Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 121927 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 227 PM EST Tue Dec 12 2017 .SYNOPSIS... Strengthening low pressure over upstate NY redevelops over northeast Massachusetts this evening, then intensifies into a gale center over New Brunswick Wed. A shot of arctic air wraps around this exiting low Wednesday and Thursday, with a period of strong winds and bitterly cold wind chills. Another low pressure may bring some light snow across south coastal areas Friday night into Saturday if it tracks close enough to the coast. Somewhat milder weather will arrive by the end of the upcoming weekend. && .NEAR TERM /THROUGH TONIGHT/... Continuing to monitor temperatures this afternoon. So far, timing of the ongoing forecast looks good. Winter Weather Advisory currently slated to expire at 4 PM. Will make a final decision on whether to extend it or not after 3 PM. Radar data showing light snow continuing across western MA. Where surface temperatures are still at or below freezing in north central MA, it appears the precipitation had temporarily waned, meaning not too much ice accretion was occurring. Potent negatively-tilted mid level shortwave crosses our region later tonight. This feature will help to drive much colder air into southern New England by Wednesday morning. Expecting plenty of clouds with this shortwave. Thinking it might just a bit too dry for widespread snow showers, but that remains a possibility. Most of the moisture looks to be trapped below 800 mb, which should keep any showers mainly on the western side of the Berkshires with a west wind. Below normal temperatures, well below freezing, tonight. Any standing water and/or slush will freeze. Thus this afternoon is your chance to remove any leftover snow/ice. && .SHORT TERM /WEDNESDAY THROUGH WEDNESDAY NIGHT/... Strong low pressure will spin across the Maritimes into Labrador through Wednesday night, continuing a strong surge of arctic air into our region. While the core of this arctic air remains north of our region, will see increasing W-NW winds with the sharp pressure gradient. 0-3 km lapse rates are near dry adiabatic, meaning excellent mixing and momentum transfer. Will probably need a Wind Advisory for portions of southern New England, especially across the higher terrain and towards the coasts. Still trying to refine the area and timing a bit more. Some leftover snow showers may linger across the E slopes of the Berkshires through midday Wednesday, with a few lingering along the S coast early. Otherwise, mainly dry across much of southern New England. Below normal temperatures with highs below freezing. Wind chill values later in the day remaining in the single digits and teens above zero. A few locations towards the Berkshires could see subzero wind chills Wednesday afternoon. Winds diminish some Wednesday night as better mixing moves offshore. Another mid level shortwave should pass by to our south. While this should mean some clouds for our region, temperatures will still fall into the teens away from the immediate coast. Lows in the 20s for there. Still very low wind chills in the single digits and teens for much of the region, with readings as low as -5 toward daybreak across the E slopes of the Berkshires. Any snow showers would be closer to the south coast of new England. Limited moisture would also limit any accumulations, which should be less than 1 inch, where it snows at all. && .LONG TERM /THURSDAY THROUGH MONDAY/... Overview... Deep H5 long wave trough sets up by mid week from the Arctic Circle southward to the mid Atlantic coast, while high amplitude ridge continues from the Northwest Territories through the Pacific Coast. This will bring the coldest air so far this season to the region along with strong W-NW winds through Thursday. As the cold pool retreats into eastern Canada late this week, appears that the western ridge may break down as a short wave moves into the Pacific NW late Fri or Sat. Also noting a weak mid level short wave that may wrap around base of the eastern Canadian cutoff low and cross the region. May also see a strengthening southern stream system try to ride up across the western Atlantic. Rather wide model solution spread at this point, so track is in question along with the possibility of the northern fringe of the precip shield approaching portions of the south coast late in the week or into the weekend. 00Z GFS is farthest W with this system, while the Canadian GGEM is further offshore. Something to keep an eye on. With rather wide solution spreads amongst the model suite from about Friday into early next week, lower forecast confidence for this portion of the forecast. Will use a blend of available model guidance into Friday, then transition over the model ensembles for the latter portion of this period. Details... Thursday... Expect the gusty W-NW winds to continue through midday Thursday, but will start to weaken as the eastern Canadian low slowly exits. Models continue to signal an H5 short wave passes S of New England. The northern fringe of the precip shield may clip the S coast Thursday morning through midday as the weak low passes close to or just S of the 40N/70W benchmark. Have carried CHC POPs across Nantucket and Martha`s Vineyard. Will see snow showers or a period of light snow for a few hours as the low passes. Little if any snow accumulations are expected. The arctic air will continue across the region, with high temps only in the upper teens and 20s across the higher inland terrain, ranging to the lower-mid 30s at the shore. As winds drop off during the day, wind chills will not be quite as harsh as on Wed. However, with mainly clear skies Thu night, temps will fall back to the single digits well inland, ranging to 20-25 across the outer Cape and islands. Friday through Saturday... Another weak short wave develops across the Great Lakes Fri, while stronger low pressure forms off the SE U.S. coast. The weak low will push E while the coastal low moves NE off the eastern seaboard. Models showing some spread with their development and track of the coastal low, with the GFS closest to the coast but still passing S and E of the 40N/70W benchmark. Could still see some light snow or snow showers push into SE Mass/E RI, but this is still up in the air for exact track and impacts later Friday into early Saturday. Should see improving conditions with slowly moderating temps Sat. Sunday and Monday... High pressure ridge builds across the region Sunday. Winds back from W-NW to SW, which will bring milder temps, actually near or just a few degrees below seasonal normals. May see another short wave bring some light precip by Sun night and continuing into Monday, but timing and track of this system is in question. && .AVIATION /19Z TUESDAY THROUGH SUNDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Wednesday Night/... Through 00Z...High confidence on trends but lower on exact details, particularly timing of precipitation type changes. MVFR CIGS across central and western areas will push E, with local IFR-LIFR at times across N central and western areas. Tonight...High confidence on trends, lower on details. MVFR CIGS/MVFR-IFR VSBYS improve to VFR by around 05Z-07Z, except lingering lower CIGS across higher terrain. Gusty W-NW winds develop. Tomorrow...Mainly VFR, with local MVFR possible in scattered SHSN. Strong winds with gusts to 40 kt. Tomorrow Night...VFR. Strong winds with gusts to 40 kt. KBOS Terminal...Moderate confidence in TAF. KBDL Terminal...Moderate confidence in TAF. Outlook /Thursday through Saturday/... Thursday: VFR. Windy with local gusts to 30 kt. Thursday Night: VFR. Breezy. Friday: VFR. Friday Night: Mainly VFR, with local IFR possible. Breezy. Slight chance SN. Saturday: VFR. Breezy. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Wednesday Night/... Tonight...Arctic cold front sweeps across the area with SE winds shifting to W-SW up to 20-30 kt. Vsby improves after evening rain/snow showers. Wednesday and Wednesday night...Gale force winds with gusts up to 45 kt. Rough seas up to 15 ft. Slight chance of snow showers Wednesday. Outlook /Thursday through Saturday/... Thursday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Local rough seas. Chance of snow showers. Thursday Night: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Local rough seas. Friday: Winds less than 25 kt. Seas locally approaching 5 ft. Chance of snow. Areas of visibility 1 to 3 nm. Friday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of seas approaching 5 ft. Chance of snow. Visibility 1 to 3 nm. Saturday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Local rough seas. Slight chance of snow. Areas of visibility 1 to 3 nm. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...Winter Weather Advisory until 4 PM EST this afternoon for MAZ002>004-008-009-026. RI...None. MARINE...Gale Warning from 9 AM Wednesday to 6 AM EST Thursday for ANZ232. Gale Warning from 9 AM Wednesday to 4 AM EST Thursday for ANZ233-234. Gale Warning from 1 PM Wednesday to 1 AM EST Thursday for ANZ230-236. Gale Warning from 10 AM Wednesday to 6 AM EST Thursday for ANZ231. Gale Warning from 8 AM Wednesday to 6 AM EST Thursday for ANZ235-237-251. Gale Warning from 6 AM Wednesday to 6 AM EST Thursday for ANZ250-254. Gale Warning from 5 AM Wednesday to 6 AM EST Thursday for ANZ255-256. && $$ SYNOPSIS...Belk/EVT NEAR TERM...Belk SHORT TERM...Belk LONG TERM...EVT AVIATION...Belk/EVT MARINE...Belk/EVT

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