Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 070019 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 719 PM EST Wed Dec 6 2017 .SYNOPSIS...
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A departing offshore cold front this evening gives way to a trend toward colder temperatures. Dry and seasonably chilly weather continues Thursday and into much of Friday. Low pressure will bring the potential for accumulating snow Friday night and Saturday across Rhode Island and Southeast Massachusetts, possibly farther inland pending exact track of low pressure offshore. Temperatures will average a bit colder than normal into early next week with perhaps a coastal storm bringing the potential for some rain and/or snow next Tuesday into Wednesday followed by bitterly cold weather.
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&& .NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...
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715 PM update... Post frontal cold air advection continues to overspread southern New England. SPC mesoanalysis has 850 temps currently +3C over Nantucket to -7C entering Berkshire county of western MA. Thus much colder than last night with lows tonight in the 20s, low 30s along the coast. Main change to the forecast was to insert more cloudiness /mid to high clouds/ across CT/RI and much of eastern MA in response to continue SW flow aloft and associated upper level jet streak. Otherwise previous forecast on track. Earlier discussion below. ================================================================ Cold air advection persisted this afternoon, as temps remained nearly steady or slowly fell. For tonight surface ridging extends into southern New England. Expecting mostly clear skies with light west winds, with some cirrus brushing us from the south this evening. Exception could be over the higher terrain of the east slopes of the Berkshires, where lake effect moisture could bring some stratocumulus. Elsewhere, low level westerly flow expected to provide a downslope component to limit spread of any stratocumulus. Expecting good radiational cooling tonight with overnight lows in the 20s, except some low to mid 30s near the shoreline.
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&& .SHORT TERM /6 AM THURSDAY MORNING THROUGH THURSDAY NIGHT/... Thursday... Weather influenced by high pressure centered to the SW of our area, as well as dry air from SW mid level flow. Just enough moisture in the low levels to produce some diurnal stratocumulus, mostly sunny skies expected. Low level winds out of the west continue to provide a downsloping component. Temperatures will remain seasonable with highs in the upper 30s to mid 40s, though some locales in the higher terrain of the east slopes of the Berkshires only reach the mid 30s. Thursday night... Dry and cold weather expected Thu night with weak surface ridging and continued SW mid level flow. Our area is in between low pressure systems, one to the NW over the Great Lakes region, and another well to the S. Mostly clear to partly cloudy skies along with light winds should allow for overnight lows in the upper teens and 20s. Could see somewhat less cold lows in the low to mid 30s along some portions of the shoreline. && .LONG TERM /FRIDAY THROUGH WEDNESDAY/... 425 PM update... Highlights... * Seasons 1st snowfall possible Fri ngt/Sat but details uncertain * Chilly with a few snow showers possible Sunday * Potential coastal storm may bring rain/snow to the region next Tue/Wed followed by bitter cold Friday... Sunshine to begin the day but mid and high clouds on the increase as approaching mid level trough over the OH valley in the morning advances eastward and backs the mid level flow. This allows offshore moisture to advect northeast into southern New England during the day from south to north. All model guidance suggest precip may come onshore toward nightfall across Nantucket and Marthas Vineyard. Model soundings across this area suggest blyr may be too warm for all snow at the onset, so a mix of rain and snow possible. Regarding temps, chilly airmass over the region with -8C at 850 mb and -4C at 925 mb 18z Friday. Model sounding suggest mixing heights almost to 850 mb so highs should range from the upper 30s to lower 40s. These temps are a few degs colder than normal for early Dec. Mid 40s possible along the south coast given southwest winds streaming across the relatively warm ocean waters that are in the mid to upper 40s. Friday night thru Saturday night... Still lots of uncertainty regarding how far northwest deep moisture and lift track from offshore frontal wave. Most of the deterministic guidance including 12z NAM/GFS/EC/Ukmet and GEFS ensembles place sharp qpf gradient over RI and southeast MA. The 12z Euro ensembles are a bit farther inland from southeast CT across RI into eastern MA. Keep in mind average model error at this time range (60-84 hrs) is about 125 miles! Thus risk for accumulating snow ranges from eastern CT to the Worcester Hills into northeast MA (northwest edge of envelope of solutions) to offshore (eastern most edge of solutions), including areas in between. To further complicate the forecast blyr and surface temps may be too warm for an all snow event over the Cape and Islands, so rain may lower the snowfall potential here. Therefore lots of uncertainty continues. Hence important to keep this portion of the forecast probabilistic rather than deterministic as all potential outcomes remain in play. As previous forecaster mention it will come down to trough amplification entering the Great Lakes late Sat/Sat night. Tracing back the jet energy that will carve out this mid level trough was coming onshore to Alaska this morning. Unfortunately 12z upper air soundings are no longer available across the state of Alaska. This may result in a different model trend beginning with tonight`s 00z guidance as this jet energy becomes better sampled by more 00z upper air soundings along with increased aircraft data at 00z vs 12z. Sunday... Potent mid level trough moves across the area with strong forcing for ascent. Steep lapse rates develop in response to -34C at 500 mb moving over the area. However limiting factor here is lack of deep layer moisture. This should limit areal coverage of any snow showers to isolated for widely scattered. Chilly, about 5-8 degs colder than normal with highs 35-40. It will becoming windy during the evening with the trough passage. Monday through Wednesday ... Monday with begin very cold start with lows in the teens and 20s. Dry weather likely prevails. Highs probably only recovering to 35- 40, possibly into the 40s along the south coast as winds become southwest. Around Tue arctic front drops southward with cyclogenesis developing along or ahead of the front. Large model spread here on location/timing of potential coastal/ocean low. Much more certain southern periphery of arctic air overspreading the region mid week with 850 temps falling to -16C to -20C! && .AVIATION /00Z THURSDAY THROUGH MONDAY/...
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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 Thursday Night/... 0015Z Update... No major changes from previous TAFs. Earlier discussion below. ================================================================== Tonight into Tomorrow...High confidence. VFR. West wind 5-15 kts during Thu, highest near the shoreline. Thursday Night...VFR. KBOS Terminal...High confidence in TAF. KBDL Terminal...High confidence in TAF. Outlook /Friday through Monday/... Friday: VFR. Breezy. High confidence. Friday Night: Mainly VFR, with areas IFR possible in SN across RI and southeast MA. Low confidence on snow shield. Saturday through Saturday Night: MVFR/IFR conditions possible in areas of snow. Low confidence on areal extent of snow shield. Sunday: Mainly MVFR-VFR in widely scattered snow showers. Becoming Breezy in the evening. Monday: Mainly VFR. Breezy.
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&& .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Thursday Night/...High confidence. Tonight and Thu...West winds persist overnight and on Thu. Lingering southerly swell keeps rough seas over the southern and eastern outer coastal waters as well as RI/BI Sounds. Could also have some gusts to 25 kts tonight on the southern outer coastal waters. Small Craft Advisory continues thru tonight and into Thu for those areas. Thursday Night...Lingering swell keeps rough seas (and SCA headlines) over the southern outer coastal waters and the waters east of Cape Cod. It may also bring the need for renewed SCA headlines for seas across the southern outer coastal waters and RI/BI Sounds. Also a chance for wind gusts up to 25 kt. Outlook /Friday through Monday/...Moderate to high Friday: Winds less than 25 kt. Seas locally approaching 5 ft. Chance of rain. Friday Night: Winds less than 25 kt. Snow likely, chance of rain. Areas of visibility 1 to 3 nm. Saturday: Winds less than 25 kt. Seas locally approaching 5 ft. Snow likely, chance of rain. Visibility 1 to 3 nm. Saturday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Chance of rain, chance of snow. Areas of visibility 1 to 3 nm. Sunday: Winds less than 25 kt. Local rough seas. Chance of rain showers, slight chance of snow showers. Areas of visibility 1 to 3 nm. Sunday Night: Winds less than 25 kt. Local rough seas. Monday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Local rough seas. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 2 PM EST Thursday for ANZ235-237- 250. Small Craft Advisory until 5 AM EST Friday for ANZ254>256. && $$ SYNOPSIS...Nocera/NMB NEAR TERM...NMB SHORT TERM...NMB LONG TERM...Nocera AVIATION...Nocera/NMB MARINE...Nocera/NMB

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