Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 211419 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1019 AM EDT Wed Mar 21 2018 .SYNOPSIS... A coastal storm passing well south of Nantucket will bring snow mainly late today into early Thursday morning with the greatest risk of heavy amounts south of the Massachusetts Turnpike. Upper low pressure lingers over the Maritimes through the weekend with potential for clouds and scattered rain/snow showers. Temperatures remain colder than normal through the period. && .NEAR TERM /UNTIL 5 PM THIS AFTERNOON/...
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1015 AM update... Some light snow and rain is moving up along the south coast but low level dry air is winning the battle further to the north and precip advancement to the north will be a slow process. Light precip will be focused near the south coast through early afternoon with minimal impact given temps above freezing and light precip resulting in melting on paved surfaces. We do expect snow to push north toward the Mass Pike mid/late afternoon while intensity ramps up across portions of CT/RI and SE MA where accum of 2-3 inches possible by evening. North of the Pike, it will likely take until late afternoon or evening near the NH border before snow begins to accumulate so minimal impact here through the daytime hours.
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&& .SHORT TERM /5 PM THIS AFTERNOON THROUGH 6 PM THURSDAY/... * Bulk of accumulating snow this evening into early Thu Am * Abnormally low confidence in snow amounts for a 24 hr forecast * Greatest risk for heavy snow/iso power outages south of Pike 1) Overview... A very challenging forecast late this afternoon and particularly tonight into early Thursday morning. An unusual spread remains in the model guidance given the event is pretty much inside 24 hours. The main issue is that surface low pressure will be tracking northeastward towards the Benchmark. However...model QPF fields vary considerably from the Ukmet/EC which have confine the threat for heavy snow to far southeast New England. Meanwhile...the GFS/NAM are much further north and indicate heavy wet snow right to the NH border. However...this guidance was also warmer across the far southeast New England coast especially the Cape/Islands where it indicated the potential for more rain than snow. The RGEM was the middle ground of the model guidance and given the tremendous uncertainty is the path we followed with this forecast. Greatest threat for heavy snow south of the MA turnpike to near the Cape Cod Canal. Appears the model guidance is struggling with an initial lead shortwave and also a lot of dry air in the 800 to 900 mb layer...north of the MA turnpike. The guidance that erodes that dry layer quicker indicates heavy snow right to the NH border...while the other guidance keeps that heavy snow threat south of the MA turnpike. The models do indicate some decent mid level banding and frontogenesis at times...but it seems very transient and mixed with some dry air. We did notice that the latest HRRR/RAP struggles to get much QPF north of the MA Pike through mid evening. Not exactly sure that is correct...but needs to be taken into consideration. 2) Snow Amounts/Timing: While some snow will impact the region through the afternoon...main impact probably not until after 5 or 6 pm this evening south of the Pike and even later north of that region. We should see the snow accumulations continue into early Thursday morning...especially eastern MA where some guidance hints at mid level banding. As we mentioned snow accumulations are very difficult...generally went 3 to 5 inches across central and northeast MA with 5 to 9 inches south of that region but localized 12 inch amounts are possible. Our lowest snowfall amounts are in northwest MA where some models show too much dry air to warrant more than 1 to 3 inches. As for headlines...opted for winter weather advisories across central/northeast MA with warnings continuing for most areas south of the Pike. We thought about downgrading Cape/MVY/BID to an advisory given marginal thermal profiles...but given uncertainty will defer to the next shift. 3) Wet Snow/Strong Winds Power Outage Potential: A well mixed northeast low level jet with decent low level lapse rates will support 40 to 60 mph winds gusts across the coastal plain. The strongest of those winds will be across the Cape/Islands and those will occur tonight. These winds alone may result in isolated power outages. In addition...the greatest threat for heavy wet snow is south of the MA turnpike where we will have the potential for isolated power outages. The snow amounts themselves along with the winds do not look as strong as what we have seen in the past couple of events. Therefore... while isolated power outages are certainly possible it does not look to be as widespread as what we have seen with the past few events. Nonetheless...if confidence in heavy snow increases this potential may have to be played up more than current indications. Later Thursday morning and afternoon... Some left over snow showers possible later Thursday morning...but temps will be rising above freezing so any road issues will quickly improve. In fact...much of the region may see highs in the lower to middle 40s Thursday afternoon. && .LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/... Highlights... * Snow showers/squalls possible late Friday/Saturday * Below average temps through the period * Mainly dry early next week with hit or miss showers Synoptic Overview & Model Preferences... Overall trends in the guidance suggest a medium to high confidence level for the extended. Broad longwave trough will remain over the region through much of the medium range. Several cut-off shortwaves will eject from the Hudson Bay region into the flow resulting in wave after wave. This will also result in reinforcing cold air keeping temperatures below average through the period. Thursday night into Sunday... Amplified cyclone which will impact our region Wednesday into Thursday will begin to pull out towards the Maritimes by Thursday evening. Lots of dry air will filter back into the region thanks to northwest flow. Another digging shortwave will get swept up in the broad longwave trough and push into the region Friday into Saturday developing a cold pool aloft. Low level moisture appears to saturate per the NAM and GFS with some guidance showing lapse rates near 6.5+ C/km. This could be enough to help trigger snow showers or snow squalls. In fact the BTV WRF model does show SNSQ parameters above one across western MA and CT. Best timing appears to be Friday evening, but axis of greatest potential could still shift. Saturday and beyond... Still chilly for the weekend as CAA takes hold of the region. Another cut-off 500 mb low will strengthen over the region. This low could bring in some showery weather across the area from off the ocean. Still a lot of details to work out as the 12z EC tries to bring offshore low closer to the coast, with the GFS being more progressive and trending towards dry weather by mid-week. Will wait until after this upcoming storm to begin to dive into this potential scenario. && .AVIATION /14Z WEDNESDAY THROUGH SUNDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Thursday/... Today...Moderate confidence. VFR conditions should deteriorate to MVFR/IFR thresholds south of the MA turnpike mainly this afternoon particularly near the south coast...but overall intensity of precipitation should be light through late afternoon. Ptype a mixture of light rain/snow with mild boundary layer temps and little impact on runways through late afternoon. Tonight...Low to moderate confidence. MVFR-IFR conditions in mainly snow with some LIFR thresholds for a time especially south of the Pike. Ptype mainly snow but some mixing possible across the Cape/Islands. Thursday...Moderate confidence. Lingering snow especially across eastern MA may result in some IFR conditions Thu AM. Otherwise...mainly MVFR conditions expected but improvement is possible to VFR across the interior by late in the afternoon. KBOS Terminal...Low to moderate confidence in TAF. MVFR conditions may develop this afternoon...but IFR conditions and bulk of accumulating snow probably holds off until after 00z. KBDL Terminal...Moderate confidence in TAF. MVFR-IFR conditions probably develop by mid to late afternoon in snow. Bulk of accumulations probably after 21z or 22z. Outlook /Thursday Night through Sunday/...Moderate Confidence. Thursday Night: Mainly VFR, with local MVFR possible. Breezy. Friday: Mainly VFR, with local MVFR possible. Slight chance SHRA, slight chance SHSN. Friday Night: Mainly VFR, with local MVFR possible. Breezy. Saturday: Mainly VFR, with local MVFR possible. Breezy. Slight chance SHRA. Saturday Night: Mainly VFR, with areas MVFR possible. Breezy. Slight chance SHSN. Sunday: Mainly VFR, with areas MVFR possible. Windy with local gusts to 30 kt. Slight chance SHRA, slight chance SHSN. && .MARINE... Short Term /through Thursday/... Today through Thursday...Moderate to high confidence. Storm warnings all waters late today and especially tonight...except for strong Gales across Boston Harbor and Narr Bay. Northeast wind gusts of 40 to 50 knots expected later today and especially tonight with good transfer over the ocean. This should yield 20 plus foot seas across our eastern waters by daybreak Thursday. Winds should diminish to gales later Thursday morning and afternoon...but high seas will continue across the waters. Outlook /Thursday Night through Sunday/...Moderate Confidence. Thursday Night: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of rough seas. Friday: Winds less than 25 kt. Areas of seas approaching 5 ft. Slight chance of rain showers. Friday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Saturday: Winds less than 25 kt. Areas of seas approaching 5 ft. Slight chance of rain showers. Saturday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Local rough seas. Slight chance of rain showers, slight chance of snow showers. Sunday: Strong winds with gusts up to 30 kt. Rough seas up to 11 ft. Slight chance of rain showers, slight chance of snow showers. && .TIDES/COASTAL FLOODING... Main concern continues to be the early Thursday morning high tide along the eastern MA coast and Nantucket. This looks to be mainly a widespread minor with perhaps pockets of low end moderate coastal flooding south of Boston. Overall...a 2 to 2.5 storm surge expected with perhaps a bit higher south of Boston. We issued a coastal flood advisory for Boston points north...while we held the current coastal flood watch south of Boston. Since its a late 2nd period event and minor to low end moderate coastal flood event...felt we could let the next shift decide. Some beach erosion likely across the ocean exposed portions of the outer-Cape and Nantucket. && .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...Winter Storm Warning until 8 AM EDT Thursday for CTZ002>004. MA...Winter Storm Warning until 8 AM EDT Thursday for MAZ012>023. Coastal Flood Watch late tonight for MAZ019-022>024. Wind Advisory until 8 AM EDT Thursday for MAZ007-014>016-018- 019. Coastal Flood Advisory from midnight tonight to 6 AM EDT Thursday for MAZ007-015-016. Winter Weather Advisory from 2 PM this afternoon to 8 AM EDT Thursday for MAZ009-011. Wind Advisory until 8 AM EDT Thursday for MAZ020-021. High Wind Warning until noon EDT Thursday for MAZ022>024. Winter Weather Advisory from 5 PM this afternoon to 8 AM EDT Thursday for MAZ004>007-026. RI...High Wind Warning until 8 AM EDT Thursday for RIZ008. Winter Storm Warning until 8 AM EDT Thursday for RIZ001>008. Wind Advisory until 8 AM EDT Thursday for RIZ005>007. MARINE...Storm Warning from 2 PM this afternoon to 9 AM EDT Thursday for ANZ231>235-237-254>256. Gale Warning until 8 AM EDT Thursday for ANZ230-236. Storm Warning from 7 PM this evening to 9 AM EDT Thursday for ANZ250-251.
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&& $$ SYNOPSIS...Frank/Dunten NEAR TERM...Frank SHORT TERM...Frank LONG TERM...Dunten AVIATION...Frank/Dunten MARINE...Frank/Dunten TIDES/COASTAL FLOODING... is the U.S. government's official web portal to all federal, state and local government web resources and services.