Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 240929 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 429 AM EST Tue Jan 24 2017 .SYNOPSIS...
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The coastal low will lift northeast, passing east of Cape Cod this evening. A ridge builds over our region briefly tomorrow. A cold front passes southeastward off the mid Atlantic coast Thursday followed by a second cold front moving through our region late in the weekend.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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Strong coastal low will meander its way northeast today to offshore Cape Cod by 00Z Wednesday as a high-amplitude ridge downstream and a cold anticyclone to its north impedes much progress. Substantial synoptic-scale lift via differential cyclonic vorticity advection and low-level isentropic ascent will continue to generate a broad area of precipitation on the north and northwest side of the surface low. The heaviest precipitation at this time was generally north of I-80, but lighter precipitation has developed in SE PA and S NJ in the past couple of hours. Some occasional instances of light snow have even occurred in spots as the combination of wet-bulbing and strong dynamical effects/lift have cooled the column sufficiently. However, for areas south of the southern Poconos, any wintry precipitation early this morning will be light, short-lived, and without impacts, particularly as surface temperatures warm after sunrise. High-resolution guidance is rather consistently showing a narrow band of precipitation developing in the coastal waters moving westward into portions of New Jersey today in an axis of enhanced deformation on the upstream side of the surface low. Although substantial precipitation is not expected, did feel the need to bump up QPF a tad in portions of northern/central NJ today given this consistent signal. In addition, with ambient ascent revolving around the low underneath cold midlevels, showery precipitation should occur in much of the area today, particularly in PA/NJ. As such, raised PoPs for the daytime hours, especially east of a Mount Pocono to Philadelphia to Atlantic City line. Precipitation should diminish across the area from southwest to northeast late this afternoon and this evening as the low pulls away. Regarding the southern Poconos...statistical guidance has rather poorly performed in this area in northwest surface flow this past month, generally being too aggressive in warming daytime temperatures. As such, I was pessimistic with temperatures here today, keeping the higher elevations of Carbon, Monroe, and Sussex (NJ) Counties near or below freezing. With light precipitation continuing this morning and at least showery precipitation possible into the afternoon, needed to make a decision regarding leaving the winter weather advisory as is or extending it through the day. Several reports of 1-2 inches of sleet overnight in these areas combined with at least some ice accumulations -- so travel will be treacherous in this area today given the pessimistic temperature forecast. Combined with additional (albeit light) wintry precipitation expected through the day, felt the need to extend the advisory here through 6 pm. As the column continues to cool this morning, should see some more snow mix in with the sleet/freezing rain in the higher elevations this morning, but this will eventually compete with drying midlevels as the day wears on. Ice crystal growth should generally cease, potentially leaving light freezing drizzle or ice pellets the main show before precipitation shuts down late this afternoon. Accumulations of snow/sleet will likely be an inch or less, with trace amounts of ice in most locations. Lower elevations (below 1000 ft MSL) will likely warm to above freezing. Regarding winds...as the surface gradient begins to relax this morning, speeds have noticeably diminished. Although occasional gusts to 45 mph may occur on the coast this morning, threat is too marginal to continue the wind advisory, so have canceled it with this morning`s package. Still will be breezy across the area today, with gusts 30+ mph likely along the coast and in the higher elevations of the Poconos.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM WEDNESDAY/...
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As the coastal low begins to move into the open Atlantic east of Cape Cod, northwest flow becomes established across the region overnight. The column will dry rapidly as this occurs shutting any remaining precipitation down quickly. Antecedent overcast conditions will likely scour out rapidly after dark. Winds should continue to diminish but will likely be elevated enough to prevent ideal radiational cooling. Temperatures will cool to the mid 20s in the southern Poconos to the mid to upper 30s near the coast.
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&& .LONG TERM /WEDNESDAY THROUGH MONDAY/...
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An updated long term section will be provided by 6 am. Attached is the previous discussion. 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.
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&& .AVIATION /09Z TUESDAY THROUGH SATURDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. MVFR/IFR ceilings and visibilities will continue most of today, combined with occasional light rain/drizzle and fog, especially this morning. Precipitation will likely become more showery this afternoon but remains possible, especially in PA/NJ. However, ceilings/visibilities are expected to gradually improve late today, likely becoming primarily MVFR to potentially VFR. Tonight, VFR is expected across the area. Winds should be north or northwesterly today with speeds 10-20 kts with occasional gusts 20-30 kts possible, with the highest gusts likely at KACY. Winds should diminish somewhat overnight but may still exceed 10 kts at most sites, at least occasionally. OUTLOOK... 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.
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&& .MARINE...
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Gale conditions are still occurring in the coastal waters near Long Beach Island, so continued the gale warning from Sandy Hook to Little Egg Inlet through this morning (with at least small craft advisory conditions expected thereafter). Small craft advisory is in effect for the remaining waters through tonight. Seas remain quite elevated (generally 12-20 feet in the coastal Atlantic), but they should diminish through the day. Rain is likely this morning, becoming more showery this afternoon before diminishing overnight. OUTLOOK... Wednesday...Sub-SCA conditions expected. Thursday through Friday...West wind gusts 25-30 KT possible. Saturday...West wind gusts 20-25 knots possible.
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&& .HYDROLOGY...
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Flood Watch was cancelled this morning as precipitation rates were too light overnight to generate any problems. Although more rain is expected today, amounts will generally be under a half inch and should not cause any problems in urban areas and on area rivers and streams.
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&& .TIDES/COASTAL FLOODING...
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A Coastal Flood Warning remains in effect through this morning along the NJ and DE coasts and along Delaware Bay. Storm surge values of 2 to 3 feet are possible at/around high tide this morning (generally 430 to 6 am). Fortunately, the astronomical tides are low. We should see tide levels this morning a bit lower than those experienced last evening. Strong wave action and the resulting beach erosion will likely compound the effects of any tidal surge. Waves breaking along the shore may be around 4 to 7 feet this morning. Based on the forecast guidance and the expected wind shift to the north then northwest, we are not anticipating any coastal flooding on the upper eastern shore of Maryland or on the tidal Delaware River. The wind shift will result in a decreasing potential for flooding along the coast and along Delaware Bay beyond this morning.
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&& .PHI WATCHES/WARNINGS/ADVISORIES...
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PA...Winter Weather Advisory until 6 PM EST this evening for PAZ054- 055. NJ...Coastal Flood Warning until 11 AM EST this morning for NJZ012>014-020>027. Winter Weather Advisory until 6 PM EST this evening for NJZ001. DE...Coastal Flood Warning until 11 AM EST this morning for DEZ002>004. MD...None. MARINE...Small Craft Advisory until 7 AM EST Wednesday for ANZ431- 452>455. Gale Warning until noon EST today for ANZ450-451. Small Craft Advisory until 1 AM EST Wednesday for ANZ430.
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&& $$ Synopsis...Johnson Near Term...CMS Short Term...CMS Long Term...Drag/Johnson Aviation...CMS/Drag/Gorse/Robertson Marine...CMS/Drag/Gorse/Robertson Hydrology...CMS Tides/Coastal Flooding...Iovino/CMS

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