527 FXUS61 KPHI 211329 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 929 AM EDT Wed Aug 21 2019 .SYNOPSIS... As a warm front lifts north of the Mid-Atlantic today, a surface lee trough will develop east of the Appalachians. A cold front will slowly approach the region Thursday, gradually shifting south of the area Friday and Saturday. High pressure is anticipated to build into the Northeast and adjacent western Atlantic this weekend. The next weather system may impact the area early next week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... For the morning update, I adjusted the cloud cover for the current hour and next few hours. The vis sat pix does show a good amount of sun south and east of I95. In addition, I also trimmed back PoPs across the same area. The next round of showers across central PA is expected to track across northeast PA and northern NJ missing our southeastern zones. Previous discussion... Several rounds of showers and thunderstorms are possible through today across the area. The first is occurring early this morning as a frontal boundary is drifting northward across the area and a short wave/vorticity impulse lifts across the area as well. Most shower and thunderstorm activity has been very sporadic in nature and this will continue through the early to mid morning hours. The second, and more organized round of showers and thunderstorms is possible by late this morning into this afternoon as another short wave/vorticity impulse move eastward across the area. This activity is mostly expected to affect northeast Pennsylvania and northern New Jersey. The next round of showers and thunderstorms, and the one with the more potential for strong to severe storms, will come later this afternoon. A lee-side/thermal trough will drift eastward from central pennsylvania and central Maryland this afternoon, while yet another short wave/vorticity impulse will slide across the area as well. This surface trough will be the focus for showers and thunderstorms to develop, and the short wave will enhance the lift across the area. CAPE values build to 2,000-3,000 J/kg, while shear will increase to 25-35 knots. The best combination of shear and instability is across the northern half of the area where SPC has a slight risk for severe weather, but there will be enough of both to increase the risk of strong to severe thunderstorms today across the entire area. Damaging winds will be the main threat across the area, although there will continue to be a threat for hail with a fair amount of CAPE above the freezing level. Another concern today will be the potential for heavy rainfall. PW values remain 1.75-2.00 inches, so any thunderstorm will be efficient rain producers. There should be decent storm motion for most storms, but if any training of storms occurs, this could lead to higher rain amounts and localized areas of flood could occur. Today will be another hot and humid day across the area. The Heat Advisory remains in effect for the urban corridor as heat index values are forecast around 100 degrees. The Heat Advisory was also expanded into portions of central and eastern New Jersey where heat index criteria is 100 and the heat index is forecast to reach 100 degrees. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM THURSDAY/... Showers and thunderstorms will be ongoing across the area as the surface trough continues to slowly move eastward, while the short wave/vorticity impulse aloft slides to the east. Once the short wave passes eastward through the evening, showers and thunderstorms will continue to dissipate through the evening and overnight. The surface trough will likely stall out across the area during the overnight, but once the short wave aloft passes east and the showers have dissipated, the remainder of the overnight is expected to be dry. There will be the potential for some areas of patchy fog later in the overnight, especially where rainfall occurs during the afternoon and evening hours. The most likely areas are northeastern Pennsylvania and northern New Jersey. && .LONG TERM /THURSDAY THROUGH TUESDAY/... There is large uncertainty with the forecast Thursday onward, as models are struggling mightily with the southward progress of the front approaching the area to start the period and the overall pattern that takes over starting this weekend. A cold front will be in the process of approaching/crossing the area Thursday and Thursday night, but it will struggle owing to boundary-parallel flow aloft. In the midlevels, a potent shortwave trough will be shifting eastward through Quebec, with a kicker vort max digging southeastward through the Midwest/Great Lakes. Models continue to be all over the place with the progress of this kicker system, with the 00z ECMWF clearly the slowest. The latest run mirrors a few of its past runs, keeping the slow-moving vortex in New England through much of the weekend. The 00z GFS is generally faster, but notably does feature yet another perturbation digging southeastward through New England on Saturday. Nevertheless, it is much faster with the first kicker, which continues its insistence of shunting the cold front to the south of the area on Friday. The 00z CMC looks fairly similar to the GFS, in general, albeit just a tad slower with the cold-frontal passage. Model consensus, including the ensembles, continues a slowing trend with the front, with more of the guidance suggesting potential for precipitation lasting through Friday night. As such, bumped PoPs a little bit more through this time frame (generally speaking), though there are some details worth noting... 1. There will likely be a lull in precipitation Thursday morning, perhaps lasting through much of the day, as transient shortwave ridging occurs upstream of the shortwave trough moving through the area today. Additionally, this will likely warm the profile just above the boundary layer, which may preclude more widespread diurnal convection during the afternoon. 2. There are increasing indications of a low-amplitude vort max moving through the region Thursday night, which may coincide with the highest precipitation chances across the CWA while the front is in our proximity. Did not necessarily increase PoPs from the previous forecast, but did broaden higher chance PoPs into most of the area for this period. Though timing is not ideal for maximum instability, the pre-frontal warm sector will maintain sufficient potential instability for convection overnight. 3. Though smaller-scale details differ considerably by Friday evening, it is possible another low-amplitude vort max travels along the front to produce another maximum in precipitation during this time frame. By this point, best chances would be in Delmarva and far southern New Jersey, so have maintained PoPs here through Saturday morning. With the front slow to cross the area on Thursday, temperatures and dew points will remain high. Could see another day of advisory-level heat indices in the urban corridor and southward/eastward. For now, held off on extending the current advisory given lingering uncertainties with frontal placement and potential for convection. Temperatures are still expected to be much cooler on Friday as the front sags southward. Presumably, high pressure will sufficiently build into the region this weekend for the area to dry out. However, the front remains in close enough proximity to linger some PoPs most of the day in the far southern CWA. Another caveat is the closed- off appearance of the upper low in the 00z ECMWF. Should this occur and be in close enough proximity to the area, may see some instability showers develop. At this point, this seems like a low-probability outcome, so have made no mention of PoPs for this scenario. However, will need to watch this potential in later updates. Large run-to-run consistency issues develop in the operational models for next week. There is some signal for a low to develop to our south, originating from weak troughing in the Gulf Coast region this weekend. With weak steering flow and an evolution primarily driven by convection, it is no wonder models are so divergent with the evolution of this feature. More consistent is the development of large-scale troughing in the northern U.S. next week, which appears to approach the Northeast by midweek. Combined with persistent/antecedent onshore flow from the surface high to our northeast, chances for precipitation appear to increase early next week, though confidence on the details is quite low at this point. Fairly confident temperatures will remain seasonable beyond the weekend for quite some time, which is most welcome as far as I am concerned. && .AVIATION /13Z WEDNESDAY THROUGH SUNDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Today...Most places are starting off VFR, but a period of MVFR visibilities and ceilings will be possible through the morning hours. All areas are expected to return to VFR by late morning and the afternoon. There is a chance of showers and thunderstorms across the area through the day today. The most likely time period is later this afternoon into this evening. Any showers or thunderstorms will likely lead to lower ceilings and/or visibility. Winds will be light and variable this morning, before shifting to the south to southwest later this morning and into this afternoon. Winds may gust 15-20 knots at times for some areas as well this afternoon. Tonight...Showers and thunderstorms will be possible this evening, before diminishing in coverage overnight. Lower conditions will be possible with any shower or thunderstorm, but mostly VFR conditions are expected for most of the night. Areas of patchy fog may develop overnight, especially where rainfall occurs during the afternoon and evening hours. The most likely areas are northeastern Pennsylvania and northern New Jersey. Winds will be mostly southwest 5-10 knots, before becoming light and variable for many locations overnight. OUTLOOK... Thursday...Mainly VFR, though brief restrictions may occur with showers/storms. West to southwest winds up to 10 kts. Thursday night and Friday...Very low-confidence forecast. Restrictions possible at times, with chances of showers/storms, especially on Thursday night. Chances become confined to south of PHL with time. Winds transitioning to north or northwest up to 10 kts or so. Friday night and Saturday...Generally VFR. Cannot rule out showers/storms south of PHL, but chances appear fairly low. North to northeast winds up to 10 kts, possibly stronger near the coast. Saturday night and Sunday...Mainly VFR with east to northeast winds gradually increasing to 10 to 15 kts on Sunday, possibly stronger near the coast. && .MARINE... A Small Craft Advisory was issued for this afternoon into this evening for the Atlantic coastal waters, as well as lower Delaware Bay. Winds are expected to increase to gusts of 25 knots or greater and seas are expected to build to 5 feet on the ocean as well. The advisory starts at 2 pm on the ocean, and 4 pm on the Delaware Bay, and ends 10 pm on the bay, and 4 am on the ocean. OUTLOOK... Thursday through Saturday...Sub-advisory winds/seas expected. Chances for showers/storms through the period, but especially Thursday night and again Friday night. Saturday night and Sunday...Sub-advisory winds/seas and mostly fair weather expected. RIP CURRENTS... With winds increasing to 15-20 knots on the ocean, and seas building to 4-5 feet with a period of 6-8 seconds, the rip current risk will be moderate for New Jersey, but remain low for Delaware. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...Heat Advisory until 8 PM EDT this evening for PAZ070-071-102- 104-106. NJ...Heat Advisory until 8 PM EDT this evening for NJZ010-012-013- 020-027. Heat Advisory until 8 PM EDT this evening for NJZ015-017>019. DE...Heat Advisory until 8 PM EDT this evening for DEZ001. MD...None. MARINE...Small Craft Advisory from 2 PM this afternoon to 2 AM EDT Thursday for ANZ450>455. Small Craft Advisory from 4 PM this afternoon to 10 PM EDT this evening for ANZ431. && $$ Synopsis...CMS Near Term...Kruzdlo/Robertson Short Term...Robertson Long Term...CMS Aviation...CMS/Robertson Marine...CMS/Robertson