Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 141159 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 659 AM EST Thu Dec 14 2017 .SYNOPSIS...
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Fast moving low pressure will track south of New England this morning bringing a light snowfall to the south coast and especially the Islands. Cold conditions will continue through the upcoming weekend. Scattered snow showers may fall across south coastal areas and the higher terrain Friday night into Saturday, along with gusty winds. Temperatures will turn milder early next week, but will also bring the chance of rain and/or snow showers. Mainly dry conditions expected Tuesday into Wednesday, but some light precipitation may linger across portions of the region.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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*** Light snow accumulations likely immediate south coast and Cape/Islands this morning *** 640 AM Update... Light snow continues to fall steadily across N CT/RI into SE Mass, with some spotty light snow as far N as KCEF-KORH and even into the northern Berkshires as seen on 11Z observations. Noting visibilities as low as 1-2SM from about a KHFD-KUUU-KEWB line, about where a band of somewhat higher reflectivities are seen on KBOX 88D radar imagery since 11Z. Temps were mainly in the teens well inland to the mid and upper 20s along the shore where dewpoints have been rising slowly over the last few hours, helping to enhance the light snow. Near term forecast pretty much on track, but have updated to bring conditions current. Temps were running a degree or two lower than forecast which was updated. Previous Discussion... Fairly robust mid level shortwave and attending sfc low tracks south of New Eng this morning. Large scale synoptic forcing assocd with this shortwave and left exit of the upper jet will combine with an area of low level frontogenesis to bring modest omega to the south coast and Islands. Cross sections indicate this forcing for ascent punches through favorable DGZ from 12z-16z with some instability above this layer which should bring a brief period of light to moderate snow this morning to the south coast and Islands. The limiting factor is dry air in low levels which will erode initial snowfall. Dewpoints currently in the single numbers across SNE, but dewpoints are rising along the immediate south coast and Islands and light snow is breaking out along the CT coast. Used a blend of global guidance and HREF ensemble guidance for QPF with 15-20:1 snow ratios to derive snowfall. Not much change from previous forecast with potential for 1-2 inches along the south coast and 2-3 inches over the islands. May see a coating to less than an inch up to BDL-PVD-TAN corridor, with just some flurries to the north where low level dry air is more pronounced with sharp drop off in omega. Snow will exit the Cape/Islands by midday with gradual clearing and increasing sunshine moving in from NW to SE during the afternoon. Another unseasonably cold day with highs mostly 25-30 degrees with some lower 30s near the south coast. Becoming blustery in the afternoon with WNW gusts 20-30 mph developing making it feel much colder.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM FRIDAY/... Tonight... Clear skies and diminishing wind as high pres moves to the mid Atlc coast with ridging extending into New Eng. This will result in good radiational cooling which will be enhanced by fresh snow cover near the coast. Lows dropping into the single numbers, except teens near the coast. Friday... Next mid level trough and shortwave approaches from the west as coastal low pres develops well to the south. Sunshine will give way to increasing clouds in the afternoon as the column moistens. Modest large scale forcing develops late in the day ahead of the mid level trough with best chance for any late day snow showers across SE New Eng assocd with a weak inverted trough and increased low level moisture. Still rather cold with 925 mb temps -8 to -10C which will yield another day with highs mostly in the 20s, but winds will be lighter and less of a factor. && .LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/... Overview... Overall, noting an active, progressive pattern in place across the lower 48, especially starting this weekend as the blocking high amplitude ridge across the western U.S. breaks down. This will bring weak mid level short waves eastward in the fast flow aloft. After several days of below normal temperatures, models continue to signal milder air starting to move into the northeast early next week. With any progressive pattern, however, noting widening model solution spread with the timing and track of weather systems especially across the northern stream, so have lower confidence with the track and timing of any systems beyond Monday. For now, will see a couple of weak lows passing near or SE of New England into this weekend, each bringing some light rain and/or snow showers but will move across rather quickly. As the upper pattern becomes nearly zonal, will start to see temperatures moderating closer to seasonal normals by early next week, though may still see some light precip into mid week mainly along coastal areas and across the E slopes of the Berkshires. Details... Friday night and Saturday... Light precipitation along the western periphery of low pressure moving well SE of the 40N/70W benchmark brings scattered snow showers Friday night. At this point, the best chance will occur along the immediate S coast, Cape Cod and the islands. Current timing suggests that the precip should push offshore by Sat morning. With light QPF amounts, less than 0.1 inches, should see less than an inch snow accumulations. As the low exits, W-NW winds will pick up as low level lapse rates increase during Saturday. Could see gusts up to 25-35 kt, highest across SE areas where the highest low level jet moves across. With temps running up to 10 degrees below seasonal normals, will see wind chill values in the teens to lower 20s during the day. Sunday and Monday... High pres ridge builds east across the eastern seaboard by Sunday morning, so winds will diminish by daybreak. Will see another cold day, however, as mid level winds continue from the NW, though will start to back to W later in the day. Another chilly day with highs from the mid-upper 20s across the higher inland terrain to the 30s across the coastal plain. A weak warm front will approach Sunday night, so may see some overrunning precip try to develop across western by around 00Z Monday, then pushing E quickly through the night and during Monday. Temps will hold mainly in the 20s except the lower-mid 30s along the immediate coast overnight as the precip develops. Thermal profiles suggest mainly light snow Sun night, but not a lot of precip to work with so not expect much in the way of snow accumulations at this point. As winds shift to SW, will see milder air finally push into the region. Expect spotty snow showers to change to rain during the morning, but may linger a bit longer across the higher inland terrain. Expect highs close to seasonal normals for mid December. Tuesday and Wednesday... With a nearly zonal mid level steering flow across most of the country, will see weak systems work eastward but tend to weaken as they approach. Not expect much in the way of precipitation either Tuesday or Wednesday, but timing and track of systems are in question with the fast flow aloft. Have mentioned only slight chance POPs across the higher inland terrain and near the coast through this timeframe, but confidence is low. && .AVIATION /12Z 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 Friday/...High confidence. Today...High confidence. Period of snow with MVFR-IFR conditions moves along the south coast and Cape/Islands through 16Z. Northern edge of MVFR from BDL-PVD with areas of IFR to the south, especially across the Cape/Islands. Mainly VFR cigs along and north of BAF-ORH-OWD. Improving conditions this afternoon with gradual clearing from NW to SE. W-NW gusts 20-25 kt develop in the afternoon, strongest higher terrain. Tonight...High confidence. VFR. Clear skies and light winds. Friday...High confidence. VFR with mid level clouds moving in during the afternoon. Areas of MVFR cigs developing late afternoon Cape/Islands. KBOS Terminal...High confidence in TAF. A few flurries possible this morning but any vsby restrictions should remain to the south. KBDL Terminal...Moderate confidence in TAF. Brief MVFR cigs/vsbys in light snow possible for a few hours this morning, but lower conditions should remain to the south. Outlook /Friday Night through Monday/... Friday Night: Mainly VFR, with local MVFR possible. Breezy. Slight chance SHSN. Saturday through Saturday Night: VFR. Windy with local gusts to 30 kt. Sunday: VFR. Sunday Night: Mainly VFR, with areas MVFR possible. Chance SHSN. Monday: Mainly VFR, with areas MVFR possible. Slight chance SHRA, slight chance SHSN.
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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/...High confidence. Today... Diminishing winds through the morning, then W-NW winds will increase this afternoon with gusts to 25-30 kt. Area of snow this morning will reduce vsbys, mainly over south coastal waters. SCA all waters. Tonight... Lingering SCA wind gusts in the evening then diminishing winds overnight as high pres builds in from the west. Friday... NW winds shift to the SW in the afternoon. Speeds below 20 kt with seas below SCA. Outlook /Friday Night through Monday/... Friday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of seas approaching 5 ft. Chance of snow showers. Saturday through Saturday Night: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Local rough seas. Sunday: Winds less than 25 kt. Seas locally approaching 5 ft. Sunday Night: Winds less than 25 kt. Slight chance of rain showers, slight chance of snow showers. Monday: Winds less than 25 kt. Slight chance of rain showers.
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&& .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from 3 PM this afternoon to 1 AM EST Friday for ANZ230-232>234. Small Craft Advisory until 4 AM EST Friday for ANZ231-251. Small Craft Advisory from 3 PM this afternoon to 4 AM EST Friday for ANZ235-237. Small Craft Advisory until 7 AM EST Friday for ANZ250-254>256. && $$ SYNOPSIS...KJC/EVT NEAR TERM...KJC/EVT SHORT TERM...KJC LONG TERM...EVT AVIATION...KJC/EVT MARINE...KJC/EVT

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