Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 212028 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 328 PM EST Sat Jan 21 2017 .SYNOPSIS...
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A warm front lifts north through the region tonight as high pressure over the Western Atlantic continues to drift offshore. A strong and complex area of low pressure will develop over the Lower Mississippi Valley on Sunday, and that will lift towards and pass through the Mid-Atlantic and Northeast for the start of the new work week. Several weak disturbances will impact the region late this week. A deep upper trough will build east by next weekend.
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&& .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
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Lots of low-level moisture, light winds, and a strong inversion above the surface will deep low clouds and fog across the area through tonight. The low clouds are expected to lower through the evening and overnight, and areas of dense fog are expected to develop. Therefore we`ve issued a Dense Fog Advisory for all of our CWA. A disturbance aloft is expected to move across the area overnight while an area of low pressure passes along a stalled out frontal boundary to our south. This could push an area of rain across portions of central/southern Delaware and Maryland, as well as southern New Jersey. These areas have a better chance of clearing out the fog earlier, so we will only issue the advisory through 5 am there.
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&& .SHORT TERM /6 AM SUNDAY MORNING THROUGH 6 PM SUNDAY/...
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Low clouds and areas of dense fog will begin the day across much of the area Sunday, with the possible exception across the southern areas of the CWA where rainfall could be ongoing and mixing out any fog. The dense fog should begin mixing out some by 10-11 am, but low clouds and light fog will once again continue through the day. The aforementioned low pressure located along the front to our south will weaken and slide offshore through the day, but the front will remain to our south. Another disturbance aloft is expected to move across the area during the day, which could help create some light rain across the area later during the day.
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&& .LONG TERM /SUNDAY NIGHT THROUGH SATURDAY/...
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Low pressure over the Mississippi Valley will slowly build to the east, through the TN Valley and towards the Mid-Atlantic Sunday night. Periods of rain will slide to the north and east throughout the night, some of which could be heavy at times, especially across the Delmarva and southern NJ. Conditions get complicated on Monday. NAM continues to carry a colder solution on Monday compared to the GFS/ECMWF, and this results in a period of wintry precip, mainly to the north of I-80, across the Poconos. Since the column of colder air is a bit deeper compared to previous runs, will carry a forecast of rain and sleet and will take out the mention of freezing rain. Another aspect of not for Monday is that both the 12Z GFS and the 12Z ECMWF almost have a secondary low forming over the western Atlantic, and that tracks to the north and east and well offshore. What this appears to be doing is taking the heavy rain with it, and as a result, the heaviest of the rain is falling over the Atlantic waters. NAM continues to have this area of heavy rain along the coast. For now, expecting the heaviest of the rain to fall from after midnight Sunday night through early Monday afternoon. By this time, the surface low will be over the Mid-Atlantic, and then will begin to move offshore by Monday evening. The overall big picture of the storm remains in place, but there are some timing issues that have to be watched. 65-75 KT LLJ will pass through the region from after midnight Sunday night through Monday afternoon. This will coincide with the heaviest rain and the strongest winds. Easterly pressure gradient tightens, and a 12-18 hour window of strong and potentially damaging winds are expected during this time. The strongest winds will be along the NJ coast, with sustained winds of 25-35 MPH with gusts 50-60 MPH. Along and to the south and east of I-95, east winds will range from 20-30 MPH with gusts up to 50 MPH. Back across the Lehigh Valley and into northern NJ, winds could gust up to 40-45 MPH. High Wind Watches remain in effect for coastal NJ, and Wind Advisories will likely be needed for much of central and southern NJ, parts of SE PA, and into northeast MD and DE. Persistent onshore flow will also result in widespread minor tidal flooding and beach erosion. Please refer to the Tides/Coastal Flooding section of the AFD for more details. Storm Total Rainfall amounts have trended downward with this rain based on the tracks of the GFS/ECMWF, with around 0.75-1.5" likely. But still cannot rule out some areas picking up close to 3". Conditions improve late Monday night as the low departs, but showers could linger into Tuesday as upper low passes through the region. Conditions finally dry out by Wednesday as high pressure builds east. After that, temps remain above normal through at least Thursday. Cold front comes through on Friday, and temps drop off, but still could be a few degrees above normal. Pattern changes comes late in the week and over the weekend as a deep upper trough builds east, bringing with it a return to near and possibly below, seasonal temperatures.
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&& .AVIATION /21Z SATURDAY THROUGH THURSDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Lots of low-level moisture, light winds, and a strong inversion above the surface area creating low clouds and fog which has lead to widespread IFR conditions. The low clouds will likely continue to lower this evening and tonight, and lead to areas of dense fog tonight with VSBYS lowering to around 1/2sm or less at times. The low VSBY will improve through the morning Sunday, but the low CIGS will remain through the day. An area of light rain could impact ACY/MIV during the morning, with additional rainfall possible during the afternoon Sunday for all areas. South-southwest winds will become light and variable tonight. Winds will increase out of the east-northeast by Sunday morning and continue through the day. OUTLOOK... Sunday night through Monday night...IFR/LIFR 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. Thursday...Isolated showers, possibly briefly lowering conditions to sub-VFR.
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&& .MARINE...
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Sub-Small Craft Advisory conditions will continue tonight into Sunday across the waters. However, areas of dense fog will continue, and the Dense Fog Advisory remains in place through Sunday morning. OUTLOOK... Sunday night through Monday night...Easterly flow quickly ramps up Sunday night. Expect gale force gusts to develop overnight Sunday night and storm-force gusts along the northern and central coastal waters by early Monday morning. Storm Watches were converted to Storm Warnings for ANZ450-453, and Gale Watches were converted to Gale Warnings for ANZ454-455, and DE Bay. Conditions improve from S to N through Monday evening. Tuesday...Lingering SCA conditions possible. Wednesday through Thursday...Brief lull in SCA conditions possible early Wednesday, and then conditions ramp up late Wednesday through Thursday.
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&& .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...
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An onshore flow is forecast to develop along the coasts of Delaware and New Jersey on Sunday as an initial low passes off the Middle Atlantic coast. Tide levels should begin to respond to the onshore flow on Sunday but they will likely remain below the coastal flooding thresholds at that time. The onshore flow is expected to strengthen on Sunday night 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.
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&& .PHI WATCHES/WARNINGS/ADVISORIES...
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PA...Dense Fog Advisory from 10 PM this evening to 10 AM EST Sunday for PAZ054-055-060>062-070-071-101>106. NJ...Dense Fog Advisory from 10 PM this evening to 10 AM EST Sunday for NJZ001-007>010-012>019. Dense Fog Advisory from 10 PM this evening to 5 AM EST Sunday for NJZ020>027. 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...Dense Fog Advisory from 10 PM this evening to 10 AM EST Sunday for DEZ001. Dense Fog Advisory from 10 PM this evening to 5 AM EST Sunday for DEZ002>004. MD...Dense Fog Advisory from 10 PM this evening to 10 AM EST Sunday for MDZ008. Dense Fog Advisory from 10 PM this evening to 5 AM EST Sunday for MDZ012-015-019-020. MARINE...Dense Fog Advisory until 10 AM EST Sunday 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 4 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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