Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 220725 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 325 AM EDT Mon May 22 2017 .SYNOPSIS...
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Low pressure tracks across southeastern Canada tonight through Monday. An associated cold front will move through our area this evening, then stall just offshore through Tuesday. An area of low pressure moves across our area Wednesday morning, followed by a cold front late Wednesday night and Thursday morning. High pressure will build in from the southwest late Friday into Saturday, then it shifts offshore into Sunday.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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We are beginning to see the first wave of showers as of 3 AM. So far rain totals have been relatively light. Expect that this will continue until the second wave, associated with a second mid level vorticity max is expected after sunrise through mid day. As far as the hazards, instability still appears meager, but perhaps enough for isolated thunderstorms along and east of the I95 corridor. The main story will be the potential for heavy rain. At this point the axis of heaviest rain looks to be east of the I 95 corridor through mid day today. Even so, the latest models have backed off a bit on QPF values. Therefore, do not expect a widespread threat, but still potential for localized flooding especially in poor drainage areas and especially if any thunderstorms develop. It still looks like there will be a sharp drop off in rain totals north and west of I95. Expect rain to taper off rather quickly late this afternoon as the cold front approaches the region.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM TUESDAY/...
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Low clouds are expected to persist through at least the first half of the night until dry air advection dominates in the wake of the cold front. Depending on how much rain areas see this morning, some fog may develop as well, but the low clouds should inhibit widespread fog development. Models have trended slightly later with the arrival of the cold front, as a result expect low clouds to linger a bit longer, and thus there will be a shorter opportunity for efficient radiational cooling, especially for the Coastal Plains. Therefore, temperatures from the I95 corridor to the east may not drop off that much overnight.
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&& .LONG TERM /TUESDAY THROUGH SUNDAY/...
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Long term discussion will be updated shortly. Summary...Times of wet weather, however warmer temperatures look to occur especially later this week and next weekend. Synoptic Setup...An amplifying upper-level trough is forecast to gradually shift eastward from the Midwest and Plains Tuesday night and Wednesday. This should move out of the East Friday as some ridging arrives and establishes high pressure off the East Coast during next weekend. At the surface, an area of potentially disorganized low pressure looks to move through Tuesday night and Wednesday. If short wave energy can consolidate more ahead of the aforementioned trough, then surface low pressure would be able to strengthen and organize. While the pattern for awhile looks unsettled, there is less certainty with the details especially for the Tuesday through Wednesday time frame. We used a model/continuity blend for Monday night through Tuesday night, then blended in the 12z WPC Guidance thereafter. Some adjustments were then made following additional collaboration with our neighboring offices. For Tuesday and Wednesday...An upper-level trough is forecast to migrate eastward from the Midwest and Mississippi Valley. This will allow for some downstream riding, however the model guidance shows several embedded short waves within the southwesterly flow aloft. These should result in surface low development into Mid Atlantic region, although the parent surface low is currently forecast to track into the Great Lakes Wednesday night. The initial surface low should develop on the stalled frontal boundary, however timing is less certain as some guidance is faster with this feature. It appears the main forcing may arrive Tuesday afternoon and night, then some drying tries to work in during Wednesday. While there is less certainty with the details, went ahead and made some adjustments to the PoPs with the highest values mainly from about I-95 on south and east later Tuesday into early Wednesday. For Thursday and Friday...As an upper-level trough shifts eastward from the Great Lakes and Ohio Valley Thursday, surface low pressure is forecast to move from the eastern Great Lakes toward northern New England. Its trailing cold front should cross our area mostly Thursday morning, however there may be a weak surface low near the triple point as it crosses our region. Some showers are expected with this system Thursday and enough instability should be realized to generate some thunderstorms and therefore a chance of thunder is included for much of the area mainly Thursday afternoon/early evening. As the trough begins to lift out Friday, a weak cold front or surface trough moves through. This in combination with cyclonic flow and some instability may generate some showers especially across the western/northern zones. The PoPs were kept on the lower side at this time. A westerly wind will increase Friday as low pressure strengthens across New England. There does not appear to be much cooling though in the wake of this system. For Saturday and Sunday...Some guidance is slower in removing the upper-level trough from the Northeast Saturday, however overall a narrow ridge should arrive as the next trough moves across the Plains and Midwest. The arrival of the ridge allows for surface high pressure to build in from the southwest before shifting offshore into Sunday. There may be a warm front lifting to our north Saturday night into early Sunday, however the opportunity for convection with it is less certain this far out especially given the main energy is currently forecast to be over the Great Lakes to the Central Plains. As of now, included a slight chance PoP for most areas Sunday and Sunday night.
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&& .AVIATION /07Z MONDAY THROUGH FRIDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Ceilings will continue to lower, with widespread IFR conditions expected after 12Z. Heavier showers look to move through the terminals from 12Z to 18Z, but chances for rain exist before and after this time frame. There is a slight chance for thunderstorms but it is too uncertain at this time to include in the TAFs. Even as the precipitation begins to taper off after 18Z, expect the low clouds to linger through the rest of the day time hours. If there are breaks in the clouds late this afternoon into the evening hours, patchy fog may develop. Conditions should begin to improve after 06Z behind a cold front which will bring in drier air. OUTLOOK... Tuesday...VFR ceilings lower to MVFR or IFR as showers develop, especially from about PHL south and east. Lower confidence on the timing. Northeast or east winds mostly 10 knots or less. Wednesday...Times of MVFR/IFR conditions along with some showers. A few thunderstorms are possible late in the day or evening. East- southeast winds 5-15 knots. Thursday and Friday...Some showers and thunderstorms around Thursday resulting in times of MVFR/IFR conditions, improving Thursday night or Friday. South to southwest winds Thursday, becoming westerly and potentially gusty on Friday.
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&& .MARINE...
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Although some gusts above 20 kt are possible this evening, we are expecting conditions to remain below SCA criteria today and tonight. By this evening, fog may develop on both the Delaware Bay and the coastal waters. However, it is uncertain at this time how widespread or dense any fog will be. Any fog that does develop is expected to dissipate in the pre-dawn hours as northerly winds increase. OUTLOOK... Tuesday...Winds may gust to near 25 knots late Tuesday across the southern zones with seas building to around 5 feet. There is lower confidence with the details given timing differences with low pressure along a stalled front. Wednesday through Friday...The winds should be below Small Craft Advisory criteria, however seas may reach or just exceed 5 feet at times on the ocean zones.
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&& .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Gorse Near Term...Johnson Short Term...Johnson Long Term...Gorse Aviation...Gorse/Johnson Marine...Gorse/Johnson

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