Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 231959 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 259 PM EST Mon Jan 23 2017 .SYNOPSIS... Strong low pressure near the Virginia Capes late today will drift northeast toward Long Island through Tuesday morning, before passing east of Cape Cod Tuesday evening. Weak high pressure crosses our region on Wednesday. A cold front passes southeastward off the mid Atlantic coast Thursday and with another cold frontal passage possible again Sunday or next Monday. && .NEAR TERM /THROUGH TONIGHT/... This section will update shortly after 330 PM. Strong low pressure continues to move northeast along the central Appalachians this morning and will move offshore of the Mid-Atlantic later this evening. Meanwhile, high pressure remains to our north across eastern Canada. A tight pressure gradient remains across the area today and will continue to lead to increasing winds. Winds have already gusted 40-50 mph and will continue to increase through this morning and into this afternoon, especially from I-95 eastward. We have upgraded the areas along the I-95 corridor south and eastward to a High Wind Warning through late this afternoon. Areas north and west of of I-95 remain in a Wind Advisory. As several vorticity impulses move across the area in the diffluent southerly flow aloft, periods of rain and drizzle will continue into this afternoon. The heaviest precipitation is expected to move in from south to north during the 18Z-06Z time frame. Models are quite variable with QPF totals. Location of maximum QPF will be highly dependent on exact placement of the coastal front, frontogenetical forcing and the axis of deformation to the northwest of the surface low, and exact influence of the low- level jet developing just downstream of the surface low. It still appears that coastal New Jersey should see the best chance of highest precipitation. Current Flood Watch looks OK for now. This storm will also impact our northern most zones with winter weather. Light snow has already been reported in some areas across Sussex, New Jersey. Temperatures are below freezing across portions of Sussex, New Jersey as well as Carbon and Monroe, Pennsylvania. So any precipitation that falls in the higher elevations of these counties will see a wintry mix of snow, sleet, and freezing rain through today. Therefore the Winter Weather Advisory continues for these counties. Snow and snow/sleet mix looks to become heavy for a period tonight. It`s very important to note that there are still subtle thermal differences between model guidance, which leads to quite a bit of uncertainty with regards to how ptypes transition through the event. Therefore, forecast confidence in snow/ice accumulations is definitely lower than what we would like to see for this close out and with a winter headline in effect. Nonetheless, it only takes a light glaze of ice to produce hazardous travel...which is enough to warrant the headline. Please see WSW for more details. && .SHORT TERM /TUESDAY/... This section will update shortly after 330 PM. The main band of precipitation is expected to be moving across the area this evening as the deepest lift associated with the strong system moves through. The highest QPF totals will likely occur during this time, with storm totals (liquid equivalent) 1-3 inches area wide, much of this occurring in the aforementioned 18Z-06Z period. As this steadier precipitation moves through, precipitation in the higher elevations of Carbon, Monroe, and Sussex County (NJ) will likely transition to sleet and wet snow. Combined with the strong winds this evening, this will make for treacherous travel in these areas. As most precipitation is expected to be sleet, antecedent ice accumulations and subsequent snow accumulations are currently expected to remain below warning thresholds. However, much uncertainty remains as to the exact evolution of precipitation type with this storm -- i.e., this forecast is of below average confidence. Synoptically-driven low-level jet stream will slowly move northeast away from the region during the evening hours. As this does so, the strongest lift should depart the area after midnight, and winds should begin to decrease. Nevertheless, the proximity of the surface low will permit winds to continue gusting up to 40 mph across much of the area, especially in New Jersey and southeastern Pennsylvania. Light precipitation will likely continue, especially with any wraparound on the northwest side of the cyclone (indicated by virtually all operational guidance, though there remain considerable discrepancies regarding exact placement). As precipitation winds down in the southern Poconos and adjacent Sussex County, NJ, precipitation may transition to light freezing rain or drizzle as ice crystals become more difficult to generate as the midlevels dry out. Temperatures are expected to stay just above freezing from the I-95 corridor southeastward to near freezing in the Lehigh Valley to below freezing in the southern Poconos and adjacent Sussex County. && .LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/... 500 MB: Weak ridging follows the departure of our coastal storm Tuesday night and Wednesday, followed by a series of short waves that carves out a positive tilt trough in the eastern USA. Temperatures: around 10 degree above normal Wednesday, 10 to 15 above normal Thursday, 5 to 10 above normal Friday, cooling to near normal Sunday and Monday. Forecast basis: Unless otherwise noted, a 50 50 blend of the 12z/23 GFS/NAM MOS guidance Tuesday night-Wednesday night, thereafter the 12z/23 GFS MEXMOS Thursday and Thursday night, and then the 15z/23 WPC guidance Friday-Monday, at times blended with the 12z/23 GFS MEXMOS guidance or the long term temps were adjusted because of the 12z/23 ECMWF 18z 2m temperatures. The dailies... Tuesday night and Wednesday: a little leftover mixed precipitation is possible but it should not last long, if there is any. Conditions dry out Tuesday night as high pressure builds into the region and moves offshore during the day Wednesday. Gusty northwest winds to 20 or 25 mph Tuesday night and Wednesday morning become west or southwest Wednesday afternoon and diminish a bit. Wednesday night and Thursday...warm air advection ahead of cold front. Maybe a few showers. Lots of uncertainty regarding whether it will rain. Blended the MEX POPS with the previously drier forecast. Later Thursday into the weekend...gradual cool down with generally dry weather expected. Breezy at times. Next Monday...by then another cold front has either passed or is passing and the GFS is the first of our usual reviewed global models to suggest low pressure forming on the front over the mid Atlantic states. To say the least, uncertainty and so we stayed with the dry WPC fcst. && .AVIATION /20Z MONDAY THROUGH SATURDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. This short term aviation section will update shortly after 330 PM. Conditions are generally MVFR this morning across the TAF sites, but are expected to lower to IFR through this afternoon and tonight. Low clouds, drizzle, and fog will continue this morning before steadier rain moves in this afternoon and this evening. There is a slight chance of some wintry precipitation tonight at KABE/KRDG, but confidence was too low to include in the TAFs at this time. Strong east to northeast winds are expected through this evening, with gusts 30-40 kts for ABE, RDG and 40-50 kts possible for ACY, ILG, MIV, PHL, and PNE through today. Winds will decrease some this evening and overnight, but remain gusty 25-35 kts. A strong low- level jet will generate easterly low-level wind shear this afternoon and evening across the entire area. Precipitation will likely diminish from south to north by late this evening or overnight, but with little recovery of ceilings/visibilities expected. GUSTS UP TO 50 KT. OUTLOOK... Tuesday night and Wednesday...VFR. NW winds 10-15 KT with gusts to 20 kt with winds backing to west in the afternoon. Late Wednesday night or Thursday...Isolated showers, possibly briefly lowering conditions to sub-VFR. Friday and Saturday...VFR. West winds 10-20 KT with gusts 25 to 30 kt. && .MARINE... This short term marine section will update shortly after 330 PM. The Storm Warning remains in place for the Atlantic waters adjacent to New Jersey and Delaware. We`ve upgraded the Delaware Bay to a Storm Warning as winds are already gusting 40-45 knots, and are expected to increase to 50 knots. The winds will diminish some this evening and overnight, but will likely remain around gale force through the night for much of the waters. OUTLOOK... Tuesday night...northwest SCA, possible low end leftover gale early in the evening. Wednesday...Sub-SCA conditions expected. Thursday through Friday...Wind gusts 25-30 KT possible. && .HYDROLOGY... Flood Watch is in effect for this afternoon and tonight for central and NE NJ, where the flooding risk is highest (heaviest rainfall and most prone to flooding issues in urban locales). 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 from late this afternoon through tonight. 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. && .TIDES/COASTAL FLOODING... A Coastal Flood Warning is in effect for this afternoon/evening along the NJ coast from Atlantic County northward to Monmouth County. The latest ETSS has trended higher with tidal forecasts at both Atlantic City and Sandy Hook. Even though ETSS keeps Sandy Hook below moderate, we think guidance is trying to play catch up. The trend has been for the duration of very strong onshore winds to extend later into the day and even into part of tonight. The timing of the peak storm surge now looks to coincide with the late afternoon/evening high tide to yield moderate tidal flooding in the warned area. A Coastal Flood Advisory remains in effect for the coastal areas of Delaware and the remaining portion of the coastal areas of New Jersey, Delaware Bay and the tidal Delaware River below the Commodore Barry Bridge area. The advisory covers the high tide cycles through this evening, but it will likely need to be extended into the following high tide early Tuesday morning. There is a concern that moderate coastal flooding develops with this afternoon/evening`s high tide but did not have as much support from the guidance as we did at Atlantic City and Sandy Hook. We will continue to closely monitor the situation. 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. The expected surge along with the wave action and the forecast rainfall will likely result in additional and more widespread minor flooding around the high tide from late this afternoon and evening. The onshore flow is expected to begin weakening from south to north tonight as low pressure passes off the Middle Atlantic coast and the tight pressure gradient ahead of the system lifts into New England. However, 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 Tuesday afternoon. 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. && .CLIMATE... Rechecking monthly normal. The January departure at Philadelphia, when all is said and done, should end up 5 degrees above normal or 10th to 17th warmest January in the period of record. .PHI WATCHES/WARNINGS/ADVISORIES... PA...Wind Advisory until 7 PM EST this evening for PAZ060>062-101- 103-105. High Wind Warning until 7 PM EST this evening for PAZ070-071- 102-104-106. Winter Weather Advisory until 11 AM EST Tuesday for PAZ054-055. NJ...Flood Watch through late tonight for NJZ008-010-012>015-020- 026. Wind Advisory until 7 PM EST this evening for NJZ007>010. Coastal Flood Warning until 11 AM EST Tuesday for NJZ012>014- 020>027. High Wind Warning until 7 PM EST this evening for NJZ015>019- 021. High Wind Warning until midnight EST tonight for NJZ012>014- 020-022>027. Coastal Flood Warning until 11 PM EST this evening for NJZ016. Coastal Flood Advisory until 4 PM EST this afternoon for NJZ016. Winter Weather Advisory until 11 AM EST Tuesday for NJZ001. DE...Coastal Flood Warning until 11 AM EST Tuesday for DEZ002>004. High Wind Warning until 7 PM EST this evening for DEZ001-002. High Wind Warning until midnight EST tonight for DEZ003-004. Coastal Flood Warning until 11 PM EST this evening for DEZ001. Coastal Flood Advisory until 4 PM EST this afternoon for DEZ001. MD...Wind Advisory until 7 PM EST this evening for MDZ008-012-015- 019-020. MARINE...Storm Warning until 1 AM EST Tuesday for ANZ452-453. Storm Warning until 4 AM EST Tuesday for ANZ450-451. Storm Warning until 10 PM EST this evening for ANZ430-431-454- 455. && $$ Synopsis...Drag 258 Near Term...CMS/Klein/Robertson Short Term...CMS/Klein Long Term...Drag 258 Aviation...CMS/Drag/Robertson/MPS 258 Marine...CMS/Drag/Klein/Robertson/MPS Hydrology... Tides/Coastal Flooding... Climate...drag 258

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