Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 110528 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1228 AM EST Mon Dec 11 2017 .SYNOPSIS... Mainly dry and cool conditions are expected through Monday. Low pressure will approach from the west late Monday night into Tuesday. This will likely bring a period of accumulating snow, mainly across interior northern Massachusetts, while any snow will quickly change to rain in the coastal plain. A shot of arctic air follows Wednesday and Thursday. Another low pressure system may bring more snow to the region sometime in the Thursday night to Friday night timeframe. && .NEAR TERM /UNTIL 6 AM THIS MORNING/... 10pm update... For the most part, forecast remains on track. However, will add an overnight risk for flurries inland as light echos are already showing up in advance of the approaching shortwave as clouds move over signaling column moistening. Also, will bump up POPs along and offshore of S coastal areas per latest mesoscale guidance beefing up chances for near shower SHRA/SHSN. Will continue to monitor this overnight. Previous discussion... Lake-effect clouds were spilling over the Berkshires into western and central MA. Expecting a mid level shortwave to arrive tonight, which will only spread more clouds from west to east across southern New England, especially after midnight. As such, not expecting good radiational cooling conditions. That said, min temperatures tonight should be below normal. Fairly dry air below 800 mb, so dry weather will prevail for much of the interior. A little different story towards the coast, and coastal waters. The arrival of colder air over the still relatively warm ocean will kickstart the ocean-effect process. This will be mainly in the form of clouds, but cannot dismiss the possibility for some showers around the Cape and islands, including Block Island. A west wind direction would place the greatest risk for any showers across the southern coastal waters. && .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM TUESDAY/... Mid level shortwave departs during Monday morning, followed by a brief period of higher pressure. A nearly stationary front may sag far enough south to reach portions of southern New England late Monday into Monday night. Mainly dry with clearing sky once any lingering morning showers move offshore. High temperatures remaining below normal. A low pressure over the Great Lakes starts to exert more influence on our weather late Monday Night. Another case of increasing clouds after midnight, with a risk for some precipitation late. Given low temperatures below freezing across pretty much all of southern New England, light snow would be expected over land, with the possibility of some rain over the ocean waters. Thinking around one inch or less across southern New England through Monday night. It will all come down to timing. A faster arrival would give more time for snow to accumulate before daybreak. Slower timing would mean even less snow. At this point, the majority of any precipitation from this low pressure would occur after daybreak Tuesday. Some impact on the Tuesday morning commute is expected, especially from Worcester county west. && .LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/... Highlights... * Accumulating snow Tue across interior N MA with any snow quickly changing to rain in the coastal plain * Arctic air and bitter cold wind chills follow Wed/Thu * More snow possible sometime Thu night to Fri night Tuesday... Accumulating snow is the main forecast issue, but latest guidance suggests focus for appreciable snow accum will be confined to interior northern MA west of I495 and especially the Berkshires. Amplifying northern stream trough across the Great Lakes will eventually become negatively tilted over the northeast. Primary low moves into the eastern Lakes while a secondary low is forecast to develop across SNE, but not in time to stop southerly low jet from advecting milder air north across SNE. Models all indicate rapid warming below 850 mb. Any snow early Tue will quickly change to rain across northern CT, RI and E MA as low level warming advances north. However, there could be an issue with a brief period of freezing rain south of the Mass Pike in the interior during the transition from snow to rain. Further into the interior, especially north of the Pike and west of I495, the snow will last through the morning but even here we expect an eventual changeover to rain in the afternoon. The exception may be across the east slopes of the Berkshires which may remain mostly snow. Preliminary snowfall forecast is for 2-5 inches across northern Worcester county to the Berkshires and possibly into the higher terrain in western Hartford county, but a few locations could reach 6 or 7 inches in the Berkshires, where decent omega in the snow growth region persists the longest. Any accum across RI and eastern MA will likely be limited to less than an inch. Winter weather advisories will likely be needed for interior northern MA, with a low probability for marginal warnings for the Berkshires. Wednesday and Thursday... Behind the departing low pres, a surge of arctic air will invade the region with blustery NW winds as deep upper trough moves through. Temps well below normal with highs mostly in the 20s and lower 30s near the south coast. The gusty winds will make it feel much colder. Mainly dry weather during this period, but can`t rule out a few flurries Wed as the upper low moves across New Eng. Thursday night into Friday night... Another amplifying northern stream trough approaches from the west with multiple shortwaves leading to the potential for some accumulating snow sometime during this period. However, low confidence on timing as models are struggling with resolving what will become the dominant shortwave. May have to watch the late Fri/Fri night period per ECWMF and EPS ensembles. Saturday and Sunday... Mainly cold and dry Sat then moderating temps Sun ahead of next upper trough approaching from the west as SW flow develops. Milder air may be accompanied by a few rain or snow showers. && .AVIATION /05Z MONDAY THROUGH FRIDAY/...
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Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tuesday/... Through today...High confidence. Mainly VFR except brief MVFR CIGS mainly across Cape/Island terminals this morning. Occasional very light SHSN/SHRA through mid day. W winds gradually shift to the SW. Tonight...Moderate confidence. VFR to start, will give way to MVFR then IFR early AM Tue as CIGS lower and light SN/FZRA moves in mainly after 06Z from W-E. SN accums would be light. Winds shift around to the E-NE. Tomorrow...Moderate confidence. Mixed MVFR and occasional IFR conditions in a mix of SN/FZRA giving way to all RA through sunrise into mid day Tue. Risk for NW MA to remain SN long enough for accums to exceed reach 3-6 inches with generally less further SE. Low risk also for a period of FZRA before the change to RA. Conditions improve after 20Z from S-N. KBOS Terminal...High confidence in TAF, lower categories don`t move in until after 06Z tonight. KBDL Terminal...High confidence in TAF. Outlook /Tuesday Night through Friday/... Tuesday: MVFR/IFR conditions with snow changing to rain. Windy along the coast with areas gusts to 30 kt. Tuesday Night: Mainly VFR, with local MVFR possible. Breezy. Slight chance SHSN. Wednesday: Mainly VFR, with local MVFR possible. Windy with gusts to 35 kt. Wednesday Night: VFR. Windy with gusts 25-35 kt. Thursday: VFR. Windy with local gusts to 30 kt. Thursday Night and Friday: MVFR/IFR conditions possible. Chance SN.
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&& .MARINE...
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Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tuesday/...High confidence. Gusty west winds gradually diminish Monday afternoon. However, rough seas will take longer to subside, especially across the southern coastal waters, where the fetch will be longer. Therefore, most Small Craft Advisories will continue into tomorrow. Chance for showers, especially across the southern coastal waters tonight into Monday morning, then again late Monday night. Outlook /Tuesday Night through Friday/... Tuesday: Low risk for gale force winds eastern waters with gusts up to 35 kt. Local rough seas. Rain likely. Tuesday Night: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Rough seas up to 9 ft. Local visibility 1 to 3 nm. Wednesday: Strong winds with gusts up to 40 kt. Rough seas up to 14 ft. Slight chance of snow showers. Visibility 1 to 3 nm. Wednesday Night: Moderate risk for gale force winds with gusts up to 40 kt. Rough seas up to 15 ft. Local visibility 1 to 3 nm. Thursday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of rough seas. Thursday Night and Friday: Winds less than 25 kt. Local rough seas. Chance of snow and rain. Visibility 1 to 3 nm.
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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 1 PM EST this afternoon for ANZ231>234-250. Small Craft Advisory until 6 PM EST this evening for ANZ235- 237. Small Craft Advisory until 7 AM EST this morning for ANZ251. Small Craft Advisory until midnight EST tonight for ANZ254>256.
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&& $$ SYNOPSIS...Belk/KJC NEAR TERM...Belk/Doody SHORT TERM...Belk LONG TERM...KJC AVIATION...KJC/Doody MARINE...Belk/KJC

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