769 FXUS61 KBOX 202351 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 751 PM EDT Tue Oct 20 2020 .SYNOPSIS... A frontal boundary will remain over or near the region tonight, providing an abundance of clouds and fog developing overnight. The front lifts north Wednesday providing decreasing cloudiness in the afternoon along with mild conditions. High pressure builds in Thursday supporting dry and warm weather, followed by cooler temperatures Friday. A cold front may bring a few brief showers late Saturday into Saturday night, but the main impact will be cooler/seasonable temperatures by Sunday. Perhaps a period of a wet weather impacts the region sometime early next week. && .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/... 740 pm update... folded in the latest observations into the first few hours of the forecast. have also added in drizzle for much of the region. looking at soundings via bufkit, all the models are showing the atmosphere from the surface up to 2500-3000ft or so becomes fully saturated overnight. suggests to me that drizzle is a pretty good bet. in fact, some of the recent obs along the south coast, where the moisture is currently most abundant, are reporting drizzle. Some of that may be some left overs of the batch of showers, but is a sign of what is expected to develop. Latest hi-res model composite reflectivity output shows light dBz values developing overnight. I'll take that as another signal of drizzle. Much of the guidance also suggests the fog may become dense across many areas. I'm not totally sold on that. Forecast soundings, although showing near calm winds at the surface (a plus for fog), also show 10-15kt of wind developing 500ft above ground in many areas. Typically that means a little bit of mixing in the boundary layer that results in more stratus than dense fog at the surface. Will go ahead and issue a short special weather statement (SPS) about fog. Honestly, if people don't immediately recognize what to do if driving in any fog, any SPS or advisory we issue isn't going to help much. 430 PM Update... Tonight... WAA scattered showers with possibly an isolated thunderstorm will continue for a few more hours over southeast MA, before low/mid level jet exits to the northeast and results in thermal and moisture advection waning. Otherwise as short wave trough over southern Quebec exits into the maritimes, in its wake 1030+ mb high noses southward thru Maine and into northeast MA overnight. This will setup a weak warm front around the Mass Pike area, as previous forecaster alluded too in earlier discussion. However other than spotty light drizzle near this boundary tonight, mainly dry weather prevails. But with dew pts ranging from 55-60 tonight, well above the normal overnight low, sufficient low level moisture for fog, locally dense to develop. Not enough confidence to hoist a dense fog advisory but definitely something later shifts will need to monitor. Given these dew pts, lows tonight only in the mid to upper 50s. Hence mild for late Oct when the avg low is 40-45. && .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/... 430 PM update... Wednesday... Short wave trough and accompanying parent low over Great Lakes exits into Ontario. This will lift warm front from over southern New England northward. Low clouds and morning fog should slow lift and burn off to at least partial sunshine during the afternoon. Although given the low sun angle of late Oct, it may take into the mid afternoon for sunshine to develop. This will impact afternoon temps. For now will go conservative and forecast highs 65-70, but could be warmer if sunshine develops sooner. Still warmer than normal with avg highs in the upper 50s and lower 60s. As warm sector advances northward, winds become southwest 10-15 mph with gusts up to 20 mph possible in the afternoon, dew pts will climb into the low and mid 60s especially across CT-RI and south of the Mass Pike, providing a touch of humidity and a warm feel to the airmass. Other than early morning patchy fog/drizzle, mainly dry weather should be featured the remainder of the day. Wed night... Cold front from parent low approaches southern New England from the northwest. Low risk for a few showers in the evening across northern MA. Otherwise these showers should dissipate with the loss of daytime heating. Thus mainly dry weather prevails along with mild temps (mid to upper 50s) as cooler air lags well behind the front. && .LONG TERM /THURSDAY THROUGH TUESDAY/... 430 pm update... 12z suite of models show good continuity from prior runs and consistency among one another through the weekend, which lends to above average forecast confidence for Thursday through Sunday. Basically looking at a dry stretch of weather through Saturday, with above normal temperatures. The warmest day still looks to be Thursday. A cold front is expected to move through late Saturday/Saturday night, with perhaps a few light showers. Sunday will see a return to more normal temperatures. GFS the last couple of days at least has been indicating a rather wet Monday and Tuesday, while the ECMWF and Canadian were much drier. 12z runs of the EC and Canadian have trended wetter, but at this point there is still plenty of model uncertainty so I trended lower on PoPs from the GFS-based guidance, but still a bit higher than the NBM model consensus which still incorporates previous models runs which were drier. For a few additional details regarding the forecast each day, keep reading if you really want to. No guarantee that it will really be worth your time though... Thursday: Models still showing 15-16C temperatures at 925mb, and with light westerly low/mid level flow with at least a little downsloping from higher terrain to our west, and a good amount of sun, we should be able to reach into the 70s in many areas away from the coast. There is still evidence of a shallow "cold" front that oozes southward across the region during the day as high pressure builds north of here. With winds eventually shifting N/NNE later in the afternoon, the immediate eastern coastal areas will be cooled off a bit more than inland areas. On the marine side of things, Wavewatch model guidance still shows a pretty hefty batch of wave energy from the storm with swell periods around 15 seconds reaching our south/southeast facing shores. Still expect the surf to start ramping up late in the day Thursday because of that. Friday: Upper ridge builds, which will keep "Epsilon" well offshore, though we should expect continued swell energy reaching the coast with building seas. Would think we'll see some rough surf all day Friday, and Saturday as well. 925mb temperatures will be 2-3C cooler than Thursday, and with low level flow more east/southeast it will end up not as warm as Thursday. However with plenty of sun and highs in the 60s (perhaps 70F around Hartford?) I for one won't be complaining. Saturday: We should sneak one more "warm" and mostly dry day in. A cold front will be moving through at some point during the day. Right now it appears it will be primarily during the afternoon and evening, so we should have time to warm up enough to touch the 70F mark in many areas. Of course, a more speedy arrival and highs will be restricted to the 60s. Not a lot of upper level forcing or deep moisture, so pretty low PoPs are still forecast. Sunday: Overall a typical fall day. Should have a good amount of drying post-front, so expecting a good deal of sun. With 925mb temperatures down to 3-5C and northeast winds, highs in the 50s will be about it. Monday/Tuesday: Models suggest that deepening southwest flow aloft will lift the late Saturday cold front northward as a warm front and become a focus for precipitation development as isentropic lift strengthens and weak areas of low pressure move along it. Where exactly that front sets up (over us? to our north/west?) dictates how wet we get. There are actually some indications in the GFS and ECMWF that the front is far enough north that some surface instability develops across CT on Monday. Guess I was in a slightly fiesty mood that I decided to add isolated thunder for parts of CT Monday...which also lines up with WFO OKX's thoughts. Kept PoPs generally in the 30-45% range, but I suspect that will change between now and then. && .AVIATION /00Z WEDNESDAY THROUGH SUNDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. 00z Update: Overall moderate confidence. Currently still some VFR out across the CT River Valley, but widespread MVFR further east, with IFR visibilities and ceilings rapidly developing across the south coast/southeast MA. All guidance indicates IFR conditions will spread north and west as the night goes on, and I see no reason to doubt that. Have all of the TAF locations ultimately going to IFR. There is uncertainty as to exactly how low the ceilings go, how low the visibilities go and when all of it will happen. So please take those error bars into account. Have visibilities going down to 1/2sm in many locations. They may end up being a little higher than forecast. Have also added in drizzle in most TAFs. It's possible it won't be everywhere. Improvement tomorrow will be slow. Given the low sun angle this time of year, and not a lot of wind to mix things around, IFR conditions will continue thru at least 16z in many locations. Even then, MVFR conditions are expected to last the rest of the day Wednesday. We may see VFR conditions develop in some inland areas Wednesday night, but IFR (low ceilings, fog) will redevelop along the south coastal areas and spread inland. Large uncertainty at this point as to how far inland. KBOS Terminal...moderate confidence KBDL Terminal...moderate confidence Outlook /Thursday through Sunday/... Thursday through Friday Night: VFR. Saturday: Mainly VFR, with local MVFR possible. Saturday Night through Sunday: VFR. Breezy. && .MARINE... 430 pm update... Tonight...any evening showers with isolated thunder should end later this evening. Then low clouds, fog and drizzle likely limit vsby overnight. Some of the fog could locally be dense. Wednesday...boundary over southern New England lifts north as a warm front, so S-SE winds become SSW and increase 10-15 kt, gusts up to 20 kt near shore possible. Low clouds, fog and drizzle in the morning will improve by midday and especially in the afternoon. Wednesday night...cold front moves across the waters. Little if any showers expected. Outlook /Thursday through Sunday/... Thursday: Winds less than 25 kt. Seas up to 5 ft. Thursday Night through Friday: Winds less than 25 kt. Areas of rough seas. Friday Night through Saturday: Winds less than 25 kt. Rough seas up to 8 ft. Saturday Night through Sunday: Winds less than 25 kt. Rough seas up to 9 ft. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...Nocera/Nash NEAR TERM...Nocera/Nash SHORT TERM...Nocera LONG TERM...Nash AVIATION...Nash MARINE...Nocera/Nash