Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 072116 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 416 PM EST Thu Dec 7 2017 .SYNOPSIS...
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Dry and seasonably chilly weather will continue into Friday. Low pressure will bring the potential for light to moderate snow accumulations Saturday and Saturday night across much of southern New England, with a mix of rain and snow confined to the Cape and Islands and portions of southeast MA. Another coastal storm may bring more snow and/or rain Tuesday into early Wednesday. This storm will be followed by unseasonably cold and windy weather late Wednesday into Thursday.
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&& .NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
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WV imagery shows tight, but fast moving vort-max just W of the region this afternoon, which should cross the region after 00Z this evening. Little fanfare is expected, except a continuation of the wind trajectory which will carry Lake effect moisture/streamers E, into W portions of the forecast area. Will continue with a low risk for SHSN or flurries through the evening. After the wave passage this evening, only a minor response with mid-upper lvl ridge building which will begin to carry upper lvl moisture associated with a stalled boundary to the S, yielding a gradual increase in upper lvl clouds, even as the lake effect moisture is essentially cutoff. Therefore, although winds do go light, not a perfect radiational cooling scenario, but give a dip in dwpts into the teens to around 20F, overnight mins should still drop into the low-mid 20s with some decoupling. Otherwise, dry forecast overnight.
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&& .SHORT TERM /6 AM FRIDAY MORNING THROUGH FRIDAY NIGHT/...
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Tomorrow... Weak mid lvl ridging and the development of confluent flow aloft will, thanks to lingering subsidence inversion, slow the moisture loading. This loading process begins from top-down, with building upper then mid lvl clouds throughout the day. PWATs end the day (as in 00Z) near 0.7 inches along extreme SE MA. However there remains a layer from the sfc to H8 with dwpt depressions at or exceeding 10F. Given this, will continue with a mainly dry forecast through much of the daylight hours, with only slight chance POPs at ACK by late afternoon and evening. However, timing continues to slow with POPs on meso-scale guidance, something to consider with future updates. Temps are expected to be cooler than today as previous forecaster noted. This is thanks to a combination of a colder start in the morning, combined with more widespread cloud cover throughout the day. This is in spite of ever so slight warm advection aloft within the modest mid lvl ridge. Tomorrow night... Continually increasing moisture through the sounding. It will take some time for the low lvl dwpt depressions to fully be overcome, however this is mostly likely across ACK and Cape Cod, closer to the src of lift along the stalled frontal boundary to the S, and within the deepest moisture as PWATs reach nearly +1-sigma by 12Z Sat. Therefore, will yield very gradual increase in POPs from SE to NW through the overnight hours such that most remain dry, but a low risk for showery precip possible across extreme SE MA toward Sat morning. Min temps much warmer than Thu night, as cloud cover and increased moisture will limit diurnal curve, yielding mainly upper 20s NW to low-mid 30s SE. P-type with the precipitation is somewhat tricky, although soundings suggest that the sfc to H92 wetbulb is near or below 0C, am not confident dynamics are strong enough to support much additional cooling. SSTs surround the Cape/Islands remain close to 10C, so this will likely help to moderate temps somewhat. Given these factors, will favor a mainly SHRA scenario, although if precip rates increase toward morning as the low pres wave to the S, develops this could yield a transition to light and generally non-accumulating snow/rain mix.
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&& .LONG TERM /SATURDAY THROUGH THURSDAY/...
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Highlights... * Light to moderate snow accum Sat/Sat night w/highest risk near the I-95 corridor * Coastal storm may bring more snow and/or rain Tue into early Wed * Blustery and unseasonably cold weather follows late Wed into Thu Overview... Anomalous ridging over the western CONUS and north Atlantic will result in persistent deep trough across eastern Canada to the Gt Lakes and northeast. Northern stream shortwave energy rotating around the western ridge and digging southward will bring risk of multiple snow events to SNE while also delivering a blast of arctic air around Wed/Thu timeframe as piece of the polar vortex breaks off and descends south into New Eng. Details... Saturday and Saturday night... Models continue to trend west with potential moderate impact snow event for Sat/Sat night. This is the result of an amplifying northern stream shortwave which digs south across the Gt Lakes and Ohio valley, producing a more robust coastal low tracking further NW and closer to the benchmark with deep moisture axis overspreading much of SNE. Confidence is increasing in at least a moderate snow event across portions of SNE as the guidance is converging toward 0.50" QPF extending back along and west of the I-95 corridor with heavier QPF to the SE. The westward trend has also introduced more warming in the boundary layer with ptype issues across Cape/Islands and possibly into far SE MA. It is still too soon to know if this westward trend will continue as the responsible shortwave at 12z today was still near the Arctic ocean and not captured within the better sampling upper air network. This may not be well sampled until the 12z Fri model suite so additional changes still likely. Given the ECMWF and GFS were in good agreement on track/intensity of the coastal low and QPF field we used a blend of these guidance sources as a basis for the forecast. This suggests heaviest snowfall may occur near and a bit west of the I-95 corridor with lesser amounts further SE and especially toward Cape Cod due to potential mixing issues or changeover to rain. We will have preliminary max forecast of 4-6 inches from NE CT and central MA to interior SE MA, a bit less toward CT valley. Snow amounts decreasing to 2 inches near the Cape Cod canal and less than an inch for outer Cape/islands where ptype issues more certain. Heaviest snow likely Sat afternoon and evening before tapering off. This scenario actually in decent agreement with EPS ensembles. No watches yet due to areal uncertainty, marginal warning event and we are still 2-3 days out in the model world. However it is important to emphasize that further changes are possible. A further west trend would bring heavier snow back across western MA and CT with rain/snow line possibly threatening BOS-PVD corridor. While a SE trend brings more snow near the coast with less interior. Sunday through Friday... Main emphasis was placed on Sat/Sat night event so we did not focus much on this portion of the forecast. Another digging northern stream shortwave may result in a potential significant coastal storm sometime Tue-Wed timeframe but pattern is complex with multiple shortwaves moving through the flow. Low confidence on timing/location and track of the storm. What we are confident about is a blast of arctic air and windy conditions following this storm later Wed into Thu.
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&& .AVIATION /20Z THURSDAY THROUGH TUESDAY/...
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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 Friday Night/... Through tonight...High confidence. VFR. W winds gust to around 20-25 kt near shore, but remain around 10 kt inland. Flurries possible in extreme NW MA. Tomorrow and tomorrow night...High confidence. VFR continues although with CIGS gradually lowering through the day. Lowest CIGS Nantucket/Cape Cod overnight, but generally only low VFR. Some light SHRA mainly ACK, outer Cape Cod through early Sat morning. Winds slowly shift to SW tomorrow afternoon, but remain at around 10 kt. KBOS Terminal...High confidence in TAF. KBDL Terminal...High confidence in TAF. Outlook /Saturday through Tuesday/... Outlook /Saturday through Tuesday/... Friday Night: Mainly VFR, with local IFR possible toward daybreak Sat. Breezy. Chance -RA/-SN across southeast New England. Saturday: IFR developing as snow overspreads the region. Rain/snow mix possible Cape/Islands. Saturday Night: MVFR/IFR conditions possible. Snow tapering off overnight. Sunday: Mainly VFR, with areas MVFR possible. Breezy. Slight chance SHSN. Sunday Night: VFR. Breezy. Monday: Mainly VFR, with local MVFR possible. Breezy.
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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 Friday Night/...High confidence. Overnight into tomorrow... W winds will increase slightly this evening, with gusts to around25 kt. Seas respond, increasing to 5-6 ft on the ocean waters by early AM. Therefore, will continue small craft advisories through about mid-morning to mid day tomorrow, until seas gradually dissipate. Tomorrow night... Light flow shifts SW. Mainly quiet boating weather except for some SHRA on the SE waters. Outlook /Saturday through Tuesday/... Friday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Chance of rain and snow late. Saturday: Winds less than 25 kt. Rain and snow likely. Local visibility 1 to 3 nm. Saturday Night: Winds less than 25 kt. Local rough seas. Rain and snow likely. Local visibility 1 to 3 nm. Sunday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Rough seas up to 10 ft. Slight chance of rain/snow showers. Areas of visibility 1 to 3 nm. Sunday Night: Moderate risk for Small Craft Advisory winds with gusts up to 25 kt. Rough seas up to 9 ft. Monday: Moderate risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of rough seas.
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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 7 AM EST Friday for ANZ231>235-237. Small Craft Advisory from 7 PM this evening to 5 AM EST Friday for ANZ230-236. Small Craft Advisory until 10 AM EST Friday for ANZ250-251. Small Craft Advisory until 1 PM EST Friday for ANZ254>256.
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&& $$ SYNOPSIS...KJC/Doody NEAR TERM...Doody SHORT TERM...Doody LONG TERM...KJC AVIATION...KJC/Doody MARINE...KJC/Doody

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