Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 210008 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 708 PM EST Fri Jan 20 2017 .SYNOPSIS... Weak low pressure passes north of the region this evening, and then weak high pressure builds east late tonight through Saturday. A strong and complex area of low pressure will impact the Mid-Atlantic from Sunday through Tuesday. Conditions briefly dry out for the mid-week period, and then unsettled weather returns to close out the work week. && .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
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The last of the steady rain was being drawn out to sea early this evening with a few showers in its wake. We should remain between two weather systems tonight. A mid level short wave trough is forecast to continue lifting northeastward over the Great Lakes while another short wave moves eastward off the Middle Atlantic coast. Low pressure at the surface is expected to weaken near Lake Huron while another weak low is anticipated to move from the North Carolina coast eastward and out to sea. Weak ridging should impact our region with a light southeast to southwest surface flow. Abundant low level moisture is forecast to remain in northeastern Maryland, Delaware, New Jersey and eastern Pennsylvania tonight with drying expected aloft. The resulting subsidence should result in lowering clouds along with the development of patchy fog and drizzle. Temperatures are anticipated to remain nearly steady or they may rise a few degrees during the night due to the southerly flow and the cloud cover.
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&& .SHORT TERM /6 AM SATURDAY MORNING THROUGH 6 PM SATURDAY/... Weak high pressure will build along the coast on Saturday and we should see mainly dry conditions across the region. The southerly flow will keep moisture up across the area and its possible some light drizzle may remain into early Saturday. Unfortunately, skies will remain pretty clouded over through the day. However, we will still see temperatures rebound a bit and rise into the upper 40s across the north to mid to upper 50s across much of the area as the warm southerly flow continues. && .LONG TERM /SATURDAY NIGHT THROUGH FRIDAY/... High pressure along the East Coast moves offshore Saturday night. During this time, closed H5 low will strengthen and develop over the Southern Plains and Gulf Coast. In addition, high pressure north of the Hudson Bay will begin to nose its way to the south and east. Dry conditions expected Saturday night, but clouds associated with the developing low will stream northward into the Mid-Atlantic and Northeast. The first wave of rain lifts to the north and east and into the Mid- Atlantic by Sunday afternoon and Sunday night. There is still uncertainty with regards to the forecast from there. 12Z NAM is the coldest solution, with the high over the Hudson Bay dropping farther to the south and east compared to the ECMWF and the GFS. This allows a shallow layer of below freezing temperatures to filter into far NW zones, generally to the north and west of I-80, and results in a prolonged period of freezing rain. Since the GFS and ECMWF have a similar position with the high and have the surface low farther north than the NAM, will side with the warmer solution. That does not rule out at least a brief period of sub-freezing temperatures across parts of the Poconos, so will mention a chance of freezing rain on Monday. Latest trends lower the potential of freezing rain, but it cannot be ruled out at this time. Waves of rain slide along the coast Monday and Monday night as that low approaches from the south and west. A tight easterly gradient develops between the high to the north and the approaching low. Combined with a 65-75 KT LLJ, can expect strong easterly winds to develop on Monday, with winds potentially gusting to 50 MPH for much of NJ to the east of I-95, and gusts potentially reaching 60 MPH along the NJ coast. Wind Advisories will likely be needed, and there is the possibility for High Wind Watches/Warnings for sometime on Monday. With high PWATs upwards of 1.5", can expect periods of moderate to locally heavy rainfall during this time. Please refer to the Hydrology section of the AFD for flooding information. Low pressure lifts through the region Monday night, and precip tapers off fairly quickly behind the departing low. Winds also diminish fairly quickly as the gradient relaxes. Few lingering showers possible on Tuesday as H5 trough/shortwaves pass through the Northeast. Dry conditions expected on Wednesday as high pressure builds through the region. A deep upper trough then approaches for the late week period. Unsettled weather is possible through the end of the work week, but at this time, does not appear to be anything widespread and organized. A return to more seasonal temperatures is expected by Friday. && .AVIATION /00Z SATURDAY THROUGH WEDNESDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Abundant low level moisture will remain in our region for tonight and Saturday. We are anticipating lowering conditions for tonight with all eight of our TAF sites dropping into the IFR range and perhaps the LIFR or VLIFR category for a time. There should be lingering light rain showers this evening, then areas of fog and drizzle overnight into Saturday morning. We will maintain the low conditions into Saturday morning with some improvement possible for Saturday afternoon. We have kept the forecast somewhat pessimistic due to the presence of a temperature inversion aloft that should keep the low level moisture from lifting and mixing significant. The TAFs indicate an improvement to MVFR for the afternoon. However, this remains a low confidence forecast. A light and variable wind tonight may settle into the southwest for Saturday around 5 knots. OUTLOOK... Saturday night...VFR. High confidence. Nearly calm winds. Sunday...Conditions deteriorating from S to N in the afternoon with IFR conditions in RA. Moderate confidence. East winds increasing to 10-15 KT. Sunday night through Monday night...IFR and lower conditions in RA. East winds 15-20 KT with 20-30 KT gusts at terminals north and west of I-95, and 20-30 KT with 30-50 KT south and east of I-95 likely on Monday. Conditions gradually improve Monday night. Moderate confidence. Tue...Scattered showers with brief sub-VFR conditions. Wednesday...VFR.
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&& .MARINE... No hazards are anticipated on the waters through Saturday. Seas remain around 4 feet along the coast with a 10 to 12 second period. Seas are gradually subsiding and should drop to around 2 to 3 feet tonight. East to southeast winds around 10 knots or less tonight will become more southerly on Saturday. OUTLOOK... Saturday night through Sunday morning...No marine headlines anticipated during this time. Seas will genly be around 2 to 3 ft with wind 10 kts or less. Sunday afternoon through Monday night...Easterly flow increases to 15-20 KT with 25-30 KT gusts by Sunday afternoon. Sunday night, east winds increase to 20-30 KT with 35-45 KT gusts by Monday morning. For northern and central ocean waters, a Storm Watch was issued for Monday for the potential for 50-60 KT gusts. Meanwhile, a Gale Watch was issued for Monday for southern ocean waters and DE Bay for the 35-45 KT gusts. Periods of moderate to heavy rain expected during this time with low VSBYs. Conditions should improve late Monday night. Tuesday...Lingering SCA conditions possible. Wednesday...Sub-SCA conditions expected. && .HYDROLOGY... Before heading into the weekend, I wanted to address the potential hydro impacts for the Sunday through Tuesday period. 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 right now 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. But keep an eye in things if more than 3.00 inches of rain come to fruition and you reside in a flood prone river basin. && .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...None. DE...None. MD...None. MARINE...Gale Watch from late Sunday night through Monday afternoon for ANZ430-431-452>455. Storm Watch from late Sunday night through Monday afternoon for ANZ450-451. && $$ Synopsis...MPS Near Term...Iovino Short Term...Meola Long Term...MPS Aviation...Iovino/MPS Marine...Meola/MPS Hydrology...Kruzdlo Tides/Coastal Flooding...Iovino is the U.S. government's official web portal to all federal, state and local government web resources and services.