Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 172141 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 541 PM EDT Tue Apr 17 2018 .SYNOPSIS... Low pressure over the Canadian Maritimes will gradually pull away from our area tonight and Wednesday. A second area of low pressure will approach from the west and move across the mid- Atlantic region on Thursday. As the low moves away to the northeast on Friday, high pressure will begin to build in from the west, continuing through the weekend and into early next week. && .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
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545 pm update: Hourly temperatures, dew points, and winds were updated through the overnight hours. Current temperatures are too warm across the area, especially northwest of the urban corridor. Dew points were a little too low, as well, and were raised several degrees overnight based on latest hi-res guidance. Finally, the winds are beginning to slacken a little earlier than anticipated, so lowered speeds/gusts for the evening hours by about 5 mph or so. No changes were required to the freeze warning with these updates. Definitely a close call in most spots, but the colder valleys/Pine Barrens in particular tend to be forecast too warm by the guidance in these regimes, so fairly confident the warning will verify in these locations. 415 pm update: Quick update to the grids to add a slight chance of sleet generally northwest of the I-295 corridor through early this evening based on several reports of ice pellets mixing in with the rain/snow showers. Previous discussion... A large broad mid/upper low, and surface low pressure will continue to drift slowly northward across New England the rest of this afternoon through tonight. As they do so, a short wave/vorticity impulse will move across the area later this afternoon into this evening. Meanwhile, an area of enhanced low- mid level lapse rates will move across the area at the same time, and combine with enhanced moisture in the low-mid levels. This has created an enhanced area of, mostly, snow showers to our west, which is expected to continue tracking eastward through the remainder of the afternoon and into this evening. Temperatures are fairly warm across much of the area, so for most places, the precipitation will likely melt and fall as rain, although there could be some snow flurries mixed in at times with the rain across portions of eastern Pennsylvania and northern New Jersey. The higher elevations of the Poconos and northwest New Jersey may actually remain all snow as the precipitation falls. However, with the temperatures being fairly warm and this late in the season, we do not expect much if any accumulation, maybe a light dusting for some higher elevations. This activity should end quickly after sunset as the lapse rates decrease and the short wave passes to the east. Once the showers end, there could be some clearing from south to north during the overnight hours. As this happens, we expect temperatures to cool across the area. Some areas will likely fall below freezing across portions of eastern Pennsylvania, southern New Jersey and northeast Maryland where the growing season has started. Therefore we have issued a Freeze warning for these areas. The rest of the area that is forecast to fall below freezing across other portions of northeast Pennsylvania and northern New Jersey have not started their growing season. There is also some areas across southeast Pennsylvania and central/southern New Jersey and much of the Delmarva where temperatures are forecast to remain above freezing. We did not issue a Frost Advisory as we expect there to be some wind overnight around 5-10 mph, and RH values may not be high enough for widespread frost to develop. Some areas could continue seeing winds gust 15-20 mph at times overnight.
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&& .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH 6 PM WEDNESDAY/... Quiet weather is expected on Wednesday as the low pressure to our north continues to drift farther north away from the area, while very brief and weak high pressure moves across the area. No precipitation is expected during the day, although skies will likely be partly to mostly cloudy. Highs will be warmer than Tuesday, but will still likely be a couple of degrees below normal. Winds will be gusty at times out of the west to southwest, with gusts 15-20 mph. && .LONG TERM /WEDNESDAY NIGHT THROUGH TUESDAY/... For the extended period, a somewhat amplified flow pattern aloft is forecast to continue over the CONUS through this week. A large scale upper trof over the eastern U.S. through the weekend may give way to some ridging early next week. At the surface, low pressure is forecast to move across the northern mid-Atlantic region Wednesday night and Thursday, accompanied by some fairly light precipitation. This will be followed by a large area of high pressure building over the area starting on Friday and continuing through the weekend and into next week. Temperatures through this period are expected to be somewhat below normal, although a moderating trend towards normal is forecast for early next week. After Thursday, little if any precip is expected for the area. On Wed night and Thu a shortwave trof will approach from the west and move through the area. Warm advection and precip will spread east across the area overnight ahead of it and continue on Thu. The best chance for precip looks to be Wed overnight into Thu morning and generally north of PHL where the forecast has likely PoPs. It looks like the lower part of the atmosphere should remain warm enough for all the precip to reach the surface as rain. A warm front will try to move north into our area ahead of the surface low, but the warm sector is expected to affect only Delmarva and south NJ. The surface low and trailing cold front will move offshore by Friday afternoon with lingering but diminishing precip behind it. Total rain amounts should be no more than about 1/3 inch at most. Although cold advection sets in behind the cold fropa, precip should remain rain until perhaps late afternoon in the higher elevation of the Poconos and vcnty. The forecast has a chance or slight chance for snow/rain showers there in the far NW through Thu night and Fri. The rest of the forecast through the weekend into early next week has high pressure building in from the NW and remaining over the NE US. No precip is expected for this time, and the cold air will gradually moderate so below normal temps over the weekend should return to near normal for Mon/Tue. && .AVIATION /22Z TUESDAY THROUGH SUNDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Rest of today...VFR conditions expected. Broken to overcast skies around 4,000-5,000 feet will continue. Showers will move into the area from the west, but are not expected to lead to significant restrictions at this time. If snow ends up being the predominant precipitation type, mainly for ABE/RDG, VSBYS may temporarily lower to 5 or 6 miles. Will not include in the TAF at this time until we get some ground truth upstream, closer to the area since temperatures are still fairly warm. The rest of the area will likely remain all rain. Tonight...VFR with showers early this evening. Clouds may diminish overnight from south to north. Winds diminish to around 10 knots in the evening. Wednesday...VFR conditions expected. Winds expected to increase and gust 15-20 knots during the morning and into the afternoon. Outlook... Wednesday night and Thursday...Periods of MVFR to IFR conditions are likely in rain and fog. Confidence high for at least MVFR. Light winds, becoming W to NW and gusting to 30 kt Thursday afternoon. Thursday night and Friday...Mainly VFR, but with NW winds gusting to 30kt. Confidence moderately high. Friday night through Sunday...VFR conditions expected, with diminishing NW winds. && .MARINE... Small Craft Advisory remain in place across the Atlantic coastal waters and Delaware Bay tonight through Wednesday morning as winds continue to gust 25 to 30 knots at times. Winds will likely drop below 25 knots for a couple of hours late this afternoon through this evening for some areas, but but a second surge of wind is expected overnight and into early Wednesday morning. Wind gusts are then expected to diminish Wednesday afternoon as they shift to the southwest and an inversion develops, plus by this time, the pressure gradient may lower some as well. Outlook... Wednesday night through Thursday morning...No marine headlines are anticipated. Thursday afternoon through Friday...SCA conditions are likely through much of this period. Gales are also possible Thursday night. Friday afternoon through Sunday...No marine headlines are anticipated. && .FIRE WEATHER... Although the relative humidity may drop to around 30 percent or lower on Wednesday and winds could be gusting in the low 20s, fuels are expected to remain moist with the rainfall on Monday and the cloudy conditions Tuesday. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...Freeze Warning from 2 AM to 8 AM EDT Wednesday for PAZ060- 101>104-106. NJ...Freeze Warning from 2 AM to 8 AM EDT Wednesday for NJZ016>022- 027. DE...None. MD...Freeze Warning from 2 AM to 8 AM EDT Wednesday for MDZ008. MARINE...Small Craft Advisory until noon EDT Wednesday for ANZ430-431- 450>455. && $$ Synopsis...AMC Near Term...CMS/Robertson Short Term...Robertson Long Term...AMC Aviation...AMC/Robertson Marine...AMC/Robertson Fire Weather...Robertson

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