Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 191330 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 930 AM EDT Sun Mar 19 2017 .SYNOPSIS... An ocean storm will intensify and track close enough to the coast to give several inches of heavy wet snow today to the outer Cape and Nantucket. Elsewhere little if any snow is expected. In addition this storm will bring very strong winds to region as well. Drier and milder temperatures follow Monday and especially Tuesday. An arctic front will bring near record cold temperatures Wed and Thu along with bitterly cold wind chills. A warming trend then develops Friday and Saturday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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930 am update... Moderate snow now being reported on Nantucket with some accumulations occurring even on paved surfaces given the intensity. Lighter snowfall has overspread Cape Cod and Marthas Vineyard. Still a very difficult forecast as decent snowfall rates will be needed for much accumulation given marginal boundary layer temps/March sun angle. Some of the latest high resolution data such as the HRRR/RAP have backed off on QPF amount. Not sure if this heavier band of snow will make it onto much of the Cape/Marthas Vineyard given some mid level dry air and distance of ocean storm. It is also uncertain how long the moderate snow will persist across Nantucket. The best chance for accumulating snow will be through about noon or 1 pm as stronger forcing will probably shift east after that time. No changes to the forecast at this time but closely monitoring trends as we may need to lower snow accums across portions of the advisory if heavier bands are unable to sustain themselves. Will have a much better idea in the next 2 hours. ================================================================= */ Highlights... * High end snow advisory event with a low risk of reaching warning level criteria for the Outer Cape and Nantucket considering heavy, wet snow and strong to damaging northerly winds. * It only takes a couple of inches of heavy, wet snow in addition to winds to start snapping branches resulting in potential power outages. * Looking at an early morning into afternoon event with a 2 to 4 inches forecast with a low risk someone may see 5 to 6 inches. WINTER WEATHER ADVISORY issued for the Outer Cape and Nantucket. * Strong to damaging northerly winds in addition to heavy, wet snow. Gusts up to 60 mph forecast across the Outer Cape and Nantucket later today continuing overnight. A combination of HIGH WIND WARNINGS and WIND ADVISORIES posted. */ Discussion... Model Preference: While consistent signals within a majority of the 19.0z forecast guidance with respect to frontogenesis and deep layer lift through snow growth regions within the wrap around comma head of deep layer moisture, going with a NAM / WRF blend with its better handing of 2m temperatures and wet-bulb with respect to the expected intensity of snowfall. Good agreement with up to date runs of the RAP / HRRR. Snowfall: With anticipated intense precipitation, likely starting off as rain initially before changing over to snow, should see low level column cooling as it becomes near-isothermal as exhibited by NAM BUFKIT sounding profiles. Focusing on the time frame of early morning into the afternoon hours for greatest impact of intense precipitation with potential 1 inch per hour snowfall rates and reduced visibility down to a half a mile, in addition to the heavy, wet consistency and possible impacts of downed limbs and power outages. Expect precipitation to taper off late with snow perhaps changing back over to rain as it becomes light. Most of the snow accumulation mainly on non-paved surfaces. However as the column cools towards freezing and/or snow picks up in intensity, will likely see snow accumulation along area roadways. Given the impacts and forecast accumulation amounts above have gone ahead and issued a WINTER WEATHER ADVISORY, mainly for the Outer Cape and Nantucket. The Upper Cape is forecast to see mainly a 1 to 2 inch event with less impact. Winds: Consensus of near-term high-res guidance has 45 to 50 mph (40 to 45 kt) northerly flow at H925 from around midday Sunday to Monday morning. With cold air advection proceeding on the back side of the comma head low should see the boundary layer mix out to around H9 allowing for the mix down of faster momentum. While just on the cusp of warning criteria, given accompanying heavy wet snow aiding in the potential for downed limbs and powerlines, have gone forward with issuing a HIGH WIND WARNING, mainly for the Outer Cape and Nantucket with less impact over the Upper Cape. A WIND ADVISORY continues for Marthas Vineyard. Sustained winds 20 to 30 mph along the immediate E MA coastline with gusts 35 to 55 mph, with again the strongest gusts potentially as high as 60 mph across the Outer Cape and Nantucket.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM MONDAY/... Northerly winds will continue to be an issue overnight initially sustained around 20 to 30 mph along the immediate MA coast with the strongest winds across the Outer Cape and Nantucket. Gusts up to 60 mph. HIGH WIND WARNINGS continue. Precipitation should be for the most part over. Perhaps some lingering snow showers over the Cape and Islands, but for the most part will see a clearing trend W to E. Lows dropping back down into the 20s with the immediate E MA coast hovering around the freezing given a component of onshore flow. && .LONG TERM /MONDAY THROUGH SATURDAY/... Highlights... * Moderating temps Mon and Tue with 50 possible for some Tue! * Arctic blast with near record cold returns middle of next week * Warming trend Friday and Saturday Details... Monday... Strong subsidence on backside of departing ocean storm will support any morning clouds giving way to sunshine by midday. Clouds may be a bit more stubborn across the outer Cape and Nantucket where northerly flow will provide ocean effect clouds. But even here sunshine should develop during the afternoon as winds turn to the northwest. Modest northerly winds early (especially Cape Cod and the islands) shift to the NW by days end and diminish. Model soundings support highs in the 40s, very close to normal. It may feel a bit cooler courtesy of a modest NNW wind 15 to 20 mph. Overall, close to normal for late March. Tuesday... Arctic front approaching from the northwest but not expecting a frontal passage until sometime Tue night or early Wed morning. Prefrontal airmass over southern New England somewhat mild with 925 mb temps at sunrise around +2C. Northwest gradient will promote good mixing and downsloping winds to support highs of 50-55 with mildest readings across CT/RI and especially southeast MA where snow cover is very minimal. This will be the first warmer than normal day since March 9th. As for high temps, guidance continues to trend warmer for Tue so will continue to blend in the warmer EC mos guidance here. Euro ensembles also showing a warm signal here with 80-90% prob of 50+ degs Tue over RI and southeast MA. Wednesday and Thursday... Arctic front comes thru early Wed morning and may be accompanied by a few snow squalls. Although at this time range difficult to say much more given mesoscale nature of potential squalls. Potential arctic short wave briefly backs the mid level flow across the northeast coast which stalls an offshore front. However arctic short wave appears to lack sufficient amplitude to back the mid level flow, therefore moisture and associated frontal boundary likely remain offshore with little to no impact for southern New England. Will need to watch this closely. Otherwise frigid airmass overspreads the region Wed/Wed night and lingers across the area into Thu. Both ensembles and deterministic guidance have 850 temps over the region plummeting to -18C to -20C Wed night! This yields temp anomalies at 850 mb and 925 mb of -2 standard deviations colder than climo. This will result in near record cold temps Wed and Thu along with bitterly cold wind chills Wed night into Thu morning. Given the magnitude of the cold airmass will lean toward the coldest guidance. Friday and Saturday... Good ensemble agreement that cold mean trough and associated arctic airmass will be progressive with height rises to follow Fri-Sat. Thus expecting a warming trend this period. Can/t rule out some precip in WAA pattern Fri and possible frontal passage Sat. && .AVIATION /13Z SUNDAY THROUGH THURSDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. 11z update... No major changes from previous TAFs. Earlier discussion below. ================================================================== Short Term /through Tonight/... Today...Moderate Confidence. MVFR cigs eroding W to E becoming VFR. N winds. However more extensive impacts along the immediate E MA coast especially Outer Cape and Nantucket. Sustained N/NE winds 20 to 30 kts, gusts up to 50 kts. Heavy, wet -SN/SN with visibility down to 1/2SM. Accumulations of around 2 to 4 inches on the Cape and Islands. SHSN possible within the I-95 corridor towards the Cape Cod Canal. Tonight... -SN/SN threat diminishes, lingering SHSN over the Cape and Islands where MVFR cigs mainly linger. VFR elsewhere. KBOS TAF... Mostly MVFR with SHSN however low confidence with respect to any snow accumulations and visibility restrictions. Blustery N winds 15 to 25 kts sustained with gusts up to 35 kts. Both MVFR and wind threats trend downward overnight. KBDL TAF... Becoming VFR late today. N winds with the possibility of gusts up to 20 kts, diminishing overnight. Outlook /Monday through Thursday/... Mon...High confidence. VFR develops inland, with MVFR and occasional IFR lingering Cape/Islands in the morning mainly due to low CIGS, but some lowered vsbys in light SN may linger as well. North Winds early shifting to NW late in the day. Meanwhile Cape/Island gusts may hold 30-45 kt Mon morning then diminishing thereafter. Tuesday and Wednesday...High confidence. Mainly VFR. Modest WNW winds Tue but increasing Wed behind arctic front. Low risk of few snow squalls and MVFR conditions accompanying the arctic front Tue night/Wed morning. Thu...High confidence for VFR and a trend toward diminishing winds. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tonight/...High confidence. An area of light to moderate rain / snow mix will pivot westward of a deepening ocean storm accompanying with strong northerly winds sustained around 25 to 35 kts with gusts up to 50 kts. Will see wave heights build as high as 16 feet on the waters E of Cape Cod. GALE WARNINGS have been issued for all waters accordingly except the southeast outer waters including Nantucket Sounds where a STORM WARNING is currently in effect with forecast wind gusts up to 50 kts. Anticipate dissipating trends across the waters beginning Monday morning. Outlook /Monday through Thursday/... Monday...High confidence. Northerly gales continue with gusts near 35 kt Mon morning. Otherwise winds slacken Monday afternoon and shift to the northwest. Low risk for some vsby restrictions early Mon in snow/rain showers eastern MA waters. Tue...high confidence. Modest WNW winds, dry weather and good vsby. Arctic front barrels across the waters early Wed morning. May be accompanied by a few snow squalls. Wed and Thu...Moderate confidence. NW winds 20 to 30 kt behind arctic front. Moderate freezing spray possible especially Wed night. && .TIDES/COASTAL FLOODING... Coastal flooding appears unlikely from this storm. The main concern would be Cape Cod Bay with strong NNE wind gusts this evening with a 1 to 2 foot storm surge. However relatively low astronomical high tides should prevent any problems. && .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...High Wind Warning from 2 PM this afternoon to 2 AM EDT Monday for MAZ022-024. Winter Weather Advisory until 5 PM EDT this afternoon for MAZ022-024. Wind Advisory until 5 AM EDT Monday for MAZ023. RI...None. MARINE...Storm Warning from 5 PM this afternoon to 2 AM EDT Monday for ANZ232. Gale Warning until 8 AM EDT Monday for ANZ231-233>235-237-250- 251-256. Gale Warning until 8 AM EDT Monday for ANZ230-236. Storm Warning from 2 PM this afternoon to 8 AM EDT Monday for ANZ254-255.
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&& $$ SYNOPSIS...Nocera/Sipprell NEAR TERM...Frank/Sipprell SHORT TERM...Sipprell LONG TERM...Nocera AVIATION...Nocera/Sipprell MARINE...Nocera/Sipprell TIDES/COASTAL FLOODING...Staff

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