Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 180853 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 453 AM EDT Tue Apr 18 2017 .SYNOPSIS... High pressure, centered over eastern Canada early this morning, will build east and southward through today before shifting toward the Canadian Maritimes tonight and Wednesday. A low pressure system crossing the lower Great Lakes region on Thursday, will move to southern New England Friday as it trails its cold front through our area. Weak high pressure will spread to our north on Saturday. Another low pressure system in the lower Ohio Valley Sunday morning should move to the mid Atlantic coast by late Monday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... An upper-level trough is forecast to quickly exit the Northeast this morning. Some ridging then quickly follows this afternoon, which drives strong surface high pressure initially centered just north of New England eastward by late in the day. This surface high extends down the East Coast into the Carolinas. The placement of the surface high will result in a low-level onshore flow, initially northeast which then turns southeast this afternoon. While the winds should be light overall especially farther inland, some gustiness should occur through the day closer to the coast as the boundary layer warming allows for increasing thermal differential between the ocean and land. This will keep the coastal communities much cooler and this cooling will spread inland at least some this afternoon. As the aforementioned upper-level trough shifts away from the Northeast this morning, the accompanying upper-level jet will also depart allowing any lingering higher level cloudiness to shift offshore. There should be enough additional drying along with increasing subsidence due to the incoming ridge to limit cloud develop during the rest of the day. The airmass will be noticeably drier today, with the exception closer to the coast due to an ocean influence. High temperatures are a MOS/continuity blend, then some of the high- res guidance was blended into the hourly temperature grids to help capture the influence of the ocean cooling for the eastern zones. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM WEDNESDAY/... A ridge axis quickly shifts to our east during the first half of tonight, and the center of surface high pressure shifts more toward the Canadian Maritimes. The surface high though still extends southwestward into the Mid Atlantic region, allowing for a continued onshore flow. As this all occurs, the mid level flow backs to westerly while the low-level flow becomes more southwesterly just to our west overnight. This allows for warm air advection to strengthen to our west. This will also result in the increase of theta-e advection. This is mainly focused to our south and west around the southwestern periphery of the surface high. Some increase in clouds are expected overnight from west to east, and while much of this should be the mid to high level variety some guidance wants to bring in areas of low clouds toward daybreak. This may be overdone given the additional moistening that needs to take place, however warming aloft beneath a cool boundary layer will result in a strengthening inversion. Much of the area should experience a quick cool down early in the evening as the ocean influence expands inland. Low temperatures were a MOS/continuity blend overall, however the hourly temperatures included some high-res guidance to hopefully better capture the cooling inland during the evening. Thereafter, radiational cooling will occur before an increase in clouds arrive late. && .LONG TERM /WEDNESDAY THROUGH MONDAY/... 500 mb: Above normal heights over the mid Atlantic states Wednesday will give way to a short wave passage across the northeast states Friday. Ridging redevelops over the weekend ahead of a sharp trough moving east from the lower Mississippi Valley. That trough cross the east coast early next week and it may be considerably sharper and a greater impact than the current progressive suite of model solutions. Temperatures: The first 17 days of April have averaged basically 6 to 7 degrees above normal. Calendar day averages Wednesday will be near normal, about 8 to 12 degrees above normal Thursday and Friday, slightly above Saturday. then normal or slightly below Sunday and Monday. Forecast basis: unless otherwise noted, a 50 50 blend of the 00z/18 GFS/NAM MOS Wednesday-Thursday except temps used the warmer GFS, MAV which was a good match for the GGEM; then GFS MEX MOS Thu night and Friday, and WPC 05z guidance Fri-next Monday. The dailies... Wednesday...small chance patchy IFR st/fog to start the day, otherwise a partly sunny start gives way to overcast skies during mid or late afternoon when scattered mid level sprinkles are possible. Southeast wind. Thursday...mild, possible patchy IFR st/fog to start the day. Otherwise all day shower chances along and north of I-80 closer to a stalled front north of our forecast area, while the remainder of our forecast area to the south of I-80 could see a period of showers and thunderstorms late at night ahead of the approaching cold front. Southwest wind. Lows Thursday night 15 degrees above normal. Friday...mild with any remaining showers moving off the coast during midday. Becoming Partly sunny during the afternoon. Southwest wind shift to northwest, gusty to 20 mph. We have checked records for Friday and with near 15C at 850 MB...its possible there will be some upper 80s in s NJ. Our current fcst reflects lagging cloud cover and sct showers ending...but should it clear sooner- during the morning...our 330 am forecast temps for Friday could be too cool by 6 degrees. Saturday...For now we`re banking on high pressure dominating but there is a chance that showers will be streaming eastward from the Ohio Valley. Northwest to north wind. Sunday-Monday...Showery rains and some of the rain could be heavy. Looks like a wintry pressure pattern with an Ohio Valley low redeveloping east of Virginia Monday and cold air damming developing. For now, the system is modeled progressive so the rain should tend to diminish or end later Monday. This may be the beginning for a period of below normal temps that we`ll need to keep the month out of the top 5 warmest Aprils on record. It probably wont be enough to prevent a top 5 to 10 warmest. See climate section at 530 AM for more info on the month as a whole. && .AVIATION /09Z TUESDAY THROUGH SATURDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Through today...VFR with a ceiling at or above 15000 feet dissipating in the morning. Light and variable to northeast winds around 10 knots, shifting to the southeast during the afternoon. Some gusts this afternoon to 20 knots possible mostly at ACY. Tonight...VFR with some increasing clouds mainly above 15000 feet late. There is a chance for some low clouds to develop toward daybreak, however this is low confidence. Southeasterly winds less than 10 knots. OUTLOOK... Wednesday...Patchy IFR st/fog possible to start, otherwise VFR cigs develop midday. MVFR or IFR conds possible at night in showers/st/fog. Southeast wind with max gusts 15-20 kt during the day. Thursday...Possible IFR st/fog to start, otherwise VFR cigs variable brief IFR conditions in st/fog/a period or two of showers and scattered thunderstorms. Southwest wind. Friday...VFR cigs variable brief IFR conditions in st/fog/showers ending during midday becoming VFR sct-bkn aoa 5000 ft during the afternoon as a southwest wind in the morning shift to northwest...gusts 20 kt. Friday night and Saturday...Mainly VFR conditions expected. Northwest to north winds. && .MARINE... Strong high pressure centered just north of the Great Lakes this morning will shift into New England later today, then toward the Canadian Maritimes tonight. This will result in a north to northeast wind today, however shifting to the southeast this afternoon and tonight. The winds will be gusty at times, however peak gusts look to be about 20 knots. Seas will average 2-4 feet (1-3 feet on Delaware Bay), and did raise wavewatch guidance a bit mainly across the southern ocean waters given the initial northeast flow. As the southeast flow occurs this afternoon and evening, some enhancement may occur nearshore and up Delaware Bay due to the initial greater temperature difference between the water and land. Overall, the conditions are expected to be below Small Craft Advisory criteria through tonight. OUTLOOK... 44009 rts around 02z/17. Wednesday...Mainly sub-advisory conditions on the area coastal waters. East to southeast winds around 10 to 15 knots. Gusts may be around 20 knots at times. Thursday and Thursday night...Sub-advisory conditions expected. Seas 2 to 4 feet with south to southwest winds around 10 knots. Fog may develop. Friday...Seas will build on Friday and may be near 5 feet on the ocean. Saturday...Seas may be near 5 feet on the outer edges of the ocean zones early but will be diminishing through the day. && .CLIMATE...
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We have checked records for Friday. Near 15C at 850 MB...its possible there will be some upper 80s in s NJ. Fridays record high for ACY is 84-1985. PHL 89-1976. Depending on wind direction ACY may be within 2F of record. April as a whole for PHL is averaging 6.2F above normal for the first 17 days. Our 330 AM fcst plus the last 3 days of the GFS 2m temps and then normal temps on the 29th and 30th...yields a positive departure of 4.2 degrees (58.2) or 4th warmest April on record. To drop out of a top 10 warmest, this projection would have to be 1.7 degrees too warm. Not likely to fall out of the top 10 warmest Aprils on record.
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&& .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Drag Near Term...Gorse Short Term...Gorse Long Term...Drag Aviation...Drag/Gorse Marine...Drag/Gorse Climate...453 A

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