732 FXUS61 KPHI 261341 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 941 AM EDT Thu Aug 26 2021 .SYNOPSIS... High pressure remains anchored off the Mid-Atlantic coast. A cold front approaches the area on Friday and slowly works its way through the region and remains south of the area over the region. That front returns back north as a warm front early next week. Another cold front approaches for the middle of next week. && .NEAR TERM /THROUGH TONIGHT/... Earlier fog has almost entirely dissipated. So now the focus turns to the heat and humidity and potential for convection today. The forecast for today is somewhat challenging, as the overall large-scale pattern is rather nebulous, with boundary- layer processes appearing to be critical in the resultant coverage of convection this afternoon. The synoptic environment features a strong midlevel ridge in the western Atlantic, elongated westward into much of the southern U.S., with faster quasi-zonal midlevel flow confined to the U.S./Canada border area. However, a weak midlevel vort max was meandering in the Ohio Valley, approaching the central Appalachians early this morning. The vortex will move little today but will likely eject several weaker perturbations eastward on the northern periphery of the aforementioned ridge. Thus, the expected pattern does imply that there may be some weak lift aloft in portions of the Mid-Atlantic today (certainly compared to yesterday). Several convection-allowing models develop a fair amount of convection this afternoon in the diurnally destabilizing environment. However, analysis of these simulations suggests that the projected boundary-layer profiles are a little too moist. This bias may make all the difference between an uncapped environment and one that is sufficiently capped to prevent more than an isolated storm or two. In comparison from the 00Z runs, the Nam Nest has backed off a bit from coverage of storms (though interestingly, it still appears to have a moist bias at least looking at the surface dewpoints). The FV3 is now showing the most coverage, while the HRRR shows only isolated coverage. Agree with the previous shift that coverage is more likely to be isolated to scattered; probably more coverage than seen yesterday, but less coverage than the more aggressive models would suggest given their expected bias. For now, think slight- chance to low- end chance PoPs will suffice, with the highest PoPs generally in the northern third to half of the CWA. With weak steering flow and deep-layer shear, storms should be disorganized and short- lived but may also be rather potent given the sufficiently dry midlevels (though not terribly dry; PWs still generally 1.6-1.8 inches). Cannot rule out gusty outflow winds and a microburst or two should higher coverage of storms occur. Other main concern of the day is obviously the heat/humidity, with forecast heat indices this afternoon right around the century mark (probably a couple degrees higher, in general, than what was observed yesterday). The dew points have mixed out a little bit more than consensus guidance has suggested the past couple of days, and I suspect this will occur again today. That said, conditions will be close enough to criteria for long enough that I am fairly confident heat-advisory conditions will be met in Montgomery, upper Bucks, and Hunterdon Counties in addition to those already included in the product. Any convection should diminish quickly with the loss of daytime heating, with another warm/humid night with patchy fog to follow. && .SHORT TERM /FRIDAY THROUGH FRIDAY NIGHT/... High pressure will be off the Mid-Atlantic coast. Abundant low level moisture will be in place with surface dew points in the 70s. A cold front with some mid-level shortwave energy approaches from the west, and showers and thunderstorms develop in the afternoon and early evening. Another hot and humid day with highs in the lower 90s and heat index values around 100 degrees. Will go ahead and extend the current Heat Advisory through Friday, and will expand it to include much of central New Jersey. PWATs will be around 2-2.25 inches, so heavy rain is possible. Given recent heavy rain, it will not take much to produce at least localized flooding. The stronger storms will be capable of damaging wind gusts. The threat for convection will last through much of the overnight. && .LONG TERM /SATURDAY THROUGH WEDNESDAY/... A prolonged period of unsettled weather is on tap for the long term period. Although temperatures back off a bit and will top off in the 80s, humidity levels will remain high as surface dew points will be in the 70s well into the start of the new week. Cold front sags south of the region on Saturday, but remains over the Mid-Atlantic before returning back north as a warm front early in the new week. Although mid-level ridge of high pressure will be over the area, some shortwave energy passes through the region and will interact with that front to touch off mainly diurnally driven convection. over the weekend and into the start of the new week. Main threat will be heavy rain due to the abundant low level moisture. Another cold front passes through the region late Monday or Tuesday. The threat for showers and thunderstorms continues. Highs during this time will mainly be in the upper 80s to around 90, before falling back into the lower 80s for the middle of next week. Humidity levels finally back off a bit, but surface dew points remain in the 60s. Tropical low pressure may bring another round of potentially heavy rain to the region towards the end of next week, but models have little consistency on its timing and strength. && .AVIATION /14Z THURSDAY THROUGH MONDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. This afternoon...Mainly VFR, but there is a chance of isolated to scattered storms, especially north of PHL. For now, have mention at ABE/TTN, but confidence is still quite low on timing/coverage/location. Winds southwest around 5 kt, with sea/bay breeze influence likely at ILG/ACY and possible at MIV. Low confidence. Tonight...Mainly VFR with patchy fog possible, which may be somewhat more prevalent than that observed early this morning. Winds light and variable. Moderate confidence. Outlook... Friday-Friday night...Patchy fog early Friday morning. Otherwise, mainly VFR. Scattered SHRA/TSRA are possible for the afternoon and through the overnight which could lead to localized restrictions. Winds mostly light and variable Friday, becoming light but steady from the northeast overnight. Moderate confidence. Saturday-Saturday night...Prevailing VFR, but localized restrictions possible in SHRA/TSRA. East or northeast wind 5 to 10 kt. Moderate confidence. Sunday-Sunday night... Prevailing VFR. Localized restrictions possible in SHRA/TSRA, but probably with less coverage than on Saturday. Winds shifting from northeast to southeast at 5 to 10 kt, then becoming light and variable overnight. Moderate confidence. Monday-Monday night... Prevailing VFR but restrictions possible in SHRA/TSRA. Southwest wind 5 to 10 kt. Low confidence. && .MARINE... Sub-advisory conditions are expected through tonight. Winds will generally be southwest from 5 to 15 kt with seas primarily below 3 feet. Outlook... Friday-Monday... No marine headlines are anticipated. Several wind shifts with gusts mainly 20 kt or less. Chances for showers and thunderstorms each day. Seas ranging from 2 to 4 ft. Rip Currents... There is a LOW risk of rip currents through Friday. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...Heat Advisory until 8 PM EDT Friday for PAZ070-071-102>106. NJ...Heat Advisory until 8 PM EDT Friday for NJZ009-010-012>015- 017>020-026-027. DE...Heat Advisory until 8 PM EDT Friday for DEZ001. MD...None. MARINE...None. && $$ Synopsis...MPS Near Term...CMS/Johnson Short Term...MPS Long Term...MPS Aviation...CMS/Johnson/MPS Marine...CMS/MPS