Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 300520 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 120 AM EDT Thu Mar 30 2017 .SYNOPSIS... Expect a clearing trend with dry weather continuing through Thursday. Low pressure from the Ohio Valley passes S of New England Saturday. This brings snow and mixed precipitation Friday into Saturday with several inches of snow possible over higher elevations north of the Mass Pike. High pressure brings dry weather Sunday and Monday, followed by another approaching storm on Tuesday. && .NEAR TERM /UNTIL 6 AM THIS MORNING/... 10pm update... The main change with this late evening update will be to adjust overnight mins slightly. Although latest IR imagery shows mainly SKC conditions now across S New England, pres gradient remains tight enough to support a continuation of 5-10kt breezes through at least midnight. Therefore, outside of the most sheltered valleys it will be difficult to fully decouple limiting the amount of time for radiational cooling during the AM hours. Therefore, mainly widespread upper 20s for mins with this update. Previous discussion... Overall quiet weather and turning chilly. High pressure building into the area will result in initial breezy NW flow with gusts up to 30 mph along the E coast of MA, especially for the outer Cape`s to gradually relax. Drying out through the atmospheric column with increasing subsidence. Should become mostly clear with colder air still in place aloft. Lows down into the 20s for most areas, with the colder conditions N/W over the high terrain, perhaps even the adjacent low lying valleys with lighter flow. && .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM FRIDAY/... Thursday... High pressure in control. Overall light winds with enhanced ridging across the region. However initial moisture and mid-level energy begins to eject atop the ridge into the NE CONUS towards late. With a slight moderation in the airmass with increasing clouds towards evening, expect it to turn out to be a nice day warming into the upper 40s to low 50s. Some scattered clouds. Could be underdoing the temperatures a degree or two, however the airmass aloft will be slightly cooler than that which was observed today. Thursday night... Potential snow / sleet especially N/W portions of S New England for the Friday morning commute. Increasing H7 diffluence collocated with building mid-level H5-7 moisture. Convergent isentropic upslope rear- ward with the nose of the H85-7 low, however models diverge on the strength of such flow with the high-res solutions a bit more robust than global models. Nevertheless, PoPs increasing to likely in N/W MA and CT especially. With increasing moistening and lift within the snow growth region, should see precipitation start out as snow. Yet with a warm nose punctuating around H7-8, considering the depth and magnitude of the cold layer below, could see snow change over to sleet. A coating up to an inch of snow is possible, with higher amounts for the N/W high terrain. Temperatures marginal, slightly below freezing, and overnight. The intensity light, perhaps some challenges with accumulation, so mainly concerned given this time of year for exposed elevated surfaces to becoming snow covered. Lows down around the low to mid 30s. N/E winds. && .LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/... Highlights * Snow sleet and freezing rain north of the Pike Fri into Sat * Rain snow and sleet changing to rain south of the Pike * Several inches of snow/sleet accumulation and light ice accumulation possible north of the Pike with greatest risk over higher elevations Friday through Saturday... Models agree on overall pattern with potent mid level low over Ohio valley tracking south of New Eng with intensifying secondary low tracking neat 40N Fri night into Sat. There are differences with the interaction of this mid level low with northern stream trof in Canada which impacts QPF and thermal profile. GFS is weaker with northern stream trof and less interaction which translates to a weaker low level jet and a bit less QPF but much colder thermal profile. GFS would suggest mostly snow north of the Pike with potential for up to a foot, especially higher elevations. However, NAM and ECMWF to a lesser extent are stronger with the northern stream trof with stronger low level jet, heavier QPF but much more aggressive with warm nose above 850 mb. This would imply less snow but a prolonged period of sleet and freezing rain. A very uncertain ptype and snowfall forecast as minor changes in thermal profile will result in much different outcomes. We are still 48-72h out in the model world so really can`t lock into any one solution as changes are likely. Due to uncertainty we used a blend of model temp profiles to derive snow and ice accum. Timing and precip types/accumulation... Initial shot of warm advection precip will overspread SNE Fri morning assocd with the mid level warm front. The column is starting out rather cold but warmer air in the 850-700 mb layer will be approaching from the SW. Expect snow or mixed snow/rain/sleet south of the Pike and especially near the coast as warmer air aloft moves up from the SW. North of the Pike expect mainly snow on Friday. Precipitation will be heavy at times late Fri and especially Fri night into early Sat as the secondary low tracks to the south and comma head moves across the region. The biggest uncertainty is how far north the warm nose aloft gets. We will mention a mix of snow sleet and freezing rain north of the pike Fri night. Precip should begin to taper off Sat afternoon as the low moves east of New Eng and may transition back to snow before ending as deeper cold air move back southward. Potential for 6+ inches of snow north of the Pike with greatest risk over higher elevations, so we issued a winter storm watch for this area. Some ice accumulation is possible as well if the warm layer advances northward. South of the Pike across CT/RI and SE MA expect amounts mainly under 2 inches and confined to the grass and trees due to temps remaining above freezing. However, this is not a high confidence forecast due to uncertainty in precip type. Gusty NE winds developing along the coast during Sat and may need wind advisories for gusts over 40 mph. Sunday onward... A low confidence forecast concerning a very active weather pattern. Evaluating upstream, persistence of a low over the Gulf of Alaska promotes downstream ridging promoting equatorward flow along its E periphery. Pacific-origin energy sheared S, cyclogenesis occurs over the 4-corners region of the SW CONUS. Pushing E it undergoes further amplification and negative tilt to the lee of higher terrain before shooting off downstream through the progressive flow. It then comes down to the environment ahead, on the strength and location of any preceding high pressure noting isallobaric / ageostrophic flow and any subsequent cold air damming above which initial overrunning in advance of C CONUS storm systems is likely to occur. Despite uncertainty with low level thermal fields coupled with the magnitude and track of synoptic features, considering the late-March into early-April timeframe, increasing length of daytime heating, as well as lack of Arctic air and near-neutral teleconnections, am left to believe that any wintry weather outcomes, will be marginal with a low to moderate impact potential. Much of the focus in N/W MA, along the high terrain as we`ve seen recently with recent weather. Initial focus on the Monday night into Tuesday timeframe. Consensus blend of forecast data preferred. Ensemble members still exhibiting a large amount of spread. && .AVIATION /05Z THURSDAY THROUGH MONDAY/...
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Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Friday/... Today...High confidence. VFR with light winds. Mid to high level cigs increasing late. Tonight...Moderate confidence. Cigs lower and thickening towards morning, but mainly after 06Z and especially N/W. -SN develops mainly after 06Z as well and slowly spreads W-E across portions of MA/CT. This will lead to a reduction rapidly to IFR/LIFR vsbys before CIGS drop below MVFR. Light NE winds. Fri...Low confidence. High confidence in a trend toward IFR/LIFR everywhere as both CIGS and vsby lower. Confidence is low mainly a transition from SN to a mix of -SN/PL/FZRA and pure RA through the day. The heaviest precip waits until Fri evening, but this mix could still occur during the daylight hours Fri. KBOS TAF...High confidence in TAF through 06Z tonight. KBDL TAF...High confidence in TAF through 06Z tonight. Outlook /Friday Night through Monday/... Friday through Saturday...Moderate confidence. Widespread IFR/LIFR expected with some improvement from west to east Sat afternoon. Snow/sleet/freezing rain north of the pike and rain/snow/sleet to the south transitioning to all rain by Fri night. Gusty easterly winds near the coast Fri night becoming NE Sat. Sunday into Monday... VFR. NW winds becoming E with time. Increasing clouds late in the period.
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&& .MARINE...
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Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Friday/... Initially breezy overnight with near-gale force gusts. Believe there to be a low risk of NW gales. Winds taper into the morning period as high pressure builds into the region. Allowing winds to become light, will see seas dampen with time resulting in the end of small craft advisories towards Thursday evening. Increasing clouds towards Friday morning as winds become N/E ahead of the next storm system. Outlook /Friday Night through Monday/... Friday-Saturday...Moderate confidence. Easterly winds below SCA Fri but increasing Fri night with risk for gale force gusts to 35 kt. Winds becoming NE Sat with gales possible. Seas building up to 10 ft over the outer waters. Sunday into Monday... Blustery NW flow initially becoming light into Monday while turning E beneath high pressure. Should see wave action diminish throughout the period.
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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...Winter Storm Watch from Friday morning through Saturday afternoon for MAZ002>006-008-010-026. RI...None. MARINE...Small Craft Advisory until 2 PM EDT this afternoon for ANZ231>234-251-255-256. Small Craft Advisory until 8 AM EDT this morning for ANZ230- 235>237. Small Craft Advisory until 8 PM EDT this evening for ANZ250- 254.
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&& $$ SYNOPSIS...KJC/Sipprell NEAR TERM...Doody/Sipprell SHORT TERM...Sipprell LONG TERM...KJC/Sipprell AVIATION...KJC/Doody MARINE...KJC/Sipprell

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