539 FXUS61 KPHI 151804 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 204 PM EDT Sat May 15 2021 .SYNOPSIS... A large area of surface high pressure will remain near the region into late next week. A trough in the upper levels of the atmosphere will cross over the region Sunday into Monday. && .NEAR TERM /THROUGH TONIGHT/... As of 200 PM, some lightning has been occurring across interior southern New Jersey. This is within an area of surface-based CAPE up to 500 J/kg based on the latest mesoanalysis. Elsewhere, activity may be just showers or some sprinkles given lower instability. Cumulus development has occurred as the convective temperatures were reached away from the coast. So far, some showers have been focused mostly on terrain features with additional development in interior southern New Jersey near a weak surface trough. The air mass is rather dry, especially on the 12z Sterling/Dulles, with more moisture on the Upton RAOB in the mid levels. Temperatures are now in the 70s, with the exception of the coast and parts of the Poconos where it is cooler. The overall flow is light and the vertical growth of the showers may be such to not result in charge separation, and showers should tend to be slow movers (pulsing). Made some tweaks to the hourly grids to keep them current based on observational trends, and also made some adjustments to the PoPs. A couple of additional short waves will move across the area overnight, however instability will wane after sunset with loss of daytime heating. Therefore, any shower activity is not expected to last much longer after sunset for most of the area with some mid level clouds persisting across parts of the area for awhile. && .SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/... A pair of mid level short wave troughs embedded in the larger northwesterly flow will cross our region during this period. The first, which is expected to cross Sunday afternoon into Sunday evening will likely be the deeper of the two, but even that trough should be filling and weakening by the time it reaches our region. None the less, this trough will probably provide the best chance for showers in our region through the next week. In addition to broader lift associated with the trough, some low level lift will be possible along a sea breeze which is likely to develop during the afternoon. One factor with how widespread rain will be will likely be moisture, as models depict precipitable water values below average for this time of year. On Monday, a weaker mid level short wave trough will cross the region. Lift with this trough is expected to be even weaker than the Sunday trough, so expect more limited coverage of rain on Monday. && .LONG TERM /TUESDAY THROUGH FRIDAY/... Broad mid level ridge should slowly build over the eastern U.S. through this period. Consequently the surface high will likely build west closer to the mid level ridge axis. There is still some question as to where the ridge axis will develop as well as if the late week trough northeast of our region and the surface high over the Hudson Bay will have much impact on the ridge over our region. With the 00Z runs, models were further east with the late week trough (previous depicting it over far eastern Canada, now more over the northern Atlantic), and depicted the Hudson Bay high further north. Consequently, going just by the operational models, a late week backdoor cold front in our region looks less likely. That being said, I'm hesitant to jump on a single model run as there is going to be some uncertainty any time we have such a significant pattern shift. Therefore, stayed close to a blend of guidance with a blend of the previous forecast. In general, we should see mostly dry conditions, though the closer we are to the periphery of the ridge, the more likely we are to see rounds of rain, with a warming trend through at least Thursday. If the cold front stays north of our area on Friday, then the warming trend could continue through the week. && .AVIATION /18Z SATURDAY THROUGH THURSDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. This afternoon...VFR overall. A couple of showers around this afternoon (very isolated thunder). Variable winds around 5 knots, becoming southeast to south-southwest up to 10 knots. A few gusts to around 15 knots possible mainly at MIV and ACY due to a sea breeze. Low confidence on a shower or thunderstorm impacting any terminal. Tonight...An early shower around, otherwise VFR ceilings at times. Southeast winds 5-10 knots early, becoming light and variable to locally calm. Moderate confidence. Sunday...VFR ceilings around. A few showers or a thunderstorm are possible especially from midday on. Light and variable winds, becoming southwesterly 5-10 knots. An afternoon sea breeze should lead to south or southeast winds 10-15 knots mainly at ACY and MIV. Low confidence if any showers/storms move over a terminal, and the timing and progression of the sea breeze. Outlook... Monday...Mostly VFR conditions are expected. There is a slight chance for showers Monday afternoon which could result in temporary ceiling and visibility restrictions if any directly impact a TAF site, but coverage is expected to be quite limited. Winds will be close to southerly, with south southwesterly more likely for the Delaware Valley and SE PA TAF sites (KPHL, KILG, KPNE, KTTN, KABE, and KRDG), while the NJ coastal TAF sites (KMIV and KACY) are more likely to be south southeasterly. Wind speeds should be 5 to 15 kt during the day, decreasing below 10 kt overnight. Moderate confidence on most aspects of the forecast, but low confidence if any showers will directly affect a TAF site. Tuesday and Wednesday...VFR conditions are expected. Winds starting light and variable, eventually settling out of the southwest with wind speeds less than 10 kt. Moderate confidence. && .MARINE... Winds and waves are expected to remain below advisory levels today and tonight. Our friends at the USCG like to remind everyone enjoying the water in the Spring that water temperatures are still in 50s. Outlook... Sunday through Wednesday...winds and seas should stay below SCA criteria through this period. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Johnson Near Term...Gorse/Robertson Short Term...Johnson Long Term...Johnson Aviation...Gorse/Johnson/Robertson Marine...Johnson/Robertson