Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 270434 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 1234 AM EDT Tue Jun 27 2017 .SYNOPSIS... A weak cold frontal passage occurs overnight and early Tuesday. High pressure will follow into the mid Atlantic states later Tuesday and Wednesday. This high anchors offshore during the end of this week. Eventually, a cold front should cross all of the mid Atlantic states late Sunday or Monday. && .NEAR TERM /UNTIL 6 AM THIS MORNING/...
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A weak front or surface trough continues to slowly shift into eastern Pennsylvania. It is with this feature along with some weak short wave energy that is helping to focus some showers. The HRRR, RAP and some other guidance continues to focus additional shower development up the I-95 corridor through the early morning hours. Based on radar trends, increased the PoPs quite a bit overnight along this corridor. While some lightning is possible due to some mainly elevated instability, any thunder should be rather isolated. Some dry air entrainment earlier may still exit in renewed showers to produce locally gusty winds. The temperature, dew point and wind grids were adjusted based on the latest observations, then the LAMP/LAV guidance was blended in to assist. No major changes were made to the low temperatures.
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&& .SHORT TERM /TUESDAY/...
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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.
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&& .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 /06Z TUESDAY THROUGH SATURDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Overnight...VFR overall. Some showers will shift northeast of KRDG and KABE through 06z, while additional showers will be near KILG, KPHL/KPNE to KTTN in about the 05z to 09z time frame. A brief drop in the local visibility/ceiling may occur with these showers. The chance of thunder appears low enough and therefore was not included in the TAFs. Light and variable winds overall. Tuesday...VFR overall. A chance for some showers and thunderstorms exists, especially in the afternoon, but confidence with respect to timing and location is too low to include in the current TAFs. Winds will increase out of the west at 10 to 12 knots with gusts up to 20 knots. Winds will start to subside a bit late in the day. 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.
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&& .MARINE...
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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 for Tuesday should result in a low risk for the development of dangerous rip currents.
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&& .EQUIPMENT...
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The nationwide network outage that affected the National Weather Service point and click web pages looks to have been now restored.
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&& .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Drag/Gaines Near Term...Gorse Short Term...CMS Long Term...Gaines Aviation...Gaines/Gorse Marine...CMS/Gaines Equipment...

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