Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 171959 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 359 PM EDT Thu May 17 2018 .SYNOPSIS...
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A nearly stationary front will extend from the central Appalachians through the central Mid-Atlantic through Friday night, with a series of surface lows moving slowly along the front. A stronger low will move from the Tennessee Valley Friday into the lower Great Lakes by Saturday, allowing the front to lift north before slowly dissipating. A cold front will approach the Atlantic coast by late Sunday into Monday before stalling out through the first half of next week. High pressure will attempt to nose its way into the Mid- Atlantic by the middle to end of next week.
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&& .NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
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Models continued the trend with the heaviest axis of precipitation further south. Surface high pressure continues to slowly build south towards our region overnight. This should be effective in shunting the heavier rain further south. The heaviest rain overnight is expected generally across Delmarva and into far southern New Jersey. See hydrology section below for information on the flooding concerns. As with last night, temperatures will likely not drop off very much thanks to the persistent cloud cover. Lows are generally expected to be in the upper 50s to lower 60s.
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&& .SHORT TERM /6 AM FRIDAY MORNING THROUGH 6 PM FRIDAY/...
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In many ways, Friday is a repeat of tonight, with the surface high and a short wave mid and upper level ridge keeping heavy rain generally confined to the southern half of our area...or along and south of a RDG-PHL-ACY line. If the stalled front edges north, there could be enough instability to support a few thunderstorms across Delmarva. Otherwise, it is more of the same with rounds of showers, possibly heavy at times. Flooding concerns continue through the day. Few changes made to the temperatures. With most of the region remaining in the cold sector and the persistent cloud cover, expect highs to be generally in the 60s across the region tomorrow.
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&& .LONG TERM /FRIDAY NIGHT THROUGH THURSDAY/...
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Friday night through Sunday... The main story during this period will be ongoing periods of heavy rain with an associated flooding threat that will last at last through Saturday as tropical moisture continues to stream north across an oscillating front. Forecast models in good overall agreement with this scenario and Flood Watch has been expanded through the day Saturday. To begin the period Friday evening, a warm front will be located near the Delmarva with a deep layer southerly flow of tropical moisture continuing to result in anomalously high PWAT values streaming north over this boundary. Lift will be also be aided by perturbations in the upper level flow moving north. Expect rain, heavy at times, to be ongoing at this time but likely heaviest from around Philly southward closer to the warm front. Heading through Friday night, warm front at the surface won`t make too much progress north however expect warm advection to be quite strong in the 850- 950 mb layer with 35 knot LLJ from the south bringing elevated warm front with some associated instability north through the forecast area. As result, rain, heavy at times will continue across southern parts of the forecast area but also expand north to affect northern and NW NJ into the southern Poconos. Some thunder will be possible, mainly over far southern areas, with the limiting factors being very warm temperatures aloft. However this same environment factor will continue to result in very efficient warm rain processes favorable for producing high rain rates. In addition, as deep layer flow aligns mainly out of the S/SW profiles continue indicate a favorable set up for backbuilding and training cells which could also enhance flood threat. So this all said, threat for widespread urban and small stream flooding along with more localized flash flooding will continue across most of the area Friday night. This could result in very poor travel conditions due to the reduced visibilities in heavy rain and any flooded roadways. Heading into Saturday, aformentioned surface warm front lifts north as weak low pressure moves from Ohio NE across the lower Great Lakes. Southerly flow of rich, tropical moisture continues across the area with lift also continuing to be aided by upper level forcing as main upper level trough begins to approach from the west. Not expecting any real sunshine due to continuing saturated conditions but weak diurnal heating in warm sector will nonetheless result in precip becoming more convective in nature increasing the chance of thunder. As such, while the rain likely won`t be quite as steady, expect that periods of rain and showers will continue across the forecast area and that these will continue to be quite heavy at times. Favorable environment will continue to persist for some backbuilding and training of cells continuing to exacerbate flooding threat area wide. It should also be noted, as we head through time, the flood threat on larger, mainstem rivers will increase as excess run-off and stream flow makes its way into these. More on this is detailed in the hydro section. Saturday night into Sunday, some changes in the pattern finally occur, at least temporarily. Upper level trough axis swings from Ontario/Quebec east into Atlantic Canada and this will drag some drier air in the mid and upper levels south. At the surface, cold front will move south toward the region Saturday night but stall across the region into Sunday. The upshot is that the more widespread soaking rains will abate during this period but the threat of more scattered showers and storms will continue. In and of itself the rain in this period may not be that significant however conditions will be so saturated that it won`t take much to exacerbate any ongoing hydro issues. Additional rain amounts for the weekend period (Friday night through Sunday) should generally be in the 1 to 1.5 inch range but locally amounts will be higher due to the convective nature of the system. Also, this is in addition to what occurs through Friday so all told, a prolonged significant rain event. Sunday night Through Thursday... Relative lull continues Sunday night into early Monday as weak ridging may try to briefly move into central PA. However the front, though fizzling, will remain near the Delmarva so we keep at least slight chances of showers in the forecast. Unfortunately by late Monday into Tuesday as the next upper level trough approaches, this will act to drive the front back to the north through the area with periods of rain and showers returning along the chance of thunderstorms by Tuesday afternoon. Beyond this point forecast confidence starts to really drop off based on recent model performance at this time range. Models indicate that front should eventually clear the area by sometime late Wednesday but the details of exactly when this occurs remains uncertain. Generally models have been showing this to occur 6-7 days out for the past couple days but then delaying the timing. For this reason keep at least slight chances of showers in the forecast through Wednesday. Beyond this time, present indications are that widespread fair, dry weather should occur next Thursday as high pressure finally moves in. Temperatures Monday through Thursday look to be near to a little above average (highs in the 70s to low 80s).
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&& .AVIATION /20Z THURSDAY THROUGH TUESDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Rest of today and tonight...Any improvements to VFR or MVFR will be brief, with conditions settling back down to IFR after 00Z. Low confidence though in the timing and flight categories. Light and variable winds should continue through the overnight hours. Tomorrow...MVFR or IFR conditions will continue through the day tomorrow. IFR conditions are more likely from KPHL and southward where heavy rain is more likely. Winds are expected to increase from the northeast to 10 to 15 mph. Moderate confidence. OUTLOOK... Friday night through Saturday night...Periods of sub-VFR with occasional showers and possibly storms, especially on Saturday. Torrential rainfall is possible with the showers. Winds east- northeast 10 to 15 kts with stronger gusts, especially near the coast Friday night. Winds become southeast/south Saturday and Saturday night, with potential for gusts continuing, especially near/southeast of PHL. Moderate confidence. Sunday through Monday...Periods of sub-VFR possible, though conditions may become predominantly VFR. Chances for showers/storms late afternoon through Sunday night and again Monday afternoon. Winds southwest or west 5 to 15 kts. Low confidence. Monday night through Tuesday...Periods of sub-VFR possible as more widespread showers and storms may return during this period. Low confidence.
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&& .MARINE...
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Winds should increase starting Friday morning. On the coastal waters especially, seas will subsequently increase as well. Small craft advisory conditions are expected beginning Friday morning. there is a small chance for gale force gusts on the coastal waters tomorrow afternoon, but that is unlikely at this time. Outlook... Small craft Advisory conditions continue Friday night through Saturday along with periods of rain, heavy at times, with local visibility restrictions likely. Saturday night through Tuesday...SCA conditions may linger Saturday night. Otherwise, expect mainly sub-advisory conditions with occasional chances for showers and storms, especially Monday night and Tuesday.
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&& .HYDROLOGY...
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Numerous rounds of showers and thunderstorms will continue across the region into the weekend. Heavy rainfall is possible during this period, with the heaviest rain falling late Thursday night through Saturday. The heaviest of the expected rain has shifted south and now resides across southern NJ and the Delmarva. The current forecast still calls for 2-4 inches of rain across much of the HSA through Saturday. This does not include the rain that has fallen the last few days. With this afternoon update, less rain is expected across northern NJ and isolated amounts up to 5 inches is expected across the southern DE and adjacent Eastern Shore counties. A variety of flooding types are all possible within the Flood Watch we issued...from low-lying and poor drainage, to flash flooding, to river flooding. All of our hydrologic forecasts can be accessed via our Advanced Hydrologic Prediction Service website. Tides are also expected to increase through the end of the week as well. This could exacerbate flooding across those areas where fresh water and tides meet.
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&& .TIDES/COASTAL FLOODING...
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Observed tidal levels as we approach low tide this afternoon are right around 1/3 to 1/2 ft above astronomical prediction. These positive anomalies are expected to remain steady through tonight, which translates to water levels at or just below minor coastal flood threshold around high tide this evening for many locations along the NJ and DE oceanfront and for later tonight along the DE Bay. Positive tidal departures should increase on Friday in response to strengthening onshore flow along the NJ and DE coasts. Coastal Flood Advisories will likely be needed for the Friday night high tide cycle with the latest total water level forecasts indicating widespread minor coastal flooding along the oceanfront and DE Bay. For the tidal portion of the DE River, spotty minor tidal flooding seems to be the most likely outcome.
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&& .PHI WATCHES/WARNINGS/ADVISORIES...
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PA...Flood Watch through Saturday afternoon for PAZ060>062-070-071- 101>106. NJ...Flood Watch through Saturday afternoon for NJZ007-009-010- 012>027. DE...Flood Watch through Saturday afternoon for DEZ001>004. MD...Flood Watch through Saturday afternoon for MDZ008-012-015-019- 020. MARINE...Small Craft Advisory from 8 AM Friday to 6 PM EDT Saturday for ANZ450>455. Small Craft Advisory from 8 AM Friday to midnight EDT Friday night for ANZ430-431.
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&& $$ Synopsis...Fitzsimmons Near Term...Johnson Short Term...Johnson Long Term...Fitzsimmons Aviation...Fitzsimmons/Johnson Marine...Fitzsimmons/Johnson Hydrology... Tides/Coastal Flooding...

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