474 FXUS61 KPHI 282040 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 440 PM EDT Wed Apr 28 2021 .SYNOPSIS... A slow moving frontal boundary will remain just north of the region through Thursday. A couple weak waves of low pressure will track along the front, before stronger low pressure pushes a strong cold front through the region on Friday. High pressure then builds in for the weekend before moving offshore early next week. Another frontal system may then affect the region by Tuesday or Wednesday. && .NEAR TERM /THROUGH THURSDAY/... The warmest day of the year (thus far) is underway across our region with temperatures generally in the mid 80s and creeping into the upper 80s at spots. A warm W-SW flow prevails over the area as we reside on the southeast side of low pressure located in the vicinity of the eastern Great Lakes. At mid-lvls the ridge axis has more or less completely flattened, with a notable northern stream perturbation passing to our north. Convection has developed upstream of the area in association with this wave and will gradually push eastward through the afternoon. However, given the trajectory of the mid- lvl/and sfc.wave north of the area coupled with the unfavorable diurnal timing expect that most of the area remains dry with only a slight chance of a few showers/thunderstorms over our NW this evening. The system's cold front will not make it into to our area and will actually begin to retreat northward overnight. A warm night is thus expected given continued warm/moist southwesterly flow and the lows in the mid 60s will likely be the warmest we have observed in some time. We will be somewhat of a lull between disturbances so would suspect we remain dry with only mid- high lvl clouds. On Thursday a stronger northern branch disturbance and associated sfc.low & cold front will gradually approach the area. The best upper-lvl forcing will reside upstream of the area most of the day before more substantial height falls arrive later in the afternoon, with the cold front likely not arriving until the evening. That being said, there will be plenty of moisture (PWATs > 1.5 inches) to interact with a few weak disturbances ejecting ahead of the main trough so will retain fairly high PoPs (particularly over the NW), but QPF amounts will be fairly meager outside of the NW where a few tenths will be possible. Can't rule out a few rumbles of thunder NW of I-95, given a few hundred J/kg of CAPE, but overall profiles don't look too impressive so not too high on this threat. Highs may be tempered somewhat by cloud cover and precipitation but assuming at least some breaks, upper 70s and lower 80s are still expected across much of the area (apart from the far north). && .SHORT TERM /THURSDAY NIGHT THROUGH SATURDAY/... The stronger synoptic low and cold front will approach the area Thursday evening/night as the initial main trough axis races quickly from the Great Lakes late Thursday and then offshore by Friday morning. Convection (potentially strong) will likely develop ahead of the cold front, but given the unfavorable diurnal timing expect storms to weaken by the time they reach our area, that being said it still bears watching. Southwesterly winds should start to increase overnight Thursday night as the surface low draws near. This will keep temperatures mild as well, with lows around 60 degrees. The strong cold front looks to cross the area around daybreak Friday with temperatures and dewpoints falling in its wake and precip chances waning. In the wake of the strong cold frontal passage, we will dry out, and attention will turn to the winds. Trends seen on the previous overnight shift have continued today, with the potential for a rather significant wind event late Friday into Friday night. In the wake of the front, a very robust shortwave trough and associated vorticity lobe will move through the region towards Friday afternoon. An additional rain shower is possible late Friday morning into the early afternoon ahead of this disturbance, but will likely be limited in coverage due to decreasing moisture. Behind this secondary disturbance, strong cold advection and a tight pressure gradient will develop as surface low pressure rapidly deepens heading into the Canadian maritimes, while high pressure encroaches from the west. 28.12z guidance has actually trended even stronger on wind fields for late Friday afternoon into Friday night. 12z NAM and GFS BUFKIT soundings are supportive of wind gusts of at least 40 to 50 mph, if not higher, Friday late afternoon and evening. Continued to trend the wind forecast upward to reflect this, though there is still room to go up a bit more. Profiles suggest that little if any stabilization of the boundary layer is likely at least during the first half of Friday night, so while the timing is not the most favorable for high winds in a traditional sense, that currently doesn't appear it will be much of a hindrance. If current trends continue, we will at least reach Wind Advisory levels by late Friday, and could potentially even be near High Wind Warning thresholds. This is worth watching closely over the next couple of days especially given leaves are now coming out on the trees, increasing the wind damage risk. By late Friday night and into Saturday, the gradient weakens rapidly as high pressure builds in just to our south. Wind gusts over 30 mph early Saturday morning will struggle to reach 20 mph by later afternoon. Highs will favor the mid 60s as a cool air mass remains in place with the northwest flow. Dry and mostly sunny conditions are expected thanks to high pressure. && .LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/... Overview... Rinse and repeat? The pattern for late in the weekend and next week is starting to look quite similar to the pattern for the current week. Model consensus is improving, though still not in complete agreement, that ridging will rebuild over the Southeast early next week. Strong surface high pressure will shift offshore, with southerly return flow developing. Meanwhile, a slow moving frontal boundary with multiple waves of low pressure tracking along it will likely set up near or north of our region. While the next ridge currently does not look quite as impressive as the one over the region this week, it is likely to bring another warm-up early next week following the cooldown early in the weekend. Rain chances will likely be on the low side to start the week, with improving chances for showers and thunderstorms with time as the frontal system moves closer and warmth and moisture increase over our region. Dailies... Saturday night-Sunday night... Mainly quiet weather appears likely with high pressure centered to our south. Saturday night will be on the cool side, with lows in the 40s. A disturbance passing to our north on Sunday will increase clouds across the far north. Can't rule out some light showers towards the Poconos and far northern New Jersey especially Sunday morning, but expecting a dry day overall. As ridging begins to build, temperatures will warm also, back into the 70s, and in fact well into the 70s in most areas. Should be a very nice day, with dry weather continuing into the overnight and lows on the warmer side of average, in the 50s. Monday-Monday night... Ridging over the East reaches its maximum amplitude, while surface high pressure shifts further offshore. Temperatures will get back into the 80s on southwest flow around the periphery of the offshore high. Latest trends are a little bit faster with the approach of the next weather system, and raise the possibility of a shower or storm threat on Monday afternoon, especially to the west. But still favoring the idea that most of the region will stay dry Monday, with better rain chances holding off until later in the week. Tuesday-Wednesday... The frontal system mentioned above will likely approach our region towards the midweek period. It's much too early to try to time it out and gauge the risk of when the best shower and thunderstorm chances will be. But the main takeaway is that while no washouts are expected, rain chances will likely increase by Tuesday. Continued warm temperatures and increasing humidity are also likely ahead of the front, with a cooldown expected once it moves through, possibly by Wednesday. Overall, a typical late spring forecast. && .AVIATION /21Z WEDNESDAY THROUGH MONDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Rest of Today...VFR. West-Southwest winds 10-15kts with period gusts to 25kts. High confidence Tonight...VFR with a slight chance of showers/thunderstorms particularly at KRDG, KABE. Light west-southwest winds around 5kts. Moderate confidence. Thursday...A chance for MVFR conditions in the morning particularly NW, and then the chance for periodic restrictions during the day in showers. Winds generally SW around 10kts with gusts to near 20 kts. Low confidence in southward extent and/or duration of sub-VFR conditions. Outlook... Thursday night... A cold front likely crosses the region sometime Thursday night, with restrictions in showers and perhaps thunderstorms possible Thursday evening. Initially SW winds around 10kts will shift to the WNW 10-15kts with some gustiness possible closer to Friday morning. Moderate confidence. Friday-Friday night... Mainly VFR. A cold front moving through early in the day may bring some scattered showers and locally MVFR conditions. Otherwise, winds will be the main story for this period. West-northwest winds of 15 to 20 kt with gusts to 25 kt on Friday morning will increase to 20 to 25 kt with gusts to 40 kt for the late day and into the overnight. Winds should gradually decrease later in the night. Moderate to high confidence. Saturday-Saturday night... VFR. West-northwest winds 10 to 15 kt with gusts to 25 kt, decreasing by late day. Overnight, winds becoming light and variable or light southwesterly. High confidence. Sunday-Sunday night... VFR. Southwest wind around 10 kt, a bit lower overnight. High confidence. Monday-Monday night... Mainly VFR. Southwest wind 5 to 10 kt. Moderate confidence. && .MARINE... Sub-SCA conditions will prevail through the day on Thursday with SW winds 10-15kts with gusts to 20kts. Wave Heights should be about 3-4 ft. Outlook... Thursday night...Small Craft Advisory conditions likely developing Thursday evening as seas increase to 5 feet or greater and winds increase to 25 knots or higher late Thursday night. Showers or thunderstorms possible. Friday-Saturday... Winds will continue to increase on Friday behind a strong cold front. A Gale Watch is now in effect from early Friday afternoon to early Saturday morning, with the strongest winds expected late Friday afternoon into the first half of Friday night. West-northwest winds of 20 to 30 kt with gusts of 40 to 45 kt are expected. A period of Storm Force conditions cannot be ruled out. Winds will start to decrease later Friday night, and conditions should quickly subside to SCA levels on Saturday morning, and below SCA levels by later Saturday afternoon. Seas 4 to 6 ft, decreasing to 2 to 4 ft by later Saturday. Saturday night-Monday... No marine headlines are anticipated. Winds will become southwest for most of this period with gusts to 20 kt. Seas mainly 3 to 4 ft. && .FIRE WEATHER... Windy and dry conditions are expected to develop late Friday through Saturday across the area. Depending on the total amount of precip that falls across the area Thursday through early Friday, favorable fire-weather conditions may develop during the late Friday through Saturday period. Northwesterly winds are forecast to gust to 30-40+ mph Friday night with maximum relative humidity values mostly in the 40s. Minimum relative humidity will likely fall to around 25-30% on Saturday with winds continuing to gust to around 20-25 mph. Fire weather partner coordination may be needed during this period. By Sunday, lighter winds and relative humidity values in the 30s should preclude an fire weather concerns. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Gale Watch from Friday afternoon through Saturday morning for ANZ430-431-450>455. && $$ Synopsis...O'Brien Near Term...Carr Short Term...Carr/O'Brien Long Term...O'Brien Aviation...Carr/O'Brien Marine...Carr/O'Brien Fire Weather...WFO PHI