601 FXUS61 KPHI 211828 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 228 PM EDT Wed Oct 21 2020 .SYNOPSIS... The weakening cold front to our west will push to the north today. A low pressure system will cross the Great Lakes region on Friday, dragging a cold front through our area late Saturday. Another system may impact the region early next week. && .NEAR TERM /THROUGH TONIGHT/... Fog and low clouds have been slow to dissipate today, which appears to be tied to a couple of factors. The first is that low-level steering flow has been slow to veer, keeping a southeasterly component to (light) surface winds through midday. As stronger mixing has slowly set in, southwesterly winds have become established where stronger clearing is present, and this trend should continue for the rest of the day. However, temperatures will likely be lower than forecasts today in southeast Pennsylvania and northern/central New Jersey, where the clouds/fog have persisted the longest. Have made some adjustments to hourly temperatures through this evening to accommodate for the latest satellite trends. The second factor for the somewhat slow dissipation of the clouds/fog is the lower sun angle, and this will be an important consideration for the forecast tomorrow (see discussion below). Tonight's main forecast challenge is fog potential again. The synoptic pattern is rather stagnant in our area at first glance, but to our north, a strong vort max will be moving east- northeast through southeast Canada. This will force the surface ridge axis offshore to progress eastward as well, which should contribute to continued veering of the surface flow. Such a change would be somewhat unfavorable for widespread/dense fog development (given the relative lack of an onshore component). Additionally, the slower dissipation of clouds today may prevent a near-surface inversion from developing as readily overnight (instead remaining more aloft), which would favor low stratus versus fog. Model soundings are indicative of such behavior, with a lack of a stronger inversion at the surface (especially NAM-based guidance). However, given the prevalence of fog this morning and similar low-level thermal profiles (along with light winds), I am expecting fog to develop across the area once again. At this point, though, I am not particularly confident regarding the density/coverage of the fog. Certainly cannot rule out widespread dense fog, but confidence is too low currently to place in the forecast. With clouds/fog, anticipating temperatures to be quite warm, with lows in the upper 50s to lower 60s across the forecast area. && .SHORT TERM /THURSDAY/... Main concern for Thursday is how quickly lower clouds and/or fog dissipate(s) and resultant effects on temperatures. Sound familiar? A particularly complicated aspect of the forecast tomorrow is that low-level steering flow will likely become more onshore during the morning (again, light, but worth noting) as a surface ridge becomes reestablished offshore. This may act to slow clearing and/or promote advection of cooler air from the ocean versus the diabatically-warmed air over land. Guidance has been trending downward for highs as a result, and this seems reasonable given the developing synoptic pattern. Additionally, the lowering sun angle as we head toward the cool season is worth considering, especially since models were a little fast to mix the boundary layer on Wednesday. One thing to watch will be a weak perturbation moving east- northeast in the faster midlevel flow to our northwest. By 18z Thursday, the low-amplitude trough will be migrating through the Saint Lawrence Valley. Some guidance is developing showers in advance of the trough in New York and possibly Pennsylvania during the afternoon. Some of these showers may clip the Poconos area and vicinity. For now, think chances are too low for measurable rain to mention in the forecast, but cannot rule out later inclusion if model trends continue. Given the unusually high spread in guidance values for highs and questions regarding more substantial warming, generally leaned the forecast toward consensus for max temperatures on Thursday. However, I did weigh colder guidance a little more highly, which results in highs generally in the low to mid 70s across the area. && .LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/... We will see a return to more of an easterly flow on Thursday and Friday. Another round of fog and low clouds (possibly drizzle) is possible Thursday night into Friday morning due to the southeast flow and advecting moisture inland from the coast. The low that moves through the Great Lakes region on Friday will start to push a cold front towards our area. This front looks like it will actually be able to make its way through our area on Saturday/Saturday night. Not a significant amount of moisture is available with the boundary as it approaches from the west but we could still see some showers move through, mainly later in the day Saturday into Saturday night. As the front starts to push through, the wind will veer to the west and then northwest. This should help to dry us out (a nice change from the very wet conditions this week) with a push of some more seasonable and cooler air arriving for Sunday. Beyond the frontal passage (which may end up remaining nearby and not fully clearing the region), things look muddled once again as the guidance struggles to find some consistency. Overall, it looks unsettled with another low pressure system potentially moving through our area sometime later Monday or Tuesday. Ahead of that system is the potential for more low clouds/fog/drizzle in the easterly flow and then we should see some more showers with the system moving through. In essence, we will once again be trending back to more of a wet pattern across the Mid- Atlantic. && .AVIATION /18Z WEDNESDAY THROUGH MONDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Through this evening...Slow improvement to VFR is expected at most TAF sites by 20z or so, with southwest winds 5 to 10 kt. However, lingering MVFR CIGs are possible. Moderate confidence. Tonight...Another round of low clouds and/or fog is anticipated tonight. Conditions may favor low stratus versus the widespread fog seen this morning, but expecting both to be problematic at most of the terminals, especially after 06z. Light and variable winds. Moderate confidence. Thursday...Morning sub-VFR CIGs/VSBYs are expected, with gradual improvement to VFR by or during the afternoon. Winds will likely be somewhat variable through the day and generally around or below 5 kt. Low confidence, especially with timing category changes. Outlook... Friday...MVFR/IFR conditions in fog/low clouds likely in the morning, possibly hanging around into the afternoon. Improving conditions to VFR possible by the afternoon. East to southeast winds less than 10 knots. Moderate confidence. Saturday...MVFR or lower conditions possible in showers as a cold front crosses the terminals, mainly during the afternoon or evening. Southerly winds veering to the west through the day and then northwest/north Saturday night. Speeds less than 10 knots. Generally moderate confidence but lower confidence on timing of showers and category changes. Sunday...MVFR or lower conditions possible in showers and low clouds as the cold front remains near the region. Northeast winds less than 10 knots. Low confidence. && .MARINE... Two primary concerns exist for the marine zones the next 24 hours. The first is fog potential tonight. Another round of dense fog is possible, but conditions are not quite as ideal tonight as a west to southwest wind becomes established. Nevertheless, some visibility restrictions are anticipated, with slow improvement on Thursday. The other concern is slowly building seas from Hurricane Epsilon. Seas will likely reach 5 feet by late morning and will continue rising during the afternoon. A small craft advisory remains in effect beginning at 11 am for the Atlantic waters. Winds will be quite light through Thursday, likely under 10 kt most of the time. Outlook... Thursday night through Sunday...A Small Craft Advisory is in effect through Friday. It will likely continue through the weekend before seas start to subside late Sunday or Monday. Seas build and should reach 5 feet by late Thursday morning on the southern ocean waters. Seas may not reach 5 feet on the northern waters until Thursday afternoon. Seas will continue to build through Friday and Saturday to 5 to 8 feet and remain elevated through the weekend. Winds will be out of an easterly direction with speeds around 10 knots, becoming west to southwest on Saturday, then northwest Saturday night and increasing to 10 to 15 knots before becoming more northerly on Sunday. Some gusts around 15 to 20 knots are possible Friday through Sunday. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory from 11 AM Thursday to 6 PM EDT Friday for ANZ450>455. && $$ Synopsis...Meola Near Term...CMS Short Term...CMS Long Term...Meola Aviation...CMS/Meola Marine...CMS/Meola