Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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684 FXUS61 KPHI 231025 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 625 AM EDT Wed Aug 23 2017 .SYNOPSIS... A cold front will slowly cross our region through the morning hours before moving off shore late today. High pressure is then expected to build towards the northeastern U.S. and remain over our region into early next week. A low pressure system off the East Coast could affect our weather mid week next week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... Cold front has slowed slightly, but should still be through the I95 corridor by mid morning. It will then likely slow its progress east thanks to increased daytime mixing and may linger on the coastal Plains (especially southern Delmarva) through mid day. Adjusted PoPs to keep a mention of showers and thunderstorms along and ahead of where we expect the front to be. Though shower activity has mostly diminished for now, expect to see some additional development ahead of the front within the next few hours as we get some initial daytime heating (though do not expect as widespread coverage as what we saw overnight). MOS guidance doesn`t appear to be handling the front and the effect on highs very well, so went with a blend of operational models which show below normal highs along and west of the fall line. Even for the eastern Coastal Plains which may stay in the warm sector until just before peak heating, clouds could limit highs through the day. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM THURSDAY/... Clearing skies and cold air advection will promote below normal temperatures across the region. The one question will be if we will see any fog development overnight. At this point, have not mentioned it in the forecast as we should also see considerable dry air advection behind the front. && .LONG TERM /THURSDAY THROUGH TUESDAY/... 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 Gulf Coast by early next week. 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. Thursday...Scattered light showers possible late Thursday in e PA with considerable afternoon cloudiness. 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. The trough aloft could result in a shower north of I-78 Friday afternoon and there should be considerable cloudiness at that time. Otherwise, for now, good weather is predicted. Tuesday...WAA overrunning clouds, if not rain, from a potential tropical remnant moving northeastward from the Gulf coast. && .AVIATION /12Z WEDNESDAY THROUGH SUNDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Any patchy fog should dissipate by 12Z. There is a small chance that isolated showers and thunderstorms could develop near KMIV and KACY before 18Z, but chance is too low to mention in the TAFs. Otherwise, VFR conditions are expected through the TAF period. The cold front will continue to move east across the region through 18Z. It should be arriving at the Delaware Valley TAF sites (including KPHL, KILG, KTTN, KPNE) near or shortly after 12Z. Behind this cold front, expect northwesterly winds around 10kt with some higher gusts especially just behind the front. After 00Z, wind speeds should diminish below 10kt. Outlook... Thursday through Sunday...VFR. Generally light winds from the N or NW, possibly becoming N-NE on Saturday and Sunday. small chance of a light shower vicinity KABE/KTTN/KRDG Thursday afternoon and Friday afternoon. && .MARINE... Winds and seas have dropped below small craft advisory criteria. Therefore, the SCA was allowed to expire at 6 AM. Winds and seas should stay below SCA criteria for the rest of today and tonight. For locations that still have southwesterly winds, expect a wind shift to northwesterly by mid day as a cold front moves through. Outlook... Thursday through Saturday...Winds and seas below SCA criteria. Sunday...For no SCA headline but chance that northeast flow will increase and cause hazardous seas to develop along the S NJ and DE coasts Sunday (5 feet). Rip Currents... We are forecasting a low risk for the formation of dangerous rip currents today as earlier elevated seas are diminishing quickly. Conditions are expected to improve (i.e. the risk should lower) through the day. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Johnson Near Term...Johnson Short Term...Johnson Long Term...Drag Aviation...Drag/Johnson Marine...Drag/Johnson

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