662 FXUS61 KBOX 161129 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 729 AM EDT Thu Apr 16 2020 .SYNOPSIS... A fast-moving weather disturbance races past southern New England today, bringing scattered rain or snow showers. Low pressure passes near or south of New England Friday night into Saturday with a period of rain for most, mixed with wet snow in the interior. Brief clearing on Sunday but the weather pattern remains unsettled through early next week. Temperatures start to moderate closer to normal by Sunday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 720 AM Update: Bands of wet snow associated with mid-level wave and cold front are reducing visibility to as low as one- half mile (most recently at Lawrence) are moving rather quickly eastward, their southward extent roughly to the Mass Pike and into Boston. Special Weather Statement in effect until 8 AM to address low visibility in some of these snow bands. Wet pavement from these but up to an inch of snow on grass appears possible. Cold front moves off the coast by late morning with brief WNW wind shift. Clearing skies though even with more sun by afternoon highs only in the 40s today. Previous discussion follows... Main portion of a mid level shortwave moves past our region today. There will also be a weak cold front, which should move offshore later this afternoon. This combination will be enough to generate some rain and snow showers, which were had already moved into the western half of southern New England. The atmosphere is not really primed for a lot of precipitation, mainly due to limited moisture. Temperatures were low enough to support snowfall. This will remain the case until daybreak, then temperatures will rise too much to maintain snow. Still thinking snow accumulations will be light, an mainly on the grassy surfaces. Starting to think there may be a second round of light rain showers along the east coast of MA early this afternoon as colder air arrives aloft. This is not a certainty, but did include a risk for these showers with the forecast. High temperatures expected to be around 10 degrees below normal. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM FRIDAY/... High pressure takes over our weather tonight, providing drier and colder weather. Low temperatures still about 10 degrees below normal. Thinking Friday will be mainly a dry day across most of southern New England as a high pressure moves offshore from the Mid Atlantic coast. Some question as to how quickly a low pressure can approach southern New England from the central Appalachians. Looking at an increasing risk for some light rain Friday afternoon, but not really certain on the precise timing. High temperatures should be slightly higher than today, but still 5-10 degrees below normal. && .LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/... Highlights... * Overall active weather pattern with several opportunities for precip. * Rain, mixed with minor accumulations of wet/dense snow mainly interior north Fri night as a surface low passes over or just south of Southern New England. While confidence has increased in precip types, snow accumulation forecast still is uncertain due to marginal temperatures. * Cooler, raw Saturday with periods of rain, clearing late. * Remains unsettled Sunday thru Tues. Best chance of storminess Monday thru Mon night, though impacts will depend on storm track. Details... Friday Night into Saturday: Surface low and attendant warm front now taking shape across the eastern Rockies is forecast to move across the northern mid- Atlantic into Southern New England Friday night into Saturday. While there remain some details that are still unclear and unresolved, confidence continues to grow in this frontal system bringing at least a period of rain to interior rain/wet snow mix with minor snow accumulations for Friday night. The slower/stronger trend in the 500 mb and surface features in the guidance over the last 24 hours has maintained itself into the 00z cycle. For Friday night, though some wet-bulb cooling should take place as precipitation arrives around mid-evening to midnight from west to east, temperatures are expected to remain marginal (in the 33-36F range). Driven primarily by warm advection resulting to a west-east band of 850 mb frontogenesis, we should see a modest thump of steadier precipitation rates and a mixover from rain to wet snow during the overnight hrs mainly for areas north and west of the Hartford-Providence corridor and N/W of I-95. Temperatures likely to remain warm enough south and east of this area to stay as predominantly rain, though in the Hartford-Providence-Boston urban areas, a period of predominantly rain mixed with wet snow appears to be the most likely outcome with slushy coatings and mainly wet pavement. Move further north and west and a period of accumulating wet snows looks more likely, especially west of I-495. On the accumulation forecast, with late-season events accumulating snows tend to be primarily confined to the higher terrain, on grassy surfaces and snow tends to accumulate better at night. This will also be a wet/dense snow with snow to liquid ratios likely under climo and closer to something less than 10:1, especially with snow microphysical parameters being sub- optimal for efficient snow growth. So for the interior western- central MA into northern parts of CT and northern RI, have shown forecast totals in the 1-3" range for the Pioneer Valley, east slopes of the Berkshires into northern Worcester County into northern sections of Tolland County. Totals then taper to less than an inch towards the Hartford-Providence corridor and into Boston Metrowest. ECMWF ensemble probabilities of 3" snowfall have increased in the interior, but important to keep in mind that these are for pure 10:1 SLR snows and this probably won't fit that situation. This should prove to be a fairly limited impact snowfall with better chances of accumulations on grass versus pavement, and while it's not out of the question that some portion of the interior may need lower-end Advisories in the coming days given the upward trend in the snow probabilities, confidence isn't yet there to hoist as yet. While late-season snows aren't unheard of, will point out that accumulating snowfall over an inch this late in the season hasn't really occurred in quite some time. To put in context for the climo sites, if an inch or more of snow verifies, the last time Boston and Providence saw such a snow accumulation this far into the calendar was back on April 28, 1987, for Worcester April 27th, 1993, and back on April 28th 1987 for Providence. So at least 27 years ago. Another question is the speed at which the system exits, which affects PoPs/Wx and temperatures into Saturday. Most of the guidance has trended slower Saturday - in fact the 00z ECMWF has a closed isoheight at 500 mb late Saturday as the system pulls offshore. While rain should predominate on Saturday, to sort of course-correct if the slower trend continues I've opted to slow the taper-down of PoPs and lowered temps significantly compared to NBM/Superblend. With east/northeast onshore flow, clouds and rain around, other than in western CT/western MA late in the day, Saturday has the look of a pretty chilly/raw day if the slower trend in guidance comes to fruition. Lows in the 30s for Friday night, with highs in the upper 30s to low- mid 40s. Sunday: Fairly active/unsettled weather pattern continues. However Sunday looks to be a relatively quiet day, as shortwave ridging/height rises aloft build across Southern New England in the wake of the Friday night - Saturday system. Mostly sunny conditions with a warming trend to temperatures (extent of the low-level warming varies across models, casting some uncertainty on highs), though mid to high clouds approach late from the W/NW associated with a cold front. Sunday Night through Monday night: Though recent model runs appear to be coming into somewhat better focus on major features for this period, the timing and track of those remains unclear. A cold front is progged to sag southward from NH/VT and eastern NY early Sunday night. Though some limited chances for showers are possible in western MA from the cold front, most models generally dissipate/wash this feature out. The more apparent feature in this period is a southern-stream shortwave trough moving quickly from the Deep South/Gulf Coast States Sunday night into Monday. Most models show a strengthening cyclone moving through the southern Appalachians and then offshore Monday into Monday night, to a track near or southeast of the 40N/70W benchmark with progged sub-980 mb sea-level pressure. Digging into the ensembles, the ECMWF ensemble mean and individual models in its membership are stronger and closer to Southern New England compared to the GEFS, but both are still far enough offshore to keep any cold-conveyor precip confined to Cape Cod, the Islands and coastal eastern MA/RI later Monday into Monday night. While at the moment there doesn't appear to be any significant threat for precip to advance across interior CT into western/central MA, we are still several days away and will have to watch if the coastal storm track were to shift even further to the coast, in turn putting those interior locations more at play for precip. Storm track will dictate p-types as well, but anticipating this to be mainly plain rain. Will thus opt for a dry forecast except for coastal eastern/southeast MA and into RI. Winds will likely pick up on the the waters later Monday, so could see a potential for marine headlines for at least the offshore marine zones. Otherwise too much uncertainty to highlight any other possible/potential impacts. Tuesday - Wednesday: Next system looks to arrive on the heels of the Monday night coastal low sometime Tuesday or Tuesday night. Northern-stream trough energy shifts SE across Southern New England, with brief subsidence early Tuesday giving way to a Clipper-type low and related cold frontal passage. This system could be starved of moisture but a shot of seasonably cold air aloft looks to be in the cards post-frontal. && .AVIATION /11Z THURSDAY THROUGH MONDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. 12z TAF Update: Today...High confidence. Short-term MVFR/IFR visby in SHRASN thru 14z mainly at BOS/BED and at ORH as cold front moves offshore. Improvement to VFR thereafter, with diurnal CU and bases 5-7 kft. Winds shift from the northwest back to the west. Expect gusty conditions as the mixed layer grows. Should see gusts generally on the order of around 20 kt. Should see the mixed layer decouple toward or just before 00Z, which will bring an end to the gusty winds. Tonight and Friday...High confidence VFR conditions. KBOS Terminal...High confidence in the TAF. KBDL Terminal...High confidence in the TAF. Outlook /Friday Night through Monday/... Friday Night: MVFR/IFR conditions possible. Breezy. RA, SN. Saturday: Mainly VFR, with areas IFR possible. Breezy. Chance RA. Saturday Night through Sunday: VFR. Breezy. Sunday Night: Mainly VFR, with local MVFR possible. Breezy. Slight chance SHRA. Monday: Mainly VFR, with areas MVFR possible. Breezy. Chance SHRA. && .MARINE... Marginal conditions for Small Craft Advisories, with gusty winds up to 25 kt with seas around 5 feet across the outer coastal waters. Winds expected to be at their strongest tonight as colder air moves over the waters. Outlook /Friday Night through Monday/... Friday Night: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of seas approaching 5 ft. Rain. Saturday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Local rough seas. Chance of rain. Saturday Night: Winds less than 25 kt. Local rough seas. Sunday: Winds less than 25 kt. Seas up to 5 ft. Sunday Night: Winds less than 25 kt. Seas up to 5 ft. Chance of rain showers. Monday: Low risk for Small Craft Advisory winds with local gusts up to 30 kt. Seas up to 5 ft. Chance of rain showers. && .HYDROLOGY... Runoff from the Monday rainstorm continues to impact area waterways. River levels on the CT River at Hartford expected to fall below flood stage this afternoon. The CT River at Middle Haddam is more complicated, with flooding expected due to the combination of river level and tidal fluctuations. River Flood Warnings have been issued for both forecast points. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 6 AM EDT Friday for ANZ232>234. Small Craft Advisory from 8 PM this evening to 8 AM EDT Friday for ANZ231-251. Small Craft Advisory until 8 PM EDT Friday for ANZ235-237-255- 256. Small Craft Advisory from 6 PM this evening to 8 PM EDT Friday for ANZ250-254. && $$ SYNOPSIS...Belk/Loconto NEAR TERM...Belk/Loconto SHORT TERM...Belk LONG TERM...Loconto AVIATION...Belk/Loconto MARINE...Belk/Loconto HYDROLOGY...Staff