Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 211757 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 1257 PM EST Wed Nov 21 2018 .SYNOPSIS... An arctic cold front may bring a few localized snow squalls later today into this evening, followed by record cold and bitter cold wind chills tonight into Thanksgiving. High pressure moving off the coast will allow temperatures to moderate some on Friday and particularly on Saturday. Dry weather should prevail most of the day Saturday, but a fast moving low pressure system will bring a period of rain Saturday night with a low risk it may begin as some ice across portions of the interior. The rain may linger into the first part of Sunday before coming to an end. A potentially stronger low pressure system may bring more unsettled weather by Monday and/or Tuesday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 1255 PM update... Scattered snow showers approaching from the north and west will move across SNE this afternoon. Low risk for a heavier snow squall but it should be isolated. Low level lapse rates are not as favorable across SNE compared to the north which will be a limiting factor. No changes to forecast. Previous discussion... Main concern for the afternoon is the risk of a few snow showers/isolated squall assocd with the arctic front along with brief strong wind gusts. Arctic front located from the St Lawrence Valley to eastern Lake Ontario with numerous snow showers developing along the front. Some heavy squalls as well with lightning noted east of Lake Ontario. This activity is in an area of steep low level lapse rates and convergence along the front. Guidance suggests the steepest 0-3km lapse rates will remain focused to the north this afternoon along with best forcing. However, enough moisture and convergence for isolated to sct snow showers to survive the trip into SNE mid/late afternoon as the front moves through. There is a low risk for an isolated heavier squall, mainly across the Berkshires and northern MA which will be closest to steeper low level lapse rates. The other concern is for brief strong wind gusts right behind the front. Soundings indicate potential for brief 40-50 mph gusts, especially over higher terrain. Will highlight this potential in SPS. Previous discussion... On the heels of the low pressure system that brought us yesterday`s rain and snow exiting to the northeast, we`re ever so briefly under the influence of some high pressure ridging extending from the surface high over the southeast U.S. Satellite imagery this morning shows mostly clear skies, though upstream we see clouds associated with mid and upper level moisture that will bring increasing cloud cover from the northwest today. Freezing temperatures that brought a black ice risk overnight will warm above freezing by early to mid morning for most locations, topping out in the low to middle 40s (upper 30s in higher elevations). This comes ahead of our next strong frontal system, an arctic cold front associated with the low pressure system currently north of the Great Lakes. As this relatively weak low tracks east along the U.S/Canada border it will cross northern New England this afternoon/evening, dragging with it our arctic front. We`ll see gusty winds increasing through the day as the pressure gradient tightens and a robust LLJ moves over. With good mixing behind the front one hazard will be brief stronger winds in the wake of the front up to 35-45 mph. Current timing of the front would be from west to east between about 2 PM and 7 PM. We`ll see a chance for scattered snow showers along and ahead of the front, but the main threat from this front will be potential for some localized but strong snow squalls. Model soundings show marginally good low level lapse rates and plenty of moisture in the lowest levels of the atmosphere, though the best forcing and steepest lapse rates of 7-8 C/km remain over northern New England. Thus, the best chance for strong snow squalls lies to our north, but will still be possible for us, especially over northwest MA, the closest proximity to the more favorable environment. These squalls would have the potential to drop quick bursts of heavy snow along with gusty winds, dropping visibilities on area highways during one of the busiest travel days of the year. Motorists are encouraged to avoid driving through these if encountered. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM THURSDAY/... Behind the front temperatures plummet under northwest wind, as a strong surface high pressure over the Great Lakes region moves toward SNE. It will be an very anomalous air mass with H85 temps around -20C. This translates to low temperatures at the surface in single digits for central and western MA with teens elsewhere. Additionally, gusty winds will mean even colder wind chill values in the single digits and even below zero. For this reason a Wind Chill Advisory has been issued for the east slopes of the Berkshires Thursday morning where wind chill values will drop to -15 F. Most of the region will be dry, with one possible exception being the outer Cape where we may see some ocean effect snow showers. The combination of arctic air temps and relatively warm SSTs in late Nov will generate impressive instability. Much of the snow would likely form offshore, but there is a chance for some showers over land given the favorable NW wind trajectory and temperature profile. Thursday will be a very cold and blustery Thanksgiving indicated by a striking pressure rise/fall couplet and strong LLJ. Given the good CAA we`ll be well mixed and model soundings show that LLJ being mixed down to the surface keeping winds gusty all day. Meanwhile high temperatures won`t make it out of the teens and 20s. Potential for ocean affect snow continues on Thursday, especially for areas east of HYA late in the day/overnight as winds become more northerly. && .LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/... Highlights... * Record low temperatures possible Thanksgiving night/early Fri am * Ocean effect snow showers/flurries may clip outer-Cape into Fri am * Dry Fri into part of Sat with moderating temperatures * Period of rain Sat night perhaps lingering into part of Sun with a low risk it may begin as a period of ice across the interior * Another round of unsettled weather likely Mon and/or Tue with odds favoring ptype mainly rain but that is not set in stone Details... Thanksgiving night... Arctic high pressure will build into the region from the west. This will allow winds to gradually diminish and with highly anomalous cold airmass in incredibly cold night is in store for the region for this time of year. Given the diminishing winds/mainly clear skies this will be the night for the radiator sites to be much colder than some of the urban centers. Expect most of the outlying locations to see low temperatures bottom out in the single digits...with perhaps even some below zero readings in the normally coldest spots of western MA. The urban heat islands of downtown Boston/Providence will see overnight low temps in the lower to middle teens. Record low temperatures may be set at some of our sites. Dry weather and mainly clear skies are anticipated for Thanksgiving night. The exception will mainly be the outer-half of Cape Cod...where the highly anomalous airmass will result in an extreme 850 to SST differential of 30C. Northwest wind trajectories and dry low level air will limit potential...but given the extreme nature of the airmass and over 1000 J/KG of ocean induced Cape will probably see some ocean effect snow showers/flurries. A coating to 1 inch of snow is certainly possible across portions of the outer- Cape with even the low risk of localized 2 inch amounts. Friday into Saturday afternoon... High pressure will move east of the region along with some upper level ridging moving in from the west. The result will be in dry weather continuing but moderating temperatures Fri and especially by Saturday afternoon. High temperatures on Friday should recover into the upper 20s to the lower 30s...which is still well below normal for this time of year. However...a big improvement from Thanksgiving Day and much less wind. While clouds will be on the increase during the day Saturday ahead of a wave of low pressure...dry weather should prevail most of the day. Milder air should continue to work into the region ahead of a shortwave trough...allowing many locations to see highs recover well into the 40s. Saturday night into Sunday... A fast moving wave of low pressure should bring a period of mainly rain to southern New England Saturday night...perhaps lingering into the first part of Sunday. However...the system looks fairly progressive so dry weather may dominate by Sunday afternoon/night. Ptype should mainly be rain given lack of Canadian high pressure and current thermal profiles. Still we will need to watch for at least the low risk for a period of ice across the interior at the onset...dependent on how cold the boundary layer is at that start. Monday and Tuesday... Appears another low pressure system...perhaps an intensifying storm will impact the region sometime Mon into Tue. Appears a fairly stout -NAO will be in place...but the antecedent airmass will not be that cold. Therefore...odds favor this being mainly a rain event. However...if we can get rapid intensification off the coast can not rule out some wintry weather getting involved. Too early to say much more other than it looks unsettled. && .AVIATION /18Z WEDNESDAY THROUGH MONDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Thursday night/...High Confidence. Through 00z... Mainly VFR cigs, but brief lower conditions in scattered snow showers. Low risk of brief IFR in a squall. W/SW gusts 20-30 kt with brief 40 kt NW gust possible late day with the arctic front, especially higher terrain. Tonight through Thursday night... Mainly VFR, but areas of MVFR cigs developing over the outer Cape late tonight into Thu evening. A few flurries possible at times over the outer Cape. NW gusts to 25-35 kt at times through Thursday, diminishing Thu night. KBOS Terminal... High confidence. KBDL Terminal... High confidence. Outlook /Thursday Night through Sunday/... Friday through Friday Night: VFR. Breezy. Saturday: Mainly VFR, with local MVFR possible. Breezy. Slight chance FZRA. Saturday Night: Mainly MVFR, with areas VFR possible. Breezy. RA, chance FZRA. Sunday: Mainly MVFR, with areas VFR possible. Breezy. Chance RA. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Thursday/...High confidence. Winds will temporarily diminish this morning while shifting to W/SW, but will increase this afternoon and especially tonight while shifting back to W then NW behind arctic front. Gusts to 35-40 kt likely, peaking tonight. Gale warnings have been issued. Vsbys may be reduced in snow showers late Wed night across waters east and NE of Cape Cod. Outlook /Thursday Night through Sunday/... Thursday Night: Moderate risk for Gale force wind gusts up to 35 kt during the evening. Areas of rough seas. A chance of snow showers. Areas of visibility 1 to 3 nm. Friday: Winds less than 25 kt. Seas locally approaching 5 ft. Friday Night through Saturday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Saturday Night: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Local rough seas. Rain. Sunday: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of rough seas. Chance of rain. && .CLIMATE... An early season shot of arctic air will likely result in some records being broken Thanksgiving into early Friday morning. November 22nd Location / Record Low / Record Minimum High Boston..........9 (1879) / 24 (1880) Hartford.......14 (1969) / 27 (1978) Providence.....16 (1987) / 30 (2008) Worcester......11 (1987) / 24 (2008) Thanksgiving Location / Record Low / Record Minimum High Boston.........11 (Nov 27, 1873) / 24 (Nov 28, 1901) Hartford.......12 (Nov 28, 2002) / 27 (Nov 23, 1989) Providence.....14 (Nov 23, 1972) / 30 (Nov 28, 1996) Worcester.......9 (Nov 23, 1989) / 22 (Nov 23, 1989) November 23rd Location / Record Low Boston.........11 (1880) Hartford.......12 (1972) Providence.....14 (1972) Worcester.......9 (1989) && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...Wind Chill Advisory from 4 AM to 10 AM EST Thursday for MAZ002- 008-009. RI...None. MARINE...Gale Warning until 7 PM EST Thursday for ANZ231>234-251. Gale Warning from 6 PM to 9 PM EST this evening for ANZ230. Small Craft Advisory until 7 PM EST Thursday for ANZ236. Gale Warning until 7 PM EST Thursday for ANZ235-237-250- 254>256. && $$ SYNOPSIS...Frank/BW NEAR TERM...KJC SHORT TERM...BW LONG TERM...Frank AVIATION...KJC/Frank MARINE...Frank/BW CLIMATE... is the U.S. government's official web portal to all federal, state and local government web resources and services.