Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 230139 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 939 PM EDT Sat Oct 22 2016 .SYNOPSIS... Strong low pressure over Maine will move north through eastern Quebec Sunday. A cold frontal passage occurs late Sunday night, then strong high pressure over central Canada gradually builds to our north by Wednesday. A frontal system will approach from the west on Thursday and probably move east of the mid Atlantic coast on Friday. && .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/... A dramatic change in our weather compared to the last several days. This is courtesy of a negatively tilted upper-level trough in the Northeast. This feature will continue to pivot to the north- northeast through tonight, driving deepening surface low pressure into eastern Canada. The light showers and sprinkles associated with the axis of the strong trough aloft will continue to shift eastward for the remainder of the night. Any showers with the axis of deformation to our north are ending and the rest of the night is expected to be dry. Otherwise, cyclonic flow and cold air advection will maintain an area of clouds although this is expected to thin out from south to north. Any clouds that linger through the night should be across the northwestern zones given some contribution by the eastern Great Lakes (i.e. lake effect). The winds will continue to be gusty into this evening especially given a tight pressure gradient, cold air advection and good low-level mixing. As we go through tonight however, the mixing should diminish especially as the thermal advection weakens or even turns into very weak warm air advection toward daybreak. As this occurs, the winds are expected to diminish some with less gustiness. If the winds drop off enough, temperatures will drop more however we are expecting at least some wind to continue through the overnight. As for the low temperatures, we used a MOS/continuity blend however did not undercut these given the expectation that a breeze remains through the night. Temperatures were raised with the updates this evening slightly based on the wind gusts and updated metar observations. && .SHORT TERM /6 AM SUNDAY MORNING THROUGH 6 PM SUNDAY/... The upper-level trough weakens across the area during Sunday as it shifts toward the Canadian Maritimes. Strong surface low pressure however well to our north-northeast combined with high pressure to our southwest will keep a tight pressure gradient across our region. This will result in a gusty west-northwest wind through the day, which should increase quite a bit by mid to late morning as the boundary layer warms. The warming aloft though may temper the true mixing a bit. The gradient does relax toward late afternoon and therefore we are expecting the winds to start diminishing. The airmass overall looks rather dry through the day, however as some warming occurs around 850 MB and with residual cyclonic flow areas of stratocumulus may result especially across the northwestern areas. Given the low-level warming expected to occur, a milder afternoon is expected. High temperatures were mostly a blend of MOS and continuity. && .LONG TERM /SUNDAY NIGHT THROUGH SATURDAY/... 500 MB: Cold closed low in Quebec weakens over the Maritimes Wednesday as a short wave amplifies over the western Great Lakes region. That short wave moves off the mid Atlantic coast Friday followed by ridging Saturday. Temperatures...October through the 21st has averaged around 4 degrees above normal. The chill during midweek will make a significant dent in that above normal departure. Calendar day averages Monday should be around 3 to 5 degrees above normal, cooling to between 5 and 10 degrees below normal Wednesday and Thursday, then warming at or above normal Friday and Saturday. Forecast basis: Unless otherwise noted, a 50 50 blend of the 12z/22 GFS/NAM MOS guidance Sunday night-Monday night, thereafter the 12z/22 GFS MEXMOS Tuesday and then the 15z/22 WPC guidance Tuesday night - Saturday, at times blended with the 12z/22 GFS MEXMOS guidance. The dailies... Sunday night...A secondary surface cold front slides through the region late (after midnight). As mentioned by previous shifts, a tight band of frontogenetic forcing along and just ahead of this front should lead to some showers. However, limited synoptic scale lift means that coverage should be limited across our region, mainly north of I-78, possibly focused along I-80. PWAT does increase to three quarter inch from the 0.3 inches 12 hours earlier. lows several degrees above normal. Confidence: above average. Monday and Tuesday...A gusty northwest flow pattern sets up to start the work week, resulting in cold air advection taking hold Monday night and Tuesday. Gusty northwest winds each afternoon to 25 mph. Diurnal cloud cover, with greatest coverage during each afternoon. Confidence: well above average. Frost or freeze will probably occur early Wednesday morning across much of eastern PA and NJ late Tuesday night as winds decrease- decouple with the approaching ridge of the Canadian centered and originating airmass. The growing season has ended in the Poconos and Warren-Sussex counties of nw NJ. GFS 2m temps, combined with the WPC guidance and 12z/22 GFS mex, suggests that we may be able to post a freeze watch followed by a freeze warning for the I-78 region northward. It appears likely that if this forecast scenario is close to reality that the growing season will likely end for much of the area are along and north of I-78. Wednesday...Sunny and rather cool with a light northerly flow. Confidence: above average. Wednesday night...If its clear through 08z/27, there would be a repeat of frost or freezing conditions, similar to that of 24 hours earlier. However, there is uncertainty regarding skycover, associated with a developing warm front to our west. Confidence: average or just below average with the primary uncertainty being skycover. Thursday...Modestly interesting for late October. Chilly high to the northeast and an approaching frontal system from the west with the associated short wave in the Great Lakes heading east southeast. thickening clouds with a cold rain possible by late in the day. Confidence: average. Rain looks quite likely for Thursday night. This appears to be a possible developing occlusion with low pressure forming on the occlusion by Friday morning along the mid Atlantic coast. Friday...Quite a bit of uncertainty. For now, have forecast the progressive solution...any morning showers should end allowing a rainfree afternoon with a solution of a surface low moving east from the mid Atlantic coast, as per WPC and 00z/22 NAEFS guidance. However, there is uncertainty and so our skycover forecast is a little heavier than what might normally be associated a progressive drying out solution. Saturday...Looks nice with high pressure dominating. && .AVIATION /02Z SUNDAY THROUGH THURSDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Tonight...VFR. West-northwest winds around 15 knots with some lingering gusts around 25 knots till around midnight. Sunday...VFR. West-northwest winds increase to 15 knots with gusts up to 30 knots by mid to late morning, then diminish late in the afternoon. Outlook... Sunday night...VFR cigs developing during the night but there is a small chance of MVFR conditions in showers after midnight, primarily for TAF sites north of KPHL. Confidence: above average. Monday through Wednesday...Mostly VFR conditions expected. Gusty northwest winds 20 to 30 kt both Monday and Tuesday afternoons. Confidence: well above average. Thursday...VFR cigs developing by midday, probably deteriorating to IFR or MVFR conds later Thursday night in showery rains. easterly wind becoming southeast to south late. Confidence: average. && .MARINE... A Gale Warning remains in effect for all zones into Sunday. Wind gusts on both the ocean and in Delaware Bay are still gusting around or just over 35 knots in spots. No changes to headlines with the 9:30 pm update. With the thermal difference between the airmass advecting into the region and the water the gale gusts may linger into the overnight in Delaware Bay. This will be reevaluatedwith the 12:30 am update to see if an extension is needed. Deepening low pressure tracking into eastern Canada will continue to circulate a colder airmass across our area. This combined with a tight pressure gradient and stronger flow is working with efficient low-level mixing. As a result, wind gusts of 35-45 knots are expected although these will diminish from south to north overnight and during Sunday. No changes were made to the ending times and trends will be monitored. A Small Craft Advisory will be needed for a time following the end of the Gale Warning. Given the strong offshore wind, seas will be elevated however the highest will be in lower Delaware Bay and then farther from the Atlantic coast. Outlook... Sunday night...SCA conditions may continue into Sunday night, especially outer waters. Confidence: average Monday and Tuesday...SCA conditions with northwest winds occasionally gusting above 25 kt, though seas may drop off below 5 ft. Confidence: well above average. Wednesday...No marine headlines anticipated. northerly winds gusting under 20 kt expected to turn northeast at night. Confidence: above average. Thursday...An SCA may be needed for the Atlantic waters Thursday night. Confidence: Below average. && .CLIMATE... Have quickly rereviewed the monthly departures. Will focus much more closely in the sunday afternoon AFD. For now...still expecting around a #15 warmest October in PHL/ABE, that said with average confidence. Will probably have a much better handle on the monthly ranking in late Sundays AFD. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Gale Warning until noon EDT Sunday for ANZ452>455. Gale Warning until 6 PM EDT Sunday for ANZ450-451. Gale Warning until 2 AM EDT Sunday for ANZ430-431. && $$ Synopsis...Drag Near Term...Gaines/Gorse Short Term...Gorse Long Term...Drag Aviation...Drag/Gaines/Gorse Marine...Drag/Gaines/Gorse Climate... is the U.S. government's official web portal to all federal, state and local government web resources and services.