Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 221628 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 1228 PM EDT Tue Aug 22 2017 .SYNOPSIS... The Mid-Atlantic region will be located today between high pressure that is parked offshore and a cold front that will be moving through the Great Lakes and Midwest states. The cold front will move into the region late tonight and slowly exit off the Delmarva coast on Wednesday. Canadian high pressure builds into the region on Thursday and should continue to be the primary influence on our weather into early next week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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Primary update this afternoon was to slow the timing of any thunderstorms that move into the area. Main thunderstorm development will likely occur from northwest to southeast after 6 pm and continue into tonight, with strong to severe storms capable of gusty winds and heavy rainfall possible. Should any isolated pre-frontal development occur, it will likely remain confined to the higher terrain to the north and west of Philadelphia. Stratus from this morning was slow to clear, so mixing has been somewhat delayed from previous forecast. Expect within the next hour or so that winds should begin to rapidly increase in speed from the south-southwest ahead of the approaching low pressure system. Deeper mixing may allow dew points to lower a few degrees, though with hot temperatures and dew points still largely in the lower to mid 70s, the heat advisory for northern DE, the Philadelphia metro corridor into NYC remains on track. Regarding severe potential, CAPE-shear parameter space is more than adequate for severe storms. MLCAPE could approach/exceed 2000 J/kg this afternoon in the western CWA, with effective bulk shear of 35 to 45 kts. With a deep layer of positive buoyancy and unusually strong veering wind profiles for this region, the potential for strong/damaging winds is present. Main limiting factor (aside from nebulous/weak ascent and a lack of near-surface convergence) is weak midlevel lapse rates, which may prevent the development of stronger negative buoyancy/cold pools in convective downdrafts, and hinder larger hail development. Nevertheless, the ambient environment is quite supportive of severe storms. The risk for hail and even an isolated tornado is nonzero, especially with any storms that remain discrete.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM WEDNESDAY/... As the aforementioned pre-frontal trough approaches the region tonight, a band or bands of loosely organized storms should approach/enter the region. However, with nocturnal stabilization commencing, the chances for severe storms are diminished in our CWA versus points to the north/west earlier in the day. Additionally, the vort max approaching the area will be weakening as it lifts northeastward into New England. One more limiting factor is the potential for fairly weak cold pools, given weak midlevel lapse rates and a deep layer of substantial atmospheric moisture. All of these factors suggest convection may struggle to survive as it progresses eastward through the CWA overnight. This looks to be especially true south of I-78, where large-scale ascent rapidly weakens. One potential modulating factor, however, is the nocturnal increase of a low-level jet, which may enhance downward momentum transport on the upstream side of linear convection. With residual instability and strong shear profiles, high-resolution guidance may be too quick to diminish convection after dark. One other factor to consider: Winds will be slow to diminish after dark, which will keep boundary layer mixing elevated. This would slow the process of increasing convective inhibition, and models do tend to forecast this process too quickly, in general. With the above in mind, kept PoPs primarily slight chance south of I- 76, with much higher chances north of I-78 through the night. The synoptic cold front should be on the doorstep late in the night, but temperatures/dew points will be quite high in advance of it. Kept forecast numbers close to MAV guidance, though confidence is low given potential impacts from convection. One other note: The progression of the convection and upstream cold front appears to be slowing with the latest set of model guidance. The updated forecast slows the progression of higher PoPs in accordance with this trend. As a result, PoPs are generally unmentionable in the far southern CWA until late in the night. && .LONG TERM /WEDNESDAY THROUGH TUESDAY/... The cold front will most likely reside along or just south/east of I- 95 by daybreak Wednesday morning. The movement of the front will then likely slow down as it progress southeastward through the Delmarva during the day as it approaches downstream ridge blocking over the western Atlantic Ocean. Accordingly, conditions should dry out rather nicely on Wednesday along and especially north/west of the I-95 corridor while locations in southeastern NJ and Delmarva will continue to be under a chance for showers. If the front slows down even more and some breaks in the clouds develop, there may be enough instability to fuel thunderstorms toward the middle Delmarva peninsula in the afternoon. Consensus from the latest suite of model guidance keeps the more favorable environment for severe storms just south of Delaware. An isolated strong to severe storm and heavy rain producers is possible if the front gets hung up north. High pressure centered over central Canada midweek shifts southeastward into the Great Lakes Thursday through Saturday, then eastward into southeastern Canada/Northeast U.S. Sunday and Monday. This expansive high will control our regional weather pattern across the Mid Atlantic during this time. Barring any major changes to the synoptic pattern, the upcoming Thursday-Monday period could potentially be only our second (virtually) rain-free stretch that spans at least five days in what has otherwise been a very wet summer. I say "virtually rain free" because 1) cannot completely rule out an isolated shower Sunday-Monday with the trough axis of the upper low expected to move through at a time when onshore flow could act to moisten the low-levels and 2) 8-13 June, which was the other 5+ day dry stretch this summer, was technically not rain free as MPO reported 0.01 inches of rain on both 10 June and 13 June. The last time all eight of our climate sites have not recorded any measurable rainfall for at least five consecutive days was 2-6 March 2017 (honorable mentions: 15-19 May except for 0.02 inches at MPO on 15 May and the previously mentioned 8-13 June period). Below normal temperatures can also be expected Thursday-Monday with highs in the mid 70s-lower 80s and lows in the 50s- lower 60s. This will provide very comfortable temperatures for outdoor activities. && .AVIATION /16Z TUESDAY THROUGH SATURDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Stratus and fog from this morning has largely cleared from the area, though transient low-level bands may occasionally but briefly lower ceilings to MVFR through the early afternoon. Primary concern at this point will be a significant impact to aviation as multiple lines of thunderstorms form well west of the area, then gradually move into the TAF sites from northwest to southeast this evening into tonight. Brief heavy downpours reducing visibility and strong gusty winds will be possible in association with the thunderstorms. Outlook... Wednesday...Predominately VFR. Any leftover showers should be isolated and end by midday for I-95 terminals, N and W. Farther S/E toward MIV and ACY, chances for showers and even a thunderstorm could linger through the afternoon as a cold front slowly progresses offshore. Localized/brief restrictions possible in thunderstorms. Winds veer from W to NW during the morning with speeds 5-10 kt. Wednesday night through Saturday...VFR. Generally light winds from the N or NW, possibly becoming N-NE on Saturday.
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&& .MARINE...
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Southerly winds will continue to increase today ahead of a low pressure system, with southerly swells and building seas in response. Speeds should reach advisory criteria by mid- afternoon and continue through tonight. Gusts may approach/exceed 30 kts during the evening hours. Seas should range from 4-6 feet this afternoon and tonight. There is a chance for at least brief SCA conditions on the Delaware Bay later this afternoon into the evening. Conditions will be monitored closely. There is a chance of showers and thunderstorms tonight, with the best chances in the New Jersey coastal waters. Winds and seas will be locally higher near any storms that occur. Outlook... Wednesday...Did not extend the SCA into Wednesday as available wave guidance shows seas in our coastal waters falling below 5 ft by daybreak. However, will still leave the door open for an extension into the morning if seas take a bit longer to subside. Winds change direction out of the NW and decrease steadily during the day behind cold fropa. Wednesday night through Saturday...Winds and seas below SCA criteria. RIP CURRENTS... There is a moderate risk of rip currents at area beaches this afternoon and evening. Conditions may approach high late this evening as southerly to southwesterly winds peak and seas increase to around 5 feet. Though short- period swell should be the primary mode through the period, an underlying longer-period swell may be present. Should this be more dominant than currently progged or should conditions worsen earlier than forecast, an upgrade to high risk may be required later today. Without question, if swimming in the waters today, use extra caution and common sense. Swim only in the presence of lifeguards, and do not swim alone! Heed any and all restrictions by area beach patrols.
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&& .PHI WATCHES/WARNINGS/ADVISORIES...
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PA...Heat Advisory until 8 PM EDT this evening for PAZ070-071-102- 104-106. NJ...Heat Advisory until 8 PM EDT this evening for NJZ010-012-015- 017>019. DE...Heat Advisory until 8 PM EDT this evening for DEZ001. MD...None. MARINE...Small Craft Advisory until 6 AM EDT Wednesday for ANZ450>455.
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&& $$ Synopsis...Klein Near Term...CMS Short Term...CMS Long Term...Klein Aviation...CMS/Klein Marine...CMS/Klein

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