863 FXUS61 KPHI 222023 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 423 PM EDT Wed Apr 22 2020 .SYNOPSIS... A warm front is anticipated to lift into our region tomorrow. Low pressure is expected to move from the Ohio River Valley tomorrow evening to the waters off Nova Scotia on Friday evening. Weak high pressure should follow for Friday night into early Saturday. Another low is forecast to progress from the Ohio River Valley to the waters off Nova Scotia during the period from Saturday night through Monday. High pressure approaching from the west is expected to build across our region on Monday and Tuesday. && .NEAR TERM /UNTIL 6 AM THURSDAY MORNING/... Amplified troughing in the Canadian Maritimes will progress slowly northeastward overnight, with northwest midlevel flow continuing across the Mid-Atlantic. A residual surface pressure gradient exists across the area today, leading to breezy conditions this afternoon. However, as the associated surface low in eastern Canada continues to weaken and lift northeastward, the pressure gradient will relax as high pressure to our northwest approaches. The region will reside in a local col (or mathematical saddle point) during the overnight hours, which will promote a rapid calming of winds during the late evening. It is at this time when temperatures are expected to drop rapidly, with conditions likely approaching freezing in the northern third to half of the area (despite increasing cloudiness and associated warm advection late). Though raw model guidance provides a fairly large range of solutions temperature-wise tonight, think the colder guidance is more likely since (1) higher-level clouds tend not to preclude stronger cooling, at least as much as modeled, in this region, (2) fewer clouds during the late evening should be timed with the weakening winds, and (3) there is good consensus regarding the timing of the nocturnal decoupling. As a result, decided to issue a freeze warning generally from Berks County to Ocean County northward and a frost advisory for southeast PA and southern NJ outside of Philadelphia and the beaches. Confidence is rather low on the specific delineation of these products, and evening trends will be monitored closely to determine if some fine- tuning will be required. The only other concern is with potential for precipitation in the Poconos late tonight. Generally suspect that the precipitation will be well to the north/west of the area, but there are indications that a weak perturbation will straddle the New York/Pennsylvania border between 06z and 12z. Cannot rule out some light rain/snow in the southern Poconos, so kept in some slight-chance PoPs during this period. Any precipitation is expected to be light with minimal impacts. && .SHORT TERM /6 AM THURSDAY MORNING THROUGH FRIDAY/... A strong midlevel vortex will eject from the central Plains on Thursday, with difluent flow downstream in the Appalachians and Mid- Atlantic. Warm-advection precipitation will develop in the Ohio/Tennessee Valleys during the day, spreading northeastward toward the East Coast by late afternoon. GFS/ECMWF timing is decently similar (generally between 18z Thursday and 00z Friday), so confidence was high enough to lower PoPs early in the day and increase them by 00z. Precipitation will ramp up considerably Thursday night, with a lengthy period of occasionally moderate to heavy rain expected as several ingredients come together: (1) an increasing low-level jet in advance of a surface low moving through the Ohio Valley, supplying anomalous moisture into the northern Mid- Atlantic (1.25-1.5 in. PWs 06z-12z Friday), (2) considerable warm advection (aided by the low-level jet) as a zonally-oriented baroclinic zone becomes established east of the low, (3) an upper- level jet streak departing the East Coast, leaving the region in the favorable right-entrance region (allowing for considerable upper- level divergence), and (4) differential cyclonic vorticity advection increasing in advance of the approaching midlevel perturbation (and coinciding with the nocturnal maximum in the low-level jet). The result is QPF on the order of 1-2 inches by midday Friday. Precipitation rates will not be excessive given the overall setup (i.e., the region generally being on the cold side of the attendant surface low); however, would not be surprised to see a few instances of nuisance-type flooding in the usual spots (especially if the QPF maximum overlies the urban corridor, a distinct possibility given the 12z model output). Precipitation may linger through the afternoon via wraparound trajectories on the northwest side of the surface low (likely tracking through the Delmarva region during the day). Decided to increase PoPs a bit for Friday afternoon as a result. However, the heaviest precipitation should be over by this time. Regarding temperatures, stuck pretty close to statistical consensus, though confidence is a little lower than average with highs on Thursday given uncertainty with the increasing cloudiness and approaching precipitation. This results in highs in the upper 40s to low 50s in the southern Poconos and northwest New Jersey and mid 50s to low 60s elsewhere. Temperatures should drop little Thursday night as clouds and precipitation will limit nocturnal cooling. Friday's temperatures will be tricky, as clouds/precipitation will likely limit heating north of the low, but temperatures may soar south of its track. Given the low's track will likely be near or over southern portions of the CWA, this suggests a large temperature gradient may exist across the area. For now, I cooled temps up north slightly and warmed them in Delmarva to increase the general gradient, but the result will likely be much sharper than the current forecast suggests. && .LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/... Overview: The active weather pattern continues into early next week. The low pressure system Sunday and Monday is expected to take a similar path as the Thursday/Friday low pressure system, bringing another chance for widespread rain across the region. Another low pressure system is expected to approach the region mid week next week. Details: Saturday...A surface ridge is expected to briefly build in on Saturday, bringing a lull in between the two low pressure systems. Temperatures will remain slightly below normal, with highs in the 50s and lower 60s. Saturday night through Monday...The next low pressure system will be crossing from the Ohio Valley to the Canadian Maritimes. We should stay mostly in the cold sector, so we expect most of the precipitation to be a steady stratiform rain. While there remains quite a bit of uncertainty with how much rain we will see. However, there is a concern that depending on how much rain we see with the Thursday/Friday system, that some small creeks and streams could already be elevated. We will continue to monitor this especially as we see how the Thursday/Friday system evolves. Monday night and Tuesday...High pressure is expected to once again briefly build in, bringing another period of tranquil weather, albeit cool conditions. Temperatures are expected to be 10 to 15 degrees below normal, with highs in the 40s and 50s. Tuesday night and Wednesday...Another low pressure system may approach the region. This one though appears to be taking a different path than the two earlier systems, with a mid and upper level trough digging southeast out of south central Canada. However, there is considerable difference between the models in how far south this trough will dig. Thus, for now, have stayed close to a blend of guidance through this period. && .AVIATION /21Z WEDNESDAY THROUGH MONDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Tonight...VFR with west to northwest winds 10 to 20 kt with gusts to 30 kt through early evening rapidly diminishing and becoming light/variable overnight. High confidence. Thursday...VFR through most of the day, but clouds will be increasing and lowering with time, and precipitation will likely move in near/after 21z with sub-VFR conditions developing fairly quickly. Light/variable winds becoming southerly around 10 kt late. Medium confidence. Thursday night...Sub-VFR conditions and rain probable with winds generally northeast (north of PHL) to southeast in Delmarva and southern New Jersey (speeds generally 5 to 15 kt). Some LLWS possible. Medium confidence. Friday...Sub-VFR persisting for much of the day, though some improvement may occur late. Rain likely, especially in the morning. Winds generally northeast to north 5 to 15 kt becoming northwest with time. Medium confidence. Outlook... Friday night and Saturday...Mostly VFR conditions are expected. Winds are expected to shift from northerly to southeasterly by mid day Saturday. Wind speeds of 5 to 15 kt. Moderate confidence. Sunday...Conditions could lower to MVFR or even IFR as rain moves into the region. Southeasterly winds may shift to northwesterly through the day with wind speeds of 5 to 10 kt. Low confidence. Monday...Conditions are expected to improve to VFR as the rain leaves the region. Northwesterly winds of 10 to 15 kt. Moderate confidence. && .MARINE... Small-craft advisory conditions continue on the waters this afternoon (and occasionally, a gust to around 35 kt has been observed on the central/southern New Jersey coast). However, winds are trending downward during the past couple of hours, and continue to expect sub-advisory conditions to become the norm on Delaware Bay by mid-evening. Conditions on the Atlantic should improve by midnight. As such, made no changes to the small craft advisory through the evening hours. Thursday through Friday...After a period of light/variable winds in the morning, an east to south wind will become established during the afternoon. Advisory-level winds may develop on the ocean by evening, especially on the Delaware/southern New Jersey waters. During this period, a steady rain should develop and continue through most of Friday. By midday, winds will initially weaken and become somewhat variable in direction, then switch to north and become a little stronger (speeds 10 to 20 kt with higher gusts), with another period of advisory-level winds possible late in the day. Seas will build to 5 feet by Thursday evening and likely remain near/above advisory criteria through Friday. Outlook... Friday night and Saturday...Elevated seas may linger on the coastal waters through this period. Otherwise, winds should stay below SCA criteria. Sunday and Monday...Winds and seas are expected to increase on all waters leading to a prolonged period of SCA conditions. && .FIRE WEATHER... The combination of continuing gusty winds for today along with relative humidities dropping below 30 percent may lead to an increased risk of wild fires. The limiting factor however will be fuels due to recent rainfall. In coordination with our partners, we've issued an SPS highlighting this threat for central and southern NJ (Mercer to Monmouth Counties southward), Delaware, Southeastern PA, and northeastern MD as this area generally saw less than a quarter of an inch of rain Tuesday. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...Freeze Warning from midnight tonight to 8 AM EDT Thursday for PAZ060>062-103-105-106. Frost Advisory from midnight tonight to 8 AM EDT Thursday for PAZ101-102-104. NJ...Freeze Warning from midnight tonight to 8 AM EDT Thursday for NJZ001-007>010-012-013-015-020. Frost Advisory from midnight tonight to 8 AM EDT Thursday for NJZ016>019-021-022-027. DE...None. MD...None. MARINE...Small Craft Advisory until 11 PM EDT this evening for ANZ450>455. Small Craft Advisory until 8 PM EDT this evening for ANZ430- 431. && $$ Synopsis...Johnson Near Term...CMS Short Term...CMS Long Term...Johnson Aviation...CMS/Johnson Marine...CMS/Johnson Fire Weather...Johnson