Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 221858 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 158 PM EST Sun Jan 22 2017 .SYNOPSIS... Low pressure over the Southern Plains will strengthen as it tracks northeastward along a stationary front today and tonight. This low will reach the central Appalachians by Monday morning before redeveloping off the North Carolina coast Monday afternoon. The coastal low is then forecast to move slowly northward up the Mid- Atlantic coast Monday night before eventually turning northeastward toward Nantucket late Tuesday. Several weak disturbances may impact the region late in the week. An upper trough will amplify over the eastern United States by next weekend. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... The Dense Fog Advisory has been allowed to expire as visibilities have improved. Another disturbance aloft is forecast to move across the area this afternoon, which could help create additional showers across the area. Otherwise, there remains a lot of moisture trapped underneath the inversion above the surface, so low clouds and areas of fog will continue into the afternoon. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM MONDAY/... A complex low pres system over the southeastern states continues its motion twd our region. Wind will increase and rain will become heavier and steadier from s to n. A wind advisory will go into effect toward the end for srn areas durg the overnight hours. It is psbl that temps cud be cold enough in the highest elevs of the Poconos for a wintry mix by daybreak Mon but little accum is expected. && .LONG TERM /MONDAY THROUGH SATURDAY/... Strong low pressure will reach the central Appalachians by Monday morning before redeveloping off the North Carolina coast Monday afternoon. The coastal low is then forecast to move slowly northward up the Mid-Atlantic coast Monday night before eventually turning northeastward toward Nantucket late Tuesday. Below is a break down of impacts from this storm... Heavy Rainfall... The window for heavy rain can now be narrowed down to Monday: more specifically early morning-early afternoon for Delmarva, late morning- afternoon for NJ and perhaps E PA (possibly lingering into early evening north of I-195 and east of I-287). Models have come into better agreement with the location of the heaviest rainfall on Monday: over the coastal plain from southern DE up to central NJ. The hi-res guidance is picking up on a coastal front that sets up only a few miles offshore on Monday. The S-N oriented axis of heaviest rain would be favored along and to the west (i.e., on the cool side) of the coastal front by about 25-50 miles. Low-level isentropic lift and moisture flux convergence will be maximized on this side of the boundary with the nose of the easterly LLJ directed at the coastline. Additional light to perhaps moderate rainfall amounts are forecast for Monday night and Tuesday. Forecast storm-total rain amounts range from less than an inch across our far western zones (from the eastern shore of MD up to Chester and Berks Co. in PA) to 1-1.5 in the Delaware Valley to 1.5- 2.25 in the coastal plain. Locally higher amounts up to 3+ are still not out of the question toward the coast. Wintry Precipitation... An overwhelming majority of models have trended colder with boundary-layer temperatures on Monday and Monday night. This looks to be in part due to the slower arrival of the precipitation from the low to our southwest, which would allow extra time for high pressure to build in from eastern Canada and thus for the low-level cold air to drain southward into northeastern PA and northwestern NJ. Accordingly, there is more of a concern for wintry precip from I-80 northward. Forecast soundings and partial thickness nomograms indicate the predominate ptype would be freezing rain Monday- Monday night. For now, icing look to be primarily confined to the higher ridges (elevations above 1000 ft) of Carbon, Monroe and Sussex (NJ) counties. If thermal profiles trend even colder, then sleet may cut down on icing from freezing rain across the far north. Regardless, sleet will mix in at times when the precip intensity picks up. A brief changeover to wet snow is also possible in the Poconos Monday afternoon-evening if a band of heavier precip makes it this far northwest and sufficiently dynamically cools the column. Forecast snow/sleet accumulations are less than 1 at elevations above 1000 ft along the I-80 corridor and 1-2 in the southern Poconos for ridges above 1700 ft in elevation. These amounts are conservative due to the high uncertainty of a changeover to snow and near-freezing surface temperatures that are marginally supportive of accumulations. Conversely, went higher with ice accumulations at the higher elevations north of I-80. These icing amounts were calculated using the FRAM model. With freezing rain likely to not begin until around or just after sunrise Monday morning, opted to hold off on any winter headlines at this point so the day shift can re-evaluate the next cycle of guidance. The primary surface low over the central Appalachians will eventually transfer it`s energy to the coast Monday afternoon and night. 00Z models have trended stronger with the coastal low (both at the surface and aloft), resulting in a well-defined deformation band on the backside of the low center on Tuesday. Precip may transition to snow or sleet before ending Tuesday afternoon across NE PA/NW NJ as winds turn out of the NW and colder air wraps around the backside of low. This Winds... High Wind Watches were upgraded to Warnings for eastern NJ. Additionally, Wind Advisories were issued farther west along the I- 95 corridor and farther south into southern DE. A tight pressure gradient is forecast to develop on Monday as low pressure deepens near the central Appalachians and high pressure noses southward from eastern Canada. There are still some timing differences between models that need to be resolved, but generally the strongest winds (50-60 mph gusts toward the Jersey shore and 40- 50 mph gusts along the I-95 corridor) are expected to develop from SE to NW during the early morning hours Monday. Winds will diminish from S to N during the afternoon and early evening as the easterly LLJ lifts northward with time. There is still a potential that Wind Advisories may need to be expanded farther to the west by a tier of counties but confidence in reaching criteria drops off rather precipitously farther inland. Tuesday night through Saturday... Conditions finally dry out Tuesday night and 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. && .AVIATION /19Z SUNDAY THROUGH FRIDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Another day of poor flying conds with IFR and LIFR continuing with low clouds and fog. Then, an area of low pres will approach from the sern CONUS and RA will pick up and get steadier and heavier from s to n tonight. Wind will also increase from the e later tonight. Wind will start off the day fairly light but will become nely around 10 kts by aftn and 15 to 20 kts by eve. Wind will gust to at least 25 kts by mrng and gusts to 30 kts especially closer to the cst can not be ruled out. OUTLOOK... Monday and Monday night...IFR/LIFR with periods of rain. Also, E winds continue to strengthen to 15-20 KT with 30-35 KT gusts at ABE/RDG, to 20-25 KT with 35-40 KT gusts at I-95 terminals, and 25-35 kt 45-50 KT gusts near ACY from early Monday morning through Monday afternoon. 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. && .MARINE... The Dense Fog Advisory was extended through 6 pm. Otherwise, conditions on the area waters are relative quiet today, but they will begin to deteriorate as low pres approaches from the sern states. Seas will increase and e to nely flow will strengthen by tonight, storm and gale warnings have been posted. OUTLOOK... Monday and Monday night...Easterly flow quickly ramps up early in the morning Monday to gale force in the DE Bay and DE coastal waters and to storm force in the NJ coastal waters. No changes to the Storm and Gale Warnings from yesterday afternoon. Winds quickly weaken across the southern waters during the afternoon (but still at SCA levels). A more gradually drop off of the wind speeds are expected farther north in the coastal waters off central NJ late in the day but should eventually fall below gale force during the mid evening hours. 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. && .HYDROLOGY... There is a potential for limited hydro impacts from the upcoming storm system. 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. The heaviest rain is expected to fall on Monday. Storm-total rainfall amounts will generally between 1-2 with locally higher amounts to 3" possible in eastern NJ. It appears poor drainage or nuisance flooding is more likely, not flash flooding or river flooding, especially if the heavier rain falls in the coastal plain as currently predicted. && .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... PA...Wind Advisory from 6 AM to 7 PM EST Monday for PAZ106. Wind Advisory from 6 AM to 4 PM EST Monday for PAZ070-071. NJ...Wind Advisory from 6 AM to 7 PM EST Monday for NJZ008-010-015- 019. High Wind Warning from 3 AM to 9 PM EST Monday for NJZ012>014- 020-026. Coastal Flood Advisory from 2 AM to 11 AM EST Monday for NJZ012>014-016-020>024-026-027. Wind Advisory from 3 AM to 2 PM EST Monday for NJZ016-021. High Wind Warning from 3 AM to 4 PM EST Monday for NJZ022>025- 027. Wind Advisory from 6 AM to 4 PM EST Monday for NJZ017-018. DE...Coastal Flood Advisory from 2 AM to 11 AM EST Monday for DEZ001>004. Wind Advisory from 3 AM to 2 PM EST Monday for DEZ001>004. MD...None. MARINE...Dense Fog Advisory until 6 PM EST this evening 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. && $$ Synopsis...Klein Near Term...Nierenberg/Robertson Short Term...Nierenberg Long Term...Klein/MPS Aviation...Klein/Nierenberg/Robertson/MPS Marine...Klein/Nierenberg/Robertson/MPS Hydrology... Tides/Coastal Flooding... is the U.S. government's official web portal to all federal, state and local government web resources and services.