Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 192029 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 329 PM EST Sun Jan 19 2020 .SYNOPSIS... A secondary cold front crosses our region during tonight with a much colder and drier air mass behind it. While low pressure tracks near the Canadian Maritimes Monday, strong high pressure from the Midwest to the Tennessee Valley gradually builds east through Tuesday. The high then settles right over our area Wednesday and Thursday before weakening and shifting to our east during Friday. Low pressure should then arrive from the southwest Saturday before moving away Sunday. && .NEAR TERM /UNTIL 6 AM MONDAY MORNING/... A shortwave trough will be moving through the Northeast tonight, and an attendant cold front will shift quickly southeastward through the CWA this evening. Snow showers downstream of the Great Lakes will continue to impinge upon the Poconos, but enhanced lift along the front may allow snow showers to penetrate farther southward into the Lehigh Valley and adjacent northern New Jersey between 6 pm and midnight. Have maintained a mention of slight chance of snow showers in these areas, with somewhat higher chances in the Poconos, where quick accumulation of a few tenths of an inch may occur in the strongest bands. As the trough moves offshore late tonight, snow showers should quickly wane, with nil PoPs across the area by daybreak. The pressure gradient will remain fairly strong overnight, so expect breezy northwest winds to continue. This should allow wind chills to fall below zero in the Poconos and into the single digits to low teens elsewhere. The winds should keep temperatures from plummeting, but they will still be quite cold. Lows will generally be in the teens to low 20s, though possibly in the single digits in the Poconos. && .SHORT TERM /6 AM MONDAY MORNING THROUGH 6 PM MONDAY/... A broad and quite strong (1044+ mb) surface high will be building into the central Plains on Monday. Meanwhile, the surface low in the Canadian Maritimes will continue to lift away from the region. The pressure gradient will remain sufficiently strong in the Mid- Atlantic (thanks to the intensity of both surface features) to generate another breezy day across the area. However, chances of precipitation should be minimal as large-scale descent encompasses much of the eastern U.S. by this time. With continued cold advection through the day, less favorable orientation of surface flow for downsloping, and overall weaker mixing, highs on Monday should be colder by several degrees. Generally used a continuity/statistical blend for temperatures, as there was excellent agreement amongst the guidance. Max temps may struggle to reach 20 in the Poconos and should barely make freezing southeast of I-95. && .LONG TERM /MONDAY NIGHT THROUGH SUNDAY/... Summary...A cold air mass Tuesday moderates from Wednesday right into the weekend. Dry conditions continue however moisture will be on the return for the weekend as a storm system arrives. Synoptic Overview...An expansive upper-level trough in the East gradually shifts offshore during Tuesday with an upper-level ridge arriving in its place. The ridge will drive strong surface high pressure eastward through Friday with it eventually weakening and shifting to our east. This pattern results in a cold air mass giving way to a warming trend for the second half of the week and including the weekend, however it could get tricky over the weekend with the next storm set to arrive. For Monday night and Tuesday...An upper-level trough will dominate the East during this time frame. At the surface, strong high pressure builds into the Tennessee and Ohio valley`s through Tuesday, however it will extend into our region with time. A north to northwest flow of cold air will be maintained as a result with temperatures below where they normally should be for the daytime highs. Plenty of dry air in place especially in the lower levels, although a northern stream upper-level trough glances our area to the north by later Tuesday. This may allow for at least some mid to high level clouds drifting through for a time. The winds will diminish through this time as the aforementioned surface high builds closer and the pressure gradient relaxes quite a bit. For Wednesday and Thursday...The axis of an upper-level ridge is forecast to slide across our region during Wednesday. This will allow surface high pressure to crest across our region as well, then as the ridge shifts east the surface high will also shift with it and weaken as we go through Thursday and especially Friday. Given the ridge aloft and some gradual light return flow becoming established, some warm air advection will result in temperatures moderating through this time frame. A significant upper-level trough across the Plains Thursday will shift eastward through Friday, with surface low development across the Tennessee to Ohio valley`s. Another high pressure system then becomes situated across eastern Canada. For Friday through Sunday...A significant upper-level trough continues to shift eastward and takes surface low pressure with it. Some ridging across the region ahead of this storm positions high pressure across eastern Canada with some of it extending down into the Mid-Atlantic region. While a milder air mass will be in place, this may change as the low-level flow turns more easterly Friday night in advance of a warm front. A secondary surface low is forecast to develop across the Carolinas to Virginia Friday night as the primary low occludes in the upper Ohio Valley. This will all result in increasing clouds especially later Friday with the chance for some precipitation increasing later Friday night and especially Saturday, some of which could be wintry especially across parts of eastern Pennsylvania to northern New Jersey. These details are much less certain as it will depend on the storm track, timing and thermal/moisture profiles. As of now, the scope of the dry air does not look as impressive compared to leading into yesterday`s (Saturday) event. As of now, the storm is forecast to be moving away as we start Sunday with improving conditions. && .AVIATION /21Z SUNDAY THROUGH FRIDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Tonight...Mainly VFR, though snow showers may make it as far south as ABE this evening (in which brief sub-VFR may occur). Northwest winds 10 to 15 kt with gusts up to 30 kt this evening becoming less frequent and lowering to around 20 kt overnight. High confidence. Monday...VFR with northwest winds 10 to 15 kt with gusts to 25 kt. High confidence. Outlook... Monday night through Wednesday...VFR. Northwesterly winds diminish to 10 knots or less Monday night and continue Tuesday before becoming light and variable into Wednesday. High confidence. Thursday...VFR. Light and variable winds to locally light southwesterly. High confidence. Friday...VFR with increasing and lowering clouds; MVFR/IFR ceilings may develop during the night along with a chance of some precipitation. Light and variable winds, becoming southeast to east around 5 knots. High confidence during the day, then low to moderate confidence at night. && .MARINE... Strong northwest winds continue over the waters this afternoon, and these are expected to continue through tonight before slowly decreasing on Monday. A few gusts may approach gale force through the overnight hours, but conditions appear too infrequent/marginal to warrant an upgrade of the small craft advisory to a gale warning. Winds on Monday appear to remain sufficiently strong for continuation of the advisory through the day. However, as offshore winds continue, seas will gradually subside below 5 feet tonight and remain below criteria on Monday. Outlook... Monday night...Northwest wind gusts to near 25 knots during the early evening will then diminish through the night. It looks on the marginal side, however a Small Craft Advisory may need to be extended into the evening hours mostly for the Atlantic coastal waters. Tuesday through Thursday...The conditions are anticipated to be below Small Craft Advisory criteria as high pressure gradually builds over our region. Friday...The conditions are anticipated to be below Small Craft Advisory criteria during the day, then the winds become southeast and east at night and start to increase. It looks like the winds remain below advisory criteria, however seas may build to around 5 feet across the Atlantic coastal waters Friday night. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 6 PM EST Monday for ANZ430-431- 450>455. && $$ Synopsis...Gorse Near Term...CMS Short Term...CMS Long Term...Gorse Aviation...CMS/Gorse Marine...CMS/Gorse

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