Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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656 FXUS61 KPHI 280147 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 947 PM EDT Mon Mar 27 2017 .SYNOPSIS... A warm front will move northward into Delmarva and southern New Jersey, stalling as it pushes into southeastern Pennsylvania this evening. Low pressure will track through the Ohio Valley tonight and cross our region Tuesday evening. High pressure will build down from Canada for Wednesday and Thursday. Another low pressure system will move through our area on Friday and Saturday. Weak high pressure returns for Sunday and Monday. && .NEAR TERM /THROUGH TUESDAY/... A frontal boundary will stall across central/southern Delmarva and southern New Jersey overnight tonight. This will keep a light easterly flow across the northern third of the area overnight tonight. An area of low pressure will drift through the Ohio River Valley and toward western Pennsylvania by daybreak. Meanwhile, southwest flow aloft will develop across the area as a mid-level trough approaches from the west as well. Several short wave/vorticity impulses will move across the area overnight tonight, which is expected to create scattered showers overnight and through daybreak. It is possible that a couple of rounds could develop. One around midnight, then another alter in the overnight and through daybreak. There is some elevated instability, so it is possible that some isolated thunder may occur overnight. With this easterly flow, areas of fog will likely develop across much of the region. This has already started in some areas, with parts of the northern coast of New Jersey developing dense fog. We`ve issued a Dense Fog Advisory for Monmouth and Ocean counties where the dense fog has already started. The rest of the area is a little tricky. This initial dense fog was likely due to clear skies and the ability to radiate quickly. As we go through the next few hours, some mid-high level clouds are expected to move into the area. In addition, scattered showers will are also expected to move across the area through the night as well. These both could help keep fog from becoming widespread dense across the area. However, we do expect areas of at least light fog to develop, some which could be locally dense at times. So it is possible that the Dense Fog Advisory may need to be expanded westward, but confidence on how widespread it will be is low at this time. && .SHORT TERM / TUESDAY THROUGH 6 AM TUESDAY/... Surface low in the Ohio Valley at 12Z Tuesday moves east along or just south of the Mason-Dixon Line into Maryland/Virginia by 00Z Wednesday as the shortwave trough begins a curve toward the southeast on the outer fringe of a subtropical ridge in the Gulf of Mexico. Degree of cooling in the midlevels combined with strong ascent downstream of the vort max will promote the development of storms in much of the Mid-Atlantic on Tuesday. Most model guidance suggests two rounds may occur. The first would occur early in the day as elevated convection along the nose of enhanced low-level isentropic ascent. After a brief lull in the late morning and early afternoon, more convection is expected to develop along/east of the low and southward- extending cold front, aided by substantial cooling in the midlevels and large-scale ascent via differential cyclonic vorticity advection. Vertical shear looks relatively weak, and MUCAPE values will generally remain at or below 1000 J/kg, especially if early-day convection eradicates preexisting instability. Prospects for severe convection look limited at best, but scattered storms are still a good bet as the rounds of precipitation move through the region, particularly in the afternoon. Best chances appear to be along and south of the Mason-Dixon Line given the track of the low and the associated lifting mechanisms. However, overall uncertainty remains given the multiple rounds of precipitation that may occur. Therefore, a general broadbrush of chance to likely PoPs was placed in the grids for now. There is better potential for locally heavy rainfall given the decent PWATs, multiple rounds of precipitation expected, and potential for localized training storms, especially if storms can align along the baroclinic zone. Though widespread flooding is unlikely, think isolated instances of nuisance short-term flooding is a possibility. This will continue to be monitored in later forecasts. Temperature forecast remains a concern with the placement of the eastward-extending warm front again a complicating factor. Stuck close to continuity and the cooler side of the guidance for now, but large errors in these values are likely given the poor handling of the frontal boundaries thus far and the increased complications from precipitation. && .LONG TERM /TUESDAY THROUGH MONDAY/... Tuesday night will continue to have some showers as the low pressure system makes its way offshore and to the east of our forecast area. Showers will decrease in coverage from west to east and should completely clear the area by late Tuesday night. As far as convection goes for Tuesday night, instability looks extremely limited and overall the best instability looks to be mainly to the south of our area. Might be a rumble of thunder or two but confidence is pretty low that we see much convection through the night. Canadian high pressure will start to push southward into our area on Wednesday and persist across the region through Thursday. While the system itself isn`t particularly strong, it should allow for us to dry out as a decent northwest flow is across the area on Wednesday. Winds will lighten up for Thursday with cooler air settling over the Mid-Atlantic. Highs will be slightly above normal on Wednesday and much closer to normal on Thursday. The next system starts to advance towards our area on Thursday. Low pressure will develop to our west and then move into the Tennessee Valley by late Thursday. AS the low continues to move towards our area, we will start to see cloud cover increase and showers will once again start to move into the area. Expect some light rain to develop in the vicinity of the approaching warm front The center of the low will cross just to the south of the region on Saturday, by around mid morning. The rain will end from west to east and we should see a little bit of clearing occur. However, there is not much of a flow behind this system and it may end up remaining pretty cloudy, especially for areas inland. Weak high pressure will again be around the region for Sunday and Monday. Models indicate that a shortwave may move through later on Sunday, which may trigger a few light showers. Confidence is low at this time. && .AVIATION /02Z TUESDAY THROUGH SATURDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Conditions improved to VFR everywhere late this afternoon into this evening, and will continue for another couple of hours. However, conditions will lower through this evening and into the overnight hours. MVFR conditions will develop after 02z, then IFR conditions will develop after 04z. It is possible that dense fog may develop overnight, but confidence is still low in dense fog for now, so we`ll keep the lowest VSBYS at 1sm for now. We expect a couple of rounds of possible showers through tonight into Tuesday. The first round is expected to develop by 06Z and move eastward through daybreak. Second round of precipitation likely after daybreak Tuesday into midday. Lightning strikes are possible, but low confidence on timing/occurrence at this time precludes their presence in TAFs. Third round of precipitation likely tomorrow afternoon/evening. Storms are more likely with this round. Winds should be generally light and variable. Conditions may improve to MVFR briefly during the day Tuesday, but likely lower again by later afternoon/evening. Winds overnight will be light, generally 5 knots or less. Any direction is expected to be vary from northeast to southeast. OUTLOOK... Tuesday night...MVFR/IFR conditions possible in showers and fog... becoming VFR late. Light winds will become north to northwest overnight around 10 knots or less. Wednesday...VFR conditions expected. Winds will be north around 10 to 15 knots, some gusts up to 25 knots possible. Thursday...VFR conditions expected. Light north winds... becoming east late afternoon. Friday...IFR or MVFR conds in periods of rain. East to southeast winds around 10 to 15 knots. Saturday...MVFR/IFR conditions early in rain... becoming VFR late. North to northeast winds around 10 knots or less. && .MARINE... Dense Fog Advisory has been issued for the New Jersey coastal waters north of Little Egg. Areas south may also develop fog overnight, but it is less certain at this time. This will be monitored closely this evening. Winds/seas will be under advisory criteria through Tuesday. Chances of showers/storms late tonight through Tuesday, likely occurring in several rounds. OUTLOOK... Tuesday night...Southerly winds will become west then northwest overnight. Speeds around 10 to 15 knots. Seas 2 to 4 feet. Wednesday...Northerly winds around 10 to 20 knots with gusts around 25 knots late. Seas increasing to around 5 feet . A SCA will likely be needed as conditions near criteria by Wednesday evening. Thursday...Decreasing seas through the day. North winds becoming easterly late in the day, mainly around 10 to 15 knots. Friday...East to southeast winds around 10 to 20 knots with gusts up to 30 knots possible. Seas will build through the afternoon, exceeding 5 feet by Friday evening. A SCA looks likely. Saturday...SCA conditions continue. Northerly winds around 10 to 15 knots. Seas will remain above 5 feet through Saturday, starting to subside a bit late. && .CLIMATE... PHL temperatures continue to project near 1 degree F below normal for the month as a whole. && .EQUIPMENT... KNEL appears to be reading 10F too warm the past couple of days. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...Dense Fog Advisory until 10 AM EDT Tuesday for NJZ013-014-020- 026. DE...None. MD...None. MARINE...Dense Fog Advisory until 10 AM EDT Tuesday for ANZ450-451. && $$ Synopsis...Meola Near Term...CMS/Robertson Short Term...CMS Long Term...Meola Aviation...CMS/Robertson/Meola Marine...CMS/Meola Climate... Equipment...

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