Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 222115 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 415 PM EST Sun Jan 22 2017 .SYNOPSIS...
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Low pressure over the Gulf Coast will drift north and east tonight and will be over the Mid-Atlantic by Monday afternoon. This low will then meander off the New Jersey coastline Monday night before departing late Tuesday. Weak high pressure builds across the region on Wednesday. Low pressure passes north of the area Wednesday night, and then a deep upper trough builds into the Eastern United States for the weekend.
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&& .NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
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Several vorticity impulses will move across the area tonight within the southwest flow aloft as a large closed low begins to move northeastward out of the southern gulf states and into the southeast states. Low clouds and fog continue this afternoon and evening, and may continue into the night. In addition, periods of light rain will move across the area through this evening and overnight as well. Temperatures are forecast to stay steady, or slowly fall through the night. However, any wintry precipitation is not expected across the far northern areas until during the day Monday. Winds are generally out of the east to northeast around 5 to 10 mph this afternoon and evening. However, as the surface low to our south begins to lift northward overnight, the pressure gradient begins to increase across the area, and winds will begin gusting 25-35 overnight and into early Monday morning.
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&& .SHORT TERM /6 AM MONDAY MORNING THROUGH 6 PM MONDAY/...
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The strong low pressure system to our south will continue to move northward toward the Mid-Atlantic through Monday. As it moves into the Mid-Atlantic, a secondary low is forecast to develop near the Virginia/North Carolina coasts, before intensifying and taking over as the main low late in the day. A strong pressure gradient will remain across the area, and winds are expected to increase across our entire forecast area. Winds of 40-50 mph are expected everywhere except coastal New Jersey, where 55 to 60 mph winds could develop. A Wind Advisory is in effect for all areas except coastal New Jersey where a High Wind Warning is in effect. Periods of rain will continue across the area as well during the day Monday as several vorticity impulses will move across the area as they swing around the closed low that will be located to our south. Periods of moderate to even heavy rainfall could develop during the day, especially across portions of eastern New Jersey. A Flood Watch is in effect for a portion of eastern New Jersey starting late in the afternoon. Another concern during the day Monday will be wintry precipitation across Carbon and Monroe, Pennsylvania and Sussex, New Jersey. Temperatures are expected to continue to lower through the day, and fall below freezing across portions of these counties, especially the higher elevations. Therefore, a wintry mix of snow, sleet, and freezing rain could develop during the day Monday. A Winter Storm Watch is in effect starting midday Monday and continuing through the day.
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&& .LONG TERM /MONDAY NIGHT THROUGH SUNDAY/...
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Surface low pressure will move into the western Atlantic waters Monday evening, where it will become nearly stationary due to a blocking high over eastern Canada and northern New England. Meanwhile, 65-75 KT LLJ will slide along the coast Monday evening, departing by daybreak Tuesday. This complex system will continue to have significant impacts for the region. Heavy rain: Persistent and deep onshore flow will usher abundant low level moisture into the region with PWATs over an inch. Meanwhile, approach of the aforementioned LLJ will result in moderate to heavy rain developing across eastern NJ. Both the NAM and the GFS indicating an area of frontogenesis along the NJ shore, corresponding to where the heaviest rain is expected. Between 1 and 3 inches of rain is possible Monday afternoon and Monday night, with locally higher amounts possibly up to 4 inches. With the low slow to depart, there could be some lingering bands of precip into Tuesday morning, but the departure of the LLJ, that is not likely. Flood watch has been issued for much of northeast NJ, but think greatest effects will be right along the shores. Winds: Tight easterly pressure gradient forms between the two aforementioned systems. Combined with the LLJ, the strongest of the winds will linger across parts of northeast NJ Monday evening, before diminishing Monday night. Most Wind Advisories and High Wind Warnings should be over by midnight Monday night, but the pressure gradient should be strong enough to result in 20-30 MPH wind gusts into much of NJ, parts of southeast PA, and possibly into DE. Winds diminish by daybreak Tuesday. Coastal Flooding: Please refer to Coastal Flooding section of the AFD below. Wintry Precip: NAM continues to be the coldest solution, but there are some signals from the GFS that enough cold air will wrap around the back side of the low to allow for a wintry mix Monday night. Enough warm air aloft will mix into the region with surface temps falling below freezing across Carbon, Monroe, and Sussex counties to allow for freezing rain, snow, and sleet Monday evening, and then that will change to freezing rain and sleet going through Monday night. For those areas, a Winter Storm Watch has been issued for the wintry weather that will impact not only Monday, but Monday night as well. Going back into the Lehigh Valley and into northern NJ, rain should mix with sleet. There is the potential for sleet to spread back farther into southern NJ and into southeast PA, but for now, will keep the frozen precip confined to northern areas. An additional 1-3 inches of snow is possible in the Watch area, as well as between 1/10 and 2/10 inch of additional ice. From Tuesday on: Although surface low departs Tuesday morning, upper level low will lag behind. With several strong shortwaves wrapping around this low, can expect another shot of precip to impact the region through Tuesday afternoon. precip should be rain for the bulk of the area, but a wintry mix is possible for Carbon and Monroe counties. Conditions dry out Tuesday night as high pressure builds into the region and moves offshore during the day Wednesday. Temps will run a few degrees above normal on Tuesday, generally in the upper 30s to mid 40s, while a return to above normal temperatures expected on Wednesday with highs in the 40s and 50s. Low pressure will organize over the Great Lakes as a deep upper trough builds into the western U.S. This portends a pattern change for the end of the week and weekend. Cold front pushes through the region Wednesday night. For now, expecting dry conditions. However, GFS is indicating an area of precip developing offshore with the passage of the front. It is possible that more organized precip can develop, but models currently keep it offshore, so will go with a mainly dry forecast with its passage. Colder air then builds into the region for the end of the week and weekend with a return to more normal temperatures. Highs will fall into the 20s and 30s over the weekend.
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&& .AVIATION /21Z SUNDAY THROUGH FRIDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. IFR conditions continue through the period with low CIGS and VSBYS. Periods of light rain, drizzle, and/or fog will continue into this evening and overnight. More steady rainfall is expected to move across the TAF sites starting Monday morning and continuing through the day. Winds generally out of the east to northeast around 5 to 10 knots will continue into this evening, before they begin to increase overnight. Wind are expected to become gusty later in the night and into daybreak Monday, then continue to increase through the day Monday. Gusts 30-40 knots are possible for most areas, with stronger gusts closer to the coast. OUTLOOK... Monday night...Strong winds and heavy rain with IFR/LIFR conditions expected for NJ terminals E of I-95 Monday evening. For the rest of the terminals, winds should diminish to 15-20 KT with gusts up to 30 KT Monday evening as winds turn to the N. Some wintry precip possible for KABE/KRDG. Tuesday...Lingering IFR conditions with rain continuing through the day. Sleet or snow may briefly mix in at ABE before ending. Gradually improving to MVFR and then VFR from S to N late in the day. Tuesday night and Wednesday...VFR. NW winds 10-15 KT with gusts to 20 kt. Thursday...Isolated showers, possibly briefly lowering conditions to sub-VFR. Friday...VFR. West winds 10-20 KT.
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&& .MARINE...
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The Dense Fog Advisory was extended through the evening hours as there is still expected to be fog across the waters. Near term guidance indicates it should begin dissipating through the evening hours. Storm and Gale Warnings remain in place across the waters. A strong low pressure system will move toward the area overnight and through Monday, increasing the pressure gradient across the area. Strong winds are expected to develop overnight tonight and continue through the day Monday. OUTLOOK... Monday night...East flow becomes NE. Sustained winds 20-30 KT with 35-45 KT gusts will linger over northern ocean waters, and then winds diminish a bit by daybreak Tuesday. For all other ocean waters and DE Bay, winds diminish to 15-20 KT with 20-30 KT gusts. Tuesday...Strong SCA conditions expected, with another round of Gales possible on the ocean waters Tuesday afternoon. Wednesday...Sub-SCA conditions expected. Thursday through Friday...Wind gusts 25-30 KT possible.
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&& .HYDROLOGY...
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Flood Watch has been issued for most of NE NJ. Impacts will be due to excessive rainfall, as between 1-3 inches of rain with locally higher amounts, possibly up to 4 inches, is expected Monday afternoon and Monday evening. River flooding should be limited 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.
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&& .TIDES/COASTAL FLOODING... A Coastal Flood Advisory (CFY) was issued for coastal communities along the DE and NJ coasts as well along the DE Bay. The CFY currently only covers the early Monday morning high tide. The CFY will likely need to be extended into Monday afternoon/evening to cover the following high tide cycle. Although local regression and ETSS/NOS model guidance does not indicate moderate coastal flooding, it is too early to rule it out. An onshore flow is forecast to develop along the coasts of Delaware and New Jersey today as an initial low passes off the Mid-Atlantic coast. Tide levels should begin to respond to the onshore flow this afternoon but they will likely remain below the coastal flooding thresholds at that time. The onshore flow is expected to strengthen tonight as another low begins to approach from the southwest and the pressure gradient tightens. A storm surge along the coasts of Delaware and New Jersey is anticipated to build to 2 to 3 feet by the early morning high tide on Monday. Increasing wave action and the resulting beach erosion will likely compound the effects of the surge. Waves breaking along the shore may build to 5 to 8 feet. Widespread minor flooding is forecast to develop along the coasts of Delaware and New Jersey, on the back bays, along Delaware Bay and along Raritan Bay. Thankfully, the astronomical tides will be relatively low on Monday since we are coming off the last quarter phase of the moon. Nevertheless, a number of roads will likely flood and become impassable for a time around high tide. The strongest of the onshore flow is forecast to occur on Monday morning. The maximum surge of 3.5 to 4.0 feet is anticipated to occur around the low tide late on Monday morning. However, water will likely remain trapped in the back bays of Delaware and New Jersey at that time. The onshore flow is expected to begin weakening from south to north on Monday afternoon as low pressure begins to arrive from the southwest and the tight pressure gradient ahead of the system lifts into New England. However, the storm surge is forecast to remain in the 2.5 to 3.5 foot range along our coastline around the high tide late on Monday afternoon. As a result, another round of widespread minor coastal flooding is expected at that time. There could be pockets of moderate flooding from Long Beach Island up to Raritan Bay depending upon the rate that the onshore flow actually begins to lessen. Lingering minor coastal flooding is again possible for the high tide early on Tuesday morning being that the flow is not expected to become fully offshore until the daylight hours on Tuesday. Based on the forecast guidance and the expected pattern we are not anticipating any coastal flooding on the upper eastern shore of Maryland. Also, the potential for anything more than spotty minor tidal flooding appears low for the tidal Delaware River above the Commodore Barry Bridge. There may be some areas of minor flooding on the portion of the tidal Delaware River that runs between New Castle County, Delaware and Salem County, New Jersey especially around Monday morning`s high tide. && .PHI WATCHES/WARNINGS/ADVISORIES...
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PA...Wind Advisory from 5 AM to 4 PM EST Monday for PAZ060>062-070- 071-101>106. Winter Storm Watch from Monday afternoon through Tuesday morning for PAZ054-055. NJ...Flood Watch from Monday afternoon through late Monday night for NJZ008-010-012>015-020-026. Wind Advisory from 5 AM to 7 PM EST Monday for NJZ007>010-015. Wind Advisory from 3 AM to 4 PM EST Monday for NJZ016-021. Coastal Flood Advisory from 3 AM to 9 PM EST Monday for NJZ012>014-020>027. High Wind Warning from 3 AM to 9 PM EST Monday for NJZ012>014- 020-026. High Wind Warning from 3 AM to 4 PM EST Monday for NJZ022>025- 027. Coastal Flood Advisory from 5 AM to 10 PM EST Monday for NJZ016. Wind Advisory from 5 AM to 4 PM EST Monday for NJZ017>019. Winter Storm Watch from Monday afternoon through Tuesday morning for NJZ001. DE...Wind Advisory from 3 AM to 4 PM EST Monday for DEZ001>004. Coastal Flood Advisory from 3 AM to 9 PM EST Monday for DEZ002>004. Coastal Flood Advisory from 5 AM to 10 PM EST Monday for DEZ001. MD...Wind Advisory from 3 AM to 4 PM EST Monday for MDZ008-012-015- 019-020. MARINE...Dense Fog Advisory until 1 AM EST Monday for ANZ430-431- 450>455. Storm Warning from 1 AM to 4 PM EST Monday for ANZ452-453. Storm Warning from 1 AM to 10 PM EST Monday for ANZ450-451. Gale Warning from 1 AM to 7 PM EST Monday for ANZ430-431-454- 455.
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&& $$ Synopsis...MPS Near Term...Robertson Short Term...Robertson Long Term...MPS Aviation...Robertson/MPS Marine...Robertson/MPS Hydrology... Tides/Coastal Flooding...

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