Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 211922 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 322 PM EDT THU JUL 21 2016 .SYNOPSIS... High pressure builds south and east of the area today and tonight. A frontal boundary is expected to move southeast into our area Friday night into Saturday, and then slowly sag south of the area through Sunday. Another frontal boundary is expected late Monday into Tuesday. High pressure should then briefly build back into the area Wednesday. && .NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
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Its been an excellent summer day...a few cumulus and winds generally under 15 mph with a few southwest gusts 15-20 mph on the interior near and west of I-95 while a strengthening southerly sea breeze is now developing along the coasts with gusts 20-25 mph possible by 5 PM. Relatively low humidity. By 5 PM we should have Day 1 of what will probably be quite an extended and notable heat wave beginning this afternoon at Philadelphia. As of 245pm max temp at phl 89. ABE and RDG both at least 90F. Tonight...Fair skies and a bit milder, in part due to a south to southwest breeze, a rising dewpoint and maybe some southeastward moving cirrus debris from the sewd moving Great Lakes thunderstorms. Forecast basis is generally a 50 50 blend of the 12z/21 GFS/NAM mos.
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&& .SHORT TERM /6 AM FRIDAY MORNING THROUGH 6 PM FRIDAY/...
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Mostly sunny, hot and humid with a southwest wind gusting 15-20 mph during the afternoon. Should reach 95-96F in Philadelphia Friday afternoon (4C warmer at 850 mb) and dewpoints around 5F warmer during the afternoon as compared to the upcoming upper 50s values of this afternoon. The result will be a max HI around 98-99F, possibly 100F for an hour. We may eventually need to issue a heat advisory for Friday afternoon, prior to the higher impact potential Excessive Heat this weekend. Thunderstorm risk?: It appears a 700mb Temperatures cap of 10C develops during the afternoon so while we cant rule out tstms... it appears if any occur...they should be confined near or N of I-78...roughly KPTW-KMJX northward and mainly late afternoon, if then. It may be that they develop and move rapidly sewd near 00z/23? Add gusty winds and heavy rain to the thunder fcst for late Fri. PWAT 1.7" so if it thunders, rain will be briefly heavy with usual poor drainage street flooding. Its all conditional on tstms developing which is the uncertainty. Forecast basis was generally a 50 50 blend of the 12z/21 GFS/NAM mos guidance, favoring the warmer GFS with support from the 2m temps of the 12z/21 NAM and ECMWF.
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&& .LONG TERM /FRIDAY NIGHT THROUGH THURSDAY/... Hot and humid conditions are continued to be forecast for much of the extended period, with Saturday, Sunday, and Monday expected to be the warmest days. Thicknesses and 925/850 mb temperatures are expected to continue to climb during this period. With the building thicknesses and 925/850 mb temperatures, daytime highs will be in the mid to upper 90s Saturday, Sunday, and Monday, followed by low to mid 90s again by Tuesday. Dangerously high heat index values over 100 degrees are expected over the weekend into Monday, and possibly into Tuesday. With several days of dangerously high heat index values expected, we have issued an Excessive Heat Watch starting Saturday and continuing into Sunday for all of our forecast area except Carbon and Monroe counties in Pennsylvania and Sussex county in New Jersey. This watch will likely need to be extended into Monday, and possibly Tuesday. Rain chances through the extended will be centered around a couple of frontal boundary passages. The first is late Friday into Friday night, and the second period being Monday into Monday night. On Friday, a pre-frontal trough will be to our west, with a frontal boundary approaching the area late in the day as well. As these features move eastward, along with the associated short wave aloft, scattered showers/thunderstorms could begin moving into the area late in the day Friday into Friday night. There is a fair amount of instability forecast with the heat and humidity, and there will be some weak shear present as well. If any thunderstorms do develop, the could produce a brief period of heavy rainfall and gusty winds. A similar situation is forecast to occur on Monday as a frontal boundary is forecast to approach late in the day into Monday night. Instability increases during the day into the evening, and as the frontal boundary and associated short wave move into the area, showers and thunderstorms may develop and move across the area. The rest of the extended forecast is expected to be dry. && .AVIATION /19Z THURSDAY THROUGH TUESDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Rest of today...VFR sct cu aoa 6000 ft in the non sea breeze influenced region. A sw wind generally under 10 kt (sct gusts 15kt) except south southeast bay and sea breezes from vcnty KACY to KMIV and KILG this afternoon. Tonight...VFR. Possible MVFR haze fog at a few taf sites near 09z/22 but confidence below average on any vsby restriction. Friday...VFR sct clouds aoa 5000 ft. A sw wind gusty 15-18kt during the afternoon. Small chc of a sewd moving gusty tstm (35kt) vcnty krdg/KABE/KTTN near or after 21z. Confidence: below avg on thunder. OUTLOOK... Friday night...Generally VFR. A chance of showers and thunderstorms which may briefly lead to lower conditions. Gusty southwest winds 15-20 knots. Saturday-Sunday...VFR. Monday-Monday night...Generally VFR. A chance of showers and thunderstorms which may briefly lead to lower conditions. Gusty southwest winds 15-20 knots.
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&& .MARINE...
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Southerly winds will gust just above 20 kt late this afternoon and evening along the northern NJ coastal waters, but are expected to remain below SCA criteria at this time. Seas should generally be 3 ft or less. Caped any gusts at 24 kt. Friday...a south-southwest wind gusty to 20 to possibly kt during the afternoon. No SCA attm since confidence on wind above 22 kt is below average at this time (attm). capped late Fri gusts at 24 kt for the time being. OUTLOOK... Friday night...Small Craft Advisory conditions possible with increasing winds ahead of an approaching cold front into Friday night. Saturday-Sunday...Sub-Small Craft Advisory conditions expected. Monday-Monday night...Near Small Craft Advisory conditions possible with increasing winds ahead of an approaching cold front. RIP CURRENTS...There should be a developing moderate risk for the formation of dangerous rip currents along the central and northern NJ shore this afternoon, due to increasing southerly sea breezes and heading into low tide. Elsewhere, only a low risk. Confidence: average. The probable risk for the formation of dangerous rip currents Friday is moderate along the NJ shore from Atlantic County northward and low for the Delaware Beaches and Cape May county. Confidence on how much southerly component is a little below average so the moderate risk may end up low enhanced. Looking further ahead, with the Atlantic Basin still devoid of tropical storms, the risk for the formation of dangerous rip currents should be Low. Water temperatures have rebounded after the massive upwelling event associated with the squall line passage late Monday. Water temps today were in the low to mid 70s, above normal for this time of year and quite a rebound from the lower 60s of Tuesday. Colder upwelling might develop again late Friday or Saturday.
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&& .CLIMATE...
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Near record (within 2f) or record warmth is anticipated for many sites on 1 or 2 days of the following listed. 23rd 24th 25th 26th PHL 101-2011 98-2011 96-1899 101-1892 ABE 99-1955 95-1999 95-1999 98-1940 ACY 105-2011 100-2011 99-2010 96-2011, 1963 ILG 100-2011 98-2011 96-1987 99-1894 TTN 104-2011 98-1910 97-1999 99-1892 GED 104-2011 99-2011 99-2010 97-2012 RDG 100-2011 96-2010 96-1999 99-1940 MPO 91-1955 91-1914 90-1999 89-1949
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&& .PHI WATCHES/WARNINGS/ADVISORIES... PA...Excessive Heat Watch from Saturday morning through Sunday afternoon for PAZ060>062-070-071-101>106. NJ...Excessive Heat Watch from Saturday morning through Sunday afternoon for NJZ007>010-012>027. DE...Excessive Heat Watch from Saturday morning through Sunday afternoon for DEZ001>004. MD...Excessive Heat Watch from Saturday morning through Sunday afternoon for MDZ008-012-015-019-020. MARINE...None. && $$ Synopsis...Robertson Near Term...Drag 322 Short Term...Drag 322 Long Term...Robertson Aviation...Drag/Robertson 322 Marine...Drag/Robertson 322 Climate...322

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