Area Forecast Discussion
Issued by NWS Boston, MA

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029 FXUS61 KBOX 222031 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 331 PM EST Wed Nov 22 2017 .SYNOPSIS... Quiet weather Thursday and Friday with dry and cool conditions. Milder air returns Sat ahead of a cold front but will have to watch a coastal storm to the south. Blustery and cooler weather follows Sunday into Monday, then dry and milder conditions again Tuesday into Wednesday. && .NEAR TERM /UNTIL 6 AM THURSDAY MORNING/... Even with KBOX currently down TBOS giving very clear picture of where the cold front is currently located, draped from about Plymouth to New Bedford and continuing an ENE speed such that it should be fully offshore of even the Cape/Islands by 2130Z (about 430 local). While this continues to mark the introduction of drier air aloft initially, marked by a clear delineation in GOES-16 high/lower cloud products, some leftover trapped lower moisture is still manifesting itself as light rainfall mainly from the Worcester Hills E. This rain shield should take through about 22Z to fully clear the region as drier air begins to entrain and erode the lower clouds. POPs and cloud timing were mainly a blend of latest HRRR ensembles. Once the skies clear and differential CAA can be realized, expect an evening burst of gusty NW winds which should tap the 30-35 H92 LLJ through about midnight local. Gusts to about 30-35 mph can be expected this evening as a result. After this, mass fields show enough of a slackening in the pres gradient to support some decoupling with clearing. Speaking of this decoupling. Although RAW 2m temps were best for hourly temps through midnight, thereafter, it is not capturing the radiational cooling potential within a cooling airmass where H92 temps drop to -5C by 12Z. Therefore, leaned most heavily on MET/MAV data for overnight mins, featuring mainly low-mid 20s outside of the urban compacts. && .SHORT TERM /6 AM THURSDAY MORNING THROUGH THURSDAY NIGHT/... Thanksgiving... Very weak ridging follows the shortwave passage overnight and ahead of weak opening wave approaching from the Great Lakes by late day. The resulting subsidence inversion suggests mainly dry conditions, and soundings support this. The inversion will also limit diurnal mixing to H92 where a slight increase in temps to near -1C should allow highs to rebound in the low 40s based on expected sunshine alone. However, given winds shift to the W based on isallobaric component with high pres shifting slightly, downsloping will allow some locations to reach the mid and even upper 40s in spite of the cool start. All-in-all, a pleasant albeit cool day to head over the river and through the woods. Tomorrow night... The weak aforementioned wave will pass mainly across N and central New England with the sfc reflection being a very weak secondary cold front. Although soundings do suggest an every so slight increase in upper lvl moisture, this is frontal passage is quite dry. Therefore, expecting little fanfare as it moves through outside of an increase in evening clouds. The primary impact will be to limit overnight diurnal temp drop off thanks to less decoupling. Mins hold mainly in the mid-upper 20s in the coolest spots. && .LONG TERM /FRIDAY THROUGH WEDNESDAY/... Highlights... * Dry and cool Fri, then warming Sat with a low risk of showers * Blustery and cooler Sun/Mon * Turning mild again Tue/Wed with mainly dry conditions Overview... Models are advertising a progressive pattern in the long term period as -AO/-NAO pattern trends to neutral. This will result in a changeable temp regime in the extended period. Amplifying northern stream trough moves into New Eng Sun which will be preceded by a warming trend, especially Sat, then colder Sun/Mon. As trough approaches models indicating coastal development S of New Eng Sat assocd with a southern stream shortwave and this will need to be watched as it could bring some rain if it trends northward. Next mid level trough moving through the Rockies and central US early next week with downstream ridging in the east will bring another warming trend through midweek. Significant model differences in the large scale flow toward the end of the period which will determine if another period of wet weather moves in by Wed or it remains dry and mild. Friday into Saturday... High pres south of New Eng moves offshore which will bring a warming trend through Sat with temps likely getting back well into 50s Sat. Dry conditions Fri, then a bit more tricky for Sat as a coastal low assocd with a southern stream shortwave is forecast to develop south of New Eng while cold front approaches from the west. NAM is a western outlier with a slug of heavy rain on Sat while GGEM has heavier rain just off the coast. ECMWF/GFS/UKMET remain offshore but several GEFS members bring some rain Sat night. Odds favor a dry day but a shift to the north and west would jeopardize a dry forecast, especially SE New Eng. Otherwise, can`t rule out a brief shower in the west as front approaches. Sunday and Monday... Deep mid level trough moves into New Eng Sun then moves to the east Mon. Expect blustery and colder conditions Sun into Mon but warm advection will commence during Mon. Below normal temps. Tuesday and Wednesday... Milder Tue with increasing SW flow, then uncertainty for Wed as GFS brings a front with showers into the region while ECMWF keeps it dry and mild. Deterministic GFS does not have support of most GEFS ensemble members which are dry so we kept a dry forecast. Above normal temps. && .AVIATION /20Z 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 this evening.. Mixed IFR to VFR conditions continue with most VFR W of the Worcester Hills and mixed MVFR/IFR east through about 21Z. After this point, rapid improvement to VFR all sites from W-E through the remainder of the evening as winds shift to 320. Gusts to around 25-30 kt possible also with this wind shift. Tonight into tomorrow night... Mainly VFR after this evening. NW winds continue to gust around 25 kt through the overnight hours but then drop off during the early morning. These winds shift to mainly W tomorrow and tomorrow night. KBOS Terminal... Mixed VFR/MVFR conditions with last batch of rain through about 21Z along with E-NE flow. Once winds shift to 320 conditions improve rapidly thereafter. With the wind shift, gust 25-30 kt are possible at times, which linger into the overnight. KBDL Terminal... Mainly VFR, winds will shift to NW no later than 20Z with gusts to around 25 kt at times lingering into the evening. Outlook /Friday through Monday/... Friday: High confidence. VFR. Friday Night: High confidence. VFR. Breezy. Saturday: Moderate confidence. Mainly VFR, with areas MVFR possible. Breezy. Slight chance SHRA. Saturday Night: Moderate confidence. Mainly VFR, with local MVFR possible. Breezy. Sunday through Monday: High confidence. VFR. Breezy. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Thursday Night/... Tonight...High confidence. NW winds increase through the evening, with gusts mainly 25-30 kt across the waters into early tomorrow morning, there is a low risk for Gale gusts mainly on the outer waters, but these will be few and far between and only support a high end small craft advisory at this time. Seas will build with the increase in winds, peaking at around 8-9 ft on the SE outer waters. Tomorrow and tomorrow night...High confidence. Winds shift to the W, with gusts generally remaining below 20 kt after sunrise. Seas however will take some time to fully diminish, and will require small craft advisories to linger much of the day. Advisories have been extended for the outer waters into tomorrow night given 5 ft lingering swells are expected. Outlook /Friday through Monday/... Friday: Winds less than 25 kt. Seas up to 5 ft. Friday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of rough seas. Saturday: Winds less than 25 kt. Seas up to 5 ft. Chance of rain showers. Saturday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Slight chance of rain showers. Sunday: Moderate risk for Small Craft Advisory winds with gusts up to 25 kt. Local rough seas. Slight chance of rain showers. Sunday Night: Moderate risk for Small Craft Advisory winds with gusts up to 25 kt. Rough seas up to 9 ft. Monday: Winds less than 25 kt. Local rough seas. && .EQUIPMENT... The KBOX radar is currently down due to an equipment failure. The radar may remain down into Thanksgiving. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 4 AM EST Thursday for ANZ230>234- 251. Small Craft Advisory until 1 AM EST Thursday for ANZ236. Small Craft Advisory until 6 PM EST Thursday for ANZ235-237. Small Craft Advisory until 7 AM EST Friday for ANZ250. Small Craft Advisory until 7 AM EST Friday for ANZ254>256. && $$ SYNOPSIS...KJC/Doody NEAR TERM...Doody SHORT TERM...Doody LONG TERM...KJC AVIATION...KJC/Doody MARINE...KJC/Doody EQUIPMENT...staff

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