Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 131341 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 841 AM EST Sat Jan 13 2018 .SYNOPSIS...
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The cold front has moved offshore this morning and its associated low pressure will continue to lift to our northeast today. The low is forecast to reach Canada`s Maritime Provinces around midday. Arctic high pressure is anticipated to build into our region from the west for Sunday and Monday. Weak low pressure is forecast to approach from the Great Lakes on Monday night and Tuesday. Low pressure off the coast may affect our weather during the middle of the new week. High pressure is expected to follow for Thursday and Friday.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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The cold front has moved offshore and cold gusty northwest winds have set in across the area. We`ve issued a Wind Advisory for the higher elevations of Carbon and Monroe, Pennsylvania and Sussex, New Jersey through 1 pm this afternoon as winds have been gusting 45-50 mph there and will likely continue over the next few hours. Even though the front has moved offshore, scattered showers are developing behind the front as another short wave/vorticity impulse is moving across the area this morning. Areas across eastern New Jersey and central/southern Delaware and Maryland are still above freezing, but will be dropping soon. So any precipitation here will start out as light rain or drizzle, before changing over to snow or sleet/snow grains. For the remainder of the area, the cold air continues to push in and they are below freezing. So any precipitation will be frozen; so snow or sleet/snow grains are expected. Any precipitation is expected to be light, so little accumulation is expected. Therefore no advisory has been issued. However, with the cold temperatures, any remaining water on roads, walkways, parking lots, and other untreated surfaces. So we will keep the SPS in effect through the afternoon.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SUNDAY/... High pressure across the Midwest states will continue to push towards the area tonight. We will still be in the push of colder air drier air being advected across the area. Skies will be clear for the most part. A few clouds across the far N/W are possible. Temperatures will fall well below normal, with single digits N/W and teens elsewhere. Wind chill values will range from 0 to -10 up across the highest elevations N/W to the single digits across Delmarva and metro Philadelphia. && .LONG TERM /SUNDAY THROUGH FRIDAY/... Sunday through Monday...As the low pressure continues to move away, we will see high pressure building into the region. The pressure gradient will remain fairly tight through at least midday Sunday before starting to relax. This will help to keep the winds up across the region. With a northerly flow, cold air will filter into the area and it will feel markedly colder than recent days. High temperatures will remain cold through Sunday and Monday. Although some moderating of the temps on Monday is expected as the high shifts to the north of our area and we see more of return flow develop across our area. Highs on Sunday will remain in the upper teens across the north to the mid to upper 20s across the rest of the area. Monday is expected to be around 5 degrees warmer than Sunday. Conditions through this period are expected to remain dry. Monday Night Through Friday... Overview... A southern stream shortwave skirts by to our south off the Carolinas Monday night. A trough across the eastern US amplifies and cuts off Wednesday into Thursday, with large model disagreement on where this occurs. Meanwhile, high amplitude ridge over western North America gradually weakens along with a lack of high latitude blocking over the Davis Strait. These features, along with additional shortwave energy feeding into the northeast US, suggest the cut off low will gradually fill and lift northeast of the Middle Atlantic by Fri. For sensible weather, an Alberta Clipper gives two opportunities for precipitation Monday night into Thursday. Mon night through Tue night...With the approach of the Alberta Clipper low, the first chance for light wintry precipitation is possible. The ECMWF keeps a surface ridge to the east in place longer compared to the GFS. Thus, the GFS begins precip as early as Mon night while the ECMWF holds off until Tue. The GEFS mean is more conservative compared to the GFS, with precip beginning across the southern Poconos and Lehigh Valley later Tue night. We continue to confine precip to the aforementioned areas for Tue night, becoming more widespread across the remainder of our region into Wed. As for precipitation type, a consensus of the guidance maintains 2-meter temperatures below freezing during this period. Model soundings indicate temperatures below freezing through the column, in addition to the mid and upper levels being saturated with respect to ice. While soundings indicate a veering low-level wind profile, suggestive of warm air advection, soundings do not indicate much in the way of a warm nose. Therefore, expect predominantly snow, with little if any mixed precip at this point. Given modeled QPF is relatively light, do not expect more than a dusting to a couple inches of snowfall potential, with the highest amounts focused northwest of the fall-line at this time. With sub-freezing temperatures, this would make roadways slick. Temperatures near seasonal levels. Wed thru Fri...Alberta Clipper moving through the lower Great Lakes with eventual redevelopment further east, but the tough question is exactly: where? The latest suite of 12Z/12 operational models continue to diverge, with little run to run consistency. The GFS develops low pressure off the NC coast Wed night and moves it northeast along the northern wall of the Gulf Stream, well south of the 40N/70W benchmark, into Thursday. The UKMET is similar. These scenarios would have little impact on our region, with the storm system remaining well offshore. Meanwhile, the ECMWF is much further west, developing low pressure east of the Virginia Capes Wed night, then northward through Cape Cod on Thu. It`s expansive precip shield on the western side of the system (a bit of a model bias at times) is also suspect. Nevertheless, this solution has greater impacts on our region in terms of wintry weather. The respective ensembles are similar, the GEFS further east and the EPS further west. But there is a lot of spread noted in the mid- level height fields of both the GEFS and EPS regarding the placement of the cutoff low, which translates into uncertainty with the location of the attendant surface system, e.g., storm track. WPC favors a blend of the GEFS and EPS ensembles, which places emphasis on more extensive redevelopment further north and east of our region, which would limit impacts. This scenario would favor a return to generally fair weather Thursday into Friday, and we continue that thinking with this forecast, with caveat that confidence is on the lower side given the uncertainty. Therefore, impactful weather is still possible, and we advise paying attention to future forecasts. Otherwise, it will be unseasonably cold Wednesday into Friday, with temperatures 5 to 10 degrees below normal. && .AVIATION /14Z SATURDAY THROUGH WEDNESDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Today...NW winds gusting up to 30-35 knots early, then 25-30 knots through the afternoon. MVFR conditions will improve to VFR through the morning and into the afternoon. Sct rain/snow showers possible which will temporarily reduce visibilities and ceilings. Tonight...VFR. Mostly SKC. Mostly NW winds around 10 knots. Outlook... Sunday...VFR conditions expected. Northwest winds around 10 to 15 knots, becoming more northerly overnight. Confidence: High. Monday...Mainly VFR conditions expected. Light north to northeast winds. Increasing mid-high clouds late. Confidence: Moderate. Monday night...Periods of light snow and IFR conditions are possible late, mainly at ABE and RDG. Light northwest winds. Tuesday through Wednesday...Periods of light snow and IFR conditions are possible across all the terminals, with the greatest potential in terms of intensity and duration at ABE and RDG. Northwest winds 5 to 10 knots. Overall, lower confidence prevails.
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&& .MARINE...
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A Gale Warning has been issued for all waters today. Winds on land are gusting 35-40 knots and are expected to transfer to the waters through this morning and early afternoon. Winds are expected to diminish through the afternoon, so we may be able to drop the Gale Warning back to a Small Craft Advisory this afternoon. Showers this morning, then fair weather. Outlook... Sunday...VFR conditions expected. Northwest winds around 10 to 15 knots, becoming more northerly overnight. Confidence: High. Monday...Mainly VFR conditions expected. Light north to northeast winds. Increasing mid-high clouds late. Confidence: Moderate. Monday night...Periods of light snow and IFR conditions are possible late, mainly at ABE and RDG. Light northwest winds. Tuesday through Wednesday...Periods of light snow and IFR conditions are possible across all the terminals, with the greatest potential in terms of intensity and duration at ABE and RDG. Northwest winds 5 to 10 knots. Overall, lower confidence prevails.
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&& .PHI WATCHES/WARNINGS/ADVISORIES...
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PA...Wind Advisory until 1 PM EST this afternoon for PAZ054-055. NJ...Wind Advisory until 1 PM EST this afternoon for NJZ001. DE...None. MD...None. MARINE...Small Craft Advisory from 6 PM this evening to 6 PM EST Sunday for ANZ450>455. Gale Warning until 6 PM EST this evening for ANZ430-431- 450>455.
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&& $$ Synopsis...Iovino Near Term...Robertson/O`Hara Short Term...O`Hara Long Term...Meola Aviation...Robertson/Meola/O`Hara Marine...Robertson/Meola/O`Hara

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