will be due to an upper-level trough arriving from the west, however a pre-frontal trough may serve as the primary focus for showers and thunderstorms. We will continue with chance PoPs with the idea for at least scattered convection possible Wednesday. As of now, the cold front should be offshore to start Thursday and high pressure centered just north of the upper Great Lakes starts to build southeastward. This should result in a cooler and drier airmass overspreading our region for Thursday. && .AVIATION /01Z FRIDAY THROUGH TUESDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. As of 9:30 PM, some MVFR ceilings are developing at RDG. TAF sites with VFR now will gradually lower to MVFR late tonight. Showers and scattered thunderstorms will develop late tonight as a warm front slowly lifts north through the terminals. Best chances will be at KABE/KRDG, but then chances spread east early Friday morning. MVFR/IFR conditions look to persist after rain ends for a period late Friday morning, and then conditions lift to VFR by early afternoon. A cold front moves into the region from the west Friday afternoon. Showers and thunderstorms with heavy rain will impact the terminals with IFR conditions, as well as the potential for small hail and gusty winds. SSE winds 5-10 KT becoming light and variable at times overnight, then S 5-10 KT Friday afternoon. OUTLOOK... Friday night...Some showers and thunderstorms move through mostly in the evening with areas of MVFR/IFR conditions for a time, then if clouds thin enough some local fog may develop. Southwest winds 5 knots or less, becoming west-northwest late. Saturday...Some local fog possible very early, otherwise VFR. Northwest to west winds around 10 knots, diminishing at night. Sunday and Monday...VFR overall with light winds. Tuesday...Mainly VFR. Winds becoming south-southwest 5-10 knots. && .MARINE... Winds and seas below SCA criteria tonight through Friday morning. Friday afternoon, a southerly pressure gradient tightens, resulting in 15-20 KT winds. With an inversion over the waters, not expecting 25 KT wind gusts. However, it does appear as if seas could build close to 5 feet for a few hours in the afternoon. Will hold off on a SCA for Friday afternoon and evening given low confidence in widespread SCA conditions developing. Scattered showers and thunderstorms will impact the waters tonight, but then widespread showers and thunderstorms with heavy rain, small hail, and gusty winds will impact the waters Friday afternoon and evening. OUTLOOK... Friday night...A southwesterly flow increases some ahead of a cold front, however forecast soundings overall indicate gusts should be below 25 knots. If the increase in winds remain long enough, seas on the ocean zones could build to 5 feet through the evening. Since it looks marginal, held off on a Small Craft Advisory. Some thunderstorms in the evening could produce locally strong wind gusts. Saturday and Sunday...The conditions are anticipated to be below Small Craft Advisory criteria as high pressure gradually builds in. Monday and Tuesday...Light winds to start Monday should become southerly, then increase some on Tuesday. While some gusts to about 20 knots are possible Tuesday, the conditions during this timeframe are anticipated to be below Small Craft Advisory criteria. RIP CURRENTS... We are forecasting a solid risk for the development of dangerous rip currents on Friday for the DE and NJ shore, given increasing southerly winds and seas. In fact, NJ shore will be on the upper end of moderate. && .TIDES/COASTAL FLOODING... Based on current trends and all available guidance, we do not anticipate minor flooding with the high tide tonight along the eastern shore of MD. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Gorse Near Term...Franck/MPS Short Term...Franck/MPS Long Term...Gorse Aviation...Franck/Gorse/MPS Marine...Franck/Gorse/MPS Tides/Coastal Flooding... is the U.S. government's official web portal to all federal, state and local government web resources and services.