Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 141955 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 355 PM EDT Mon Aug 14 2017 .SYNOPSIS... High pressure will weaken tonight. A weak warm front positioned across the western Atlantic and Carolinas will slowly push northward toward the region tomorrow and tomorrow night. At the same time and much further offshore, Tropical Storm Gert is expected to pass to our south and east. A cold front will approach the region from the northwest on Wednesday and track as far south as Reading, Trenton, and Belmar. The front will retreat Thursday into Friday, but may be slow to clear the area as high pressure blocks its progression northward. A cold front, from the west, is then expected to cross the region Friday night into Saturday. High pressure will build on Sunday and Monday. && .NEAR TERM /UNTIL 6 AM TUESDAY MORNING/... Southwesterly midlevel flow poleward of TC Gert in the western Atlantic is aiding in the progression of several perturbations moving into the Mid-Atlantic region during the next several hours. The most prominent vort max is approaching the area now, with light rain and/or showers attendant to the strongest (not to be confused with strong) lift just downstream of it. Models have really struggled with the evolution of the precipitation, both in terms of timing (overall, too slow) and intensity (overall, tending to diminish the precipitation too quickly). The HRRR and 12Z NAM Nest have generally converged on a solution, bringing light showers through and northwest of the I-95 corridor during the next few hours. The main question mark will be the intensity of the showers on the southwest fringe of the precipitation. Here, the showers may have marginal buoyancy to work with as the stronger instability approaches the area from the south on the upstream side of the precipitation shield. The last several runs of the HRRR and the 12Z NAM Nest indicate stronger convective cells on the southwest side, and current radar observations indeed suggest such an evolution is occurring. The question, however, will be if enough instability is present for lightning. Model soundings are not promising in this regard, with very thin positive buoyancy present (owing to modest low- and midlevel lapse rates and a rather shallow layer through which parcels may rise). With this in mind, think the chances of thunder are too low for mention at this point...though it certainly cannot be ruled out entirely. The QPF trend has been upward with the main band of precipitation as it moves through this evening/overnight, but with large-scale ascent generally weak (and weakening as upper-level confluent flow mitigates effects of differential cyclonic vorticity advection) and the mode/intensity of the showers unclear ... not biting too much on the heavier precipitation amounts at this point. Think a tenth to a quarter inch in the main swath of precipitation is possible, with locally higher amounts if stronger convective cells develop. A weaker perturbation upstream and poleward of the evening vort max will sweep through the Northeast and northern Mid-Atlantic tonight. As this perturbation moves through, there may be some enhancement of showers in the northern CWA (generally north of I-78) later tonight. The forecast generally features slight chance to chance PoPs across the area, highest in the I-95 corridor to the Lehigh Valley this evening through around midnight or so, then confined to the Poconos and vicinity after midnight, tied to the aforementioned perturbations. Conditions will be increasingly humid tonight, with lows near to slightly above seasonal averages. Skies should be cloudy, and winds will be light. Some fog may develop late tonight, but confidence was too low for inclusion at this point. However, this may need to be added in later forecasts once the evolution of this evening`s precipitation is determined. && .SHORT TERM /6 AM TUESDAY MORNING THROUGH 6 PM TUESDAY/... As TC Gert makes its move northward and then northeastward through the western Atlantic, a well-timed longer-wave trough will be swinging southeastward into the Great Lakes region and the adjacent Northeast. This will be enough to keep Gert safely away from the coast. Meanwhile, the southern stream will continue to eject perturbations northeastward into the Mid-Atlantic on the northwest fringe of the subtropical high in the Atlantic. With onshore flow to the northwest of Gert, low clouds will likely be stubborn to rise/dissipate during the day in the region. MOS is considerably lower with max temperatures on Tuesday, and with onshore flow expected to continue much of the day, think the lower temperatures are a decent bet. Meanwhile, large-scale lift in advance of the southern-stream perturbations should generate scattered showers and storms during the afternoon. Instability will increase (even with the potential for low clouds to hang around through the morning, at the very least), as large-scale lift will cool the midlevels fairly effectively. This will be especially true south of an equivalent potential temperature gradient that will slowly lift northward into the Delmarva Peninsula and adjacent areas during the day (just how far north this baroclinic zone reaches will be a big forecast concern during this period). With the above in mind, slight chances for storms are present tomorrow afternoon with any showers that form. Models are indicating the strongest precipitation signal south of I- 76, and this is generally where the highest PoPs reside. The mode of precipitation is a bit of a question (some have fairly isolated convective cells or convective clusters -- e.g., the 12Z WRF- ARW/NMM; others have fairly light showers with little in the way of convection -- e.g., the 12Z NAM Nest). With such varying solutions, obviously not confident of tomorrow`s precipitation evolution in general and specifics like thunder chances and QPF in particular. The forecast is a general broadbrush of slight-to-chance PoPs, slight chance of thunder, and cloudy skies for the region tomorrow, with gradual improvement in the northwest CWA as dry air advection approaches upstream of the main trough swinging through the Northeast during the afternoon. Forecast max temps are slightly below seasonal averages and are generally a blend of MAV/MET guidance. && .LONG TERM /TUESDAY NIGHT THROUGH MONDAY/... Overview...The period starts with a stationary front draped across the western Atlantic and Carolinas, a cold front across the Great Lakes and TC Gert well east of Cape Hatteras. The cold front will work to the south and east and arrive in our region Wednesday night. It will pause across the CWA on Thursday. Confidence is low as to how far south this front will get, but right now it looks like a Reading to Trenton to Belmar line. The front will retreat north on Friday as another cold front works through the Great Lakes and Ohio Valley. Confidence is once again low on how far north the front retreats. The cold front to our west will work through the region Friday night or Saturday. Surface high pressure behind this front will cross into the region on Sunday and anchor up into Monday. Temperatures....For reference, KPHL normals are mid 80s and upper 60s. Temperatures will be above normal on Wednesday, about normal on Thursday, above normal on Friday, about normal on Saturday, Sunday, and Monday. The warmest and most humid day of the week appears to be Friday just ahead of the cold frontal passage. No extreme temperatures are expected through the period with mainly just slightly above or slightly below if not normal. Precipitation...POP confidence is low into Saturday and depends on how far the coastal warm front edges up into our area from the south Tuesday night, how far south the cold front drops into the area on Wednesday night and how far the same front retreats northward Thursday into Friday. The closer the proximity and duration of these boundaries, the more unsettled it will be. For now, we`ll paint small POPS Tuesday evening into Wednesday night across portions of the CWA. Coverage should be more widespread on Thursday into Friday night. Rainfall rates will also increase Thursday night into Friday as PWs exceed 2 inches. Small POPS will continue on Saturday and Sunday, then nil POPS Monday. Winds...Benign through the period, mainly 15 mph or less. Highest speeds right along the coast. Impacts...TC Gert is forecast to pass well to our south and east on Tuesday and Tuesday night. Swells from the system will likely bring at least a moderate risk for the development of dangerous rip currents Tuesday into Wednesday as well as the possibility of SCA conditions on the ocean. These concerns are addressed below. && .AVIATION /20Z MONDAY THROUGH SATURDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Increasing confidence that patchy light rain or showers will move through most of the terminals through this evening. Given the patchy nature of the rain/showers, VCSH and TEMPO groups are included for now along/northwest of I-95. General timing is through 06Z. Mostly VFR expected, even with the precipitation, but CIGs are forecast to lower slowly to 5-10 kft during this period. Sub-VFR CIGs are expected near/after 06Z as somewhat variable winds become more easterly. Sub-VFR conditions may prevail through Tuesday morning. Winds below 10 kts are forecast through the period. OUTLOOK... Tuesday night through Wednesday night...Mainly VFR. MVFR possible in late night and early morning low clouds and fog. Small chance of a late afternoon or evening Thunderstorm across southern NJ and the Delmarva both nights. Thursday through Friday...Mainly VFR, but MVFR possible in showers and thunderstorms. Late night and early morning low clouds and fog possible. Saturday...Some morning MVFR possible, then VFR behind an exiting cold front. && .MARINE... Seas will be slowly building during the next 24 hours as Tropical Storm Gert makes its closest approach to the area. It will remain well offshore, but longer-period swells should continue to increase during the period. Seas will approach advisory criteria late tomorrow afternoon, but uncertainty with onset and the somewhat marginal nature of the seas preclude the issuance of an advisory at this point. A chance of showers exists tonight and tomorrow, and there is a chance of lightning on Tuesday. Winds/seas will be erratic near any showers/storms that occur. Additionally, there may be some visibility restrictions late tonight, though widespread/dense fog is not anticipated. Winds should generally be from the east or southeast around or under 10 kts. RIP CURRENTS... The rip current risk remains moderate through this evening as onshore winds around 10 kts combine with gradually increasing swell during the period thanks to Tropical Storm Gert. Conditions are expected to worsen somewhat tomorrow, as longer-period swell with higher seas will occur. Swim only in the presence of lifeguards where safety prevails. Otherwise, you`re on your own with any rescue potentially critically delayed. There have been numerous drowning fatalities this year along the NJ coast. OUTLOOK... Tuesday night...SCA seas possible due to Gert Tuesday night and Wednesday. Otherwise sub-SCA conditions expected through this week. Rip currents...TC is forecast to pass well to our south and east Tuesday night into Wednesday. Swells from the system will likely bring at least a moderate risk for the development of dangerous rip currents Tuesday night into Wednesday. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Kruzdlo Near Term...CMS Short Term...CMS Long Term...Kruzdlo Aviation...CMS/Kruzdlo Marine...CMS/Kruzdlo

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