Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 240209 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1009 PM EDT Fri Sep 23 2016 .SYNOPSIS... A cold front will cross the region this evening, bringing lower temperatures. A band of rain showers is also expected behind this front later this evening into tonight, but amounts will be light. Cool and dry weather will follow this weekend into early next week. A slow-moving low pressure and cold front could lead to some showers late next week. && .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/... 10pm update... Widespread shra have developed in the deep moisture behind a cold front which is currently moving into RI/CT and SE MA this hour. 1.5 inch PWATS are enough to allow as much as 0.20-0.30 of total rainfall as these shra move through. Not as much as is needed of course, but even this little bit well help. HRRR/RAP capturing this band well, so POPs were updated toward their solutions. These should be mostly offshore by about 2AM or so. Previous discussion... Short-term guidance, such as the HRRR and RAP, continues to indicate a higher likelihood of at least some lightly measuring showers. Will continue with a brief period of likely PoPs this evening. Band of rain showers will continue to press from north to south with band of mid level frontogenesis/moisture behind a cold front. Much cooler air will continue to work into the region tonight and northeast winds will be a bit gusty along the coast. && .SHORT TERM /6 AM SATURDAY MORNING THROUGH SATURDAY NIGHT/... Drier air will move in from the northwest, allowing for skies to become mostly sunny during the morning. Mostly clear skies continue through Saturday night. Below normal temperatures during this time as well. If winds can diminish enough, there is the remote possibility of some frost across portions of the east slopes of the Berkshires Saturday night, particularly in Franklin county. Will need to monitor this, but think it is a low probability event at this time. However, a fall-like atmosphere will be in place with 850T between +3C and +5C. High temps should still recover well into the 60s to near 70 given lots of sunshine, but certainly be our first true taste of early fall. && .LONG TERM /SUNDAY THROUGH FRIDAY/... Highlights... * Dry and cool weather continues through Monday with patchy frost possible for some locations Sunday night. * Precipitation chances increase Monday night thru Wednesday night, with a return to seasonable temperatures. Overview and model preferences... Omega block over the Mississippi Valley continues to translate east and gradually deamplify through the middle of the upcoming week. The 23/12Z model suite begins to diverge around 12Z Monday, with the ECMWF notably weaker, and thus more progressive, with its handling of the closed low over the Great Lakes. We leaned toward the slower and more amplified GFS solution, given the interaction of several pieces of energy, as the ECMWF may be breaking down the block too quickly. But confidence in the details is below average, and will remain so, until the models better resolve competing shortwave energy interacting with the closed low. Details... Sunday and Monday... Northwest flow aloft will keep the region dry during this time frame, as Canadian high pressure crests across the region by Monday morning. Given the combo of a dry airmass in place, as well as a decoupled boundary layer, we anticipate ideal radiational cooling conditions to occur Sunday night. Thus, we went below guidance on temperatures, which may bottom out in the low to mid 30s for portions of the region. In particular, the interior locations northwest of the Mass Pike and I-495, as well as interior southeast Massachusetts, have the potential for their first frost of the season. Otherwise, there will be an increase in mid and high level cloudiness on Monday, as a warm front approaches the region. Monday night thru Wednesday... There is still uncertainty with the timing of a cold front during this time frame, with the more progressive ECMWF keying in on Tuesday morning, while the GFS holds off until Wednesday afternoon. We leaned more toward the GFS, and focused PoPs in the Monday night to Wednesday morning period. We also kept consistency with the previous forecast, with the greatest chance of precip focused in the late Tuesday thru early Wednesday time frame, when the deepest moisture and strongest dynamics coincide. This system does not have much in the way of moisture to work with, as Precipitable Water values only rise to 1.5 inches, so rainfall will be generally light. We also cannot rule out a rumble of thunder along the south coast Monday night into early Tuesday, as some elevated instability traverses this area. Thu and Fri... Still some uncertainty regarding how quickly the upper low departs the region. We maintained NIL to low PoPs, especially considering the amount of moisture along the back side of this feature. && .AVIATION /02Z SATURDAY THROUGH WEDNESDAY/... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Tonight...Moderate to high confidence. A period of MVFR CIGS and a band of showers are likely behind a cold front. Northeast wind gusts of 20 to 25 knots expected along the coast. Saturday...High confidence. Mainly VFR. Any leftover MVFR Cigs, and perhaps some patchy early morning fog in the typical locations, will improve to VFR by mid morning. Saturday night...High confidence. VFR. KBOS TAF...High confidence in TAF and trends, slightly lower confidence in exact timing, particularly of showers. KBDL TAF...High confidence in TAF and trends, slightly lower confidence in exact timing, particularly of showers. Outlook /Sunday through Wednesday/... Sun thru Mon...High confidence. Predominantly VFR. Northwest winds may gust to 25 kt near shore on Sunday. Mon night thru Wed...Moderate confidence. Localized MVFR conditions possible in showers, mainly Tue night into Wed, with uncertainty with regard to timing. Otherwise VFR. && .MARINE... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Tonight...High confidence. A strong cold front should result in a period of northeast wind gusts up to 25 knots, and 3 to 6 foot seas across our open waters, early this evening into the overnight hours. Small craft headlines posted for all waters except Narragansett Bay. Saturday into Saturday night...High confidence. Leftover 25 kt wind gusts diminish by mid morning as pressure gradient weakens. However, small craft headlines will continue across our outer waters into mid afternoon from leftover swell. These seas should finally drop below small craft advisory thresholds by late afternoon. Increasing north to northwest winds with a stronger surge of colder air over the waters. Not confident there will be a long period of sub advisory winds. So, will continue the small craft advisory into Saturday night across the outer coastal waters. Outlook /Sunday through Wednesday/... Sunday and Sunday night...High confidence. Small craft advisory conditions prevail. Northwest wind gusts around 25 knots Sunday, with seas 5 to 6 feet expected into Sunday night. Monday... Conditions are expected to remain below Small Craft Advisory criteria with high pressure building over the waters. Monday night thru Wednesday... An approaching storm system will lead to a period of southerly flow over the waters during this time frame. Small Craft Advisory criteria is possible during this period, however, the timing is uncertain. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 8 AM EDT Saturday for ANZ231>235- 237-251. Small Craft Advisory until 11 PM EDT this evening for ANZ230. Small Craft Advisory until 6 AM EDT Sunday for ANZ250-254>256. && $$ SYNOPSIS...Belk/99 NEAR TERM...Belk/Doody SHORT TERM...Belk LONG TERM...99 AVIATION...Belk/99 MARINE...Belk/99

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