Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 212016 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 316 PM EST Tue Nov 21 2017 .SYNOPSIS... A cold frontal boundary will move east through our region tonight and during the day on Wednesday. High pressure will then build eastward into the region for Thursday and Friday before giving way to an approaching cold front on Saturday. Another area of high pressure will build into the region for the early part of next week. && .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/... A deep upper trough, along with a sharp cold front out ahead of an Arctic airmass, will continue to track eastward through the Great Lakes and Ohio Valley tonight. Meanwhile, weak low pressure over the eastern Gulf of Mexico will lift to the north and east and move off the Mid-Atlantic coast tonight. Several shortwaves will move into the Northeast and Mid-Atlantic region out ahead of the approaching trough, and mesoscale banding of precip will develop mainly across DE and southeast NJ. Models also picking up on a new low forming just off the Eastern Seaboard sometime late tonight and into daybreak Wednesday. There are still differences among the global and short range models. The first bands of precip will move into the Delmarva and southern NJ just after midnight tonight, and then the bulk of the precip moves in shortly thereafter. The big question is where this banding sets up. Latest HRRR showing bulk of precip holding off until after daybreak Wednesday, and even much farther west than most models seem to indicate. HiRes-ARW keeps the bulk of the precip across southeastern zones, generally to the east of I-95. GFS, NAM, and ECMWF bring the precip a bit closer to I- 95 in southern NJ and southeast PA, but not as far west as the HRRR. Another question will be the timing of onset of precip. It is currently fairly dry with surface dewpoints in the upper 20s to low 30s, so it will take some time to moisten up the column to support rainfall. Temps will remain well above freezing where it is expected to rain, so no ptype issues expected. Based on these trends, will go ahead and bring the swath of likely PoPs to and just west of the I-95 corridor and bring swath of measurable QPF a bit to the west as well. For the most part, heaviest precip should be on the order of 1/4" to 1/3" QPF. && .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH 6 PM WEDNESDAY/... Rain continues, mainly along and south and east of I-95 corridor, Wednesday morning as low pressure lifts out to sea. Rainfall should wind down during the late morning hours, but with rapid intensification of that low offshore, possible for some wraparound precip to make it back to the NJ coast around lunchtime. Not much additional QPF expected, but would not be surprised if there was some additional couple of hours longer than forecast. Meanwhile, cold front works its way through the region from late morning through the afternoon. Front should be through the Fall Line by noon, and then will clear the coast by mid-afternoon. With tight NW pressure gradient developing and onset of CAA, winds will increase to 15-20 MPH with gusts up to 30 MPH from midday on. Temperatures will be turning cooler than today, especially in the Poconos, but the coldest air will hold off on arriving until Wednesday night. Highs top off in the low to mid 50s, except in the Poconos, where highs will be in the 40s. && .LONG TERM /WEDNESDAY NIGHT THROUGH TUESDAY/... Wednesday night through Friday night: High pressure will be in place during this period with mostly sunny skies. Temperatures will slowly warm as winds go from northwesterly ahead of the high pressure to southwesterly on the backside of the high pressure. Highs will generally be in the 40`s with some spots in the 50`s on Friday. Overnight lows will drop down into the 20`s and 30`s. Model guidance may be a bit warm in the Pine Barrens and in the Delaware Water Gap given a favorable radiational cooling setup. Saturday and Saturday night: Some limited transport of moisture is possible ahead of an approaching cold front. Some lift along the front may result in a few showers. Another feature to watch is an area of low pressure expected to track from near Florida northeastward to just offshore of the Carolinas. Some ensemble members do bring a period of rain northward into the region. However, the general consenus is for the cold front to continue to push the low pressure further out to sea. With warm air advection ahead of the front, temperatures have the potential to be warmer than the ensemble average which has been coming up the past few runs. Highs could reach 60 in many locations Saturday before turning colder Sunday behind the front. Northwest winds at night should keep lows warmer and generally in the 30`s. Sunday through Tuesday: A fairly strong push of cold air advection will occur behind the front. This will lead to another period of more winter like temperatures across the region with stronger northwest winds, gusting perhaps to 25 mph on Sunday. Some moisture associated with a pocket of mid-level vorticity on the backside of a departing trough may spark a few rain or snow showers across most of the region Sunday and Sunday night. Higher terrain could act as another lifting agent as well for the moisture. This outcome however depends on how progressive the trough is moving through the area. Highs Sunday and Monday will generally be in the 40`s and warming into the 50`s on Tuesday. Overnight lows will be in the 20`s and 30`s. && .AVIATION /20Z TUESDAY THROUGH SUNDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Tonight...SW winds 10-15 KT with gusts up to 20 KT rapidly diminish to less than 10 KT this evening, and then become LGT/VRB late tonight. VFR through most of tonight. BKN-OVC deck moves in from south to north mainly after 06Z. MVFR conditions in -RA/BR expected for terminals to the south and east of I-95, mainly at the KMIV and KACY TAF sites, and there may also be a period of IFR conditions late tonight as well. There may be a brief period of MVFR conditions in -RA extending back towards the I-95 terminals of KTTN-KPNE-KPHL- KILG after 07Z. Will use PROB30 for those sites given uncertainty. Wednesday...Rain continues in the morning and gradually tapers off through 12-15Z. Conditions lift to VFR. Winds become NW 5-10 KT after 12Z, and then increase to 15-20 KT with gusts up to 25 KT after 16Z. Outlook... Wednesday night through Friday night: VFR, northwesterly winds becoming westerly by Thursday but at or under 10 knots. High confidence. Saturday and Saturday night: Sub-VFR possible with some showers. Southerly wind gusts 10-15 knots. Medium confidence. Sunday and Sunday night: VFR, winds shifting to northwesterly with gusts from 20-25 knots in the afternoon. Medium confidence. && .MARINE... Will cancel the SCA for DE Bay as wind gusts have fallen below 25 KT. Wind gusts should fall below 25 KT on DE Bay this evening, and then SCA ends on DE ocean waters later this evening as pressure gradient relaxes. Conditions on NJ ocean waters should remain above SCA criteria for most of tonight. Rain moves in from the south late tonight and will continue through Wednesday morning before moving out to sea. A cold front moves across the waters Wednesday afternoon, and then NW winds increase to 15-20 KT with 25-30 KT gusts. Went ahead and issued a SCA for DE Bay as current SCA was cancelled, but a new SCA for the ocean waters will be issued for at least Wednesday afternoon when current SCA expires. Outlook... Wednesday night through Saturday night: Sub-SCA seas and winds from Thursday onward. West winds shifting to southerly Friday into Saturday with gusts of around 15 knots at times. Any westerly wind gusts near 25 knots will diminish quickly Wednesday evening but seas may linger near five feet through the entire night. Sunday and Sunday night: Winds shifting to northwest with 25-30 knot gusts possible at times. Seas building but look to stay under five feet at this time. && .PHI WATCHES/WARNINGS/ADVISORIES...
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PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 4 AM EST Wednesday for ANZ450>453. Small Craft Advisory until 10 PM EST this evening for ANZ454- 455. Small Craft Advisory from noon to 6 PM EST Wednesday for ANZ430-431.
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