Area Forecast Discussion
Issued by NWS Boston, MA

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757 FXUS61 KBOX 231508 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1008 AM EST Mon Jan 23 2017 .SYNOPSIS... Moist onshore winds will result in rain and snow showers today, but some sleet and even patchy freezing drizzle will be possible across interior northern Massachusetts. A significant storm will impact the region tonight into Tuesday, resulting in a wide variety of weather from snow and/or ice across the interior, a period of strong to damaging wind gusts along with heavy rain along the coastal plain. High pressure builds in behind the departing storm and brings dry mild weather Wednesday and Thursday. A cold front crosses New England Thursday followed by seasonably cool temperatures and blustery winds. && .NEAR TERM /UNTIL 4 PM THIS AFTERNOON/... 10 AM update... Strengthening low level ENE jet resulting in numerous rain and snow showers across much of the region this morning. This is despite strong mid level subsidence from high amplitude upper level ridge over New England. This subsidence has resulted in some breaks in the overcast from time to time. However as low level jet continues to increase today low level moisture will win out and continue to result in numerous rain/snow showers. However with temps above freezing any light snow accumulations will be confined to non-paved surfaces. The exception will be across the high terrain where temps near freezing may result in a coating of snowfall especially on secondary roads. Still could see some freezing rain/drizzle at times across the high terrain as mid level dry air advects into the snow growth region from time to time today. Otherwise expect a windy day especially along the coast with ENE winds increasing up to 45-50 mph...35 to 40 mph elsewhere. These strong winds off the ocean combined with cool temps in the mid to upper 30s to perhaps low 40s over Cape Cod and the islands will provide a damp/raw feel today. Previous forecast captures these details nicely so no major changes to the forecast with this update. ======================================================================= Previous Discussion... Large high pressure across eastern Canada will combined with low pressure slowly lifting northeast into the North Carolina/Southern Virginia areas. While strongest forcing/deepest moisture will remain south of our region today, moist low level northeast flow will result in scattered light rain showers along with areas of drizzle through late afternoon in most areas. Ptype will mainly be liquid, except across the high terrain of the Worcester Hills/East slopes of the Berks as well as interior northeast MA where a mixture of light sleet/light snow showers and perhaps some freezing drizzle will be possible. Any accums will be quite light through late afternoon, but untreated roads may be slippery in this region. Temps will mainly be in the 30s and it will feel quite raw as northeast winds increase to between 30 and 40 mph. && .SHORT TERM /4 PM THIS AFTERNOON THROUGH 6 PM TUESDAY/... *** Significant storm will bring a wide variety of hazards to the region tonight into Tuesday with greatest uncertainty revolving around ptype across the interior *** Many hazards to consider as moisture laden strong low pressure system tracks up the New England coast. Will break things down in more detail below about tonight into Tuesday. 1) Timing: Strong low pressure lifting northeast will allow moderate to heavy precipitation to quickly overspread the region from south to north roughly between 5 and 8 pm this evening, potentially impacting part of the evening rush hour. Precipitation will continue heavy at times through the overnight hours. The low pressure system will slowly lift northeast on Tuesday. While the forcing will not be as strong as tonight, closed mid level low/s south of the region will continue to result in steady precipitation on Tuesday. 2) Ptypes and Snow/Ice Amounts: The greatest uncertainty with this forecast revolves around precipitation types across the interior. The models have a pocket of cold air arriving in the 850 to 900 mb layer, as temps drop below -5C on most guidance north of the Pike for a time this evening. However, soundings are close to isothermal in the 700 to 800 mb layer. The ECMWF/GFS are colder and suggest several inches of accumulating snow across interior northern MA and east slopes of the Berks, while the NAM/RGEM are just a bit milder resulting in mainly moderate to heavy sleet. It is a very difficult forecast because a slight change in thermal profiles/ptype intensity will make a big difference in the overall outcomes. Given the uncertainty, have blended the forecast guidance for now. Have gone with 2 to 4 inches of snow/sleet across interior northern MA. If we trend colder certainly may see 6+ inches of snow near the NH border, a milder solution may result in an inch or two of mainly sleet. Further south across the higher terrain of northern CT and into southwest/south Central MA, soundings show mainly sleet. The sleet may be heavy for a time as well, so certainly may see an inch or so of sleet accumulation. Opted to extend the Winter Weather Advisory into this region, given potential for a fair amount of sleet/slippery travel. Along the Boston to Providence corridor, some sleet may initially be mixed with the rain but should not be too big of an issue with temps well above freezing. Lastly, 850T will warm well above freezing in most locations by Tuesday morning changing ptype to rain in most locales. However, surface temperatures should remain around freezing along the high terrain of the east slopes of the Berks and Worcester Hills given surface low tracking well southeast of the region. This should result in the risk for freezing rain. Ice accretion of one tenth to one quarter of an inch will be possible in these locations. Slippery travel will continue through Tuesday and winter weather advisories will likely need to be extended. 3) Heavy Rain Across Eastern MA/RI: The other concern will be across eastern MA/RI where a very strong easterly low level jet 4 to 5 standard deviations above normal along with PWATS 2+ above normal. This will result in a widespread heavy precipitation event. The heaviest of this precipitation in the form of rain should be across eastern MA/RI where the above factors interact with a coastal front. This should result in 1.50 to 3" of rain in this region with perhaps localized higher amounts. Any amounts over 3 inches of rain will bring the risk for localized urban flooding. Will also need to keep an eye on some small streams if the 3+ inch amounts are realized. Therefore, have continued the Flood Watch although this does not appear to be a situation for widespread flooding. 4) Strong To Potentially Damaging Wind Gusts: The guidance continues to be in very good agreement in showing an easterly 925 mb jet 4 to 5 standard deviations above normal. This should bring a period of strong northeast wind gusts of 35 to 50 mph inland, and between 50 and 60 mph along the coast with the strongest of that across the Cape/Islands. While it will become windy this afternoon, the strongest of the winds will occur overnight under the core of the strong easterly low level jet. Will go with Wind Advisories for most areas except northwest MA and High Wind Warnings Cape/Islands as well as the Plymouth County Coast and Cape Ann. These winds may result in a few downed trees/large limbs and isolated power outages. Fortunately, the mixing will not be as extreme as we often see in colder Noreasters along the coast, which should prevent potentially even stronger winds and more widespread damage. && .LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/... Big Picture... West Atlantic ridge in place midweek will shift out to sea. Pacific high pressure builds east into the Western USA, kicking an upper trough east to the Eastern USA over the weekend. Contour heights at 500 mb will be above average during midweek, then below average Friday through Sunday. This means mild weather midweek followed by colder weather over the weekend. Shortwave embedded in the upper trough will move northeast with the trough, crossing New England during Thursday. Diminishing upper jet associated with this shortwave sweeps east and crosses New England Wednesday night and Thursday, then remains well to our south over the weekend. Details... Tuesday night... Coastal storm Southeast of Cape Cod in the evening departs toward the Maritimes overnight. Lingering chance of showers through midnight, but the upper trough shifts east of us at that time so expect any leftover showers to taper off at that time. Winds shift from the Northwest overnight and draw colder air into Southern New England. We stayed close to consensus for model min temps, mid 20s to mid 30s. Wednesday-Thursday... Atlantic high builds and brings dry weather. Northwest winds trailing the departing storm will weaken, then turn southwest in the afternoon. Any cold air that moved in behind the storm will be pushed out again with the wind shift. A weak cold front moves across on Thursday. Left exit region of the upper jet moves across New England as the front moves through. This would bring upper venting to go along with any low level convergence along the front. But moisture fields are limited, with most moisture at or below 800 mb. So the forecast will anticipate clouds but little/no precip. Best chance of any precip will be over northwest Massachusetts. Cold advection behind the front will improve mixing, and bring winds gusts to 25 knots during the afternoon. Temps aloft support max temps in the 40s to around 50 each day. Friday through Sunday... Broad cyclonic flow with cold pool clouds and a cooling trend on temps aloft. Temps aloft trend from around -8C Friday to -11C Sunday, so max sfc temps trend from either side of 40F Friday to the 30s on Sunday. Northwest flow through the period with cold advection mixing a few stronger gusts each day. && .AVIATION /15Z MONDAY THROUGH FRIDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. 15z Update... Not much change from 12z TAFs. Persistence probably the best forecast today. Thus not much change from current conditions. However at times conditions may briefly improve to marginal MVFR- VFR across eastern MA and RI. Otherwise scattered rain/snow showers continue today. Not expecting any accumulation on paved surfaces given temps above freezing. Except high terrain such as KORH where a coating or so is possible given the colder temps. Earlier discussion below. ======================================================================== Short Term /through Tuesday/... Today...Moderate to high confidence. Low end MVFR to IFR conditions should persist in low clouds/scattered light showers and areas of drizzle today. Some light snow showers/sleet will be possible in the high terrain of the Worcester Hills/East slopes of the Berks with perhaps even a touch of freezing drizzle. Northeast wind gusts should increase to between 30 and 40 knots by afternoon. Tonight...Moderate to high confidence. Mainly IFR condition with localized LIFR conditions. Heaviest precipitation tonight, which will mainly be in the form of rain along the coastal plain with perhaps some sleet mixed in this evening. A period of moderate to heavy sleet is likely for interior MA and perhaps into portions of northern CT, with some snow possible for a time across interior northern MA. Northeast wind gusts of 30 to 40 knots expected across the interior with 40 to 50 knots on the immediate coast. Tuesday...Moderate to high confidence. Low end MVFR to IFR conditions persist. Ptype will have transitioned to mainly rain, except pockets of freezing rain possible across the high terrain of the Worcester Hills/East slopes of the Berkshires. Still windy early in the morning along the coast, but should see gusts diminish to between 20 and 25 knots during the afternoon. KBOS TAF...Moderate to high confidence in TAF. KBDL TAF...Moderate to high confidence in TAF. Period of sleet may arrive at the terminal between 21z and 23z. Outlook /Tuesday Night through Friday/...Moderate confidence. Tuesday night... Coastal storm moves off to the Maritimes. Northeast winds turn from the Northwest overnight. Winds may gust 20 to 25 knots along the eastern coast, but will be lighter inland. Wednesday through Friday... VFR. An approaching cold front on Thursday may bring a period of MVFR cigs/vsbys in rain showers for parts of Northwest Mass. West winds behind the cold front may gust to 25 knots Thursday afternoon and Friday. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. *** Storm Force Wind Gusts and 20 to 25+ foot seas across our outer-waters tonight into early Tuesday will result in a dangerous storm for mariners *** 10 AM Update... Not much change from previous forecast. ENE winds continue to increase today with gales over the southern waters spreading northward this afternoon. By sunset wind speeds will increase to near 50 kt along the southern waters of RI and about 40 kt elsewhere. Rain/snow showers will limit vsby to about 3 miles at times. Earlier discussion below. ==================================================================== Today and Tuesday...Moderate to high confidence. Pressure gradient will increase through tonight as strong low pressure tracks up the new England coast and combines with the high across eastern Canada. Northeast winds will increase to strong gales through the afternoon, and then to storm force easterly wind gusts tonight between 45 and 55 knots. Powerful winds combined with long easterly fetch will result in 20 to 25+ foot seas tonight into Tuesday morning. Winds should gradually diminish during the day Tuesday as low level jet lifts to the north. Seas will gradually diminish, but this will be a slower process. Outlook /Tuesday Night through Friday/...Moderate confidence. Tuesday night... Storm center southeast of Nantucket in the evening moves northeast to the Maritimes overnight. Rain and fog will diminish and vsbys improve. Winds shift from the northwest and diminish overnight with gusts near 25 knots. Seas start the evening 8 to 13 feet and diminish during the night as winds shift to offshore and diminish. Small Craft Advisory will be needed during this period. Wednesday through Friday... NW winds diminish Wednesday and back from the WSW. Lingering gusts to 25 knots. A cold front sweeps across the waters Thursday, followed by colder air and gusty west winds near 25 knots. Colder air and gusty winds will continue into Friday as well. Seas will diminish through the period, but with 5 to 6 foot seas lingering on the outer waters. Small Craft Advisory will be needed for some or all of this period. && .TIDES/COASTAL FLOODING... Timing of strongest winds has been delayed by a few hours on 00z model runs, so we will likewise delay timing of highest storm surge for both this evening`s and Tuesday morning`s high tide cycles. For East Coastal MA including Cape Cod, Martha`s Vineyard, and Nantucket, tide gauges are showing around a 1 foot surge at both Boston and Nantucket early this morning which is well above model storm surge guidance. We still think we will see around a 2-foot storm surge with this evening`s high tide, which would result in pockets of minor coastal flooding (not widespread), despite low astronomical tides (8.2 feet at Boston and 2.4 feet at Nantucket). A Coastal Flood Advisory will be issued for the evening high tide. There is more concern for Tuesday morning`s high tide, when astronomical tides are a bit higher (9.4 feet at Boston and 3.3 feet at Nantucket). Once again model storm surge guidance is far too low, only indicating a 1 to 2-foot storm surge. Given strength of wind field, fetch, and duration we feel a 3 to perhaps even 4-foot storm surge is more likely. Combined with seas of over 20 feet offshore, widespread minor coastal flooding is expected but there is also the potential for pockets of moderate coastal flooding, especially near Newburyport, Scituate, and possibly Gloucester and Nantucket. A Coastal Flood Watch will be issued for all of eastern MA for the Tuesday morning high tide. In addition, due to the prolonged and strong easterly flow, there is also the concern for minor coastal flooding Tuesday morning along portions of the South Coast, most notably Newport, Westerly, and Block Island. Since confidence is a bit lower, we will hold off on any headlines there, but a Coastal Flood Advisory may also need to be issued in later forecasts. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...Wind Advisory from 1 PM this afternoon to 7 AM EST Tuesday for CTZ002>004. Winter Weather Advisory from 4 PM this afternoon to 7 AM EST Tuesday for CTZ002-003. MA...High Wind Warning from 1 PM this afternoon to 7 AM EST Tuesday for MAZ007-019-022>024. Flood Watch from 1 PM EST this afternoon through Tuesday morning for MAZ005>007-013>021. Coastal Flood Watch from late tonight through Tuesday morning for MAZ007-015-016-019. Coastal Flood Advisory from 6 PM to 10 PM EST this evening for MAZ007-015-016-019. Wind Advisory from 1 PM this afternoon to 7 AM EST Tuesday for MAZ004>006-009-011>018-020-021-026. Winter Weather Advisory from 4 PM this afternoon to 7 AM EST Tuesday for MAZ002>006-008>012-026. Coastal Flood Watch Tuesday morning for MAZ022>024. Coastal Flood Advisory from 7 PM to 11 PM EST this evening for MAZ022>024. RI...High Wind Warning from 1 PM this afternoon to 7 AM EST Tuesday for RIZ008. Flood Watch from 1 PM EST this afternoon through Tuesday morning for RIZ001>007. Wind Advisory from 1 PM this afternoon to 7 AM EST Tuesday for RIZ001>007. MARINE...Storm Warning from 1 PM this afternoon to 4 AM EST Tuesday for ANZ231-232. Storm Warning from 1 PM this afternoon to 3 AM EST Tuesday for ANZ233-234. Gale Warning until 7 AM EST Tuesday for ANZ230. Gale Warning until 4 AM EST Tuesday for ANZ236. Storm Warning until 2 AM EST Tuesday for ANZ235-237. Storm Warning until 7 AM EST Tuesday for ANZ250-254. Storm Warning from 2 PM this afternoon to 6 AM EST Tuesday for ANZ251. Storm Warning until 4 AM EST Tuesday for ANZ255-256. && $$ SYNOPSIS...WTB/Frank NEAR TERM...WTB/Frank/Nocera SHORT TERM...Frank LONG TERM...WTB AVIATION...WTB/Frank/Nocera MARINE...WTB/Frank/Nocera TIDES/COASTAL FLOODING...

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