Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 131133 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 633 AM EST Wed Dec 13 2017 .SYNOPSIS... High pressure will slide across to the south of the region today. A clipper system will cross the region late tonight into Thursday. High pressure will return for Thursday night and Friday. A low pressure system will move up the Mid Atlantic coast on Friday, remaining well offshore. A brief return to high pressure is expected for the weekend. Another system will affect the region early next week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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630 am update: Forecast is in good shape, and no major changes were made at this time. Previous discussion... The winds are howling this morning as strong cold-air advection upstream of the cold front that passed through the area yesterday continues. Boundary-layer mixing and advection have been sufficient enough to generate advisory-level gusts near Delaware Bay throughout the night, and I expect these to continue through the morning hours. There may be a need to extend the advisory a few hours based on forecast soundings (accounting for a low bias in the model-forecast gusts so far), but confidence is rather low since the trend is distinctly downward with the winds during the afternoon. I will be watching wind observations closely the next few hours to determine if the advisory needs an areal expansion as well. A distinct surge in the winds occurred between 10 pm and midnight, which was strongly coincident with the most intense pressure rise tendencies, but I expect another surge near daybreak as diurnal heating begins and the surface pressure gradient increases. A particular concern is the higher terrain in the northern CWA, which the models are highlighting as nearing advisory-level this morning. Winds should begin decreasing during the mid and late afternoon hours, but will probably remain gusty through sunset. Other main question today is the potential for occasional snow showers this morning in the southern Poconos and adjacent far northwest New Jersey. Models have so far overdone the snow showers in this area (with model biases with snow accumulation at least on the order of one inch on average during the past 24 hours). A rather strong midlevel vort max moves southeastward through the northern Mid-Atlantic this morning, so felt the need to retain some PoPs in this area, but snow accumulations should remain below a half inch, if anything occurs at all. Given the winds and cold-air advection (the latter of which will be weakening through the day), today will be uncomfortably cold. I nudged temperatures downward across the area, particularly given the somewhat colder look to statistical MOS output with the 00Z data. In general, I kept temperatures about a degree or two below consensus of ECS/MAV/MET but did throw in hi-res model output for hourly temperatures as well given their much better performance overnight versus the statistical guidance (a typical result in rapid changes to temperature regimes). Wind chills will remain in the single digits in the Poconos (and start below zero this morning), with the rest of the area struggling to reach 20. Clouds may hang around the northern CWA for much of the day, but the rest of the area should see some sunshine. Increasing cloudiness near sunset will be possible as the next system makes a rapid approach to the area.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM THURSDAY/... Clipper low should be entering the Mid-Atlantic region at the start of the period, just downstream of a pivoting midlevel vort max in the Great Lakes region. The surface low will weaken as it approaches the spine of the Appalachians and redevelop offshore during the overnight period, which suggests that the attendant precipitation shield should be weakening as it approaches the CWA and strengthening once it leaves the CWA (sorry, snow lovers). Nevertheless, there will be snow overnight, but the combination of waning dynamics, dry antecedent near-surface air, and orographic shadowing should preclude snow amounts from exceeding a couple inches to the northwest of the Fall Line and should generally be under an inch to the southeast. Little if any snowfall is forecast south of the Mason-Dixon Line. The snow should enter the region late in the evening and be on the decrease by daybreak. Despite the light amounts, the timing (and the subfreezing temperatures) suggests that there will be impacts with the morning commute on Thursday. For messaging purposes, it is important to stress that the lack of a winter weather advisory (owing to light snow amounts) does not equate to a lack of impacts. Some slick roads and treacherous travel can be expected should the snow materialize as currently forecast. Temperatures will not fall much from highs on Wednesday but should drop a little bit during the evening hours, especially before the denser cloud cover invades the region. Temperatures may reach their minima prior to midnight before a slow rise in the warm-air advection preceding the surface low`s trek/redevelopment offshore through the night. && .LONG TERM /THURSDAY THROUGH TUESDAY/... An unsettled pattern continues through the extended period with several waves of low pressure slated to move through the region. Thursday...The clipper system quickly crosses the region early Thursday, along with a strong shortwave. Snow will be falling early and may make for a slick morning commute. The snow will start to taper off through Thursday morning and we should see it end by early afternoon across the forecast area. Winds may be gusty along the coast, especially in the morning, but should subside through the day as the low moves further offshore. Daytime highs will be fairly cold across the area. Highs will range from the lower 20s across the southern Poconos and northwest New Jersey to lower 40s across southern Delaware and extreme southern New Jersey. Thursday night through Friday night...High pressure will make its way into the area overnight and cross the region on Friday. A coastal low develops along the Carolina coast and makes its way up the Mid Atlantic coast. Additionally, a couple of shortwaves will rotate through the area on Friday. The models continue to show this system remaining offshore and out of our area. However, so moisture may reach into our area and we could see some snow fall towards the afternoon and into the evening. Some mixing along the coast will be possible but it looks cold enough that the majority of the area will see only snow fall. Snow should end from west to east Friday night and fairly quickly as the system moves further away from the area and out to sea. Saturday through Sunday morning...High pressure will slide across the eastern United States on Saturday, bringing us a nice day of quiet weather. The weather should remain quiet through at least Sunday morning. Warmer air will start to make its way towards the region this weekend but we won`t start to feel the difference until Sunday. Temperatures are expected to be in the 30s to lower 40s on Saturday and in the upper 30s to upper 40s on Sunday. Sunday afternoon through Tuesday...Another system will approach the region sometime around Sunday night or Monday. The models diverge with how to handle another clipper system to our north Sunday and a developing low to our south. For now, will continue to mention the chance for precipitation starting Sunday afternoon through Monday. Warm air will move into the region in the southwesterly flow and we may see a period of rain or mixed precipitation through the region. Temperatures look to remain cold over the northwestern zones and likely to remain all snow there. Plenty of uncertainty exists bringing lower confidence through this period. && .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. Wednesday...VFR with 260-310 degree wind directions. Speeds 12 to 25 kts with gusts frequently at or above 30 kts, especially in the morning. Very high confidence. Wednesday night...Sub-VFR CIGs and especially VSBYs should be expected with snow likely to most of the terminals through portions of the night (general timing between 02Z and 10Z, with longer durations likely north of PHL). Runway accumulations may occur, but should be around an inch or less. Winds will begin from the southwest around 10 kts but should become light and variable from late evening onward. Medium confidence in CIGs/VSBYs (with low confidence in occurrence/timing of snow); high confidence in winds. Outlook... Thursday...MVFR or lower conditions possible in snow through Thursday afternoon. Snow should clear out by 18Z and then mainly VFR conditions expected. West to northwest winds around 10 to 15 knots. Confidence: Moderate Friday...Mainly VFR conditions expected. MVFR or lower possible in snow showers Friday afternoon and evening. Light winds. Confidence: Low to Moderate Saturday...Mainly VFR conditions expected. West to southwest winds around 10 to 15 knots possible with higher gusts, mainly at KACY and KMIV. Confidence: Low to Moderate Sunday...Mainly VFR conditions expected. MVFR conditions in snow/rain showers possible Sunday afternoon. Southwest winds around 5 to 10 knots. Confidence: Low
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&& .MARINE...
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630 am update: Gales continue across all marine zones at this time, with the peak in the gusts likely between now and noon. Latest model forecasts a brief (around 6 hours) lull in winds (still exceeding advisory thresholds) before increasing to near or just above gale force off the Delaware coast again (albeit from southwest winds rather than west/northwest), while areas farther north likely stay at advisory levels. Seas are running about a foot above model forecasts, so made some adjustments to these through the late evening hours to account for this. Previous discussion... Gale warning continues for all of the waters today, though I did end the gale warning at 6 pm for the Delaware Atlantic coastal waters given faster forecast timing of winds decreasing late this afternoon. Gales are in full swing currently, with gusts in Lower Delaware Bay not far from storm warning criteria. These gusts (forecast to remain just shy of storm warning thresholds) should peak this morning and decrease rather quickly this afternoon. At least small craft advisory conditions are expected tonight as winds become more southwesterly in advance of the next system. Gusts may reach gale-force again in the Delaware coastal waters after midnight, but confidence is rather low at this time. Some snow may develop, generally after midnight and primarily for the New Jersey coastal waters, with visibility restrictions likely should the snow materialize. Outlook... Thursday ...Small Craft Advisory conditions continue through Thursday. Both winds and seas are expected to drop through the day, falling below SCA criteria by Thursday evening. Thursday night through Friday...Sub-advisory conditions are expected. Friday night through Saturday...Small Craft Advisory conditions possible. West to northwest winds will gusts around 25 knots possible. Seas will increase and near 5 feet. Sunday...Sub-advisory conditions expected. Winds may near 25 knots and seas may near 5 feet late Sunday.
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&& .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...Wind Advisory until 10 AM EST this morning for NJZ021>025. DE...Wind Advisory until 10 AM EST this morning for DEZ002>004. MD...None. MARINE...Gale Warning until midnight EST tonight for ANZ450>453. Gale Warning until 6 PM EST this evening for ANZ430-431-454- 455. && $$ Synopsis...Meola Near Term...CMS Short Term...CMS Long Term...Meola Aviation...CMS/Meola Marine...CMS/Meola

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