Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 110553 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 1253 AM EST Mon Dec 11 2017 .SYNOPSIS... Weak low pressure will pass by well to our north tonight, followed by high pressure on Monday. A stronger low will also pass by to the north on Tuesday. The associated cold front will usher in very cold air for Tuesday night and Wednesday. Another weak low may cross the area on Thursday or Friday, followed by high pressure and more moderate temperatures heading into next weekend. && .NEAR TERM /UNTIL 6 AM THIS MORNING/...
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12:30 am update: Made a few minor tweaks to winds and sky cover overnight and generally increased temperatures by a degree or so across the area. Otherwise, the forecast is doing fine. Starting to see a couple of light snow observations in the far northwestern CWA, but anything more than flurries or a dusting looks highly unlikely. No changes to PoPs/Wx grids at this time. Previous discussion... Cloudiness will increase tonight ahead of a stout mid-level shortwave and accompanying surface trough. Deeper columnar moisture should be confined from the Lehigh Valley and southern Poconos in PA eastward into northern NJ. There will be sufficient lift for the development of precip, but it`s expected to be spotty and light. A few snow showers have already developed in the Berks and western Chester area, but no accumulation is expected with these showers, only flurries. With the mid and upper levels saturated with respect to ice, and the atmosphere at or below freezing, snow will be the primary precip type. The fast movement of this system gives the aforementioned area a 3-5 hour window for some spotty, light snowfall. Snow ratios are 10-12:1 and only expecting up to several hundredths of an inch of QPF, so a dusting to a half inch is possible, generally from Sussex County NJ southwestward into Berks County PA. Cannot rule out isolated one inch amount amounts, especially the southern Poconos. Given the expectation of a potentially bigger event (see long term section below), the snowfall map on our website remains centered on that event, and does not reflect any accumulation whatsoever for tonight. Tonight`s snowfall expectations are far short of advisory criteria. Therefore it will be highlighted in the HWO, and if need be, handled with a Special Weather Statement (SPS). We held off on issuing a SPS for now, given some uncertainty in the temporal and spatial extent of the event.
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&& .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM TODAY/... Any snow showers are expected to wrap up by sunrise. Gradual clearing will take place from west to east during the morning, with additional stratocumulus cloud development during the afternoon, especially northwest of the fall line. Temperatures will be a few degrees warmer compared to Sunday, but still below average. In addition, the airmass will be more stable, so westerly wind gusts will be capped around 15 mph. && .LONG TERM /TONIGHT THROUGH SUNDAY/...
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The extended forecast period features a continuing large- scale trof over the eastern US, although the trof is forecast to become much flatter toward the end of the week and into next weekend. There will be a number of shortwv trofs rotating more or less rapidly through the main trof, with associated surface lows and frontal systems. Timing of the shortwaves becomes more doubtful at longer ranges. Also, their rapid movement means limited access to moisture and precip production. One of the more potent systems is forecast to approach the mid- Atlantic Monday night and then move through the area on Tuesday. Low pressure is forecast to intensify as it moves past to the north into New England, and its associated very strong cold front will cross the area on Tuesday afternoon. Ahead of the front Monday night into Tuesday morning, a broad area of UVV is forecast to develop, likely leading to some light precip. P-type is an issue, with mostly snow over the Poconos and Lehigh Valley, but a mixed bag or just rain farther S/E. 2 to 3 inches of snow could accumulate in the Poconos. All precip will change to snow by late Tuesday as cold air blasts in. Tuesday night through Wednesday night will be dominated by very gusty NW winds and strong CAA, as the deepening surface low moves slowly NE through New England and into Canada. Wind chills in the teens and single digits are likely. A clipper type system is expected to pass near the mid_atlantic sometime Thu-Fri, although the timing and track are rather uncertain at this time. This system could bring some light snow to the area, as temperatures are forecast to remain quite cold. Current guidance suggests a moderating temperature trend for next weekend with temps perhaps returning to near normal.
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&& .AVIATION /06Z MONDAY THROUGH FRIDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Tonight...Generally VFR with CIGS around/above 5000 feet (sitting around 10 kft currently but a slight lowering is likely before clouds scatter out near/after daybreak). Some flurries are possible, especially at RDG/ABE/TTN, but little in the way of impacts is expected. Winds generally light southwest. High confidence. Monday...VFR with west winds 5 to 15 kts. High confidence. OUTLOOK... Monday night...Sub-VFR conditions possible, especially north of PHL, with a wintry mix of precipitation possible. Winds generally light and predominantly southerly. Low confidence. Tuesday...Sub-VFR conditions possible, especially early in the day, with wintry mix possible generally north of PHL. Winds becoming west and increasing to 15 to 25 kts with higher gusts after cold frontal passage. Confidence medium in general evolution but low on timing. Tuesday night and Wednesday...Generally VFR with very strong west to northwest winds likely. Speeds 15 to 25 kts with gusts to 40 kts possible. Medium confidence. Thursday and Friday...Generally VFR but local MVFR possible in rain or snow showers. West to southwest winds 5 to 15 kts with potentially higher gusts. Medium confidence.
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&& .MARINE...
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Winds remain near or just above advisory criteria on the Atlantic waters, so the small craft advisory was not changed with this update. Gusts are near criteria on Lower Delaware Bay, but given the overall downward trend expected for the rest of tonight, decided not to issue anything here for now. Another advisory will be needed after midnight Monday night/Tuesday morning for all of the waters, but will let the current advisory play out before issuing another one. OUTLOOK... Monday night and Tuesday...Generally sub-SCA conditions Monday night, but advisory conditions likely by Tuesday as southwest winds increase and seas build. After cold frontal passage late on Tuesday, winds will switch to west or northwest and become quite strong. Tuesday night and Wednesday...Gale-force west or northwest winds quite probable. Wednesday night...Residual gale conditions becoming advisory- level conditions during this period. Thursday and Friday...Sub-advisory conditions are forecast at this time.
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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 6 AM EST early this morning for ANZ450>455.
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&& $$ Synopsis...AMC Near Term...CMS/Franck/Johnson Short Term...Franck Long Term...AMC Aviation...AMC/CMS Marine...AMC/CMS

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