Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 241930 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 330 PM EDT Thu May 24 2018 .SYNOPSIS...
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High pressure across our region will gradually shift offshore Friday. A weak cold front moves southward across at least parts of our area late Saturday night and Sunday, then stalls in our area through Monday. A secondary cold front moves through during Tuesday, followed by high pressure Tuesday night before it moves offshore during Wednesday. A warm front may approach on Thursday.
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&& .NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
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High pressure centered over the Great Lakes/Ohio Valley continues to build east tonight. Clear skies and light winds on tap for tonight, and this should result in radiational cooling away from the urban centers. Lows will range from the mid 50s in most of NJ and eastern PA, upper 50s in the Delmarva, and in the lower 60s along the I-95 corridor from Trenton to Philly. Patchy radiational fog is possible, mainly in the sheltered valleys.
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&& .SHORT TERM /6 AM FRIDAY MORNING THROUGH 6 PM FRIDAY/...
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High pressure continues to build east on Friday. Winds shift to more of a southwest flow and tap into some Gulf of Mexico moisture. Surface dewpoints will rise into the 50s and humidity levels along with temperatures will creep up a bit. Afternoon sea breezes will likely not make as much westward progression due to the increasing offshore wind. Most likely areas that could sea breeze will be right along the NJ coast. Warmer temperatures expected with highs generally in the mid to upper 80s, except a bit cooler in the mountains and along the coast.
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&& .LONG TERM /FRIDAY NIGHT THROUGH THURSDAY/...
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Summary...Very warm and more humid over the weekend, although some cooling should occur Sunday into Monday. Increasing chances of showers/thunder during the holiday weekend, with perhaps some drying as we go through next week. Synoptic Overview...The flow aloft is forecast to become more blocky in nature with a closed low in the West and a closed low in the vicinity of the central Gulf Coast. Meanwhile, a ridge is near the Plains with some troughing sliding near the Great Lakes and Northeast. This offers not much change, however the details become less clear especially with the extent of impacts associated with a cold front sliding southward from New England later in the weekend and early next week. In addition, tropical moisture is forecast to surge northward from the Gulf of Mexico associated with the aforementioned trough aloft and surface low development. This all occurs as a ridge remains situated off the Southeast U.S. coast. The theme in the guidance continues to show increasing chances for convection during the holiday weekend. It does not appear to be a washout, however there looks to be times where it will be convectively active. The trough in eastern Canada and the Northeast may then amplify enough during Monday and Tuesday along with the passage of a cold front to push the deeper moisture to our south and east. For Friday night...High pressure at the surface is forecast to be centered in the western Atlantic. The flow across the Mid-Atlantic is more zonal, however a short wave trough is forecast to be shifting east-southeastward from the Midwest and upper Great Lakes. A continued southwesterly flow will allow for increasing moisture advection therefore it will start to become a bit more humid through the night. A precipitable water surge to around 1.5 inches is forecast to overspread at least the southern half of the area toward daybreak Saturday. The gradual increase in surface dew points will result in a warmer night, and some higher level clouds may cross the area at times. For Saturday and Sunday...A ridge remains centered off the Southeast U.S. coast with a ridge into the Plains and closed low out West. Some northern stream energy however allows for a short wave trough to slide eastward from the Great Lakes region while a more pronounced upper-level trough moves across the Canadian Maritimes. The latter has surface low pressure tied to it and a trailing cold front is forecast to settle southward into our area. The forecast challenge continues to figure out how far south this front gets Sunday and how strong the southwestward push from high pressure is for a time. Prior to this, southwesterly flow will maintain very warm and humid air across the region. A plume of precipitable water around 2 inches overspreads the area Saturday. Within this plume, increasing instability along with some northern stream short wave energy should initiate some convection especially from late morning through the afternoon on Saturday. This should be mostly focused across the western zones to start, potentially tied to a weak lee side trough and terrain circulations, then shift eastward through the day. Convection on Saturday could be enhanced some if the short wave trough from the west ends up providing more support. There will be additional convection on Sunday especially in the afternoon and evening as the surface cold front sags southward which should provide a bit more convergence. While the deeper tropical moisture plume may reside just to our south, plenty of moisture in our area combined with possible slower storm motion and training may result in a local very heavy rain/flood threat. The extent of this is of lower confidence at this time. For Monday and Tuesday...The overall pattern does not change much, although there may be more of an influence of an upper-level trough in the Northeast. While a residual frontal zone should be in our vicinity Monday, low pressure is forecast to track well to our north with another cold front moving through during Tuesday. Some convection should occur especially Monday, however the extent of it is less certain. There will be a tropical moisture plume still just to our south, and if this is more involved across our area then an enhancement to daytime convection is possible. Despite another cold front moving through on Tuesday, showers/thunder may end up being more limited especially if a short wave trough is a little faster and allows for some drying to occur from the northwest. For Wednesday and Thursday...A trough aloft may still be across the Gulf Coast beneath a ridge to its north. The ridge across parts of the Great Lakes should shift eastward some, allowing surface high pressure to build into our area. A front will be lurking to our south with tropical moisture residing to its south, therefore the placement of this will be key regarding any shower/thunder chances for parts of our area. More uncertainty at this time frame as convective changes will depend on the track of a trough aloft as well as a plume of tropical moisture as high pressure shifts offshore.
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&& .AVIATION /20Z THURSDAY THROUGH TUESDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. This afternoon...VFR and clear skies will continue. With high pressure building in from the NW, N winds should gradually weaken and may become light SW by evening. Another factor is the sea breeze front which is already apparent on radar as of early afternoon. This will cause a wind shift to the SE at around 10 kt at ACY by 19z or so, and also MIV a couple hours later. This sea breeze could reach PHL and nearby terminals around 23z to 00z. High confidence except for sea breeze timing. Tonight...VFR with clear skies continues. With high pressure offshore, winds will be SW around 5 kt or less. High confidence. Friday...Still VFR and mostly clear skies. High pressure moves farther offshore and SW winds will increase to 10-15 knots with possible gusts up to 20 knots. Moderately high confidence. Outlook... Friday night...VFR. Southwest winds less than 10 knots. Saturday and Sunday...Times of Sub-VFR conditions with some showers and thunderstorms, especially each afternoon and evening. Monday and Tuesday...Sub-VFR conditions possible at times due to a chance of some showers and thunderstorms.
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&& .MARINE...
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Winds shifting to the SE through this evening at less than 10 kt, and will become SW tonight. Pressure gradient tightens on Friday, but think there will be a strong enough inversion to keep wind gusts capped at sub-SCA criteria. Best chances for 25 kt wind gusts Friday afternoon will be in northern NJ ocean waters, where the Ambrose Jet may result in locally stronger wind gusts. Seas on ocean 2-4 feet. Outlook... Friday night and Saturday...Southwesterly winds may gust to near 25 knots at times. The overall mixing should be limited some due to a much warmer airmass moving over the cooler waters. Seas should be 4 feet or less. Sunday and Monday...The conditions should be below Small Craft Advisory criteria, however a cold front from the north sags into our area and this will have an impact on the wind direction. Tuesday...The conditions should be below Small Craft Advisory criteria. Rip Currents... The risk of rip currents through this evening is low. Despite an onshore component to the flow with speeds 8-12 kt, wave heights and swells are forecast to be near 2 ft.
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&& .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Gorse Near Term...MPS Short Term...MPS Long Term...Gorse Aviation...Gorse/MPS Marine...Gorse/MPS

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