Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 242038 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 338 PM EST Fri Feb 24 2017 .SYNOPSIS... A strong cold front will move through the Mid-Atlantic on Saturday. High pressure will build into the region on Sunday before moving into the Atlantic on Monday. A surface low will develop in the central U.S. on Tuesday and move northeast to the Great Lakes by Wednesday and southeast Canada by Thursday. An associated warm front is forecast to move through the Northeast on Wednesday before another strong cold front sweeps across the East Coast on Thursday. && .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/... Big question tonight will be when and where low clouds and fog develop. Winds are expected to shift out of the southeast overnight. This slight shift with more of a onshore component, could result in low clouds advecting from the ocean on shore. Having said that, guidance was fairly poor with the extent of low clouds this morning, so not sure if it will be quite as widespread as what some guidance indicates. With the onshore flow and potential cloud cover, temperatures should stay quite mild, with low generally in the 50s across the region. && .SHORT TERM /6 AM SATURDAY MORNING THROUGH 6 PM SATURDAY/... The first question will be how quickly early low clouds will dissipate. Even if clouds persist into late morning though, models have recently had a cold bias in warm air advection events, so have gone slightly above guidance for highs during the day. If clouds hang on longer or if the cold front moves in earlier, current forecast would be too high. The big story in the short term will be the cold front. Latest guidance suggests it should be moving across our area from 18Z (for far western Berks Co and the southern Poconos) to just approaching the shore by evening. Forecast still looks on track for a line of showers with embedded thunderstorms along and just behind the front. The biggest question mark with thunderstorms will be surface based or mixed layer instability. CAPE values will be marginal at best so if we do end up being cooler than forecast, instability (especially surface based instability) could be negligible. Given the model cold bias though, I think we will have enough instability to support thunderstorms. Primary hazard continues to be gusty winds given the high shear environment (0-6km bulk shear values could be above 50kt). Will continue mention in the HWO and add mention to the forecast for this potential. Although there could be brief periods of heavy rain with the line, the showers and storms should be moving through fast enough to limit any flooding risk. && .LONG TERM /SATURDAY NIGHT THROUGH FRIDAY/... New long term section will be issued by 430 PM. Previous long term discussion: Active pattern in the Sunday thru Thursday time frame. Temperatures cool to seasonal levels on Sunday, with above average temperatures returning for the Monday thru Thursday time period, and additional chances for precipitation. Overall, Sunday`s forecast is high confidence. Moving into next week, there remains considerable uncertainty with regard to timing and coverage of precipitation, as well as high temperatures. Sunday... We continue to expect a return to fair weather on Sunday, along with near normal temperatures, and gusty northwest winds. Looking ahead to Monday thru Thursday: several shortwaves will interact with a warm front that sets up near our region late Monday into Tuesday, followed by a cold frontal passage Wednesday into Thursday. Timing and coverage of precip continues to remain uncertain, so the forecast reflects a general slight chance of showers during this time frame. && .AVIATION /21Z FRIDAY THROUGH WEDNESDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. VFR conditions are expected to continue through at least 03Z. After 03Z, as winds shift to more southeasterly (resulting in more of an onshore component), could see low stratus move over the TAF sites. As it does so, could also see some fog development underneath the stratus. Timing is uncertain, so have mentioned the period of greatest risk (08 to 14Z) in the the TAFs. After 14Z, any fog should dissipate and ceilings should begin to slowly lift. The next concern will be after 18Z as a cold front approaches from the west. An abrupt shift to westerly winds, widespread showers (and associated restrictions in visibility), and scattered thunderstorms are possible with and just behind the front. KRDG and KABE may see this between 18 and 21Z, while the rest of the TAF sites will be impacted 21Z or later. Outlook... Saturday night and Sunday...Conditions quickly improving to VFR behind the cold front, with showers/storms moving off the coast. West or northwest winds 10-20 knots with gusts to 30 knots likely. Above average confidence. Sunday night and Monday...VFR with west winds around 10 kts becoming southwest by Monday afternoon. Average confidence. Monday night through Wednesday...Periods of showers with associated sub-VFR conditions possible, especially during the day Tuesday and again Wednesday afternoon. Primarily light southerly or southwesterly winds, but a more easterly component may develop on Tuesday north of a developing warm front, and gustier southwest winds may occur Tuesday night or Wednesday as the warm front moves through the region. Very low confidence on timing and occurrence. && .MARINE... Seas will slowly build overnight and should be above 5 feet on the ocean waters near or shortly after sunrise. Winds will also increase after sunrise with gusts above 20kt possible. The strongest winds (and an abrupt shift to westerly winds) are expected to come with and behind a cold front which will move over the Delaware Bay late tomorrow afternoon. The front is not expected to arrive on the ocean waters before tomorrow evening. Outlook... Saturday night...Cold front will move through the coastal waters with showers/storms likely. Strong/gusty winds near storms. Small craft advisory (SCA) conditions likely as winds become west or northwest behind the front. Gusts may approach gale force. Average confidence. Sunday...Continued SCA conditions likely with winds slowly diminishing during the afternoon. Above average confidence. Sunday night and Monday...Sub-SCA conditions likely. Above average confidence. Monday night through Wednesday...Generally sub-SCA expected, though winds and seas may approach criteria after potential warm frontal passage Tuesday night or Wednesday. Periods of rain/showers likely, especially during the day Tuesday and again Wednesday afternoon. Very low confidence. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory from 7 AM Saturday to 6 PM EST Sunday for ANZ450>455. Small Craft Advisory from 6 PM Saturday to 6 PM EST Sunday for ANZ430-431. && $$ Synopsis...CMS Near Term...Johnson Short Term...Johnson Long Term...CMS/Franck Aviation...CMS/Johnson Marine...CMS/Johnson is the U.S. government's official web portal to all federal, state and local government web resources and services.