Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 221151 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 651 AM EST Wed Nov 22 2017 .SYNOPSIS... 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. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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At 630 am, a band of showers was moving through the urban corridor, extending southward through Delmarva. This will be the main show for the metropolitan area, with much higher rainfall totals farther to the southeast, where showers have occurred for much of the night. Current PoPs/Wx grids look good and made no changes with this update. Stronger convective cores exist within the larger band of showers, so keeping mention of slight chance of thunder remains a good idea. Several lightning strikes have occurred overnight, especially off the coast, but also for areas of southern Maryland/Delaware and eastern New Jersey. A few sites in Delmarva and southeast New Jersey will exceed 0.75 inches, with some localized 1"+ totals already recorded. Most hi-res guidance ended up doing reasonably well with the event, though generally biased westward with the QPE maximum. Some light showers may occur in the southern Poconos and vicinity this morning in association with the upstream vort max moving through the Northeast today. Temperatures are generally too warm for snow, but some of the colder spots in Carbon/Monroe/Sussex (NJ) Counties may see a few flakes before all is said and done. Impacts will be negligible given the very light/very brief duration of the precip. Latest hi-res guidance keeps showers lingering along the coast through late morning, so clouds may stick around longer in these locations. Otherwise, drying/some clearing should be expected farther to the west, with northwest winds becoming gusty this afternoon as a cold front moves through the area. Temperatures will remain fairly steady/mild this morning as cold-air advection lags the front but should trend downward (slowly) this afternoon. Made no changes to temperature forecast with this update. Still a little uncertain about cloud cover later this morning and this afternoon. The model forecasts look overly optimistic, but given the fairly strong agreement, I am hesitant to deviate in the negative direction. Still, a longer lasting scattered/broken stratocumulus deck would not surprise me, especially if winds/mixing are stronger than forecast (with the residual moisture from this morning`s rain contributing to generation/maintenance of the clouds underneath midlevel drying/descent). 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 /12Z WEDNESDAY THROUGH SUNDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Today...From KPHL northwestward, a few showers possible through 14Z, with brief sub-VFR CIGs/VSBYs with the heavier precip. For MIV/ACY, more prolonged sub-VFR conditions are likely in heavier showers through the morning. Thereafter, A SCT-BKN deck around 3500-5000 feet may continue through 18Z or so across the area. Light southwest winds will veer quickly to northwest and become gusty this morning (speeds 10-15 kts with gusts to 25 kts during the afternoon). Confidence low to medium this morning; high this afternoon. Frequent updates to the TAFs are likely during from 12Z to 15Z as precipitation moves through. Tonight...VFR with northwest winds diminishing. 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.
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&& .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...

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