Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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516 FXUS61 KPHI 210914 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 414 AM EST Sat Jan 21 2017 .SYNOPSIS... Weak high pressure will build across our region today. A strong and complex area of low pressure will impact the Mid-Atlantic region Sunday through Tuesday. Multiple waves of low pressure will likely track far enough to our north to keep conditions dry for the second half of next week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... Wk high pres will be over the region today. So dry conds are expected. However, there is abundant low level moisture trapped beneath an inversion, so expect plenty of clouds with fog, especially this mrng. Temps will be abv nrml, into the 50s in many locations. However, with abundant cloud cover and moisture will go a degree or two below guid, especially since guid has been too high of late. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SUNDAY/... The aforementioned high pres moves off the cst tonight. Then, a complex area of low pres over the srn plains and sern CONUS begins to move twd the region. By late tonight/erly Sun mrng, the guid begins to diverges. The GFS and NAM are dry thru the end of the pd. However, the ECMWF does bring some light precip to the Delmarva and srn NJ by daybreak Sun. The CMC and SREF bring it even further n sooner. For now, will just bring some low chc precip to the srn areas, with the caveat that it cud be further n and pops may need to be raised. Nevertheless, qpf shud be light during this time. && .LONG TERM /SUNDAY THROUGH FRIDAY/... The main focus in the long term is the strong coastal low this is expected to impact the area late this weekend into early next week. Overall models from last night`s 00Z runs have come into better agreement with the track and intensity of the low, but continue to offer a variety of solutions (which is not much of a surprise given the complexity of the forecast with so many different players involved). Accordingly, there is high confidence that the forecast area will see some impacts from this storm in terms of soaking rain, strong winds, tidal flooding and beach erosion. However, uncertainty regarding the timing and magnitude of the above impacts, as well as the possibility of wintry precip to mix in across far northern zones, remains until the smaller- scale details are able to be resolved. Here is a break down of impacts: Rainfall... The faster envelope of guidance indicates widespread rain for most of the area Sunday. A few of these faster solutions, including the 00Z Canadian, are pronounced with coastal cyclogenesis to our south along a warm front early on in the event. The coastal low, which would be located well ahead of the primary low that would likely still be located back over the Mid South, would result in a round of heavy rain (0.75-1.5) on Sunday, north of the warm front across Delmarva and southern NJ. On the other end of the spectrum, some of the slower solutions, including the 00Z GFS and NAM, hold off the onset of the steadier rain until Sunday afternoon for Delmarva region and Sunday night farther north across eastern PA and much of NJ. PoPs for Sunday were not increase with the early morning forecast update and were even adjusted downward across the northern half of the CWA. PoPs were raised to 100 percent Sunday night into Monday with periods of rain expected. Although subject to change, we are currently projecting the a greatest potential for heavy rainfall to be Sunday evening-Monday morning from the DE Bay region southward (Delmarva and far southern NJ), late Sunday night-Monday afternoon farther north across southeastern PA/central NJ, and Monday morning- Monday evening for northeastern PA and northwestern NJ. This is when the warm front moves northward through the region and strong lift/moisture transport off the Atlantic is aided by 65-75 kt easterly winds with a low-level jet streak. There is a potential for precipitation to linger into Monday night and even into part of Tuesday as the closed upper low passes overhead and the primary surface low transfers its energy to a secondary coastal low off the Delmarva coast. Models show light QPF during this part of the forecast but with a well-defined TROWAL overhead, cannot rule out a continuation of steadier/heavier rain. We are still thinking storm-total rainfall amounts will generally be between 1-3. Locally higher amounts of 4+ are possible, which would lead to more significant flooding issues if it were to occur. Conversely, rainfall amounts may be under an inch in some areas if the dry slot from this system is quicker to advance northward and shut off precip later on Monday. Additional discussion about the potential for inland flooding are included in the hydrology section below. Winds... The strongest winds with this storm are most likely to occur sometime overnight Sunday night into Monday afternoon. A tight pressure gradient is forecast to develop during this time as the low deepens near the central Appalachians and high pressure tries to build southward from eastern Canada. A High Wind Watch was issued for eastern NJ where there is a potential for easterly wind gusts of 50-60 mph. Confidence in high wind criteria being met is along the coast. Wind advisories may be needed farther inland toward the I-95 corridor. Wintry Precipitation... There is a potential for freezing rain in Carbon and Monroe Counties in PA and Sussex County, NJ on Monday and Monday night as colder air in the boundary layer drains southward in response to Canadian high pressure trying to build southward. The current thinking is ice accretion will be confined to the highest elevations (above 1500 ft) in these counties. Forecast temperatures weighted more toward the colder NAM/ECMWF/Canadian solutions, which yield surface temperatures around 32F (and wet-bulb temperatures not much lower) along the higher elevation ridges. These marginally cold temperatures and higher precip rates are not particularly favorable for a significant icing event. Ice accumulations were capped at one- tenth inch. Precip may transition to snow or sleet before ending on Tuesday as colder air wraps around the backside of H8 low. Will likely need deep lift to dynamically cool the column sufficiently for a change over to snow. Low confidence in this happening at this time since the low is forecast to weaken on Tuesday as it transfers its energy offshore. Coastal Flooding/Beach Erosion... See the tides/coastal flooding section for more details. Tuesday night through Friday... Multiple waves of low pressure will likely track far enough to our north to keep most of the area dry. The first system will track northeastward through the Great Lakes and into southeastern Canada on Wednesday. W-NW flow behind this system will gradually draw colder air into the region. We may see lake effect snow/rain showers reach the Poconos Wednesday night through Friday. && .AVIATION /09Z SATURDAY THROUGH WEDNESDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Conds were genly IFR and LIFR erly this AM as abundant low level moisture kept low clouds and fog over the area. This trend is expected to continue thru most of the day. There could be some improvement durg the aftn (to MVFR), and have gone with an optimistic fcst, but some guid suggests this will not occur. Then there will be a return to similar conds tonight with IFR and LIFR, with the psbl exception of KACY. Wind will genly be light, 5 kts or less. OUTLOOK... Sunday...Rain spreads in from S to N. Still some uncertainty in the timing of the rain, but once it arrives, conditions should quickly deteriorate to IFR. E winds increasing to 10-15 KT. Sunday night through Monday night...IFR/LIFR likely with periods of rain. Also, E winds continue to strengthen to 15-20 KT with 25-30 KT gusts at ABE/RDG, 25-30 KT with 35-40 KT along I-95 terminals, and the potential for 40-50 kt gusts near ACY late Sunday night through Monday afternoon. Tuesday...Lingering MVFR/IFR conditions with showers for at least the morning. Gradually improving conditions from SW to NE later in the day. Tuesday night and Wednesday...VFR. NW winds 10-15 kt with gusts to 20 kt. && .MARINE... No marine headlines are expected thru the near and short term pds. Seas will genly be in the 2 to 3 foot range. Winds will genly be 10 kts or less. OUTLOOK... Sunday morning...Winds and seas below SCA. Sunday afternoon through Monday night...Easterly flow quickly ramps up late in the day Sunday into Sunday night. Expect gale force gusts to develop overnight Sunday night and storm-force gusts in the in our northern coastal waters by early Monday morning. The Storm Watch was expanded southward to include all of the coastal waters of NJ. The strongest winds are expected Monday morning across the southern coastal waters and DE Bay and prolonged into Monday afternoon for the northern coastal waters. Tuesday...Lingering SCA conditions possible. Wednesday...Winds and seas below SCA criteria. && .HYDROLOGY... There is a potential for hydro impacts from the upcoming storm system Sunday through Tuesday. In short, we don`t think there will be any river flooding unless basin-wide rainfall averages climb above three inches. The most sensitive responding rivers still appear to be the Millstone and the Rancocas. The entire region has some positive things working in its favor...precipitation has been below normal the last 30 days, there`s really no consequential snow cover to speak of, there`s no ice on area waterways, streamflows are either running at or below normal as is soil moisture, and last but not least, the ground isn`t frozen. We think all the above will help mitigate the flood threat. We feel the latest MMEFS runs are running a bit hot. They show some impacts with about 2.50 inches of rainfall. But taking into account the above mentioned items and the fact that the rains will fall over a 24 to 36 period, we feel more rain will be needed for river flooding. MARFC in-house contingency runs agree with this. As of now, it appears poor drainage or nuisance flooding is more likely, not flash flooding or river flooding with storm- total rainfall amounts will generally between 1-3. There would be more concern for flooding if we start seeing amounts greater than 3 inches. With some of the wetter outlier model solutions showing localized amounts near 4, it is still too soon to rule out more significant flooding. && .TIDES/COASTAL FLOODING... It continues to appear as though an onshore flow will develop on Sunday, then it should strengthen on Sunday night. The onshore flow is forecast to be strong on Monday morning and it may begin to weaken gradually from south to north along the coasts of Delaware and New Jersey on Monday afternoon and Monday evening as low pressure approaches from the southwest and the pressure gradient relaxes. We will continue to monitor the two high tide cycles on Monday. However, the astronomical tides are rather low at that time so we are anticipating mostly minor flooding. Any flooding may be enhanced by the wave action. Breaking waves are forecast to be in the 5 to 8 foot range. There is the potential for localized moderate flooding, especially from Long Beach Island up to Raritan Bay, based on the latest guidance. The first high tide on Monday occurs in the early morning hours along the oceanfront and it would require a surge of about 2.0 to 2.5 feet to reach the minor flooding threshold. The second high tide which occurs in the late afternoon along the oceanfront would require a surge of about 3.0 feet to reach the minor flooding threshold. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...High Wind Watch from late Sunday night through Monday evening for NJZ012>014-020-026. High Wind Watch from late Sunday night through Monday afternoon for NJZ022>025-027. DE...None. MD...None. MARINE...Storm Watch from late Sunday night through Monday afternoon for ANZ452-453. Storm Watch from late Sunday night through Monday evening for ANZ450-451. Gale Watch from late Sunday night through Monday afternoon for ANZ430-431-454-455. && $$ Synopsis...Klein Near Term...Nierenberg Short Term...Nierenberg Long Term...Klein Aviation...Klein/Nierenberg Marine...Klein/Nierenberg Hydrology...Kruzdlo/Klein Tides/Coastal Flooding...Iovino/Klein is the U.S. government's official web portal to all federal, state and local government web resources and services.