Area Forecast Discussion
Issued by NWS Boston, MA

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910 FXUS61 KBOX 242045 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 445 PM EDT Fri Mar 24 2017 .SYNOPSIS... High pressure moves farther offshore tonight bringing a southwest flow of milder air to Southern New England. A cold front will push south across the region Saturday. The front will stall Saturday night, then will try to push northward Sunday and Monday. Periods of light rain will fall Saturday, then a mix of wintry weather will develop Sunday night into early Monday. Rain may fall heavily at times later Monday as low pressure passes across the region, then continues into Tuesday ahead of an approaching cold front. Colder air lags Wednesday into Thursday with breezy NW winds. Another chance for disturbed weather Friday into the weekend given the continued active weather pattern. && .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/... Warm front and its precipitation are moving off to the northeast. Southwest winds about 2000-3000 feet aloft reached 50 knots during the afternoon and will slowly diminish tonight. Even so, that will bring potential for gusts to 30 knots through the evening. Upper jet passes north of New England overnight, carrying a wave of low pressure across Quebec. This will in turn move the front south as a cold front after midnight. Low level convergence may generate light showers that reach Northern Mass by morning, but otherwise expect little/no precipitation in Southern New England tonight due to southwest flow and no low level forcing. The mild flow of air will keep temperatures from falling much. There may be a slight cooling this evening, but overall trend through the night should be steady or slowly rising. && .SHORT TERM /6 AM SATURDAY MORNING THROUGH SATURDAY NIGHT/... Saturday... Cold front sweeps across the region first thing in the morning. High pressure building in behind the front will turn winds from the north Saturday morning and from the northeast in the afternoon. The upper jet referenced earlier will be to our northeast Saturday with our area under the right entrance region. This may maintain upper venting over/near the front while precipitable water values along/south of the front remain at 0.75 to 1.0 inches, which is well above normal for mid-late March...the 90 pct exceed value is about 0.86. This should be enough to support a chance of light rain, along and a little behind the front. This rain shifts south of New England toward evening. With clouds and potential light rain, expect temps to climb just a little, with max values in the 40s. Saturday night... High pressure from Canada builds south into New England and brings colder air. Expect dry weather most of the night. Dew points will not fall as much as with most highs because our low level flow will be out of the east-northeast, more of a marine flow. Expect these dew points to be in the mid 20s to around 30. As such, we expect min temps to be in the upper 20s to mid 30s. With the ridge building in as it shifts toward the Maritimes, the models show a developing cold air damming signature Saturday night. This suggests the low level cold air will become establish in advance of the next weather system. && .LONG TERM /SUNDAY THROUGH FRIDAY/... Highlights... * A period of wintry precip is possible Sunday night into Monday morning * Unsettled weather pattern will continue into Wednesday * Improving conditions during the later half of next week Overview... Confidence with the overall pattern across the northeast U.S. appears continues to improve, but still issues with details in timing of weather systems as well as thermal pattern especially this weekend into early next week based on 12Z model suite and ensembles. Active pattern continues for the region with split flow aloft and several shortwaves ejecting across the lower 48 from the Pacific. Model guidances continues to signal the deamplification of H5 cutoff low over the midwest, becoming an open wave as it moves across the region in the steering flow late this weekend into early next week. With confluent flow aloft, high pressure persists over northern New England keeping cool surface temperatures which will result in a wintry mix of precip at times late this weekend into early next week. A second wave will follow a similar path from the Plains towards the northeast before meshing or partially interacting with the northern jet stream on Tuesday. This will result in another system for New England Tues/Wed with high pressure to follow. Details... Sunday...Moderate confidence. A stalled front along the mid Atlantic coast will try to work northward as high pressure ridge shifts SE into the Gulf of Maine. Winds aloft are mainly SW, which will allow some low level moisture well ahead of low pressure over the midwest to work slowly N during the day as ridge will shift slowly E. Marginal thermal fields and good cold air damming pattern remains across the region, which will lead to patchy mixed rain, freezing rain and/or sleet across the interior for early Sunday, but they are shallow so temps should rise above freezing during the morning, However, some sleet/freezing rain may linger through midday across the higher terrain. Temps will recover to the upper 30s to around 40 well inland, to the lower 40s elsewhere thanks to low level onshore flow off the milder ocean. Sunday night through Monday night...Moderate confidence. Stalled front to the S tries to shift back N as a warm front late Sunday-Monday, while surface low pressure works toward the Great Lakes. Good overrunning works across the region, with H85-H7 temps rising to +5C to +6C during this timeframe. Shallow low level cold air damming remains in place, though, with temps falling the upper 20s to around freezing. This will cause mixed precipitation to develop during the nighttime hours both nights. Looks like the best shot for widespread light sleet and/or freezing rain will occur Sunday night into early Monday morning. Noting mainly freezing rain signatures on both NAM and GFS BUFKIT soundings, can`t rule out sleet especially across the higher terrain of the E slopes of the Berkshires and northern Worcester hills. At this point, could see up to 0.1 inches ice accretion, but this remains dependent upon how long the cold air remains in place as well as the low pressure passage and the QPF amounts that move across. Low pressure passes across the region during Monday afternoon/ evening so expect good shot of precip to work across. This will coincide with increased PWAT values of around an inch and, along with continued confluent pattern, could see around 0.5 to 0.7 inches QPF which will move across when temps should be above freezing. Also noting some marginal instability across N CT/RI/SE Mass moving across Monday afternoon, with total totals around 50 and TQ values in the upper teens. At this point, with E-SE low level flow continuing, not expecting any thunder at this time. Precip should slowly taper off as low moves offshore Monday evening. With the marginal thermal pattern remaining in place, could see another round of light freezing rain, though looks to be primarily across the higher terrain of N central Mass after midnight through the early morning hours of Tue. Tuesday into Wednesday... Continued risk of freezing rain early on, if not at a minimum a cold rain is forecast. Associated deeper mid to upper level low over the S Hudson Bay region with a cascading, deamplifying open wave trough through the S-stream yields an elongated area of low pressure across the E CONUS with secondary low development across the Mid-Atlantic. Initial over-running moist ascent transitioning to frontogenetical, synoptic forcing ahead of a sweeping cold front. However the colder air lags. So altogether, PoP chances through Tuesday night, clearing into Wednesday. A cool, damp day especially for N/NE MA initially, gradually warming, with a decent slug of rain. The cold air lagging along with clearing Wednesday, may end up mild as conditions dry. Thursday... High pressure in control however the crux of the colder airmass is drawn S to the E of New England as the early to mid week low over the Hudson Bay region of Canada undergoes deeper cyclogenesis across the N Atlantic. Given a mix of sun and clouds, and that being a late March sun, leaning mild with more seasonable conditions. Cold air advection proceeding in the low levels along with an enhancement of flow aloft, expect some gusty NW winds the the boundary layer mixing up to around H85. Friday into the weekend... Confluence within the mid to upper levels of N/S streams across the NE CONUS continues. Any closed lows emerging out of the SW CONUS undergo deamplification. Scenario for continued over- running setups to which the thermal profiles across the region require scrutiny. Conflicting signals of low pressure evolution associated with weak wave impulses neighbored against high pressure out of Canada. A lot can change and thus a low confidence forecast. Only confidence is that we`ll continue to see an active weather pattern into the long term period. && .AVIATION /20Z FRIDAY THROUGH WEDNESDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Saturday Night/...Moderate confidence. Tonight... Mainly VFR but with possible MVFR late at night in northern MA. Southwest winds will gust 25-30 knots early, then diminish overnight. Saturday... A cold front moves north to south across the region during the morning. An area of light rain may develop, with cigs and vsbys lowering to MVFR. Winds shift from the north in the morning and northeast during afternoon/evening. Saturday night... Mainly VFR as high pressure builds from the north. MVFR will lurk to our southwest, preparing to move north again. East flow through the night. KBOS TAF...High confidence in TAF. KBDL TAF...High confidence in TAF. Outlook /Sunday through Wednesday/... Sunday-Sunday night...Low to moderate confidence. VFR CIGS/VSBYS early Sunday. CIGS lower to MVFR-IFR mainly across the interior by midday Sunday and continue Sunday night. Mainly VFR VSBYS Sun, then lower to MVFR-IFR Sun night in rain and patchy fog. Areas of -FZRA/-PL develop across the interior Sun night, especially across the higher terrain. Monday-Monday night...Low to moderate confidence. Any leftover -FZRA/-PL Monday morning will end across higher terrain. Otherwise MVFR-IFR CIGS in light rain. VSBYS should be mainly VFR, but may see local MVFR at times. Low chance for -FZRA Mon night across higher terrain of N Central Mass, mainly N of Route 2. Tuesday...Low confidence. MVFR-IFR mix with -RA/RA, low risk FZRA early over N/NE MA. N/E winds turning out of the N/W. Wednesday...Moderate confidence. Improving. Cigs lifting VFR. N/W winds continue. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Saturday Night/... Tonight...High confidence. Southwest winds gusting 25-30 knots early will diminish overnight, and become west toward morning. Rough seas linger across the outer coastal waters, and ocean-exposed south coastal sounds. Gales early on the eastern outer waters. Small Craft Advisories will lower on Boston Harbor and Narragansett Bay, but linger on the remaining waters. Saturday...High confidence. West winds shift from the north in the morning and early afternoon as a cold front moves north to south. Winds then shift from the northeast by afternoon as high pressure moves in from Quebec. Winds will be less than 25 knots through the day. Seas will linger at 5 feet on the southern outer waters and parts of RI Sound much of the day. Small Craft Advisory will be needed where seas linger near 5 feet. Saturday night...Moderate-High confidence. Northeast winds turn from due east overnight. Speeds remain below 20 knots through the night. Seas will remain less than 5 feet through the night. Outlook /Sunday through Wednesday/... Sunday-Sunday night...Moderate confidence. E-SE winds will increase especially on the southern outer waters. Expect gusts up to around 25 kt on the open waters. Seas may reach 5 ft on the outer waters S of RI briefly Sunday afternoon/evening. Visibility reductions in rain and patchy fog will develop Sun afternoon and continue Sun night. Monday-Monday night...Low to moderate confidence. E-SE winds continue. Winds gust to around 25 kt mainly over the eastern waters as seas build to around 5 ft. May need small craft headlines there. Local visibility restrictions continue in rain and patchy fog mainly during Monday. Tuesday through Wednesday...Low to moderate confidence. Rain continues across the waters with E winds. Could see some visibility restrictions ahead of a cold front sweeping the region late into Wednesday morning. Behind the front winds become more westerly and could see some gusts. Waves over 5 feet possible on the outer waters. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 6 AM EDT Saturday for ANZ232>235- 237. Small Craft Advisory until 8 PM EDT this evening for ANZ230. Small Craft Advisory until midnight EDT tonight for ANZ231-251. Small Craft Advisory until 10 PM EDT this evening for ANZ236. Gale Warning until 10 PM EDT this evening for ANZ250-254. Small Craft Advisory until 8 PM EDT Saturday for ANZ255-256. && $$ SYNOPSIS...WTB/Sipprell/EVT NEAR TERM...WTB SHORT TERM...WTB LONG TERM...Sipprell/EVT AVIATION...WTB/Sipprell/EVT MARINE...WTB/Sipprell/EVT is the U.S. government's official web portal to all federal, state and local government web resources and services.