Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 270738 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 338 AM EDT Tue Jun 27 2017 .SYNOPSIS...
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A weak cold front passes across the mid Atlantic states today. High pressure will follow tonight and Wednesday. This high moves offshore during the end of the week. Eventually, another weak cold front will cross the mid Atlantic states Saturday night or Sunday.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/... An upper-level trough is forecast to sharpen eastward today as it moves into the Northeast and Mid Atlantic. This will drive weak low pressure well to our north, however a few weak cold fronts or surface troughs will continue to push eastward. The first one early this morning has teamed up with just enough instability and lift to focus an area of showers and some thunder primarily up the I-95 corridor. It appears that the Chesapeake Bay played a role in this, helping to boost the MLCAPE just enough to get lightning going. This activity is expected to move northeast of our area through about 12z/8 AM. We continue with an area of likely PoPs in this corridor. We then turn our attention to the west. A more pronounced area of short wave energy is forecast to arrive from west to east late this morning and through the afternoon. This looks to be accompanied by either a weak cold front or surface trough. While the trough aloft is forecast to be sharpening some as it shifts eastward along with gradual cooling aloft, the bulk of the lift is more focused to our north. The majority of the model guidance is therefore showing much of the convection just to our north. There should be however enough moisture and at least some instability through the day to pop a few showers or thunderstorms. Since the overall coverage is less certain still, we kept PoPs in the slight chance to low chance range. High temperatures are a blend of continuity, MOS and some high-res guidance. Given lower heights and thickness values, the afternoon high temperatures are forecast to be a bit cooler than yesterday. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM WEDNESDAY/... The axis of an upper-level trough is forecast to move across our area during the overnight. Any shower or thunder is expected to end early as the main short wave energy is offshore, then some increase in the subsidence should begin to overspread the area as surface high pressure starts to arrive toward daybreak. The sky is anticipated to become mostly clear as a result with any clouds due to daytime heating flattening and then dissipating in the evening. Low temperatures are mostly a blend of continuity and MOS. && .LONG TERM /WEDNESDAY 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. Wednesday 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 /08Z TUESDAY THROUGH SATURDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Through today...VFR overall. An area of showers and embedded thunderstorms will move across the KILG, KPHL/KPNE to KTTN areas through about 09z. The visibility can be significantly reduced for a short period of time in any heavy shower/storm. Isolated to perhaps scattered showers/storms are possible again this afternoon, but confidence regarding timing is low and therefore a mention is not included in the TAFs. Mostly light and variable winds, becoming westerly and increasing to around 10 knots late this morning and afternoon. Some local gusts up to 20 knots this afternoon are possible. Tonight...Any shower/storm ends early, otherwise VFR with any lingering clouds tending to dissipate. West to northwest winds mainly 5 knots or less. OUTLOOK... Wednesday 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... The conditions are expected to remain below Small Craft Advisory through tonight. A southwesterly wind will become westerly late in the day and tonight as a weak cold front moves offshore. This offshore flow will increase at times tonight with some gusts to near 20 knots. A few isolated thunderstorms will be possible mainly this afternoon. 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 today should result once again in a low risk for the development of dangerous rip currents. && .CLIMATE...
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Discussion 5am.
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&& .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Drag 337 am Near Term...Gorse Short Term...Gorse Long Term...Drag Aviation...Drag/Gorse Marine...Drag/Gorse Climate...

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