Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 182330 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 730 PM EDT Wed Apr 18 2018 .SYNOPSIS... A area of low pressure will move across the area tonight into Thursday, along with its associated occluded, warm, and cold fronts. This low will drift into the Canadian Maritimes through Friday, which will keep northwest flow across the area with a couple of dry surface troughs moving across the east coast. High pressure will build into the Mid Atlantic and Northeast Saturday through Sunday, before moving offshore Monday into Tuesday. A coastal low may move north along the coast and affect the area during the middle of next week. && .NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...
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700 pm update: Main changes to the forecast were to modify PoPs tonight based on latest hi-res guidance, which generally indicate a period of light rain/sprinkles/showers from 03Z to 10Z generally north of the I-276/I-195 corridors. The HRRR suggests a briefer period of rain but potentially with heavier rainfall rates, whereas the WRF-ARW/NMM and NAM Nest suggest a longer duration of lighter precipitation. My thinking is that the briefer duration of the precipitation is more reasonable (especially given upstream radar trends), though I am not quite convinced of the wetter solutions yet (despite the rather potent look to the vort max, along with the close proximity of the surface low). Increased PoPs slightly to the north of the I-276/I-195 corridors, but reduced them to the south (and remaining unmentionable south of an Easton, MD, to Wildwood, NJ, line). Other main change was to include some chances for (light) snow to mix in with the rain late tonight in the Poconos (and far northwest New Jersey), where temperatures will become cold enough (and profiles will cool sufficiently) for the white stuff to show up. Will update storm total snow grids accordingly, though amounts are expected to be very light (and generally only at the highest elevations). Previous discussion... Surface low pressure will approach the region from the west tonight, moving into our area late tonight. Cloud cover will increase quickly tonight as the low moves closer to the area. Some light rain will fall, in advance of the approaching low but amounts are expected to remain light, with southern areas likely remaining dry through daybreak. Overnight lows will drop into the mid to upper 30s across the northern zones to mid to upper 40s across the southern zones.
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&& .SHORT TERM /6 AM THURSDAY MORNING THROUGH 6 PM THURSDAY/...
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700 pm update: As the surface low moves offshore tomorrow morning, should see a brief break in precipitation before instability showers/sprinkles move in during the late morning and afternoon as the main trough/vort max move(s) into the region. Precipitation amounts should be very light (and will likely be a mix of rain and snow in the Poconos), but would not be surprised to see some snowflakes mix in with the showers farther southeast. Trended forecast in that direction a little bit, but not too much given the expected temperatures/low-level thermal profiles. Tweaked PoPs to match the timing of the instability showers, as depicted by the NAM Nest, WRF-ARW/NMM, and HREF. Also slowed the onset of the stronger winds by a couple of hours, but did not change the magnitudes that much given the model-derived mixing profiles. Previous discussion... Low pressure will cross the region tomorrow, dragging a cold front through during the late morning/early afternoon. Some light rain will fall across the region as the low crosses the area. Amounts will be fairly light, especially across southern areas, with only a tenth of an inch or less expected. As temperatures cool across the higher terrain, a few flurries or light snow showers may occur as the precipitation starts to come to an end toward Thursday evening. Extensive cloud cover across the northern half of the forecast area will keep much cooler and only into the upper 30s to mid to upper 40s. Once you get down towards Philadelphia and points south, skies should see a few breaks. Highs into the upper 50s to lower 60s are expected. Even with the sun peeking out, it will feel cooler through the day as the winds will really start to ramp up by late morning/early afternoon. Winds will be our of the north to northwest at around 15 to 25 mph with gusts up to 35 mph possible.
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&& .LONG TERM /THURSDAY NIGHT THROUGH WEDNESDAY/... A quiet start to the extended forecast for the end of the week, through this weekend, into early next week. A return of unsettled weather is possible by the middle to end of next week. On Thursday night, the low pressure that affects the area during the day will continue to move northward into the Canadian Maritimes. Most of any precipitation associated with the low will move to our north with the low pressure. However, there could continue to be some snow showers across the higher elevations, especially during the evening hours as the low-mid level lapse rates may be high enough to interact with some left over moisture and the passing short wave. These showers will dissipate through the night into Friday morning as the moisture dissipates across the area. On Friday, the low will remain across the Canadian Maritimes, while surface trough will likely cross the area. The trough will be dry, and only present a shift in wind direction and possibly some cloud cover, as it will likely not having much of an airmass change. Winds during the day will be generally out of the northwest, and gust 25- 30 mph, occasionally up to 35 mph at times. Dry and breezy conditions will continue into Friday night as well. On Saturday, dry conditions will continue as another dry surface trough will move across the area, before high pressure builds into the area overnight. Winds will gust around 20 mph at times, but it will not be as windy as Friday. High pressure builds across the Mid Atlantic and northeast Sunday, before moving offshore my Monday. Dry conditions will continue for Sunday and Monday. As we move into Tuesday and Wednesday of next week, a coastal low is forecast to lift northward along the east coast. There is timing differences between the GFS, ECMWF, and Canadian. With the timing differences, there is the potential for rainfall starting as early as Tuesday, with the most likely period looking to be Wednesday Wednesday night. Temperatures across the area are forecast to be above freezing, so the precipitation would be all rain. && .AVIATION /23Z WEDNESDAY THROUGH MONDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Tonight...VFR to start with, but CIGs are expected to develop rapidly and descend after 03Z. By 09Z, may see RDG/ABE flirt with MVFR criteria. Light rain or showers are forecast near/north of RDG/ABE/TTN, but may have a difficult time reaching PNE/PHL/ILG and are not expected to affect MIV/ACY. Low VFR to MVFR expected at most sites by daybreak. Winds will become light/variable quickly this evening. Moderate confidence. Thursday...Low VFR to MVFR conditions are anticipated at most sites, especially during the morning, as drizzle/patchy fog may occur near RDG/ABE/TTN and light showers subsequently move through the Philly terminals northward and westward through mid afternoon. CIGs will likely rise a little bit during the afternoon as winds become westerly/northwesterly and gusty by late morning and continuing during the afternoon (10-20 kts with gusts 25-30+ kts). Moderate confidence. OUTLOOK... Thursday night...MVFR conditions possible early, before improving to VFR overnight. Gusty northwest winds 20-30 knots. Friday-Monday...VFR through the period. Gusty northwest winds 20-30 knots Friday; 20-25 knots Friday night; and 15-20 knots Saturday.
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&& .MARINE... A Gale Watch is in effect for all of our waters from 2PM Thursday through 8AM Friday. Tonight...Quiet conditions on the area waters with southerly winds around 10 to 15 knots and seas around 2 to 4 feet. Thursday...Winds will pick up out of the northwest on Thursday and become particularly gusty during the afternoon and evening. Soundings show a strong, deep layer of 35+ knots over the area waters and mixing should be fairly efficient by the afternoon. Sustained winds of 15 to 25 knots with gusts up to 35 knots. Seas will also increase to around 4 to 5 feet by Thursday afternoon. OUTLOOK... Thursday night...Gale Watch in effect for all the waters as winds could gust up to 35 knots through the night. Friday-Friday night...Small Craft Advisory conditions likely. Saturday-Monday...Sub Small Craft Advisory conditions exected. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Gale Watch from Thursday afternoon through Friday morning for ANZ430-431-450>455. && $$ Synopsis...Robertson Near Term...CMS/Meola Short Term...CMS/Meola Long Term...Robertson Aviation...CMS/Robertson/Meola Marine...Robertson/Meola

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