928 FXUS61 KPHI 200140 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 940 PM EDT Mon Apr 19 2021 .SYNOPSIS... A weak upper level wave will move out to sea overnight. A brief stint of high pressure will lead to a quiet Tuesday before a surface low organizes to our west. This low will pull a strong cold front across the region on Wednesday. High pressure will build eastward behind this system for the remainder of the week. && .NEAR TERM /THROUGH TUESDAY/... A mid level short wave trough was moving off our coast and out to sea this evening. It will continue to take most of the lingering clouds with it. Surface high pressure is forecast to nose into our region from the southwest overnight. As a result, we are anticipating a mostly clear sky and a light wind. Low temperatures should favor the 40s. Tuesday will be the nicest day of the week. With high pressure off the mid-Atlantic coast, strong WAA will be underway with a west to southwest flow. Could be a bit breezy, especially going into the afternoon, with winds generally 10-15 mph with gusts to 20 mph. Highs will generally be in the lower 70s throughout. && .SHORT TERM /TUESDAY NIGHT THROUGH THURSDAY/... The potential for severe weather continues Wednesday. A potent shortwave trough will dive southeast across the Midwest Tuesday night. A strengthening 250 mb jet just to our southwest will work in unison with this wave to promote strong cyclogenesis across the central Appalachians into Wednesday morning. With rather impressive PVA ahead of this wave and the aforementioned upper-level forcing, the large scale forcing will most certainly be in place for this feature. The northern origins of the trailing surface high from central Canada will usher in a seasonable cold airmass, which is expected to provide efficient surface lift along the cold front. While instability isn't expected to be noticeably impressive, strong low-level (speed) shear will support the development of linear storm features. Hi-res guidance is currently forecasting 50-70 knots of 0- 3 km shear. Combined with up to 1000 J/kg of MLCAPE, this will be more than sufficient to support the development of linear storm segments. Completely agree with the 3 km NAM in the forecast of a long QLCS feature moving across the entire CWA early Wednesday afternoon. Waiting as we move into the HRRR and ARW time frames for added confidence but there is good agreement amongst the global guidance so don't expect significant change at this point. A few showers and a rumble or two of thunder is possible, especially along and north of I-78 Wednesday morning. The more clearing that occurs, the better the surface heating and larger the instability that can be reached. A slower movement of the surface low, with a longer period for strengthening would likely lead to a greater severe threat as we would sit in the warm sector longer. This is one facet that remains to be worked out so to speak. Climatologically, small hail is possible with any of the storms, but strong to damaging winds will be the main risk with these storms. A strong 850 mb LLJ will advect ample moisture ahead of the front, which will lead to locally heavy downpours. Forward motion of the front should preclude widespread flooding, but can't rule out localized urban and poor drainage flooding with half to locally one inch of rainfall. Highs Wednesday will warm into the upper 60s to the south, with low to mid 60s north of I-78. Clearer skies will lead to warmer highs. Temperatures will plummet behind the front with lows in the low to mid 30s by Thursday morning. A breezy night behind the front should prohibit frost, but went ahead and added patchy frost into areas where the frost-freeze program has started. More of a freeze potential as wind will work against frost potential. A much quieter and cooler day Thursday with highs in the upper 40s north to lower 50s south with partly cloudy skies. It will remain quite breezy with westerly winds gusts from 20 to 30 mph. && .LONG TERM /THURSDAY NIGHT THROUGH MONDAY/... The weather pattern looks to stay fairly busy through the long term as one system will continue to depart early in the period with a brief respite of fair weather Friday into Saturday before the next potential system arrives for later in the weekend. In terms of the details, it will remain a bit breezy Thursday night through Friday as deep low pressure continues to lift north through Atlantic Canada while high pressure sets up to our south. The airmass over the region moderates though so Friday looks to see seasonable temperatures under mainly sunny skies. Friday night into Saturday, high pressure moves off the east coast as an upper level trough moves in to our west with the next low developing near the SE CONUS. There is a lot of uncertainty regarding how this system tracks with the ECMWF and to a lesser degree the GFS have it moving up the coast and bringing widespread rain and gusty winds late Saturday, Saturday night, and into Sunday. Meanwhile the latest GEM is much weaker and surpressed and has the system missing us to the south. Didn't deviate much from the NBM so am forecasting likely POPs for the majority of the area Saturday night which is when, according to current indications, the system look to make its closest pass. The aformentioned system should move out by late Sunday with dry weather looking to start next week. && .AVIATION /02Z TUESDAY THROUGH SATURDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Overnight...VFR under a mostly clear sky. Isolated areas of fog are possible but they will likely not affect the TAF sites. Southwest wind 6 knots or less, becoming variable at most locations. High confidence. Tuesday...VFR with scattered clouds. Southwest wind increasing to around 10 knots with gusts near 20 knots. High confidence. Outlook... Wednesday...VFR initially with sub-VFR conditions with showers and thunderstorms in the afternoon. A return to VFR near or after 00Z from west to east as a cold front crosses the region. Southwest winds from 10 to 15 knots with gusts from 25 to 30 knots. Winds turn westerly behind the front. Moderate confidence. Thursday...VFR with westerly winds from 15 to 20 knots and gusts from 25 to 30 knots. High confidence. Thursday night through Friday...Mainly VFR with W/NW winds 10 to 15 knots with gusts of 20 to 25 knots possible both Thursday evening and again by around midday Friday through the afternoon. High confidence. Friday night through Saturday...Mainly VFR with west winds around 5 knots Friday night becoming SW 5 to 10 knots Saturday. The next system could arrive very late in the day or at night bringing rain and associated restrictions. Moderate confidence. && .MARINE... Sub-Small Craft Advisory conditions on tap through Tuesday. SW winds 10-15 kt. Gusts up to 20 kt will develop Tuesday afternoon. Seas generally at or below 3 feet. Outlook... Wednesday...SCA conditions developing into the afternoon as southwesterly winds gust from 25 to 30 knots. Seas from 3 to 5 feet. Wednesday night and Thursday...Gale force wind gusts likely out of the northwest. Seas from 3 to 5 feet. Thursday night through Friday...SCA conditions likely Thursday night into early Friday before winds subside below SCA levels by late Friday. Friday night through Saturday...Conditions should be mainly tranquil on the seas through this period with Sub SCA winds/seas. However the next system could bring increasing winds and waves by Saturday night. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Davis Near Term...Iovino/MPS Short Term...Davis Long Term...Fitzsimmons Aviation...Davis/Fitzsimmons/Iovino Marine...Davis/Fitzsimmons/MPS