Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 141711 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 1211 PM EST Thu Dec 14 2017 .SYNOPSIS... Low pressure will move quickly to the north and east of the area this morning. High pressure will build in from the west for the rest of today through Friday. A low pressure system will move up the Mid Atlantic coast on Friday, remaining well offshore. High pressure then returns for the weekend. A weak low pressure system will move through Sunday night into Monday with another system expected around midweek. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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Midday update...Changes to the hourly temperatures were needed to account for the more pronounced drop in temperatures that occurred behind the cold front late this morning. Expect temperatures to not change a whole lot at current readings through mid afternoon. Previous discussion...Mid morning surface analysis shows an area of low pressure located about 250 miles east of the NJ coast or 100 miles south of Cape Cod. A cold front with this system was moving southeastward across Delmarva and southern NJ. The low pressure system and associated mid-level shortwave trough that brought light snowfall to most of the area last night and early this morning will move farther away from the area today while high pressure builds in from the west. Subsidence in its wake will lead to a sunny afternoon after we deal with a few hours of CAA stratocu late this morning immediately in wake of the fropa. West-northwest winds will become breezy late this morning and afternoon in response to a tightening pressure gradient between the downstream low and upstream high pressure. Hourly temperatures were updated for the 930 AM ESTF to take into account a non-diurnal temperature curve: Temperatures should struggle to rise much during the afternoon when CAA behind the front strengthens enough to predominately offset diurnal heating. Forecast highs are in the low to mid 20s for the Poconos, around freezing in the Lehigh Valley, upper 30s to lower 40s for the I-95 corridor and mid 40s for far southern NJ, southern DE and adjacent eastern MD (with these highs occurring by around midday in many spots).
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM FRIDAY/... High pressure moves into the Mid-Atlantic tonight, and this will cause the winds to subside after dark. Mostly clear skies to start the night will set the stage for a sharp drop in temperatures during the evening, likely exacerbated in areas where remaining snow exists (i.e., especially in the north). Generally went below guidance for lows, though with increasing cloudiness likely to occur late, there is some uncertainty here. The increasing cloud cover will be aided by a 250-mb jet streak encompassing much of the eastern U.S. downstream of another midlevel vort max entering the Great Lakes region. The main question is timing, with faster approach of the clouds likely resulting in warmer lows. Current forecasts are for an increase rapidly after midnight, which should be ample time for temperatures to plummet in the evening. Speaking of lows, they are forecast to range from single digits in the Poconos to around 20 in the Philly metro area. Bundle up. && .LONG TERM /FRIDAY THROUGH WEDNESDAY/... Friday through Friday night...High pressure will make its way into the area overnight and cross the region on Friday. A coastal low develops along the Carolina coast and makes its way up the Mid Atlantic coast. The models continue to show this system remaining offshore and out of our area. However, some moisture may reach into our area and we could see some snow/rain fall towards the afternoon and into the evening. Additionally, a couple of shortwaves will rotate through the area on Friday with a another clipper system staying to the north of the area. Guidance shows the moisture from the clipper drying out before it reaches the region but with a strong northwest flow, we could see some streamers make it from the lakes to our area. Saturday through Sunday morning...High pressure slides across the eastern United States Saturday, bringing some beautiful late fall weather. The weather should remain quiet through at least early Sunday before the next system arrives. Saturday may be a bit windy, especially along the coast. Warmer air filters in from the southwest but we won`t really start to feel the difference until Sunday. Temperatures are expected to be in the 30s to lower 40s on Saturday and in the upper 30s to upper 40s on Sunday. Sunday afternoon through Monday...A weak low pressure system will approach the region sometime around Sunday night into Monday. While the models are not too robust with moisture, it seems more likely underdone and we expect a period of rain or mixed precipitation through the region. However, with differences in timing and strength of the low, confidence remains low. Tuesday through Wednesday...The models diverge with how to handle a strong low pressure system to our north and the attendant cold front. Indications are we will see some precipitation occur sometime in the Tuesday/Wednesday timeframe but confidence is low with respect to coverage and timing. && .AVIATION /17Z THURSDAY THROUGH MONDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Today...SN had ended across the area earlier this morning. With a W-NW winds increasing late this morning, BLSN could lead to localized/brief reductions in visibility to MVFR where fresh snow fell early this morning. This has been occurring so far this morning at the terminals north of PHL. Gusty west or northwest winds will occur from 15Z to 22Z for most of the area (speeds 10-15 kts with gusts 20-25 kts or so). High confidence. Tonight...VFR with northwest winds around or below 10 kts. Increasing mid and high clouds late. High confidence. Outlook... Friday...Mainly VFR conditions expected. MVFR or lower possible in snow showers Friday afternoon and evening. Light winds. Confidence: Moderate Saturday...Mainly VFR conditions expected. West winds around 10 to 15 knots possible. Gusts up to 20 knots possible, mainly at KACY and KMIV. Confidence: Moderate Sunday through Monday...Mainly VFR conditions expected. MVFR conditions in snow/rain showers possible Sunday afternoon through Monday. Southwest winds around 5 to 10 knots. Confidence: Low && .MARINE...
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GLW for ANZ454-455 was allowed to expire at noon EST and then replaced with a SCA for the afternoon and evening. Elsewhere, a SCA is in effect thru the afternoon for the DE Bay waters and thru this evening for the coastal NJ waters. Seas should remain elevated in the 4-7 ft range for the coastal waters this afternoon, but as northwest flow develops this afternoon, there should be a diminishing trend this evening. Outlook... Friday...Sub-advisory conditions are expected. Friday night through Saturday night...Small Craft Advisory conditions possible. West to northwest winds with gusts around 25 to 30 knots. Gale force gusts possible but confidence is low at this time. Seas will increase and near 5 feet on Saturday. Sunday through Monday...Sub-advisory conditions expected.
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&& .PHI WATCHES/WARNINGS/ADVISORIES...
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PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 10 PM EST this evening for ANZ450>455. Small Craft Advisory until 6 PM EST this evening for ANZ430- 431.
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&& $$ Synopsis...Meola Near Term...Klein Short Term...CMS Long Term...Meola Aviation...CMS/Klein/Meola Marine...CMS/Klein/Meola

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