Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 221942 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 342 PM EDT Tue Aug 22 2017 .SYNOPSIS... A cold front crossing the upper Ohio Valley this afternoon will move into our region late tonight and slowly exit off the Delmarva coast Wednesday afternoon. Canadian high pressure builds into the region on Thursday and should continue to be the primary influence on our weather into early next week. Two low pressure systems should be near the Gulf coast states this weekend and eventually turn northeast early next week, possibly affecting our weather around Tuesday. && .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
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Heat indicies have reached 100-102 ILG to PHL PNE TTN. A severe thunderstorm watch may issue by 4 PM. Initial issue this afternoon is the heat across the area in broad but increasing southerly flow ahead of approaching low pressure. Moisture advection has been tempered only slightly by deep mixing allowing dew points to drop a few degrees this afternoon. None the less, heat indices are close to 100 F across portions of northern Delaware, the Philadelphia metros northeastward into NYC. A heat advisory remains in effect through 8 pm. Cold front and pre-frontal convection reach the area this evening, with the latest discussion from SPC indicating a severe thunderstorm will likely be issued for the forecast area. Convection has already formed into several short line segments with severe wind gusts reported. Given veering wind profiles with 30-35 kt of deep layer shear, any discrete cells ahead of the main lines may be capable of rotation and perhaps a brief tornado. Otherwise, deeper mixing east of higher terrain may enhance damaging wind gusts in part due to evaporative cooling. Given the warm/moist conditions ahead of the front, diurnal cooling will be slow, which may allow convection to continue in severity for at least a few hours after sunset before gradually diminishing in intensity. The cold front is expected to very slowly move through from northwest to southeast starting after midnight, but will be slower to move through coastal areas. As such, a chance for showers and thunderstorms will continue through early morning.
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&& .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH 6 PM WEDNESDAY/...
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Any ongoing showers and thunderstorms near the coast will slowly move eastward towards the waters through the afternoon as the cold front continues its surge eastward. Humidity should fall to more comfortable levels later in the day, more so by evening as a secondary front pushes through the region.
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&& .LONG TERM /WEDNESDAY NIGHT THROUGH TUESDAY/...
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500 MB: a -2SD trough begins the long term period in the Great Lakes region, then weakens slightly as it moves into the northeast Friday- Monday. Thereafter, we will monitor the northeastward progress of what should be a split flow low of tropical origins, having moved ashore along the west or central Gulf Coast this weekend. Temperatures: Calendar day averages thursday...near or slightly below normal, then 2 to 6 degrees below normal daily Friday-Tuesday. Forecast basis: a 50 50 blend of 12z/22 GFS/NAM MOS was used Wednesday night-Thursday night, then the 12z/22 GFS MEXMOS was applied Friday and thereafter the 15z/22 WPC D4-8 gridded elements of max/min T, 12 hr POP 6 hrly dew/wind/sky. Wednesday night and Thursday...Fair and more seasonable with light flow with possible patchy countryside fog. Friday-Monday...High pressure 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. We 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. Tuesday...WAA overrunning clouds, if not rain from a potential tropical remnant moving northeastward from the Gulf coast.
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&& .AVIATION /20Z TUESDAY THROUGH SUNDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Cloud bases have risen to around 4000 ft this afternoon, and may occasionally go broken across TAF sites through the remainder of the afternoon and evening. 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. The cold front moves through overnight with a wind shift from southwest to west-northwest. There may be a few lingering showers and thunderstorms ahead of the front through Wednesday afternoon, primarily for KPHL, KPNE, KTTN, KMIV and KACY before conditions clear from west to east by evening. Outlook... Wednesday night...VFR except possible mvfr or ifr fog patches toward dawn Thursday with nearly calm wind. Thursday 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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Strengthening southerly flow is allowing seas to gradually build this afternoon, which should peak around 3 to 6 feet across the ocean waters and 3 to 4 feet across the Delaware Bay. Strongest gusts are expected through this evening around 25-30 kt, and will gradually diminish overnight. Seas will gradually subside into Wednesday as a cold front moves through the area and the pressure gradient begins to subside. There will be a chance of showers and thunderstorms tonight into tomorrow afternoon. These storms may locally reduce visibility and produce wind gusts to near gale-force. 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. 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. Outlook for Wednesday... Low, possibly bordering moderate based on what develops this evening for residual effects on Wednesday.
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&& .PHI WATCHES/WARNINGS/ADVISORIES... 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 ANZ430-431- 450>455. && $$ Synopsis...Drag Near Term...99 341 Short Term...99 341 Long Term...Drag Aviation...Drag/99 341 Marine...Drag/99 341

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