Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 060742 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 342 AM EDT Thu Aug 6 2020 .SYNOPSIS... A weak stationary front will straddle the area through Friday, and a wave of low pressure will lift slowly northeastward along it from the Mid-Atlantic today to near Cape Cod by Friday night. High pressure will build into the Northeast this weekend into early next week. A cold front will approach the region slowly around the middle of next week. && .NEAR TERM /THROUGH TODAY/... What will likely be about a 48 hour stretch of unsettled weather is getting underway this morning. Synoptically, a weak frontal boundary remains draped from northeast to southwest just about across the center of the region. Mid and upper level troughing is present across the Great Lakes, with a fairly strong upper jet streak in place to our north over New England aiding in producing some large scale ascent. A convectively enhanced vorticity impulse is riding up along the frontal boundary this morning and is helping to produce an area of showers and thunderstorms over Virginia. CAM guidance generally agrees that one or more clusters of showers and thunderstorms will continue northeastward this morning, affecting mainly the southern third of the region, tracking along a gradient region of MLLCAPE. These storms could produce heavy rain given PWAT values will be surging above 1.5 inches this morning as this wave approaches. In addition, should this convection become surface based, which is possible, gusty winds may also occur. Again the focus for activity this morning should be mainly near and especially south of the Philadelphia metro area. Following the morning to midday convection, there may be a general lull for a good part of the afternoon given a lack of forcing and some subsidence in the wake of the initial vorticity impulse. However, we will then need to watch for additional convection to fire by late in the day as the next impulse begins to approach from the southwest. Once again, the southern portion of the area would be favored for this since the best forcing will be there. Once again, locally heavy rain will be possible, along with strong to potentially damaging winds in any taller storm cores given increasing DCAPE values. How widespread and vigorous this convection will be is unclear, however, due to questions about how much instability will build, the timing of the arrival of better forcing, and how much if at all the morning convection plays into overturning the atmosphere. SPC SWODY1 maintains a marginal risk across the far southern portion of the area, though could see some stronger storms extend a little further north by this evening. Have issued a Flash Flood Watch for a portion of southeast PA, adjacent areas of southern NJ, and all of our Delmarva counties. The watch runs through Friday morning and so will cover potential flooding threats from convection this morning and this evening/tonight (more on that below). Flooding potential may extend further northward with time, but will discuss that more below. Given what should be fairly extensive cloud cover today, expecting highs only in the upper 70s to low 80s. If we get a little more sunshine could see some mid 80s. Light easterly flow expected. && .SHORT TERM /TONIGHT THROUGH FRIDAY NIGHT/... A similar theme continues from tonight right on through Friday night. The frontal boundary remains near stationary over the region while the upper trough to the west slowly approaches. In addition, multiple additional convectively enhanced shortwaves ride up along the frontal boundary. Eventually, towards late Friday night or early Saturday, the upper trough axis moves overhead and should finally kick that boundary offshore as high pressure starts to build in. So eventually we will trend drier. Before we get rid of the boundary, however, we face a continued challenging and unsettled forecast. Starting out, tonight, convection will likely be ongoing towards sunset, mainly focused in the southern portion of the region as that next impulse approaches. There is a wide spread in both global and hi res guidance on how tonight`s convection fares as it progresses northeast. Suspect it will remain fairly active through much of the night given that a ribbon of instability remains in place. Any severe weather threat should diminish after dark, but there are some indications for a continued heavy rain threat into the overnight hours. This will continue to be monitored. Eventually, this overnight activity should move offshore towards Friday morning. It still does not end there, as with more energy coming up from the southwest and the upper trough to the west also approaching, yet another round of convection is likely by later Friday into Friday night. And once again, with PWAT values remaining anomalously high (nearly 2"), areas of heavy rain are possible. The severe threat Friday looks fairly low though cannot rule out some gusty winds once again. This time, activity may be focused further north or at least more broadly distributed given the position of the approaching trough, but plenty of uncertainty with that as well. In light of all this, it is very possible that the Flash Flood Watch will need to be extended both further out in time as well as further north and west to cover the heavy rainfall threats from tonight through Friday night. This is especially true over the remainder of eastern PA where FFG is very low, but just not confident enough yet in coverage of heavy rain to expand it any further. Long story short, expect the watch that is currently in effect to evolve in both its spatial and temporal extent over the coming day or so as confidence on details improves. Outside of the rain chances, temperatures Friday should be similar to Thursday given a very similar synoptic setup. Winds remaining light with prevailing easterly flow. && .LONG TERM /SATURDAY THROUGH WEDNESDAY/... Typical summertime pattern expected in the medium range, with the northern Mid-Atlantic mainly on the southern fringe of stronger midlevel flow through the period. After a fairly potent shortwave trough moves east of the region on Saturday, a period of low-amplitude west-northwest midlevel flow should occur before a series of perturbations eject eastward from the base of larger-scale troughing in central Canada and the northern U.S. next week. The trough will slowly progress to Quebec by Wednesday/Thursday, with an attendant front approaching the Northeast during this time. Surface return flow from a departing high will allow heat and humidity to return to the area following a near-average temperature day Saturday, with increasing chances for diurnally-driven convection Monday through Wednesday. Of particular interest for Saturday is the speed with which the shortwave trough departs the area. Deterministic models suggest the trough axis may linger through portions of the day, which will likely allow for instability showers (possibly storms) to occur through the afternoon. Have bumped PoPs upward for most areas. Not expecting a washout, though, and any convection should move east of the area by evening (if not before then). Expecting a rather dry period Saturday night through Sunday night as surface high pressure builds into the area. Cannot rule out a stray shower or storm on Sunday afternoon (perhaps via sea breezes), so included low-chance PoPs for this possibility. Otherwise, this will be a quiescent period. As return flow becomes more established early next week, expecting coverage of diurnally-driven convection to increase each day (Monday through Wednesday) in the CWA. A key factor in what ultimately transpires will be the timing/track of predecessor perturbations from the main trough in central North America. My suspicion is that these perturbations may track mostly north of the area, which would result in fairly low coverage of convection on Monday in particular. As the front approaches the area Tuesday and Wednesday, large-scale ascent will slowly increase, though the 00z ECMWF specifically indicates the strongest ascent will be well north of the region. The 00z GFS appears more aggressive in the southward progress of subsequent perturbations and resultant lift. Given the overall strength of the trough, I have a hard time buying the drier look to the ECMWF during the midweek period as the front slowly ekes its way into the area. Have maintained higher daily PoPs Tuesday and Wednesday. Temperatures about 3-8 degrees above seasonal averages should be expected through this period. Looks like the late-week period may be interesting, as the front may stall near or in the area, with deterministic models showing multiple rounds of convection occurring in proximity to it. Certainly a heavy-rain threat from a pattern recognition perspective. && .AVIATION /07Z THURSDAY THROUGH MONDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Through 12z... Mainly VFR but with increasing and lowering clouds. Development of MVFR conditions with showers and embedded thunder is possible right towards 12z especially near and south of ILG. May be some patchy fog also but this should be limited in coverage. Winds light and variable. Moderate to high confidence. Today... Mainly VFR initially. For areas north of PHL, VFR should be prevailing today. Near and south of PHL, a combination of VFR and MVFR is expected. In addition, scattered showers and thunderstorms are possible through much of the day which may contain occasional IFR conditions. Winds mainly from the E with speeds 5 to 10 kt, but some tstms may contain brief gusty winds. Overall, low to moderate confidence in the forecast, with the lowest confidence regarding the extent and timing of thunderstorms. Tonight... Prevailing MVFR with scattered to widespread showers and thunderstorms expected. Flight categories may fluctuate substantially especially in and near any heavier showers or thunderstorms. Winds mostly light and variable but some storms could contain brief gusty winds. Low to moderate confidence. Outlook... Friday-Friday night... Continued unsettled conditions expected. Frequent opportunities for showers and thunderstorms with associated sub-VFR conditions. Winds mainly from the E or NE through the period with speeds less than 10 kt, and may be light and variable at times especially Friday night. Some tstms may contain brief gusty winds. Moderate confidence in overall trend, low confidence in details. Saturday through Sunday night...Mainly VFR. Isolated/scattered showers/storms possible on Saturday, with any restrictions brief. Cannot rule out a stray shower or storm during the afternoon/evening hours on Sunday, but coverage should be sparse. Patchy fog may occur each night. Light winds through the period. Moderate confidence. Monday...Mainly VFR. A slight chance of thunderstorms in the afternoon with brief restrictions possible. Southwest winds 5 to 15 kt. Moderate confidence. && .MARINE... Through Friday night... Sub-SCA conditions expected. Seas 2 to 4 ft. Winds mainly E or SE at 5 to 10 kt, but may turn light northerly on Friday night. Frequent opportunities for showers and thunderstorms are expected with locally higher winds and seas in thunderstorms. Outlook... Saturday through Monday: Sub-advisory winds/seas expected through the period. Cannot rule out a stray shower or storm during the afternoon/evening hours, but chances appear rather low overall. Rip Currents... Seas continue to subside in the wake of Isaias. A low risk for the formation of dangerous rip currents is expected through Friday. && .EQUIPMENT... The NOAA Weather Radio All-Hazards broadcast from Hibernia Park Pennsylvania (Station WNG-704) remains off the air. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...Flash Flood Watch through Friday morning for PAZ070-071- 101>104. NJ...Flash Flood Watch through Friday morning for NJZ016>018-021. DE...Flash Flood Watch through Friday morning for DEZ001>004. MD...Flash Flood Watch through Friday morning for MDZ008-012-015- 019-020. MARINE...None. && $$ Synopsis...CMS Near Term...O`Brien Short Term...O`Brien Long Term...CMS Aviation...CMS/O`Brien Marine...CMS/O`Brien Equipment...WFO PHI is the U.S. government's official web portal to all federal, state and local government web resources and services.