Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 102104 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 404 PM EST Sun Dec 10 2017 .SYNOPSIS...
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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.
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&& .NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
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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. 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 MONDAY MORNING THROUGH 6 PM MONDAY/...
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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.
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&& .LONG TERM /MONDAY NIGHT THROUGH SUNDAY/...
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The extended forecast period features a continuing large- scale trof over the eastern UW, 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 /21Z SUNDAY THROUGH FRIDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Tonight...Potential for a brief period of MVFR at ABE and TTN due to lower ceilings, especially in any snow showers, late tonight. Elsewhere, VFR. Winds west-southwest AOB 10 kts. Confidence: average. Monday...VFR. Southwest winds 10G15 kts. Confidence: above average. 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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SCA continues through 5PM this evening for the Delaware Bay and 6AM Monday for the NJ Waters. We also extended the SCA for the DE Waters until 6AM Monday. While seas have fallen to AOB 5FT, winds gusts to near 25 kts are expected through the advisory period at the afore- mentioned areas. Both winds and seas are expected to subside on Monday. 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 Monday for ANZ450>455. Small Craft Advisory until 5 PM EST this afternoon for ANZ430- 431.
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&& $$ Synopsis...AMC Near Term...Franck Short Term...Franck Long Term...AMC Aviation...AMC/Franck Marine...AMC/Franck

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