Area Forecast Discussion
Issued by NWS Boston, MA

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720 FXUS61 KBOX 170953 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 453 AM EST Sun Dec 17 2017 .SYNOPSIS... Cold and dry today but with light winds as high pressure builds across the region. A weak wave of low pressure may bring spotty light snow to the region late tonight into Monday morning, mixed with rain along the south coast. Temperatures will be milder on Tuesday ahead of a cold front, then turning blustery and colder Wednesday into Thursday. A wave of low pressure which may track to our west will bring the potential for a wintry mix/rain sometime Friday and/or Saturday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... High pres builds into New Eng today bringing lots of sunshine, although some high clouds will being to move in from the SW this afternoon. Light NW winds and shallow mixing will result in a cold day. 925 mb temps -8 to -10C so highs will be mostly upper 20s to lower 30s, which is around 10 degrees below normal. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM MONDAY/... Tonight... High pres gradually retreats to the NE as weak warm front approaches from the south. Weak warm advection develops which will bring increasing clouds. Initially, low levels will be quite dry in the evening, then gradually moisten after midnight from SW to NE. As low level moisture increases, some patchy light snow is possible, especially in western New Eng. Lows mainly in the teens to lower 20s. Monday... The weak warm front will lift north into SNE bringing lots of clouds with some light snow possible, especially in the morning. Any snow would change to rain along the south coast as milder air advances northward. Forcing is weak so not much precip and any accum would be limited to a coating to less than an inch. During the afternoon, it looks mostly dry although can`t rule out some spotty light precip given the moist low levels. Given that the ice crystal growth region is fairly moist through the day, any precip would likely be in the form of light snow vs freezing drizzle where temps remain near freezing. It appears the warm front will get hung up near the south coast as a weak wave develops on the front. This will result in a north-south gradient of temps. Highs may climb into the lower 40s near the immediate south coast but cooler as you head north with coldest temps near or a bit below freezing in northern and especially northeast MA. && .LONG TERM /MONDAY NIGHT THROUGH SATURDAY/... Highlights... * Dry conditions Monday night into Tuesday, then rain and/or snow showers across NW Mass Tue afternoon/evening * Mainly dry and much milder Tue * Blustery and colder with mainly dry weather Wed into Thu * A period of snow and/or wintry mix possible Fri morning to midday, changing to rain Overview... 00Z model suite continues to signal progressive steering flow across the lower 48 through about the middle of this week as a nearly zonal northern stream flow is in place. Models then transition around the late Tue as H5 trough begins to dig across central Canada into Hudson Bay, while another long wave trough moves into the Pacific NW. The central Canadian trough progresses across the region Wed, but appears the weak surface reflection will remain far enough N of the region for mainly dry conditions. Models showing another high amplitude ridge from Alaska south across the eastern Pacific by late this week. As the downstream long wave trough digs from the Great Lakes to the desert SW, noting another short wave in the northern stream flow with another shot of arctic air trying to work south from the North Pole into the northern Plains states into early next weekend. Model solution spread increases, since each member handling this feature somewhat differently, but the GFS/GGEM and ECMWF all appear to have a similar idea in digging a long wave trough across the central U.S. late this week into the holiday weekend, with moderating temperatures possible by Fri-Sat. A lot of questions with the potential of low pressure and the approach of an arctic front sometime next weekend, but with wide model solution spread due to the timing and track of this potential feature, have low forecast confidence beyond Thursday. Details... Monday night and Tuesday... Temps will hold steady Mon night, mainly in the upper 20s to around 30 away from the coast and lower-mid 30s at the shoreline. With light S-SW wind flow, expect dewpoints close to the temps so will see patchy fog develop as well. For now, have carried a dry forecast Mon night. Can`t rule out some rain and/or snow showers across NW Mass especially across the higher terrain of the northern Berkshires, which may continue into Tuesday but changing over to rain. Expect highs on Tue mainly in the 40s, possibly touching 50 in a few spots along the S coast. Tuesday night... As a cold front approaches Tue night, most of the precip will linger across the E slopes of the Berkshires Tue night. Main energy/moisture remains closer to the parent low over central Quebec, so looks like best shot for precip will be well N of the region but can`t rule out a few stray showers mainly across N Mass. A cold front will cross the region Tue night into early Wed with perhaps a few brief rain/snow showers. High temps will recover well into the 30s to around 40, but a well mixed atmosphere should result in northwest wind gusts of 30 to 40 mph developing Wednesday and making it feel colder. Wednesday through Thursday night... The cold front should exit the coast by around 12Z on Wednesday with W-NW winds. Will see excellent low level mixing, so winds will be gusty. Could see gusts up to 30-40 mph Wed afternoon. Highs will range from only 30-35 degrees across the higher inland terrain up to the lower 40s along the immediate coast. Expect continued dry conditions on Thursday, but it will even colder with highs only in the mid to upper 20s well inland ranging to the lower-mid 30s at the coast. Expect light west winds. CLouds will increase Thu night with a few snow showers reaching into far NW Mass toward daybreak. Friday and Saturday... Low confidence forecast continues. As long wave trough digs south out of central Canada into the northern Plains, will see backing mid level steering flow to SW, along with increasing H5 heights. Arctic air starts to push S across Hudson Bay during the weekend, with the arctic cold pool pushing S. So, as high pressure exits Friday morning, leading edge of precip ahead of an approaching warm front will bring light snow during Fri. Should see transition to a mix of sleet and rain during the morning across central and western areas, with temps already above freezing as the precip moves eastward Fri morning. Some question whether snow or even a little freezing rain may linger across the higher terrain of the northern Berkshires Fri evening, but eventually should change over to all rain with a non-diurnal temp trend Fri night with S-SW wind flow in place. Next question will be how quickly the arctic front will push out of the Mississippi valley into the Appalachians. Wide variance in track and timing of the front toward the region, but at this point should see mainly rain on Saturday with mild temps. Highs look to approach 10 degrees above seasonal normals. && .AVIATION /10Z SUNDAY THROUGH THURSDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Monday/...High confidence. Today...VFR. Tonight...Mainly VFR with gradually lowering cigs, becoming MVFR in western New Eng after midnight. Patchy light snow possible. Monday...Widespread MVFR cigs. Chance of light snow or rain. KBOS Terminal...High confidence. KBDL Terminal...High confidence. Outlook /Monday Night through Thursday/... Monday Night: Mainly VFR, with local IFR possible. Patchy BR. Tuesday: Mainly VFR, with local MVFR possible. Windy with local gusts to 30 kt. Tuesday Night through Wednesday: VFR. Windy with local gusts to 30 kt. Wednesday Night through Thursday: VFR. Breezy. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Monday/...High confidence. Today...NW winds diminishing through the morning to 15 kt or less. Seas subsiding below 5 ft outer waters this morning. Tonight...Light winds and seas. Monday...Winds becoming SW and increasing in the afternoon but gusts below 20 kt. Seas below SCA. Chance of light snow changing to rain. Outlook /Monday Night through Thursday/... Monday Night: Winds less than 25 kt. Tuesday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Local rough seas. Tuesday Night: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of rough seas. Wednesday: Low risk for gale force winds with gusts up to 35 kt. Areas of rough seas. Wednesday Night: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Local rough seas. Thursday: Winds less than 25 kt. Seas locally approaching 5 ft. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 7 AM EST this morning for ANZ254>256. && $$ SYNOPSIS...KJC/EVT NEAR TERM...KJC SHORT TERM...KJC LONG TERM...EVT AVIATION...KJC/EVT MARINE...KJC/EVT

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