Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 122350 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 650 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 Wednesday. A shot of arctic air wraps around this exiting low into Thursday, with a period of strong winds and bitterly cold wind chills, ahead of which a period of light snow is possible. Another low pressure may bring some light snow around Friday night. Cold and dry once again for Saturday. A wintry mix possible by Sunday. A milder weather pattern on the horizon into next week. && .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/... 7 pm update... Tapering down. Couple of things we`re closely monitoring: 1.) 2m temperatures, expected to come down quickly as cold air advection proceeds with increasing W winds, freezing any liquid, yielding a black ice threat / impact. 2.) Drier air encroaching and eroding from the SW, subsiding and netting the conclusion of precipitation...with moist low levels and dry in the dendritic growth zone, do we need to worry about freezing drizzle? Or perhaps the clearing trend continues as clouds break and lift, dewpoints dropping. Already it seems that drier air is working its way in, the W flow subsiding over the high terrain E into the CT Valley. There are reports of conditions improving with ceilings lifting as dew- points begin to drop from the W. Some concern as to lingering precipitation in the low levels the radar may overshoot. A lot of mid-level energy still rotating and low to mid level moisture for a time before drier air works its way in. Ahead of the H5 negatively tilted trof beneath good cyclonic flow, will need to watch observations closely if additional short-fused headlines are needed for possible freezing drizzle, but otherwise will drop the WINTER WEATHER ADVISORY as planned at 7p. As the prior forecaster noted, the drying taking place within the mid-levels, moisture is trapped below H8, and to the W of the Berkshires has a tough time coming E over the high terrain, subsiding. Again, watching such mesoscale processes closely. SPECIAL WEATHER STATEMENT for overnight freezing of liquid on roads posing a black ice threat / impact continues till 10p. May need to extend this into the Wednesday AM commute. Will allow later shifts to evaluate. Well below-average temperatures over- night with lows in the teens to 20s. && .SHORT TERM /6 AM WEDNESDAY MORNING 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 TUESDAY/... */ Highlights... - Skirting clipper low with light snow Thursday - Cold and dry Thursday night into Friday - Another shot of snow shower activity going into Friday night - Cold and dry again for Saturday - Potential wintry mix event for Sunday - Early on cold, light snow followed by a warmer trend next week */ Overview... Cold, active weather pattern persists into the weekend, beginning to relax into the following week. H5 trof pattern maintains over NE N America round which disturbances rotate, at times capturing S-stream energy, quickly escaping E with lesser downstream traffic. Looking at a series of clipper systems which will bring periods of wintry weather followed by gusty winds that aid in preserving the below- average temperature pattern we have seen as of late. Strength of individual disturbances is in question which will be broken down below, however their forecast quick movement and short residency, looking at brief impacts and leaning towards less impressive / deep storm development in vicinity of New England. Certain they`ll keep on reloading negative H85 temperature anomalies across the region. But perhaps a warming trend towards late December? The wave breaking over the NW Pacific as a stout jet of milder air pushes E into W N America, colder air is shunted back N as the H5 pattern across the CONUS relaxes, becoming more zonal. While still within the periphery of the polar jet, seemingly looking at a return of an ebb and flow pattern with a nod towards shots of cooler air out of the N rather than S-stream dominance. Continued confidence in operational forecast guidance out through 72-96 hours, thereafter preference to ensemble means. Wintry weather followed by cold and blustery themes into the weekend then a gradual warm-up into next week with potential for mixed precip events early on. */ Discussion... Thursday... Skirting clipper low, potential for wintry weather. Consensus of forecast guidance has a near-miss, the crux of the forcing offshore associated with positive differential vorticity advection, stronger ascent of greater moisture availability. However, evaluation along isentropes and the environment within the snow growth region, there are indications of weak lift and available moisture along decent SW to NE frontogenetical banding around H7-5. Can not rule out light snow chances spreading as far N/W of the MA/VT/NH border. Not sold on just saying zero based on synoptics. Consensus of guidance has the dry, subsiding wedge further N over N New England. Thus like to keep chance PoPs for all of S New England, likely along the S coast. Amounts light, thinking little impact. The EPS probabilities paint a decent picture of outcomes with respect to 24-hour 0.01 liquid. A mainly early-half of the day event, tapering into afternoon/eve. Nantucket having the best shot of seeing appreciable accumulation of precipitation. Late Thursday into Thursday night... Cold and dry. Initially blustery, storm system quickly steering out as high pressure builds in. Gales briefly on the waters, all else being good. Flirting with noteworthy wind chills yet seemingly prior to the crux of cold air with single digit lows into Friday morning. That being said any liquid from melt along roads will easily freeze. Friday into Friday night... Possible snow shower activity, otherwise maintained cold. Positively-tilted H5 trof axis quickly sweeping across the region yields little threat and impact to the region. Moisture present but lacking deep synoptic lift. Agree with the bulk of development across the better baroclinic zone well offshore as the H5 trof undergoes neutral tilt out across the Atlantic. Lack of mid-upper level curvature until further downstream. Chance PoPs with energy transfer from N-stream clipper to S-stream wave along the better baroclinic zone. Mainly snow showers and looking at light accumulations. Saturday into Saturday night... Blustery and cold. High pressure settling in behind exiting storm, would expect ocean-effect stratus / showers near the coast, perhaps some Lake effect snow shower activity getting into N/W MA before deeper subsidence and drier air builds in. Sunday into Sunday night... Potential mixed precip storm. Monitoring cold air damming as there is the possibility of over-running precipitation. Gradually warming as the warm front lifts N but also as the dry slot seemingly wraps in, looking at potential change overs before all is said and done. Too early to get on particulars but something to watch. Into next week... Some final clipper systems, shots of cold, then the warming trend begins as we get back into an ebb and flow pattern. With clipper and cold, the usual shot of snow shower activity enhanced by the Lakes, followed by blustery winds and the typical offshore ocean-effect stratus / shower activity. The H5 trof relaxing, we get back into S flow and a warmer trend. && .AVIATION /00Z WEDNESDAY 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/... Tonight... CIGs improving to VFR as winds increase out of the W towards morning. RA/SN mix concludes along with impacts to VSBY. Wednesday... SCT-BKN 050 with strong, gusty W winds up around 35-40 kts along the high terrain and along the coast, a bit less elsewhere. Cross wind issues for N-S oriented runways expected. Wednesday night... Low-end VFR CIGs erode becoming SKC however marine stratus holds close to the coast. W winds tapering though will remain strong along the E-coast up around 30-35 kts into Thursday morning. KBOS Terminal... Still some -SHRA moving through the region till around 1z which may yield some TEMPO VSBY impacts with brief RA. Otherwise will see improvement to VFR with increasing W winds towards morning. KBDL Terminal... Low end VFR CIGs near-term, but should erode and lift as down- sloping W winds prevail off the higher terrain. Increasing gusts into the Wednesday AM push but the stronger winds expected into midday. Outlook /Thursday through Sunday/... Thursday: VFR. Breezy. Chance SN. Thursday Night: VFR. Breezy. Friday: VFR. Friday Night: Mainly VFR, with areas MVFR possible. Breezy. Slight chance SHSN. Saturday: Mainly VFR, with local MVFR possible. Breezy. Slight chance SHSN. Saturday Night: VFR. Breezy. Sunday: VFR. Breezy. Chance SHSN. && .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 Sunday/... Thursday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Local rough seas. Chance of snow. Thursday Night: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of rough seas. Friday: Winds less than 25 kt. Areas of seas approaching 5 ft. Friday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Chance of snow showers. Local visibility 1 to 3 nm. Saturday: Moderate risk for Small Craft Advisory winds with local gusts up to 30 kt. Local rough seas. Slight chance of snow showers. Saturday Night: Moderate risk for Small Craft Advisory winds with local gusts up to 30 kt. Local rough seas. Sunday: Winds less than 25 kt. Seas locally approaching 5 ft. Chance of rain showers. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...Wind Advisory from noon Wednesday to midnight EST Wednesday night for MAZ007-019-022>024. Wind Advisory from 10 AM to 8 PM EST Wednesday for MAZ002-008- 009. RI...Wind Advisory from noon Wednesday to midnight EST Wednesday night for RIZ008. MARINE...Gale Warning from 8 AM Wednesday to 6 AM EST Thursday for ANZ232-251. Gale Warning from 8 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 6 AM Wednesday to 6 AM EST Thursday for ANZ235-237-250-254. Gale Warning from 5 AM Wednesday to 6 AM EST Thursday for ANZ255-256. && $$ SYNOPSIS...Belk/Sipprell NEAR TERM...Belk/Sipprell SHORT TERM...Belk LONG TERM...Sipprell AVIATION...Belk/Sipprell MARINE...Belk/Sipprell is the U.S. government's official web portal to all federal, state and local government web resources and services.