Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 182344 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 644 PM EST Wed Jan 18 2017 .SYNOPSIS... Low pressure over the western Atlantic waters will continue to drift out to sea tonight. Meanwhile, high pressure over the Midwest will build east and then move offshore at the end of the work week. Weak low pressure will impact the region this weekend, and then a stronger and more complex area of low pressure will impact the Mid- Atlantic and Northeast early in the new week. && .NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...
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Water vapor imagery shows potent vort max moving east of the Carolina Coast this evening. Descent on the upstream side has finally shut the precipitation down completely but has also reinforced a low-level inversion, with nearly saturated conditions below the inversion. Persistent broken to overcast skies continue across the area this evening, which have slowed the nocturnal cooling process. Needed to adjust temperature and dew point grids to account for this. So far...not confident enough to increase minimum temperatures, as there is still potential for late night clearing, at least in the southern CWA. However, latest model guidance is just a tad slower with this clearing, and given an overall model bias of scouring near-surface moisture too quickly, my suspicion is that lows may be slightly higher than forecast overall. The key will be timing an upstream vort max currently over southwest Quebec. As it progresses southeast overnight, upstream descent (exacerbated by transient higher-amplitude ridging across the Midwest and Ohio Valley) will likely dry out the lower levels. Current timing appears to be near or after daybreak based on the latest model guidance. Winds will be light and either variable or with a predominant north or northwest direction overnight. Given the relatively calm conditions and ambient moisture in place, patchy fog remains a possibility. In favorably cold locations, this could be in the form of freezing fog, and will continue to monitor this potential through the night.
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&& .SHORT TERM /6 AM THURSDAY MORNING THROUGH 6 PM THURSDAY/... Patchy fog that does develop could linger into mid-morning, given maintenance of the low-level inversion with a building ridge aloft, and subsequent high pressure at the surface and light winds. The deeper moisture will continue moving north of the area, with some lingering clouds north of I-78 in the morning. Otherwise, a mostly sunny day develops, with some scattered clouds below the low-level inversion. Dry weather is expected, with no support for precipitation. As for high temperatures, generally 10 to 15 degrees above normal, from the low 40s in the Poconos to mid 50s in Delmarva. Northwest winds 5 to 10 mph. && .LONG TERM /THURSDAY NIGHT THROUGH WEDNESDAY/... A prolonged period of warm and wet weather is on tap for the Long Term period. Broad area of high pressure along the Eastern Seaboard will move offshore Thursday night as weakening low pressure approaches from the west. Weak secondary low will form over the Mid-Atlantic, and the primary low will dissipate as is tracks from the Great Lakes into eastern Canada. This will send a quick shot of precip into the Mid-Atlantic and Northeast, but not expecting much more than 0.25" QPF or so. Best chances will be from midday Friday through Friday evening. Conditions dry out after midnight Friday night. One concern is timing of onset of precip for the Poconos and areas to the north of I-80. If precip develops before noon on Friday, surface temps may not yet have risen above freezing, and there could be a brief initial period of rain/freezing rain. Otherwise, with onshore flow, temperatures will be warm enough for precip to be all rain. Weak high pressure then builds across the region Friday night and Saturday. Patchy fog may develop Friday night and Saturday morning, as with weak flow, there will not be much of a chance for the moisture to scour out. Conditions will otherwise be dry and mild on Saturday with highs in the upper 40s to mid 50s. High pressure moves offshore Saturday night. Meanwhile, an upper trough will move into the West Coast sometime early in the weekend and will become a closed upper low over the Southern Plains and Gulf Coast states during the day Sunday. As this system organizes and develops, it will send waves of precip out ahead of it and into much of the East Coast. The first wave will develop over the TN Valley and lift towards the area on Sunday. Both the GFS and the ECMWF bring measurable precip into the region by Sunday afternoon, and then the bulk of the rain approaches Sunday night and Monday as the main system lifts into the Mid-Atlantic region. An area of concern: high pressure will nose its way into eastern Canada and northern New England. Depending on the placement of the high, winds could take on more of a NE/E flow as opposed to an E/SE flow. This could then usher a shallow area of cold air into mainly northern zones, and would result in a period of wintry weather sometime Sunday night and Monday morning. Latest track, based on both the GFS/ECMWF, keeps temperatures above freezing, so will keep precip as all rain throughout the CWA. There may be a brief break in the precip late Monday morning, but the main low will lift through the region Monday afternoon and evening, bringing another round of potentially moderate to locally heavy rainfall. With deep onshore flow, PWATs will build to around 1.5". Pressure gradient tightens up, and with strong low level jet passing through, can expect E winds to increase to 15-25 MPH with 25- 35 MPH gusts, mainly on Monday. Rainfall ends fairly quickly from SW to NE Monday night through Tuesday morning. Behind that departing system, a deep area of low pressure will be entrenched over the northern Great Lakes region, and several shortwaves will rotate around the low and into the local area. This may touch off some showers during the mid-week period. During this time, Arctic air remains bottled up to the north. Temperatures will average above normal, with highs nearly 10 degrees above normal. && .AVIATION /00Z THURSDAY THROUGH MONDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Latest guidance is considerably more pessimistic with ceilings overnight. TAFs were modified to lower ceilings generally to MVFR for the overnight hours. Visibilities should predominantly be VFR but there is a chance of some patchy fog, particularly at KMIV and KABE. Winds should be generally light and variable. Confidence is below average given the tendency for ceilings to remain right around the 3000-foot level. Thursday...Predominantly VFR conditions. There may be some SCT030 ceilings developing around noon. Northwest winds 5 to 10 knots. OUTLOOK... Thursday night...VFR. LGT/VRB winds less than 5 KT. High confidence. Friday through Friday night...Sub-VFR conditions possible in -RA. SE winds 5-10 KT on Friday become SW less than 5 KT Friday night. Moderate confidence. Fog possible Friday night. Saturday...Improving conditions Saturday morning. VFR. West winds 5 KT or less. Saturday night...LGT/VRB winds. Mainly VFR, but cannot rule out passing shower. Sunday...E winds 10-15 KT. IFR conditions possible in RA. Sunday night through Monday...IFR and lower conditions possible in RA. E winds 15-20 KT with 20-30 KT gusts. LLWS possible with 60-65 KT LLJ Monday afternoon.
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&& .MARINE...
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Advisory conditions are no longer occurring across Delaware Bay and the Atlantic coastal waters. The Small Craft Advisory was cancelled with this update. Expecting decreasing northwest winds and lowering seas through Thursday as high pressure traverses the area. OUTLOOK... Thursday night through Sunday morning...A weak pressure gradient will result in sub-SCA conditions during this time. VSBY restrictions possible in rain on Friday, and in fog Friday night and Saturday morning. Sunday afternoon through Monday night...Easterly flow increases to 15-20 KT with 25-30 KT gusts starting Sunday afternoon, and then gale force gusts of 35-40 KT likely Monday and Monday night. Moderate to heavy rain with low VSBY expected. Conditions improve late Monday night, and winds decrease to sub-SCA levels. However, seas on the ocean should remain above SCA criteria. Tuesday...Ocean seas remain above SCA criteria.
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&& .PHI WATCHES/WARNINGS/ADVISORIES...
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PA...None. NJ...None. DE...None. MD...None. MARINE...None.
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&& $$ Synopsis...MPS Near Term...CMS Short Term...Franck Long Term...MPS Aviation...CMS/Franck/MPS Marine...CMS/Franck/MPS

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