Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 220953 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 453 AM EST Wed Nov 22 2017 .SYNOPSIS...
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A cold front will move across the area early this morning while an area of low pressure moves northeastward offshore the Mid Atlantic coast. High pressure will build toward the area tonight, and control the weather Thursday and Friday. A cold front is forecast to move across the area on Saturday while an area of low pressure moves northeastward offshore the Mid Atlantic coast. High pressure builds across the area Monday, then offshore Tuesday.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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500 am update: Two main bands of precipitation are occurring at this time. The first is along and just offshore the New Jersey coast, with quite a bit of training going on (fortunately offshore). Given that this has set up mostly off the coast, the heaviest precipitation should remain there with this main band. Totals in southern Delaware and eastern New Jersey have generally been a quarter to a half inch (locally higher), with is generally about two-thirds of what the consensus hi-res guidance was showing up to this point. Nevertheless, with more upstream precipitation, thinking totals of a half to three- quarters of an inch in these areas is plausible, and the updated QPF is not much different than the previous forecast. The second band of showers is over and near Chesapeake Bay, and this appears to be strongly associated with right-rear quadrant jet dynamics as the 250-mb jet streak lifts into the Northeast this morning. This area of precipitation has had much better success extending northward toward the Mason-Dixon Line. Forward trajectories take this precipitation generally along and southeast of I-95 this morning, which seems to be well sampled by the latest HRRR. The 06Z NAM Nest continues to look very unrealistic with the evolution of the precipitation, and the model has performed pretty badly with the event overall. Did not incorporate it into the updated PoPs/Wx grids. Previous discussion... Rain event is underway across the area, with an area of showers moving through Delmarva northeastward through eastern New Jersey early this morning. A few lightning strikes have already been observed within the stronger convective cores, so the models appear to be simulating the influx of buoyant air reasonably well. Where the models continue to struggle is with the lighter precipitation that may or may not develop on the northwest fringe of the more convective showers. The recent HRRR simulations suggest very little precip may occur west of Wilmington and Philadelphia with a band of light rain developing somewhere northeast of Philadelphia (possibly as far west as the Lehigh Valley, possibly as far east as not in Pennsylvania at all). The NAM Nest provides little usable guidance, as it has basically no clue regarding the convection occurring right now southwest of Atlantic City. With the rapid growth of the precipitation still ongoing given the continuing improvement of the synoptic-scale dynamics with the approaching trough to the west phasing with a northeastward- moving perturbation to the south, still quite difficult to determine just how far west the precipitation will make it. However, based on current trends (both radar and model), general thinking is that I- 295 is going to be a reasonable approximation to the sharp cutoff in QPF (maybe a little to the east of there even). With the precipitation rates seen so far, localized 1"+ totals are plausible southeast of a Dover to Asbury Park line. In fact, hourly accumulations in central/southern Delaware are already nearing this threshold. To the northwest, mere hundredths of an inch may occur in the greater I-95 corridor (though confidence remains pretty low, especially northeast of Philly). With time, the precipitation will make a slow eastward jog through the morning, with any lingering showers exiting the coast around noon. Drying rapidly occurs for the rest of the day, and cold-air advection begins in earnest as winds veer to northwest and become gusty this afternoon. Models may be underdoing cloud cover during the day, as mixing near the surface and residual low-level moisture keep a scattered to broken stratocumulus deck around. Trended the forecast more pessimistic, but this is low confidence. Temperatures will struggle to climb, though they may do so for a time this morning as stronger cold-air advection trails the wind shift. Forecast temperatures are heavily weighted to MET/ECS MOS, as the MAV looked too warm. Safe travels today.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM THURSDAY/... Upstream of the potent vort max moving through the region today, a surface high will build into the Appalachians overnight. With clear skies and a decreasing surface pressure gradient, temperatures will be much colder tonight. Forecast lows are several degrees below seasonal averages, ranging from around 20 in the southern Poconos to around 30 in the urban corridor and at the coast. Northwest winds will be decreasing during the period, possibly becoming light and variable late. The main uncertainty with the forecast is how quickly the winds will die down, with the potential for some temperature error if winds diminish faster than progged (i.e., it would be colder). For now, I used a model blend, as I do not have much of a feel for how quickly the winds decouple tonight. With the trend for the past day being subtly slower with the incoming high (and departing vort max), it may take a bit longer for the winds to diminish than the models suggest. && .LONG TERM /THURSDAY THROUGH TUESDAY/... Fairly quiet weather is expected for most of the extended period through next Tuesday, with a chance of isolated showers this weekend. High pressure is forecast to build across the area Thursday into Friday and bring dry weather to the region. Temperatures are forecast to be near or slightly below normal Thursday, before warming back to near or slightly above normal Friday. The high builds offshore Friday night in advance of an approaching cold front coming through the area Saturday. Also, an area of low pressure is forecast to be moving northeastward offshore of the Mid- Atlantic coast on Saturday. This low is currently expected to remain far enough offshore to not bring major effects to the area Saturday. However, as the front moves through Saturday there is a chance of isolated/scattered showers across the area. For Saturday night into Sunday, the low moves farther to our northeast with high pressure to our west. This will keep a strong northwest flow across the area Saturday night and Sunday. It is possible that some isolated lake effect showers could make their way across our area Saturday night into Sunday in the northwest flow. Temperatures remain above normal Saturday before the front moves through the area, but falls back to near normal or slightly below for Sunday. By Sunday night, high pressure begins to approach from the west and the flow turns more northerly, cutting off the lake effect showers potential across our area. The high pressure fully builds across the area Monday, then pushes offshore by Tuesday. This will bring dry weather back to the area early next week. Temperatures are expected to be near normal or slightly below for Monday, then warm back to above normal Tuesday as return flow develops. && .AVIATION /10Z WEDNESDAY THROUGH SUNDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Through this morning...Generally VFR from KPHL northwestward with a few showers possible, but CIGs likely holding predominantly above MVFR. For MIV/ACY, brief sub-VFR conditions (particularly CIGs) are likely in heavier showers, which will continue through around 15Z with rapid progression offshore thereafter. A SCT-BKN deck around 3500-5000 feet may continue through the morning across the area, with winds veering to northwest and becoming gusty around noon (speeds 10-15 kts with gusts to 25 kts). Confidence low to medium. Frequent updates to the TAFs are likely during this time frame. This afternoon...Northwest winds continue, with a diminishing trend after 21Z. VFR conditions should continue. Confidence high. Tonight...VFR with northwest winds continuing to diminish. Confidence high. OUTLOOK... Thursday-Friday night...VFR expected. Westerly winds Thursday- early Friday. Becoming southwest later Friday into Friday night. High confidence. Saturday-Sunday...Generally VFR. Isolated showers possible which may occasionally lower conditions. Southwest winds early Saturday, becoming northwest and gusting 20-25 knots Saturday night into Sunday. Moderate confidence on showers and sub-VFR conditions, high confidence on winds. && .MARINE... No changes to the small craft advisories this morning as northwest winds will pick up late this morning and exceed criteria easily this afternoon and evening. Seas will likely hover around 5 feet through tonight, as well. Showers, possibly with a couple lightning strikes, are expected through the morning hours, before improvement this afternoon. OUTLOOK... Thursday-Saturday...Sub Small Craft Advisory conditions expected, although seas may be around 4 feet Thursday before subsiding. Saturday night-Sunday...Small Craft Advisory conditions likely behind cold frontal passage. && .EQUIPMENT... The KDOX radar remains out of service. Replacement parts are expected to arrive on Wednesday, November 22nd. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 6 AM EST Thursday for ANZ450>455. Small Craft Advisory from noon today to midnight EST tonight for ANZ430-431. && $$ Synopsis...Robertson Near Term...CMS Short Term...CMS Long Term...Robertson Aviation...CMS/Robertson Marine...CMS/Robertson Equipment...Gorse

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