Area Forecast Discussion
Issued by NWS Boston, MA

Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
-- Highlight Changed Discussion --
-- Discussion containing changed information from previous version are highlighted. --
000 FXUS61 KBOX 151404 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 1004 AM EDT Thu Apr 15 2021 .SYNOPSIS... Rain develops across western Massachusetts and northern CT slowly spreads east this afternoon into early this evening. Widespread rain, heavy at times, is expected tonight into Friday with rain changing to accumulating snow in the Berkshires and Worcester Hills. A period of wet snow is also possible in some of the lower elevations of interior southern New England. Gradual improvement is expected over the weekend as the storm exits out to sea, with drier, milder weather likely Sunday. Warmer weather and rain chances return for the first half of next week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 1000 am update... going forecast for the rest of the morning into the afternoon in pretty good. All I did was incorporate in the latest observations into the trends for the next 3 hours. Otherwise it`s all about the slow moving bowling ball of an upper low clearly visible on water vapor over the eastern Great Lakes. Generally light rain continues across the western portions of MA and CT. Latest HRRR run still suggests the precipitation will only slowly move east and take until late this afternoon to reach eastern MA. Given radar trends with precipitation moving more northward than eastward, that all makes sense. Current forecast already has this timing, so I will leave it untouched. Otherwise we will be carefully interrogating the 12z guidance as it comes in to see what adjustments need to be made in a very tricky forecast where subtle changes in temperatures, precipitation rates, track of the low pressure all have significant impacts as to what will end up happening. 7 am update... Showers have made their way into western Massachusetts and Connecticut this morning, and coverage will only increase through the day, if slowly from west to east. No significant changes to the forecast for the 7 am update except to bring things in line with observations. Previous Discussion... Deep closed upper level across the eastern Great Lakes will slowly move east today and be located in southwest NY by early evening. In response, the forcing for ascent will increase and allow rain showers to develop this morning across western MA/CT. Expect the rain to become widespread in this region during the afternoon. Meanwhile, the blocky pattern and upper level ridging will keep the weather dry this morning across eastern MA & RI. Showers should gradually advance into central MA/RI around mid afternoon, but far eastern MA may remain dry other than a spot shower or two until late afternoon/early evening. High temperatures today will range from the upper 40s to around 50 across interior southern New England, but readings should drop a few degrees during the afternoon. Portions of interior eastern MA & RI may see highs top out in the lower to middle 50s, where some peeks of sunshine are possible this morning. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM FRIDAY/... * Significant Elevation Snow Expected in the Berkshires and possibly the northern Worcester Hills * A small/slushy accumulation is possible in some lower elevations This is a text book meteorological setup for a late season snowstorm across the high terrain of southern New England tonight and Friday. However, given very marginal thermal profiles and a high dependence on intensity/track specific snow accumulations are uncertain. We will break things down a bit more below. A deepening closed mid level low will approach from the west tonight and track across the region on Friday. This is a classic setup for heavy precipitation with a deep moisture plume and very strong forcing. The strong omega will result in dynamic cooling with thermal profiles becoming isothermal tonight into Friday across the high terrain. Rain should flip to snow this evening across the Berkshires and a bit later tonight across the northern Worcester Hills. 925T dropping below 0C in this region give us confidence in ptype changing to wet snow, which will fall heavy at times. Some wet snow is even possible in the lower elevations of interior southern New England very late tonight and Friday during the heaviest precipitation. Total precipitation should be on the order of 1 to 2.50 inches, which will continue through Friday afternoon. The guidance has trended a bit further southeast with the mid level low track over the past 24 hours. Therefore, we upgraded to a Winter Storm Warning for a good portion of the east slopes of the Berkshires. Heavy Wet snow of 4 to 8 inches is anticipated with localized amounts up to 10 inches possible at elevations near and above 1500 feet. Given the southeast trend; we opted to issue a Winter Storm Watch for the northern Worcester Hills. This will give the next shift a look at the 12z guidance and either upgrade to a warning or replace with a Winter Weather Advisory. This should be a classic wet snow event with surface temperatures in the 32-33 degree range. Therefore, some power outages will be possible where snow amounts reach or exceed 6 inches. Despite the recent mild weather; roads will become snow covered in the high terrain given the intensity expected. A few inches of snow are also possible in the CT Hills and possibly even some small/slushy accumulations in some lower elevations of interior southern New England. Heavy precipitation intensity may be enough to flip some lower elevations to a period of wet snow. This will be highly dependent on the track of the mid level low and its associated dryslot. We still are favoring mainly heavy rain across eastern MA/RI. However, some of our latest guidance indicates that there could even be a burst of wet snow for a time on Friday if the system tracks as far southeast as the NAM indicates. This is certainly something that later shifts will have to watch closely. So to sum up...significant wet snow accumulations are anticipated for portions of the high terrain. Amounts are highly dependent on banding and precipitation intensity with very marginal temperatures. SLR/s will be on the order of 7-9:1 in the higher elevations and even less if any accumulations down in the lower elevations. Later shifts will certainly need to fine tune snow accumulations and Ptype given this very fragile setup. && .LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/... Friday night and Saturday... The mid level blocking pattern continues, meaning a slow exit of the low overhead during this time period. The further in time the better chance for drier conditions, but wraparound moisture on the backside of the low lingers overhead bringing rain showers into at least mid Saturday morning (perhaps Saturday afternoon if some of the slower guidance verifies). Either way, cloudcover dominates, with maybe some breaks of sun. Winds, too, will remain elevated, especially Friday night, then tapering off through the day Saturday as the surface low exits and the pressure gradient relaxes. Overall not the best Spring day, with scattered showers, breezy winds, and highs that don`t make it out of the 40s. Sunday and Monday... Feeling less optimistic about a much nicer day Sunday than I did 24 hours ago (i.e. things are trending cooler and more unsettled). However, expecting far from a washout, we just may have some showers out there. Mid level flow still indicates a brief ridge Saturday night, but it`s looking weaker and more transient now, with nearly zonal flow continuing Sunday. As such, ripple or two in the flow will bring cloudcover and the risk of some light scattered showers. Model soundings even indicate a bit of instability under a modest 850 mb cold pool. It remains most likely that the majority of locations stay dry, though. This as the low level airmass will generally be moderating from Saturday, giving highs back in the 50s. Monday is more of the same in terms of the difficult to predict shortwaves in zonal flow which may bring periods of rain and clouds. We will see improvement in the temperatures as flow becomes more westerly and 850 mb temps finally rise above 0C. This should translate to highs at the surface in the upper 50s and low 60s. Tuesday and Wednesday... The main story in this period will be the surge in temperatures on southerly flow ahead of the next shortwave trough. Along with that, though, comes unsettled weather and rain chances each day; too soon to nail down who sees rain and when, but we`re looking at temperatures in the upper 60s for a time. && .AVIATION /14Z THURSDAY THROUGH MONDAY/... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. 12Z update... Today...High Confidence. MVFR to localized IFR conditions across the interior with cigs/vsbys trending lower as the day wears on. Meanwhile, VFR conditions persist for much of the day across eastern New England. Rain expands across western MA/CT this morning and slowly moves eastward this afternoon. However, steady rain may not reach the eastern MA coast until early evening. E winds of 10 to 15 knots with gusts around 20 knots. Tonight and Friday...Moderate Confidence. IFR to localized LIFR conditions dominate tonight into Friday. Widespread rain will transition to wet snow across the Berkshires and Worcester Hills tonight and continue into Friday. Rain elsewhere may transition to a period of wet snow even in the lower elevations of the interior during times of heavier precipitation overnight and Friday. E winds will gust between 20 and 30 knots on Friday with the strongest of those winds along the coast. KBOS TAF...Moderate Confidence in TAF. KBDL TAF...Moderate Confidence in TAF. Outlook /Friday Night through Monday/... Friday Night: MVFR/IFR conditions possible. Breezy. Chance RA. Saturday: Mainly VFR, with local IFR possible. Breezy. Slight chance RA. Saturday Night: Mainly VFR, with local MVFR possible. Breezy. Sunday: Mainly VFR, with local MVFR possible. Slight chance RA. Sunday Night: VFR. Monday: VFR. Slight chance RA. && .MARINE... Today through Friday...High Confidence. Slow moving intensifying low pressure later today and particularly tonight into Friday will result in strong SCA to Gale force easterly wind gusts across our waters. We opted to go with strong small craft headlines for our western waters and Gale Warnings for our eastern waters. Seas should build to between 7 and 13 feet across our open waters. Outlook /Friday Night through Monday/... Friday Night: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Rough seas up to 12 ft. Chance of rain. Saturday: Moderate risk for Small Craft Advisory winds with gusts up to 25 kt. Rough seas up to 10 ft. Slight chance of rain. Saturday Night: Winds less than 25 kt. Local rough seas. Sunday: Winds less than 25 kt. Areas of seas approaching 5 ft. Slight chance of rain. Sunday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Slight chance of rain. Monday: Winds less than 25 kt. Slight chance of rain. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...Winter Storm Watch from 8 PM EDT this evening through Friday evening for MAZ004-026. Winter Weather Advisory from 8 PM this evening to 8 PM EDT Friday for MAZ003-009-010. Winter Storm Warning from 8 PM this evening to 8 PM EDT Friday for MAZ002-008. RI...None. MARINE...Gale Warning from 5 AM to 2 PM EDT Friday for ANZ231-232-250- 251-254-255. Small Craft Advisory from 2 PM this afternoon to 8 PM EDT Friday for ANZ233>235-237-256. Small Craft Advisory from 5 AM to 8 PM EDT Friday for ANZ230- 236. && $$ SYNOPSIS...Frank/BW NEAR TERM...Frank/BW/Nash SHORT TERM...Frank LONG TERM...BW AVIATION...Frank/BW MARINE...Frank/BW is the U.S. government's official web portal to all federal, state and local government web resources and services.