Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 231922 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 322 PM EDT Wed May 23 2018 .SYNOPSIS...
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High pressure builds across our region through Thursday before shifting offshore Friday. A cold front moves southward across our area late Saturday night through Sunday, then stalls in our vicinity before another cold front moves through by later Tuesday. High pressure should then build in for Wednesday.
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&& .NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...
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Some weak mid-level shortwave energy moves off the coast early this evening. Surface high pressure centered over the Great Lakes will slowly build east tonight. Any lingering clouds across the region will continue to scatter out through this evening. Skies then become clear after sunset with loss of diurnal heating and with a drier airmass building east. Surface dewpoints will gradually fall into the 50s through the overnight. NW winds around 10 mph will become light and variable tonight as well. With clear skies and light to calm winds, can expect decent radiational cooling away from the urban centers. Lows tonight will be in the 50s for most of NJ and southeast PA, and in the 60s in/around Philly and in the Delmarva.
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&& .SHORT TERM /6 AM THURSDAY MORNING THROUGH 6 PM THURSDAY/...
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High pressure slowly builds east on Thursday. With strong subsidence ahead of the high, can expect abundant sunshine and nary a cloud in the sky. Light north winds will give way to afternoon sea breezes. Highs will top off in the lower 80s throughout, though temps at the coasts will cool off fairly dramatically behind any sea breezes.
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&& .LONG TERM /THURSDAY NIGHT THROUGH WEDNESDAY/...
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Summary...Very warm and also dry for the end of the week, however the humidity increases over the weekend and early next week. Increasing chances of showers/thunder for the second half of the holiday weekend. Synoptic Overview...An upper-level trough across the Northeast to the Canadian Maritimes is forecast to shift east allowing the flow aloft to turn more westerly in the East through the weekend. The details become less certain as we go through the weekend and early next week. It appears that a ridge off the Southeast U.S. coast tries to expand northwestward some over the weekend, while a trough aloft over the Gulf of Mexico slowly lifts northward across the Gulf Coast states later in the weekend and early next week. The flow aloft looks to become more blocky with time with some semblance of a developing omega block by early next week. There looks to be a conduit for moisture to stream northward into our region over the weekend and early next week. In addition, the pattern suggests low pressure tracks well to our north over the weekend which looks to force a backdoor front into our area. Given weaker flow, this boundary may stall in our vicinity and provide a focus for additional convective development from later in the weekend through early next week. This will also depend on embedded short wave energy and timing. While the details become less certain as we go through the second half of the holiday weekend, increasing potential for showers/thunder looks to occur. For Thursday night and Friday...As an upper-level trough moves out of the Northeast, ridging to the west will maintain surface high pressure across our region. This surface high though is forecast to shift offshore later Thursday night and Friday. The flow aloft starts to go more westerly Friday and that may open the door for some high level clouds. It will be much warmer with most of the area getting into the 80s Friday afternoon. The dew points are anticipated to be low enough to keep the humidity in check. While a southwesterly breeze increases during the day Friday, ample heating on land may allow for the surface winds to back a bit to the south- southeast resulting in an afternoon sea breeze. For Saturday and Sunday...The evolution of the flow aloft becomes a bit muddled as we go through the holiday weekend. A closed low across the West while a more westerly flow is over the Mid-Atlantic. Surface high pressure in the western Atlantic will result in strengthening southwesterly flow, continuing a very warm airmass in place through Saturday. Short wave energy within the more westerly flow aloft may result in at least some convective development Saturday afternoon primarily across the western zones. This may ultimately be tied to terrain and/or a weak lee side trough. Low pressure moving through the Canadian Maritimes later Saturday is forecast to force a southward moving cold front. This front should arrive in our area late Saturday night and Sunday, although a surface high could be retreating faster and therefore the southwestward push of cooler air may not be as robust. Another short wave trough though could be tracking just north of the Great Lakes, and this could slow the southward progression of the backdoor front. There appears to be a better chance for some convection Sunday as the front settles south and slows some. In addition, some tropical moisture from a disturbance in the eastern Gulf of Mexico looks to get drawn northward and potentially interact with the weak southward moving cold front. This along with some embedded energy associated with a short wave trough to our north may enhance the convective potential Sunday afternoon with locally heavy rain possible. 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, weak low pressure is forecast to track 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 southwest, 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. For Wednesday...A closed low may still be across the Gulf Coast beneath a ridge to its north. The ridge across parts of the Midwest and western Great Lakes puts surface high pressure to the east which is currently forecast to be building into our area. A front will be lurking to our south with deep tropical moisture residing to its south, therefore the placement of this will be key regarding any shower/thunder chances for parts of our area.
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&& .AVIATION /19Z WEDNESDAY THROUGH MONDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Tonight...Mostly VFR/SKC. Some patchy ground fog possible at outlying terminals, but confidence is not high enough to mention in the 18Z TAF. NW winds around 10 kt will diminish to 5 kt or less. Thursday...VFR/SKC. N winds less than 10 kt, backing to the west in the afternoon. Sea breezes likely at KMIV/KACY, turning winds SE in the afternoon. OUTLOOK... Thursday night and Friday...VFR. Light southwest winds or becoming light and variable, then southwest winds Friday increase to 10-15 knots with possible gusts up to 20 knots. Saturday and Sunday...Mainly VFR Saturday and Saturday night with a few showers and thunderstorms possible. Sub-VFR conditions should occur at times Sunday with developing showers and thunderstorms especially in the afternoon. Monday...Sub-VFR conditions possible due to a chance of some showers and thunderstorms.
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&& .MARINE...
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Sub-SCA conditions with fair weather on tap tonight and Thursday. Seas on the ocean will be mostly 2 to 3 ft. OUTLOOK... Thursday night...The conditions are anticipated to be below Small Craft Advisory criteria. Friday and Saturday...Southwesterly flow increases allowing the seas to build some. A much warmer airmass will be moving over the cooler waters, therefore strongest gusts may be along the coast. It is possible that wind gusts near 25 knots at times. Sunday and Monday...The conditions should be below Small Craft Advisory criteria.
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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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