Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 212118 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 418 PM EST Tue Feb 21 2017 .SYNOPSIS...
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High pressure will remain anchored just off the eastern seaboard through midweek. A cold front approaching from the northwest will likely dissipate as it moves into the region late Thursday. Low pressure organizing over the Southern Great Plains is forecast to strengthen while it tracks northeastward across the Midwest and Great Lakes region late in the week. Meanwhile, a warm front ahead of this low will lift northward through the Mid Atlantic on Friday. This system is expected to bring a strong cold front through the area on Saturday. High pressure builds in from the west late in the weekend before eventually moving offshore early next week.
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&& .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/... An active upper air pattern, which features a trough well north of the Great Lakes through tonight that weakens some. This has surface low pressure with it and a cold front that extends southward. A closed low is gradually moving across the Gulf Coast states and northern Gulf of Mexico. Meanwhile, a narrow but high amplitude ridge axis is centered along the East Coast this afternoon and this will weaken some as it shifts eastward through tonight. Our sensible weather is driven by surface high pressure that will continue to shift eastward away from our coast. This will maintain a southerly flow regime along with some warm air advection aloft. A cold front well to our west is forecast to weaken as it moves east and nearly dissipates toward morning. This weakening is due to the main energy sliding well to our north through tonight, and the thermal gradient washing out. There still remains an area of short wave energy preceding it late tonight, and the associated weak lift may be enough to produce a few showers mainly across the northern areas. Farther south, some sprinkles cannot be ruled out near daybreak. There remains plenty of cloud cover to our southwest this afternoon, although some thinning is noted. While clouds should dominate overall tonight, the thicker cloud cover looks to hold off until after Midnight. This along with some warming aloft and increase in the surface dew points should keep temperatures from dropping as much tonight. The far southern areas may see temperatures creep up some toward daybreak as the low-level flow veers more from the southwest. Low temperatures were mostly a MOS/continuity blend. && .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH 6 PM WEDNESDAY/... The upper-level flow will be more zonal Wednesday in response to energy amplifying across the West, and a strong closed low near the northeastern Gulf of Mexico. This flow regime will drive weak low pressure up across the Midwest and upper Great Lakes region, pulling a developing warm front to our northwest by the end of the day. This setup will feature warm air advection, which will be aided by the low-level flow becoming more southwesterly. The more notable warm air advection is forecast to be from the Ohio Valley on westward. Backing up a bit, a weakening short wave traversing the area in the morning may produce a few showers (mainly north) and sprinkles. Given the warming aloft and not a lot of mixing, cloud cover may hold in through about Midday before some clearing occurs. The amount of cloud cover will have an impact on the afternoon temperatures, however boundary layer warmth may start to win out again and therefore went with some of the warmer guidance (close to continuity). Despite it turning warmer once again, it looks like highs should fall short of records. && .LONG TERM /WEDNESDAY NIGHT THROUGH TUESDAY/...
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High pressure will remain anchored off the Mid-Atlantic coast through the remainder of the week. Broad south to southwesterly return flow around the high result in unseasonably warm temperatures through Saturday. The persistent cold bias that has been noted with the model guidance during this warm spell was taken into account when populating forecast high temperature grids through Saturday. Temperatures will be close to early May climo (60s north/low 70s south) on Thursday and Friday and for mid April climo (upper 50s in the Poconos, 60s elsewhere) on Saturday. We will continue to highlight daily record highs for Thursday and Friday in the climate section below. Forecast confidence for temperatures trends lower on Friday with the potential for a warm front to get hung up across our far northern zones. Also, a fair amount of mid- and high-level moisture advected around the backside of the high signals the possibility that our temperatures may wind up being too warm (by several degrees) in a few spots that see more persistent cloud cover, especially on Friday and Saturday. The region should remain dry Wednesday night and Thursday morning. An isolated shower is possible over inland areas Thursday afternoon with a decaying cold front that arrives from the northwest. Low chance PoPs were advertised for Thursday night with a warm front lifting through the region. A low risk for showers persist into Friday over northeastern PA and northwestern NJ to take into account the possibility of the warm front getting temporarily hung up across our far northern zones on Friday. Precip amounts will be very light. Low pressure over Plains is forecast to deepen as it tracks to the northeast across the Midwest late in the work week. The low will then lift north of the Great Lakes region on Saturday. A strong cold front with this system looks to move through the forecast area on Saturday. Chances for showers ahead of the front increase Friday night into Saturday. PoPs are highest (70 percent) Saturday afternoon and early evening in concert with the fropa. A slight chance (chance in the coastal plain) for thunderstorms continues to be included in the forecast for Saturday afternoon. A much colder and drier airmass will move in behind the front Saturday night-Sunday. Forecast highs on Sunday are 20 degrees colder than Saturday but they will still be near or even a few degrees above normal. Mainly dry weather looks to continue into early next week. However, a weak disturbance may move through Monday, which could result in isolated showers (cold enough for snow showers across the higher terrain in northeastern PA and northwestern NJ. Slightly above normal temperatures are forecast for Monday and Tuesday.
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&& .AVIATION /21Z TUESDAY THROUGH SUNDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Rest of this afternoon...VFR with any ceiling above 15000 feet. Southeast to south winds mostly 10 knots or less, becoming locally southwest. Tonight...VFR overall. Ceilings should lower to 8000-10000 feet, however there is a chance for MVFR ceilings developing toward daybreak from near ABE on northward. Some showers are possible after 08z mainly near and north of a line from ABE to TTN. Southerly winds 5 knots or less, or locally light and variable. Wednesday...Local MVFR ceilings possible to start mainly from ABE northward, otherwise VFR ceilings. A few showers or sprinkles may be around mainly in the morning. Southwest winds 5-10 knots. OUTLOOK... Wednesday night and Thursday early morning...Mainly VFR in the evening. Areas of fog may result in IFR conditions during the overnight and early morning hours. Thursday late morning and afternoon...VFR. SW winds gusting to 20 kt during peak heating. Thursday night and Friday...Mainly VFR but there is a potential for MVFR or IFR late Thursday night with fog/low clouds and light showers. The best chance for flight restrictions due to weather would be at northern terminals (along and north of a warm front). Friday night and Saturday...Increasing chances for showers along with MVFR/IFR conditions during the period. The potential for heavier showers and embedded thunderstorms exist Saturday afternoon ahead of a cold front. Saturday night and Sunday...Becoming VFR from west to east in wake of fropa Saturday evening. Gusty W-NW winds expected as well.
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&& .MARINE...
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The conditions are expected to remain below Small Craft Advisory criteria through Wednesday. High pressure will continue to shift offshore through tonight into Wednesday. This will allow the surface flow to turn from southeast to south and southwest tonight and Wednesday. Some slight enhancement may occur into this evening up Delaware Bay within the southeast flow. OUTLOOK... Wednesday night through Saturday...SW winds 10-20 kt but with very stable thermal profiles and limited mixing weak lapse rates, higher gusts to SCA levels are not expected. Potential for areas of dense fog over the waters with dewpoints increasing well into the 40s and even lower 50s. Tstms possible late Saturday ahead of a cold front Saturday night and Sunday...NW winds increase behind a cold front. High likelihood of a SCA. There is also a chance for gales with gusts possibly exceeding 30 kt. Seas look to build to 4-7 feet in our coastal waters.
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&& .CLIMATE...
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High temperature records for Wednesday-Friday. Although most of these records will probably remain intact, the greatest potential to break a few of these records will be on Thursday. Wednesday 2/22 Thursday 2/23 Friday 2/24 ACY 68-1991 72-1985 75-1985 PHL 68-1997/1974 75-1874 74-1985 ILG 69-1997/1974 72-1985 78-1985 ABE 68-1974 71-1985 76-1985 TTN 66-1997 74-1874 74-1985 GED 72-1997 67-1985/1990 72-1961 RDG 70-1974 72/1932/1922 77-1985 MPO 56-1997 60-1977 60-1984
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&& .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Klein Near Term...Gorse Short Term...Gorse Long Term...Klein Aviation...Gorse/Klein Marine...Gorse/Klein Climate...

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