Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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088 FXUS61 KPHI 280027 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 827 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/... Warm front has plagued the forecast today, with a shortwave trough moving through the St. Lawrence Valley and an attendant surface low weakening in southeastern Ontario unable to force the boundary northward through the CWA today. Instead, the front has reached a line from Belmar to Philadelphia, with cool/cloudy conditions to the north and mostly cloudy and warm conditions to the south. As usual, models are struggling to realize the temperature gradient across the boundary, with temps generally about 3-8 degrees too warm to the north of the boundary and up to 5 degrees to cold to the south. Expect the temperature uncertainty to continue into the evening as the evolution of the front remains one of the primary forecast concerns overnight. Forecast lows are generally mid 40s north to mid/upper 50s south, with much below average confidence. Cyclogenesis in the Mid-South is working to counter any progress the warm front can make this afternoon/evening, as pressure falls along the baroclinic zone in the Ohio Valley will impede the warm front`s poleward movement. A small but potent vorticity maximum in southern Missouri will move eastward to Ohio/Kentucky by 12Z Tuesday. Large-scale ascent will increase downstream into the Mid-Atlantic tonight. Precipitation will begin to develop later tonight as isentropic lift increases. Midlevel cooling will also allow for elevated instability to increase, so a couple rumbles of thunder may occur with any showers that develop. Latest high-resolution guidance brings in a first round of precipitation from around midnight to around daybreak, primarily confined to the present location of the warm front. The last few runs of the HRRR have suggested fairly decent precipitation rates may develop with the strongest storms, and have converged in a general corridor where the convection tracks (roughly along and up to 50 miles southeast of I-95). However, there is less than stellar support from other convection- allowing models, so uncertainty remains high. As such, kept PoPs in the chance category for now, but did make the highest PoPs in the aforementioned corridor during the overnight hours. Another concern overnight is the fog. With plenty of low-level moisture in place and the seemingly ever-present warm front straddling the CWA, patchy fog seems like a good bet. Winds are expected to be light tonight, which will only aid in its development. However, the presence of precipitation complicates the forecast a bit, so not completely sold on the prospect of widespread/dense fog. Nevertheless, this remains a distinct possibility, and its development will be monitored closely this evening. && .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 /00Z TUESDAY THROUGH SATURDAY/...
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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.
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&& .MARINE... Potential for more dense fog tonight; however, presence of precipitation complicates issuance of an advisory at this point. 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... Georgetown 3/28 RER max is 80 set in 1960. 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...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Meola Near Term...CMS Short Term...CMS Long Term...Meola Aviation...CMS/Meola/Robertson Marine...CMS/Meola Climate... Equipment... is the U.S. government's official web portal to all federal, state and local government web resources and services.