Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 121833 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 233 PM EDT Mon Mar 12 2018 .SYNOPSIS... Seasonable and mainly dry but mostly cloudy through this afternoon. Offshore storm tonight through Tuesday night, perhaps lingering into Wednesday, will deliver accumulating snow, strong to damaging winds, coastal and marine impacts. Blustery and chilly weather continues Thursday and Friday followed by moderating temperatures this weekend. Dry weather probably dominates late in the week through next weekend. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 150 pm Update... Radar is showing isolated light returns from the Worcester Hills and east, onshore flow is saturating the lower levels, enabling the development of isolated light rain/snow showers. Little if any accumulation is expected with these showers this afternoon. Mostly cloudy skies hang on for the remainder of the day. Forecast temps look on track, given cloud cover not expecting much more of an increase in temps this pm. Previous discussion follows... High pressure to the north keeping conditions mainly dry today. Mostly sunny skies into early afternoon for much of the area, with high level clouds will start to move into our area as the afternoon progresses. Scattered stratocumulus developing off east coastal MA are drifting into eastern MA with the N/E low level winds, so a mix of sun and clouds expected near the east coastal shoreline. One thing to watch is the potential for a few isolated rain/snow showers this afternoon as the mid-levels saturate. Appears to be enough LICAPE with delta T`s nearing 13C at 850mb. Cross sections and BUFKIT soundings also show the potential for a few sct rain/snow showers esp across the northern half of the region. Max temperatures near normal. && .SHORT TERM /6 PM THIS EVENING THROUGH TUESDAY NIGHT/... Monday Night into Tuesday... */ Highlights... - Moderate to heavy snow, some locations seeing upwards of a foot to perhaps two feet total accumulation - 1-3"/hr snowfall rates at times, plus winds could yield blizzard conditions with visibility less than a quarter of a mile - Strong to damaging winds, E gusts up around 65 mph possible for E/SE MA coast, brunt of the winds around Tuesday morning - Impacts to the Tuesday AM commute with difficult, near- impossible travel. - Especially the Tuesday morning high tide, strong E onshore flow yielding around a 2 to 3 foot surge, splashover, minor coastal flood issues, especially ocean-facing shores, vulnerable areas impacted by earlier storm systems Potent snow storm forecast, potential blizzard. 12/00Z guidance has come into better agreement, but still far from perfect. Trend with most of this guidance suite, except for the NAM and CMC was to shift the primary low pressure about 30-50 miles farther offshore. This will mean a colder solution, and more snow, especially across portions of eastern MA. Expecting cyclogenesis to really get underway tonight, resulting in a pronounced lower frontogenetical band in the 850-700 mb layer. As with previous thinking, this band looks like it will establish itself near the Cape Cod Canal late tonight into Tuesday. The difference now, is temperatures should be low enough to support snow. Thinking SE MA will have some of the highest totals. */ Uncertainty... Still some uncertainty in the track of this low pressure tonight. It remains possible for a shift closer to the coast tonight, but we won`t really know that with high confidence until the storm begins to strengthen, and we know where that takes place. Will continue to favor a consensus approach with the track of this low pressure. Thermal profiles will also be an issue, particularly towards the Cape and Islands. While the guidance favors a colder solution, MOS temperatures temperatures have rather large dewpoint depressions for such heavy precipitation. We tried to correct for this wet-bulb effect with our temperatures tonight into Tuesday. This assumption could be a source of error, resulting in snow accumulation where temperatures would be too high to support significant accumulations. Most concerned about this issue on Nantucket and the outer Cape, which was one of the more significant changes with this package. */ Snow... All lift and forcing mechanisms in mind, intensity of snow with 1-3" per hour snowfall rates, dragging down, manifesting its own cold, an alteration to snow ratios, calling for 12-18" of snow from Worcester County east in MA and much of RI. Generally 6-12" of snow elsewhere. Current thinking is the highest snow totals occur across SE MA, and could approach 20" in spots. This area may change with later forecasts as confidence in the track increases. Winter Storm Warnings issued for all of southern New England. Did give some thought to Blizzard Warnings, but since only have moderate confidence in the precise storm track, prefer to let the next shift take another look, */ Winds... Roughly +3-4 standard deviation 925-850 mb east winds. Low level lapse rates will mean sufficient mixing, when combined with precipitation drag, to generate very gusty winds along the east coast of MA, particularly the Cape and Islands. Altogether, forecasting N/NE winds up around 65 mph, sustained winds as high as around 40 mph. High Wind Watch continues for the east MA coastline, including Cape Cod and Islands, because did not have sufficient confidence for a Blizzard warning at this time. Again, the brunt of the winds will be around Tuesday morning. Evaluating snow ratios, expecting a dry/fluffy snow across much of the interior. The latest trends mean lower risk for heavy/wet snow across portions of SE MA, but the risk is not zero. Areas such as Plymouth County and the Cape and islands are at the greatest risk for a heavy/wet snow plus strong winds, yielding some potential tree damage and power issues. Tuesday night... Nor`easter departs, resulting in slowly improving conditions from SW to NE. Snow showers and clouds may linger along and north of the Mass Pike. && .LONG TERM /WEDNESDAY THROUGH SUNDAY/... Highlights... * A few snow squalls possible Wednesday * Blustery and chilly Thu/Fri with mainly dry weather * Moderating temps next weekend with mainly dry weather expected Wednesday... While the bulk of the steady/heavy snow will probably have come to an end by early Tuesday evening with the departing Noreaster... scattered snow showers may linger into Wed. In fact, we may see a few localized heavier snow squalls with the focus for them Wed/Wed evening. Despite surface low lifting into the Maritimes, upper level energy/cold pool aloft appears to linger across the region. So while scattered snow showers will linger Tue night/Wed...steep low level lapse rates and enough low level moisture may result in a few heavier snow squalls. Greatest risk Wed afternoon and evening as diurnal heating especially this time of year will assist with development given cold temperatures aloft. Thursday and Friday... Northwest flow aloft will result in blustery/chilly temperatures Thu/Fri. Highs should be well up into the 30s to near 40...which is below normal for the middle of March. Mainly dry weather anticipated although can not rule out a spot snow shower or two on Thursday. Saturday and Sunday... Upper trough lifts east of the region allowing temperatures to moderate some over the weekend. Still uncertainty on the extent of the moderation and how quickly this happens. Dry weather probably dominates next weekend...but low confidence given this forecast is 6 to 7 days into the future. && .AVIATION /18Z MONDAY THROUGH SATURDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tuesday Night/...High confidence. This afternoon... Mainly VFR to start with BKN/OVC CIGs 035-040, but CIGs lowering to MVFR later this afternoon/towards 00Z. Isolated light RW/SW generally near and east of FIT-ORH-SFZ. Little or no accumulation. Tonight into Tuesday... CIGs/VSBYs deteriorating tonight with SN/+SN. Possible RA/SN mix out over CHH/ACK. MVFR to start, lowering to IFR-VLIFR from 06Z-10Z and lasting for much of Tuesday. N/NE winds strongest with gusts up to 60 kts around Tuesday morning along east coastal MA. Blizzard conditions possible with snowfall rates of 1-3 inches per hour, requiring Airport Weather Warnings. Expect accumulation on all runways. Tuesday Night...Improvement from S to N. VFR likely towards the S coast by daybreak Wed. MVFR CIGS farther north, with local IFR across the higher terrain. KBOS Terminal...High confidence in TAF trends. Tues AM Push will be impacted. KBDL Terminal...High confidence in TAF trends. Tues AM Push will be impacted. Outlook /Wednesday through Friday/... Wednesday: Mainly VFR, with areas MVFR possible. Windy with gusts to 35 kt. Chance SHRA, chance SHSN. Wednesday Night: Mainly VFR, with local MVFR possible. Windy with areas gusts to 30 kt. Slight chance SHSN. Thursday: Mainly VFR, with local MVFR possible. Windy with local gusts to 30 kt. Thursday Night through Friday: VFR. Breezy. && .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 Tuesday/...High confidence. A rapidly intensifying storm will pass just SE of the 40N/70W benchmark on Tuesday. This storm is expected to achieve a central pressure as low as 960-970 mb. We expect storm force winds over most of our waters with gusts as high as 60 kts over the SE outer waters. Waves will build to between 20 and 25 feet by late Tuesday morning and possibly to 25 to 30 feet east of Cape Cod and Nantucket. Snow along the near-shore waters will reduce visibility to well below 1 mile late tonight and much of Tuesday. Outlook /Tuesday Night through Friday/...Moderate confidence. Tuesday Night and Wednesday: Strong winds with gusts 35 to 40 kt. Rough seas up to 15 ft. Winds and seas gradually subsiding. Slight chance of rain showers offshore and chance of snow showers near shore. Wednesday Night: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Rough seas up to 11 ft. Slight chance of rain showers, slight chance of snow showers. Thursday: Strong winds with gusts up to 30 kt. Areas of rough seas. Thursday Night: Moderate risk for Small Craft Advisory winds with local gusts up to 30 kt. Local rough seas. Friday: Moderate risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of rough seas.
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The Tuesday morning high tide is the one of concern. Astronomical high tides are quite low for early this week, and there is fairly limited time to enable the build up of surge and waves by the time of high tide. Nevertheless, extremely rapid intensification of a coastal storm and associated rapid increase in surge and waves have raised concern for the Tuesday morning high tide cycle along east coastal Massachusetts. Latest computer model runs point to a very intense storm that will pass just southeast of the 40N/70W benchmark. The combination of slightly stronger conditions and concern for existing shoreline vulnerability have prompted us to upgrade the Coastal Flood Advisory along the Hull to Plymouth and Sandwich to Dennis stretch of coastline to a Coastal Flood Warning. A Coastal Flood Warning remains in effect for the ocean side of Cape Cod and Nantucket. Ocean Side Cape Cod and Nantucket... We are fairly confident now of coastal impacts reaching moderate levels. We anticipate the storm surge in this area to increase rapidly to near 3.5 feet about Nantucket and along the ocean side of Cape Cod from Truro to Chatham, including Wellfleet, Eastham, and Orleans. That surge combined with waves that may reach 25 to 30 feet just east of Cape Cod and Nantucket will produce areas of moderate coastal flooding and severe erosion. We anticipate impacts to be exacerbated by vulnerabilities from the prior two storms. Hull to Plymouth and Sandwich to Dennis... Along this stretch of coastline we anticipate minor to moderate coastal flooding and areas of significant erosion. We are forecasting a storm surge of near 3 feet except 3 to 3.5 feet along the Sandwich to Dennis coastline and 15 to 20 foot waves across Massachusetts Bay and Stellwagen Bank. Here too, vulnerabilities created from prior storms may exacerbate impacts. Ocean Exposed Coastline north of Boston to Salisbury... Along this reach of ocean exposed Massachusetts coastline, we anticipate areas of minor coastal flooding and splash over with a storm surge around 2.5 feet and waves around or a little over 15 feet east of Cape Ann by the time of the Tuesday morning high tide. As is the case along the rest of the Massachusetts ocean exposed east coast, impacts could be more significant due to the current vulnerability of the coastline.
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&& .BOX WATCHES/WARNINGS/ADVISORIES... CT...Winter Storm Warning from 11 PM this evening to 8 PM EDT Tuesday for CTZ002>004. MA...Winter Storm Warning from 11 PM this evening to 8 PM EDT Tuesday for MAZ002>006-008>017-020-024-026. Coastal Flood Warning from 8 AM to noon EDT Tuesday for MAZ019- 022-024. Blizzard Warning from 11 PM this evening to 8 PM EDT Tuesday for MAZ007-018-019-021>023. High Wind Warning from 2 AM to 8 PM EDT Tuesday for MAZ024. Coastal Flood Advisory from 8 AM to noon EDT Tuesday for MAZ007-023. RI...Winter Storm Warning from 11 PM this evening to 8 PM EDT Tuesday for RIZ001>008. MARINE...Storm Warning from 4 AM to 8 PM EDT Tuesday for ANZ231-232-250- 251. Storm Warning from 4 AM to 6 PM EDT Tuesday for ANZ233-234. Gale Warning from midnight tonight to 6 PM EDT Tuesday for ANZ230-236. Storm Warning from 1 AM to 6 PM EDT Tuesday for ANZ235-237. Storm Warning from 1 AM to 8 PM EDT Tuesday for ANZ254>256. && $$ SYNOPSIS...Belk NEAR TERM...Belk/NMB SHORT TERM...Belk LONG TERM...Frank/Dunten AVIATION...Belk/Frank/Dunten/NMB MARINE...Frank/Dunten/Thompson TIDES/COASTAL FLOODING...Thompson is the U.S. government's official web portal to all federal, state and local government web resources and services.