187 FXUS61 KPHI 281129 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 629 AM EST Sat Nov 28 2020 .SYNOPSIS... High pressure will build towards the area this weekend, before eventually moving offshore Sunday afternoon. A complex low pressure system will move in from the southwest on Monday and will slowly lift north of the area on Tuesday. High Pressure will then build in towards the middle of next week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 630 am update: Some sprinkles have progressed through the northern half of the area early this morning. Have added mention to the grids through mid-morning, but have not added mentionable PoPs. Rest of forecast appears on track. Previous discussion follows... A shortwave trough is moving into the Northeast early this morning, with extensive mid-to-low level clouds near and in advance of the vort max. There are some precipitation echoes in New York and Pennsylvania in association with the larger-scale lift attendant to this perturbation. Models remain quite hesitant to bring measurable QPF to our CWA this morning when the strongest lift will move into the region. Although sprinkles are possible in some locations early this morning, measurable precipitation is unlikely, so PoPs remain at or below 10 percent. The main effect from this system for our area will be winds, and even the winds will not be especially noteworthy. Model soundings and statistical guidance do suggest gusts of 20 to 25 mph during the afternoon, so have increased the winds in our forecast somewhat. Variable cloudiness is anticipated, as the lowest clouds will likely be on the decrease with cold/dry advection increasing today. However, midlevel cloudiness attendant to the trough will probably be present through at least the morning, and instability clouds are expected after stronger mixing commences. Highs today should be slightly above seasonal averages, as the west to west-northwest winds that develop by afternoon should provide some downsloping to counter the cold advection that occurs. Basically used a statistical consensus for max temperatures, with most guidance in good agreement. Resulting values are in the mid to upper 40s in the Poconos and vicinity (where clouds may be more persistent), and 50s to around 60 elsewhere (highest in Delmarva). && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SUNDAY/... The shortwave trough will swing quickly offshore this afternoon and evening, leaving broad midlevel ridging in its wake for the eastern U.S. A surface high will barrel eastward into the Mid- Atlantic tonight, moving offshore on Sunday. Winds will diminish after sunset, and may become light and variable overnight. With clearing skies from the upstream large-scale descent, expect a good night of radiational cooling across the area. Trended low temperatures down a degree or two in most spots, especially outside of the urban corridor. If winds decouple quickly, low temperatures may be a few degrees lower than forecast, but I am a little hesitant to go much lower than guidance given that the surface high does look to move south of the area and that its timing is not overly favorable for quick evening decoupling. Winds will be lighter on Sunday, with a transition in direction from west to south. Mostly sunny skies are expected to start the day, with perhaps a gradual increase in high cloudiness during the afternoon as the early week system begins to influence the area. Currently think maximum temperatures will be similar to those seen today. && .LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/... Overview... As mentioned in previous discussions a hemispheric pattern change is expected next week, with the overall synoptic pattern becoming characterized by a longwave trough over eastern North America, and a high amplitude ridge over the west. In terms of specifics, a southern stream low (at present a cutoff low over the desert southwest) will constructively phase with a northern shortwave in the vicinity of the Ohio Valley/Great Lakes Sunday night into Monday. This system will become a closed low which will only slowly lift north through mid-week, with the cold core making its closest approach to our area on Tuesday. The system's surface low will pass to our west Monday into Tuesday with High Pressure then returning for the middle of the week. Another shortwave will rotate cyclonically around the western periphery of this initial low towards the middle of next week, with another area of low pressure potentially approaching our area from the south late week into next weekend. Dailies... Sunday night-Monday... This will be the most active period of the extended as the strengthening southern stream low makes it closest approach to our area. Rain chances will begin to increase toward daybreak Monday as warm advection aloft gets into gear, with rainfall overspreading the entire region Monday morning as the low approaches. There is a fairly robust synoptic setup for locally moderate-heavy rainfall on Monday as we will find ourselves in the left exit region of the southern- stream UL jet (although we are a bit removed of the right entrance of the northern jet, so we don't get the complimentary jet interaction), while at the low-levels intense warm air/moisture advection will occur driven by a strong LLJ (850 flow approaching 60-70kts near the coast). Guidance continues to indicate some surface-based instability present late Monday morning into Monday afternoon which yields potential for some shallow convection. Did add slight-chance thunder to the forecast to account for this, and may also need to watch for a low-topped severe threat (given the very strong wind fields aloft). QPF amounts are generally similar to the previous forecast (e.g. in the 0.75-1.5 inch range), which (if accurate) would be a good soaking but would likely not lead to many significant hydro impacts. Beyond the rainfall potential, winds will need to be watched with this system due to the aforementioned LLJ. Per usual in these warm advective, southerly flow regimes the main question is if any of this anomalously-high momentum will mix down to the surface. For this package generally went with peak overland gusts in the 25-35kt range for Monday, but sustained winds and gusts will probably reach wind advisory criteria along and near the coast (where the LLJ is strongest and low-lvl stability lowest). On a somewhat- related note, went on the higher end of guidance for MaxTs and dewpoints on Monday, as guidance tends to underestimate the thermodynamic response in these setups. Monday night-Tuesday... The first cold front associated this system looks to cross the area Monday evening, and the system's dry slot then moves into our area. This should result in any rainfall tapering off, with temperatures steadily cooling through the night. While steady rain and showers are generally expected to end overnight, thermodynamic profiles are supportive of drizzle, so I added mention of this to the forecast. Heading into Tuesday, we may see some light precipitation as the mid- lvl cold core approaches, but do not expect anything significant (and ptype would likely remain as rain/drizzle). It will be noticeably cooler on Tuesday with highs generally in the upper 40s. As the colder and drier air filters in late in the day, there may be a brief window of elevated winds primarily during the evening if boundary layer mixing occurs. Updated the wind forecast to reflect this. Wednesday-Thursday... A respite from active weather is expected in this time period as the low lifts northward and high pressure builds in from the south. Wednesday will be seasonably cool with highs only in the low to mid 40s, while temps should warm up about 5 degrees on Thursday. Another low potentially approaches from the south Friday/Saturday but the track/timing of this system is still uncertain. && .AVIATION /12Z SATURDAY THROUGH WEDNESDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Today...VFR with variable cloudiness, though cloud cover will likely diminish this afternoon. Any CIGs generally expected to be 4-10 kft. West to west-northwest winds will increase during the morning, becoming around 10-15 kt during the afternoon. Gusts to 20 kt or so possible. Moderate confidence. Tonight...VFR with west to northwest winds up to 10 kt in the evening becoming light and more variable during the overnight hours. High confidence. Outlook... Sunday-Sunday night... VFR although CIGs will likely start to gradually lower Sunday night as a storm system approaches. Winds mostly light and variable on Sunday but becoming mainly light south or southwest. Overnight, a 5 to 10 kt southeast wind should develop. High confidence. Monday-Monday night... Sub-VFR conditions expected much of the day Monday in rain, which may be heavy at times. Conditions may turn drier overnight but lingering restrictions are possible. Southeast winds during the day could gust 25 to 35 kt, with a period of higher gusts possible especially near the coast. LLWS will also be possible, particularly at sites where wind gusts are weaker. Winds should diminish overnight. Moderate confidence. Tuesday-Tuesday night... Mainly MVFR or VFR. Winds becoming south or southwest at 5 to 15 kt. Higher gusts possible from 21Z to 03Z. Moderate confidence. Wednesday... VFR, with W-SW winds 10-15kts. Moderate confidence. && .MARINE... West to northwest winds will increase this morning to 10 to 20 kt. Cannot rule out a gust or two to 25 kt this afternoon, but think conditions will remain predominantly sub-advisory. Winds are expected to diminish slowly tonight. Seas generally near or below 3 feet are expected through tonight. Outlook... Sunday...Sub-advisory winds/seas expected. Monday... S-SE winds increase considerably Monday morning to gale force with gusts to 40-45kts on the ocean waters. Cannot rule out a brief period of storm force gusts up to 50 kts during the late afternoon or evening. Seas will also increase into the 8-12ft range. Rain expected with thunderstorms possible. Monday night-Wednesday... SCA conditions expected for more or less the entirety of this period with SW wind gusts remaining around 25kts and seas only gradually decreasing from around 8 ft Monday night to 5-6ft on Wednesday. && .TIDES/COASTAL FLOODING... With the strong storm system affecting the Northeast and Mid- Atlantic on Monday and the full moon early Monday morning, the threat for coastal flooding will be elevated on the Atlantic coast, Delaware Bay, the tidal Delaware River, and the eastern shore of Chesapeake Bay. The main high tide of interest is Monday morning (the higher of the two astronomical tides), but this will also be a little on the early side relative to the increasing onshore winds. Nevertheless, guidance is suggesting at least some potential for spotty to widespread minor coastal flooding for the Monday morning high tide. We have somewhat increased concern for Delaware Bay, the tidal Delaware River, and the eastern shore of Chesapeake Bay, where strong south to southeast winds and later high tides relative to the Atlantic coast both would favor higher observed anomalies. The latest forecast goes well above consensus in these areas for the Monday high tides, with some spots reaching minor flooding. Will be monitoring model trends closely, as timing and track changes of the surface low will be critical in determining the ultimate coastal flooding impacts from this storm. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Carr Near Term...CMS Short Term...CMS Long Term...Carr/Staarmann Aviation...CMS/Carr/Staarmann Marine...CMS/Carr/Staarmann Tides/Coastal Flooding...CMS