Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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186 FXUS61 KPHI 261937 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 337 PM EDT Mon Jun 26 2017 .SYNOPSIS... High pressure will build eastward into the region tonight and throughout the day Tuesday. This high pressure system will become anchored offshore through the middle and later parts of this week. A cold frontal boundary is expected to approach from the northwest this weekend into early next week. && .NEAR TERM /UNTIL 6 AM TUESDAY MORNING/... So far, precipitation has failed to develop in the area, and with stronger large-scale ascent to the west in closer proximity to an approaching shortwave trough currently in the Great Lakes region, decided to pull PoPs for the rest of the afternoon and evening. I`m not sure a spotty shower can be completely ruled out, but satellite trends indicate a rather benign-looking cumulus field at the moment, with more pronounced vertical ascent well to the northwest of the area. A weak perturbation moves through the region around sunset, but lift in association with it seems rather muted at the moment. Tonight, the aforementioned shortwave trough marches east, reaching western New York by 12Z Tuesday. The 12Z GFS/RGEM and (to varying degrees) some of the higher-resolution guidance indicate a weak predecessor vort max moving southwest to northeast through the area during the overnight hours. Spotty showers may develop in proximity to this perturbation, moving rapidly northeastward just downstream of it. The 12Z NAM, however, looks quite dry, and the very weak lift along with some run-to-run continuity issues with the HRRR precluded me from including PoPs at this point. However, this will be something to watch during the overnight hours. I did increase cloud cover in close proximity to the track of this perturbation as a start, though this may be somewhat overdone. Overnight temperatures look to be rather cool as the shortwave trough lowers heights/thicknesses across the region. Forecast lows are around 50 in the southern Poconos, in the middle to upper 50s in more rural locations elsewhere, and in the lower 60s on the coast and in the urban corridor. && .SHORT TERM /6 AM TUESDAY MORNING THROUGH 6 PM TUESDAY/... The vort max in western New York at 12Z Tuesday pivots northeastward during the day, with a northeast-to-southwest oriented lobe of higher vorticity approaching the Mid-Atlantic region during the afternoon. There will be an attendant surface trough moving through the area during the day. Weak convergence along the associated boundary during peak heating may initiate a spotty shower or storm during the day, but guidance looks fairly tame in producing much precipitation across the area. Nevertheless, the addition of large- scale ascent in advance of the trough and the rather cold midlevels suggest that instability may be sufficient to initiate a few storms. As such, I felt that guidance was a little too tame with precipitation prospects tomorrow, especially north of the Mason- Dixon Line (in closer proximity to stronger large-scale ascent). Kept PoPs on the higher side of consensus (which still keeps it at slight chance or chance). The 12Z NAM looks too dry, whereas the 12Z RGEM seems a more reasonable outcome with the stronger large-scale lift in play. Used a blend of 12Z GFS/RGEM/WRF guidance as a starting point tomorrow for Wx/PoPs, but confidence is pretty low given that the strongest dynamics are well to the north of the area. One limiting factor appears to be the somewhat limited vertical extent of conditional instability/parcel buoyancy (upper levels cool rather slowly, and the boundary layer is relatively dry), which will prevent CAPE from becoming too substantial across the region (generally well below 500 J/kg across the region). Forecast temps are a blend of MAV/MET and continuity, with highs expected to be a little cooler than today owing to increased cloud cover and the closer proximity of the passing vort max. && .LONG TERM /TUESDAY NIGHT THROUGH MONDAY/... Highlights: 1. Heat index values near 100 possible for the urban corridor Friday and Saturday. 2. Highest chance of thunderstorms right now centered on later Saturday into Sunday with more uncertainty regarding any strong to severe storms. Tuesday night through Thursday night: High pressure will be moving across eastward across our region in this timeframe. This will begin a warming trend across the region as winds become southwesterly on the backside of the high. Humidity will slowly start to increase as well after one last refreshing day Wednesday. Wednesday looking at Bufkit soundings look to be fairly favorable for mixing which may lower RH values more than what model guidance has at this point. Soundings also show an increasing potential for wind gusts near 20 mph on Thursday. Friday through Saturday: This period largely diverts from blended model and ensemble guidance outside of the ECMWF suite. It appears the GFS is overdeveloping convection given the overall set-up below and normal biases for pattern progression that can be to quick. Mid-level ridging will continue along the east coast in this timeframe. As a result, an approaching cold front will be kept to the northwest. With development of a CAP marked by several hundred J/KG of CIN and 700 mb temps modeled near 9C it will be hard to develop any thunderstorms across the region with the front still well to our northwest. The higher terrain northwest of the I-95 fall line may be able to overcome this enough for an isolated thunderstorm or two to fire with peak heating. The main story will be the increasing heat and humidity. Looking at the modeled temperatures at 850 and 925, low to mid 90`s are entirely possible across the urban corridor both days with slightly cooler temperatures elsewhere. A continued increase in humidity will keep lows from falling much below the low and mid 70`s at night in spots. Heat index values near 100 are possible as well. Be prepared to drink plenty of water, wear light weight and loose clothing along with having a plan to spend time in air-conditioning this weekend to beat the heat. Due to some uncertainties with thunderstorm coverage, frontal timing and the region more accustomed to heat will leave this out of the HWO for now. Wind gusts from the southwest near 20 mph are currently expected for both afternoons as well. Saturday night through Monday: The cold front mentioned in the Friday-Saturday timeframe will continue to push southeast into the region eroding any CAP along with acting as a trigger for higher coverage of showers and thunderstorms Saturday night and Sunday. Heavy downpours will be possible with thunderstorms due to the very moist airmass expected to be in place. While instability looks sufficient for thunderstorms, the amount of shear currently modeled is limited for any widespread strong to severe storms at this time. The front is likely to dissipate by Monday leading to a typical summertime set-up with isolated pop-up thunderstorms. && .AVIATION /19Z MONDAY THROUGH SATURDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. VFR through the period. SCT-BKN cumulus deck around 6000-8000 feet this afternoon, with winds west or southwest 5 to 15 kts with occasional gusts to 20 kts. Tonight, skies may briefly clear for a time after sunset and then begin to increase late evening onward, especially northwest of KPHL. Winds mostly light and variable. VFR CIGs likely tomorrow with isolated to widely scattered storms possible. Coverage looks too sparse and confidence in occurrence is not high enough for TAF inclusion at this time. Winds should pick up tomorrow afternoon, with gusts to 20 kts possible. OUTLOOK... Tuesday night through Thursday: VFR. West wind 10 knots or less through Wednesday shifting to southeast from 10-15 knots on Thursday with gusts around 20 knots in the afternoon. Thursday night through Saturday: Mainly VFR. Southerly winds 10-15 knots on average. Any isolated thunderstorm may briefly lower celings and visibilities later Friday and Saturday during the late afternoon and evening. Gusts around 20 knots in the afternoons. && .MARINE... Sub-advisory conditions are expected through the period. Winds will primarily be in the southwest quadrant, with speeds generally 5 to 15 kts with occasional gusts to 20 kts or so. There is a slight chance of storms tomorrow, especially in the Delaware coastal waters, as a low develops offshore and a weak surface trough moves through the region during the day. Coverage of any precipitation should be sparse at best. OUTLOOK... Seas are likely to increase to around or just over five feet with southerly flow sometime on Thursday. These may continue into Saturday. Otherwise, seas under five feet. Some wind gusts around 25 knots are currently likely from the southerly flow late Thursday through Saturday. .RIP CURRENTS... The forecast conditions through this evening should result in a low risk for the development of dangerous rip currents. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Gaines Near Term...CMS Short Term...CMS Long Term...Gaines Aviation...CMS/Gaines Marine...CMS/Gaines

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