Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 211057 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 557 AM EST Wed Nov 21 2018 .SYNOPSIS... A strong cold front will move through the region this afternoon and evening. Cold high pressure will follow for Thanksgiving and Friday. A surface low will move near or south of the region on Saturday and Saturday night, followed quickly by another one on Monday. High pressure may nose into the region by the middle of next week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... Temperatures this morning are bottoming out in the 20 in the more rural areas of the forecast area, and otherwise in the low 30s. Temperatures will warm up going through the morning after sunrise. Otherwise, cold front currently moving through the Great Lakes and upper Midwest will track to the south and east today. Strong high pressure over central Canada will build to the south and east and will be over the upper Great Lakes by this evening. That cold front works its way south through the region later this afternoon. The passage of that front will be accompanied with a 90-100 kt jet streak along a 40-45 kt low level jet. In addition, some mid-level shortwave energy will pass through the region later this afternoon, but the strongest shortwaves will stay to the north and east of the local forecast area. However, there should be enough instability to touch off some snow showers. Coverage should be limited to mainly the Poconos and northern NJ, and if instability is stronger than models are suggesting, there could even be a few isolated snow squalls in Carbon and Monroe counties. For now, will cap PoPs at slight chance for the Lehigh Valley, southern Poconos, and northern NJ, and low end chance at northern potions of Carbon and Monroe counties. Not expecting much in the way of widespread snowfall, generally less than 1/4", but parts of the southern Poconos could get 1/2-1" or so, possibly higher in those isolated snow squalls. Behind the passage of the cold front, west winds increase to 15-20 mph with 20-30 mph gusts. Highs today will top off in the 30s in the southern Poconos and northern NJ, and otherwise in the low to mid 40s for most of NJ and southeast PA. Southern NJ and southern Delmarva will get into the mid and upper 40s. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM THURSDAY/... Cold front works its way south through the region tonight. The bulk of the upper level energy will be north and east of the region, so any isolated snow showers will move into the Delaware Valley and up to the I-95 corridor in NJ to Philly. Although the bulk of the models keep the region dry, but the WRF-ARW forecasts reflectivity down through central NJ and southeast PA. Will only carry slight chance PoPs. Again, not expecting much in the way of snow accumulation, perhaps a trace or so. But this will arrive around the evening commute. Temps begin to fall off behind the passage of the cold front, but the true Arctic air will arrive after midnight with the passage of a secondary cold front. Winds diminish to 10-15 mph with gusts up to 20 mph in the evening, then winds shift to the northwest and diminish to 5-10 mph after midnight. Lows tonight drop into the single digits in the Poconos, and otherwise in the teens and low 20s for most of the region, and into the mid 20s in southern Delmarva. Wind chills will be in the single digits/teens for most of NJ and Delmarva, and as low as -10 in the southern Poconos. && .LONG TERM /THURSDAY THROUGH TUESDAY/... The medium-range forecast features a very cold end to the week, a couple of lows affecting the region this weekend into early next week (accompanied by a warmup), and a colder/drier finish. Thursday and Friday will be mostly sunny but very cold, with some record low maximum and minimum temperatures likely Thursday and Thursday night. Winds will remain breezy on Thanksgiving Day, so the wind chills will be low (likely near zero in the Poconos; mainly in the single digits and lower teens north of I-78 and teens to the south). Winds will diminish Thursday night, but this will only allow temps to crash. Forecast highs are 20 to 30 degrees below seasonal averages, with lows around 15 to 25 degrees below seasonal averages. Friday should be warmer as the surface high associated with the arctic cold begins to move offshore, establishing some semblance of return flow. However, max temps will still likely be around or below freezing north of I-78 and just above freezing to the south. Midlevel ridging moves through the region Friday night and Saturday, and this will allow for a substantial warmup (with temps on Friday night nearly steady and back near seasonal averages for highs Saturday). However, a surface low in the Southeast will develop and move off the East Coast by Saturday afternoon, with warm-advection precipitation breaking out in much of the eastern U.S. 00z operational runs remain in fairly large disagreement, with the ECMWF remaining much farther north with the track of the low than the GFS, with the CMC splitting the difference to some degree. Think a consensus approach is best, with the ECMWF likely too weak with the polar blocking downstream and the GFS likely exhibiting its progressive/south bias with these systems. Temperatures will be warming on Saturday, with models continuing to suggest the precipitation holds off until most/all of the CWA is above freezing. Cannot rule out a brief period of messy/wintry precipitation at the onset (Saturday morning), but this is looking less and less likely as the event approaches. Confidence is not high enough to remove this potential from the forecast, but this is increasingly looking like a low-probability scenario. How much rain falls is dependent on the proximity of the low`s track, with the ECMWF/CMC suggesting a rather decent 1-2+ inch QPF event for most of the area. The GFS is considerably lower with amounts. Recent 1-2 inch rainfall events have led to a few flooding issues, so if the heavier QPF materializes, some localized flooding should be anticipated. The first system moves out by Sunday morning, with a brief lull Sunday afternoon. However, a strong surface low to the west will lift northeastward to the Great Lakes by Sunday evening and slow/stall as it approaches the blocky ridge to the northeast. This leads to triple-point low development downstream near the Mid- Atlantic/New England coast on Monday. The ECMWF remains rather progressive, though the GFS/CMC have trended faster as well. This may very well lead to a rather quick-hitting second (mostly) rain event on Monday, but the blockiness to the north has me worried that the effects could last longer than model consensus is suggesting. It may also turn windy as the coastal manifestation of the surface low moves northeastward away from the area. Will need to watch this system closely, as the pattern does not need to change too much for this to be a fairly impactful system. For now, temperatures look rather warm for most of the event, so aside from some snow showers on the cold side of the system as the northwest flow sets in, rain is primarily forecast. Trended PoPs downward and more showery on Tuesday as the low lifts northeastward into New England and beyond. Temperatures should be colder as the eastern North America trough reloads through midweek. && .AVIATION /12Z WEDNESDAY THROUGH SUNDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Today...VFR. BKN CIGs develop this afternoon at 8000-10000 feet. A few snow showers with brief sub-VFR conditions are possible at KRDG/KABE from 21Z- 00Z, but confidence is too low to include in the TAF. West winds 5-10 kt, becoming NW 15-20 kt with 20-25 kt gusts this afternoon. Tonight...A few isolated snow showers may move through KRDG/KABE/KTTN/KPNE/KPHL from 00-03Z this evening with brief sub-VFR conditions with the passage of the cold front, but confidence is too low to include in the TAF at this time. NW winds 15-20 kt with 25 kt gusts this evening veer closer to the north at 10-15 kt with gusts up to 20 kt after midnight tonight. OUTLOOK... Thursday: VFR with northwest winds 10 to 15 kts with gusts to 25+ kts. High confidence. Thursday night and Friday: VFR with light winds. High confidence. Friday night: Increasing cloudiness, with sub-VFR conditions possible, especially late. Light easterly winds. Moderate confidence. Saturday and Saturday night: Periods of sub-VFR likely with a good chance of rain. Primarily east winds 5 to 15 kts, with potentially higher gusts near the coast. Winds possibly switching to a more northerly direction Saturday night. Moderate confidence. Sunday: May see a period of improvement to VFR with north winds 5 to 15 kts, possibly with a few higher gusts. Moderate confidence. && .MARINE... Will go ahead and carry a Small Craft Advisory for all waters for today and tonight. There will be a brief lull in SCA conditions this morning, but then winds ramp back up in the afternoon. A few gale gusts are possible on the ocean waters late this afternoon and this evening, but at this time, it does not look as if gale gusts will be widespread enough or last long enough to warrant issuing a Gale Warning. NW winds 15-20 kt with 25-30 kt gusts will continue through tonight. Seas on the ocean will build to 3 to 5 feet tonight. A few snow showers are possible on northern NJ ocean waters late this afternoon and this evening. Light freezing spray is possible late tonight on the northern ocean waters. OUTLOOK... Thursday: Small craft advisory in effect. Gales cannot be ruled out, especially in the morning. Light freezing spray possible. Thursday night: Lingering advisory-level west-northwest winds diminishing to sub-advisory levels with time. Friday and Friday night: Sub-advisory winds/seas expected. Saturday: East to southeast winds will likely increase to advisory levels, with seas approaching/exceeding 5 feet. Rain likely. Sunday: Winds switch to northwest and may hover near advisory levels. && .CLIMATE... There is the potential for near record low maximum AND low minimum temperatures for the Thanksgiving holiday. Here are the record coldest high and low temperatures across our climate sites for Thanksgiving day, accounting for the change in date. (Note, this is not records for November 22 itself): Thanksgiving record low minimum: Atlantic City, NJ: 10 (1989) Allentown, PA: 16 (2000) Georgetown, DE: 20 (1996) Mount Pocono, PA: 1 (1903) Philadelphia, PA: 20 (1901) Reading, PA: 15 (1903) Trenton, NJ: 20 (1930) Wilmington , DE: 18 (1996) Thanksgiving record low maximum: Atlantic City, NJ: 31 (1996) Allentown, PA: 27 (1996) Georgetown, DE: 27 (1989) Mount Pocono, PA: 20 (1901) Philadelphia, PA: 27 (1901) Reading, PA: 30 (1989) Trenton, NJ: 30 (1930) Wilmington , DE: 29 (1996) Here is a list of the record low temperatures for November 22 and November 23: Site November 22 November 23 Allentown 15/2014 16/2000, 1964 Atlantic City 16/2014, 1964 10/1989 Georgetown 16/1987, 1969 16/1984, 1964 Mount Pocono 5/1969 6/2000, 1964 Philadelphia 14/1980 10/1880 Reading 16/1969 16/1972 Trenton 15/1880 16/1880 Wilmington 17/1964 17/1964 && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 6 PM EST Thursday for ANZ430-431- 450>455. && $$ Synopsis...CMS Near Term...MPS Short Term...MPS Long Term...CMS Aviation...CMS/MPS Marine...CMS/MPS Climate... is the U.S. government's official web portal to all federal, state and local government web resources and services.