Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 111759 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1259 PM EST Mon Dec 11 2017 .SYNOPSIS... Dry weather this afternoon will be followed by some rain on Tuesday, which will begin as a period snow/ice across portions of interior southern New England. A shot of arctic air follows Wednesday and Thursday with a period of strong winds and bitterly cold wind chills. Another low pressure system may bring more snow to the region sometime in the Thursday night to Friday night timeframe, if it tracks close enough to the coast. Somewhat milder weather will arrive by the end of the upcoming weekend. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 1 PM update... Last of the solid cloud cover was exiting Chatham and Nantucket at 1 PM. Otherwise just scattered daytime cumulus was the rule. Already seeing some high level cirrus invading the Berkshires ahead of the next system. Winds were slowly diminishing, but still gusting up to 20 mph along the east coast. Temperatures on track for highs in the upper 30s to lower 40s. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM TUESDAY/... *** Mixed Wintry Precip Event Late Tonight into Tomorrow *** Setup... Dual, unphased shortwaves will generate double barreled low pres situation late Mon night into Tue. The leading wave is currently sliding from Saskatchewan into the N Plains, while the secondary wave slides out of Manitoba into N Ontario. The fact that these remain unphased, allows the initial parent low pres, developing the great lakes to transfer energy to a secondary warm frontal low pres which will slide across central New England or along Gulf of Maine. In either case, developing 40+ kt H92 LLJ will push fully into S New England Tue morning, allowing a nose of warm air between 975-900hPa. This allows the +2C isotherm to reach near the NH/MA border, using an average of available guidance, with slightly cooler temps along the MA/VT border. Given this, precip onset within overrunning ahead of the warm front should start as mainly SN, but then gradually give way to liquid precipitation from SE to NW, with a low risk for mainly SN especially extreme NW MA and into the Berkshires. Thermal profiles are tricky, with more on that below. Latest trends are also slower as the unphased wave takes on a negative tilt and low dwpt depressions take time to overcome tonight, suggesting overall lower QPF than previous runs. Thermal profiles... While soundings mainly H85 and above remain below 0C, the nose of warm air associated with the LLJ falls mainly in the lower 100hPa. Mean temps in this layer rise above 0C to about route +2C between 12Z and 15Z Tue. Given the dwpt depressions, wet- bulbing would suggest light SN at precip onset, Sfc temps are likely to warm slower, thanks to a lingering snowpack. Areas closest to coastlines could start as a frozen mix.. Not a classic damming signature, but low lvl mass fields do show some weak inverted ridging and mesoscale guidance does have light drainage flow particularly in the CT valley and Worcester Hills. In any case, this is a typical scenario where guidance warms the sfc/2m temps too quickly. The transition to all liquid should generally occur no later than mid day, except in the higher terrain of NW MA, which could remain cool enough for SN/frozen precip. QPF/Snowfall/Ice... As mentioned above, thanks to a dry column initially overnight, timing of precip onset have slowed and the resulting overall QPF is lower, totaling around 0.30-0.50 Given this, there may be less SN at onset with some areas starting as RA/FZRA. The warm influence also favors lower ratio snows, therefore, have lowered expected snowfall through. The FZRA/SN thinking with this update is backed up by latest WPC Day 2 data. Ice accums would be light as the warm air should translate to the sfc mainly a tenth or so at best, as sfc temps are not expected to be marginal, and not in the 20s, in which ice accumulation is more efficient. Snowfall totals are now lighter, with the best chance for advisory lvl snowfall in the high terrain of NW MA, where the low lvl warm air will be slowest to arrive, if it even ever shifts above 0C. 3-6 inches is the latest forecast for NW MA but with another set of guidance still to come, these are still likely to change a bit. Timing/impact... Precip, likely mainly in the form of light snow, gradually transitioning to a mix of FZRA and RA begins mainly after 1AM from W to E during the early morning hours Tue, with widespread precipitation likely between 7AM and 1PM, and continuing to change form wintry precip to then gradually ending from S-N mainly after 3PM through 7PM. The Tue AM commute has the highest risk for impact, as SN or even a wintry mix could impact portions of the commute as the warm air build in aloft. Untreated sfc could become slippery thanks to a light SN accums and/or light icing. By the afternoon commute, precip still occurring, but mainly rain outside of the higher terrain of MA particularly. With all these factors, will hoist Winter Weather Advisories for portions of western and central Massachusetts, where confidence is highest in snowfall totals in the realm of advisory levels, and the best chance for cold air damming supporting some ice accumulations. These may need to be expanded based on new data today. && .LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/... Highlights... * Low risk for a few brief snow squalls late Tue night/Wed * Arctic air Wed into Thu w/strong winds and bitter cold wind chills * Period of snow possible sometime in the Thu night-Fri night period * Milder weather by the end of the weekend Details... Tuesday night through Thursday.. An arctic cold front will move across the region Tue night/Wed accompanied with impressive shortwave energy. A few brief passing snow showers are likely to impact the region with the arctic front. The bigger question is do we see a few localized intense snow squalls. The shortwave energy is pretty dynamic with some steep 1000 to 700 mb lapse rates. However, moisture is the wild card and it is usually a limiting factor for this activity making it over the Berks. We do think there is at least a low risk for a couple. The other issue will be a period of strong winds. Bufkit indicates very deep mixing with steep lapse rates and an impressive 850 mb westerly jet. This will bring the potential for westerly wind gusts of 40 to 50 mph Wed into Wed night and wind headlines may eventually be requited. Lastly...very cold air will be ushered into the region with highs Wed and Thu mainly in the 20s to around 30. Lows Wed night/early Thu am should be well down into the teens with some single digit readings possible across western MA. This should result in wind chill values between 5 above and 5 below zero. Thursday night into Friday night.. This portion of the forecast remains uncertain. Another piece of shortwave energy swings down from the northwest and allows for an area of low pressure to develop over the ocean. The ensembles differ considerably as to where this low develops. Some solutions depicted are fairly close to our region, while others are further south and east. This will determine if we see a period of snow sometime in the Thursday night to Friday night time range or precipitation remains out over the ocean. Will just have to wait this one out and see how models trend over the next few days. Saturday and Sunday... The upper trough lifts out of our region allowing more west to southwest flow aloft. If guidance stays the course most of the weekend will feature mainly dry weather with moderating temperatures by Sunday. && .AVIATION /18Z MONDAY THROUGH SATURDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tuesday/... Through this afternoon...High confidence. VFR. Tonight...Moderate confidence. VFR to start, will give way to MVFR then IFR early AM Tue as CIGS lower and light SN/FZRA moves in mainly after 06Z from W-E. SN accums would be light. Winds shift around to the E-NE. Tomorrow...Moderate confidence. Mixed MVFR and occasional IFR conditions in a mix of SN/FZRA giving way to all RA through sunrise into mid day Tue. Risk for NW MA to remain SN long enough for accums to exceed reach 3-6 inches with generally less further SE. Low risk also for a period of FZRA before the change to RA. Conditions improve after 20Z from S-N. KBOS Terminal...High confidence in TAF. KBDL Terminal...High confidence in TAF. Outlook /Tuesday Night through Friday/... Tuesday Night: MVFR/IFR conditions possible. Windy with local gusts to 30 kt. Chance SHSN, slight chance SHRA. Wednesday: MVFR/IFR conditions possible. Strong winds with gusts to 45 kt. Chance SHSN. Wednesday Night: VFR. Strong winds with gusts to 40 kt. Thursday: VFR. Windy with areas gusts to 30 kt. Thursday Night: Mainly VFR, with areas IFR possible. Breezy. Chance SN, slight chance SHSN. Friday: MVFR/IFR conditions possible. Breezy. Chance SN. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tuesday/... This afternoon and Tonight...High confidence. Winds are slowly diminishing. Seas will slowly below SCA criteria over the southern waters by this evening. Quiet boating weather tonight, but that will be very temporary. Tomorrow...Moderate confidence. Winds gradually shift around to the E. Gusts 25-30 kt expected on the S waters with 35-40 kt possible on the E waters. Gale Watches will be issued, with the need for small craft advisories where Gales are not observed. Otherwise, rainy conditions expected on the waters. Outlook /Tuesday Night through Friday/... Tuesday Night: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Rough seas up to 12 ft. Chance of rain showers, chance of snow showers. Local visibility 1 to 3 nm. Wednesday: Moderate risk for gale force winds with gusts up to 45 kt. Rough seas up to 15 ft. Chance of snow showers. Visibility 1 to 3 nm. Wednesday Night: Moderate risk for gale force winds with gusts up to 40 kt. Rough seas up to 16 ft. Local visibility 1 to 3 nm. Thursday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Rough seas up to 12 ft. Thursday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Chance of snow, slight chance of snow showers. Visibility 1 to 3 nm. Friday: Winds less than 25 kt. Areas of seas approaching 5 ft. Chance of snow, chance of rain. Visibility 1 to 3 nm. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...Winter Weather Advisory from 4 AM to 4 PM EST Tuesday for MAZ002>004-008>012-026. RI...None. MARINE...Small Craft Advisory until 1 PM EST this afternoon for ANZ231>234-250. Small Craft Advisory until 6 PM EST this evening for ANZ235- 237. Gale Watch from Tuesday morning through Tuesday evening for ANZ250-251-254. Small Craft Advisory until midnight EST tonight for ANZ254>256. && $$ SYNOPSIS...Field/Frank/Doody NEAR TERM...Field SHORT TERM...Doody LONG TERM...Frank AVIATION...Field/Frank/Doody MARINE...Field/Frank/Doody is the U.S. government's official web portal to all federal, state and local government web resources and services.