FXUS61 KPHI 030800 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 400 AM EDT Sat May 3 2025 .SYNOPSIS... Warm temperatures persist through today. A cold front will gradually work its way into the region tonight and stall across the region Sunday. Several waves of low pressure will be around the region through Tuesday bringing a prolonged period of unsettled weather. The complex system will gradually move out of the region Tuesday night and Wednesday. A cold front will then settle southward into the region Thursday into Friday. && .NEAR TERM /THROUGH TONIGHT/... A surface low is situated across portions of Quebec this morning, with a cold front trailing it into western New York and Pennsylvania and into portions of the Ohio Valley. The entire forecast area remains in the warm sector, and therefore temperatures will not drop much through sunrise. Lows will generally be in the mid 60s across most of the area, with upper 50s/lower 60s in the Poconos into northern New Jersey. After sunrise, temperatures will quickly begin to climb with much of the area seeing a period of partly to mostly sunny skies and increasing southwesterly flow. It will be a breezy day across the area, with winds generally 10-15 mph with gusts 25-30 mph. Afternoon highs will once again climb into the mid 80s across the Delmarva and near/southeast of I-95. Northwest of I-95, a bit more cloud cover will likely keep temperatures a bit more limited, though still near 80. Into this afternoon, a 500 mb trough will be in the process of closing off over central portions of the US as it tracks generally eastward toward the area. A shortwave with enhanced southwesterly mid-level flow will overspread the entire forecast region, with significant height falls during the late afternoon and evening hours. This feature will provide notable ascent across the area ahead of the approaching cold front. Scattered thunderstorm development is anticipated this afternoon in the pre-frontal region, with locations a bit closer to the front during peak heating more likely to see this development. Consequently, the greatest chance for storm development this afternoon appears to be across our Pennsylvania and northern New Jersey counties, primarily northwest of I-95. With strong afternoon heating, sfc-3 km lapse rates will become quite steep, exceeding 8 C/km across the area. Forecast soundings across eastern Pennsylvania into northern New Jersey indicate MLCAPE reaching or exceeding 1,500 J/kg. Wind profiles are progged to be fairly unidirectional, but enhanced winds aloft will contribute to deep layer shear around 35 kt. Overall, the environment will favor storm organization, with the strongest storms capable of producing damaging wind gusts and sporadic instances of hail. Also of note, tall and skinny CAPE profiles are noted in forecast soundings, with near saturation through the entire vertical column. Forecast soundings also indicate PWATs near 1.5", which is well above normal for this time of year. These factors indicate that any storms may produce very heavy rainfall and there will be at least some risk for flash flooding. The Storm Prediction Center has placed eastern Pennsylvania into northern New Jersey in a Slight Risk for severe thunderstorms this afternoon/evening, with the Weather Prediction Center placing generally the same region in a Marginal Risk for excessive rainfall. Farther south and east but away from the marine influence near the coast, isolated storms will still be possible. Any storms that manage to form in this environment will still have severe weather potential, but storm coverage will be much lower farther from the front. Tonight, the environment will slowly become less favorable for strong storms, but locations from the Lehigh Valley north and west may continue to see showers and storms. I-95 southeastward may remain mostly dry. With the front still remaining northwest of the area through the night, another warm night will be on tap, with lows remaining in the mid 60s. && .SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/... An upper level low will become nearly stationary over the Ohio Valley Sunday through Monday night. Surface low pressure will be mostly stationary just to the east, with the cold front moving in from the northwest tonight stalling out for a time before possibly making some more progress southward across the region as the surface low strengthens Monday. Pulses of upper level energy rotating around the closed low aloft will lead to intervals of showers and thunderstorms, but with plenty of cloud cover and winds turning southeasterly and then easterly, it looks like CAPE will be minimal, leading to mostly a heavy rain vs a severe threat. The increasingly onshore, upslope flow will lead to concentration of heavier rain across areas northwest of the I-95 corridor, where several inches may fall by dawn on Tuesday. Heavier rain risks appear to decrease progressively to the south and east, with minimal risk along the southern NJ and DE shores. As far as timing of rainfall, one main pulse looks likely to cross the region late Sunday into Sunday evening, then a bit of a break with lighter and more occasional showers late Sunday night into Monday before another, likely heavier round with the main surface low's progression into the area late Monday into Monday night. While some elevated CAPE likely leads to isolated to scattered thunder, again the limited surface warmth with marine layer increasingly influential likely prevents significant severe risk, with flooding again the main concern. Highs will be in the 70s Sunday, but may drop into the 60s with more of a marine push on easterly flow by Monday. Lows may stay in the 60s again Sunday night, but the greater marine influence may nudge them into the 50s by Monday night. && .LONG TERM /TUESDAY THROUGH FRIDAY/... Low pressure both at the surface and aloft finally makes noticeable progress north and east on Tuesday, with a gradual decrease in POPs Tuesday into Tuesday night. As the system pulls thru and winds turn back westerly, we should warm up a bit compared to Monday, with highs rising back into the 70s for most, though with plenty of clouds still around. Weak high pressure builds in for Wednesday, with minimal chance of precip. Still uncertain about just how quickly the system completely clears out, so have some lingering chance POPS north, but sub-slight chance south. Should gain a few more degrees in highs, but again mainly 70s. Guidance then has a cold front drop southward from Canada Thursady into Friday, with a return of chance POPS CWA-wide Thursday. Timing of the front may cause them to linger into Friday. Highs again mainly 70s for Thursday, but may get stuck in the 60s behind the front Friday. && .AVIATION /08Z SATURDAY THROUGH WEDNESDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Today...Mainly VFR. Southwest wind near 5 kt increasing to 10-15 kt with gusts 20-25 kt after 14-15Z. Scattered showers and storms expected to form after 18Z, primarily impacting KRDG/KABE, but perhaps as far south as the I-95 terminals. Reductions to sub-VFR are likely with any storm, though confidence is not high enough to narrow down a specific timeframe yet. Moderate confidence, with low confidence in timing/placement of any reductions below VFR. Tonight...VFR during the evening and much of the overnight hours. South-southwest wind around 5 kt shifting to southeast overnight. Ceilings will begin to lower towards 12Z from west to east, with MVFR or perhaps IFR becoming increasingly likely. Moderate confidence. Outlook... Significant periods of sub-VFR conditions with some intervals of IFR mixed in are expected Sunday through Tuesday as complex low pressure brings multiple rounds of showers and even a few thunderstorms. Predominantly VFR conditions are more likely to return as the system moves out on Wednesday. && .MARINE... A Small Craft Advisory will be in effect for the Atlantic coastal waters from 1 PM this afternoon to 7 AM Sunday. South wind 15-20 kt with gusts to 30 kt and seas 4-6 ft expected. Rain showers and isolated storms are also possible tonight. Outlook... Lingering waves around 5 feet may result in a need to extend the Small Craft Advisory on the ocean waters for a few hours into Sunday morning, but confidence is not that high just yet. Otherwise, conditions should generally subside below SCA criteria Sunday thru Sunday night. An area of low pressure crossing the region later Monday into Tuesday may result in a return of SCA conditions, mainly on the ocean waters. Conditions should then again subside below SCA levels on Wednesday. Aside from winds and seas near SCA levels, the main concern Sunday through Tuesday will be showers and occasional thunderstorms. The end of the showery conditions is more likely Wednesday. && .HYDROLOGY... A significant period of wet weather looks to begin late today and persist into at least Tuesday. Several inches of rain may fall in some locations, however timing and location will be critical to determine if any flood issues arise. If the rain is spread out over several days, little flooding may occur, but heavy convective rains concentrated in any location and shorter time period may result in significant issues. Fortunately we are coming off an extended dry spell, particularly northwest of I-95 where indications suggest the heaviest rains may end up falling. For the time being will continue to monitor trends in guidance. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory from 1 PM this afternoon to 7 AM EDT Sunday for ANZ450>455. && $$ SYNOPSIS...Cooper/RCM NEAR TERM...Cooper SHORT TERM...RCM LONG TERM...RCM AVIATION...Cooper/RCM MARINE...Cooper/RCM HYDROLOGY...PHI