Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 210059 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 759 PM EST Fri Jan 20 2017 .SYNOPSIS... A weakening weather system may produce some spotty light freezing rain across northern Massachusetts tonight. Otherwise, tranquil weather is expected Saturday into Sunday. A significant coastal storm will push toward the region Sunday night, bringing a mix of rain and/or snow, along with some icing conditions possible inland. This storm may bring strong to possibly damaging winds to coastal areas, and be potentially dangerous for mariners late Monday into Tuesday. High pressure brings dry weather for Wednesday. && .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
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725 PM Update... Band of light showers riding across Long Island and the immediate S coast of CT at 00Z on latest KBOX 88D radar and OBS appearing to be dissipating as it moves E into drier air. Also noting some very light spotty precip across Hartford county northwestward into the Albany area. Do not see any ground truth to these areas at this time, but can not rule out some sprinkles from these areas. Note light rain at KDXR and light drizzle at KHPN, as well as a couple more light rain reports on western Long Island at 00Z. Appears that the 18Z GFS may be closest to capturing this precip, but even that appears to be a bit too strong as the area moves into dry air and remnants of ridge axis across the region. Have updated to bring conditions current and lowered POPs to mainly slight chance through the night. Temps running mainly in the 30s, with temps close to freezing across the higher terrain, and around 40 along the coast and across the lower CT valley at 00Z. With cloud cover prevalent across the region, do not expect temps to fall much overnight. If anything, mins will probably occur over the next few hours before holding steady or slowly rise. Have updated remainder of near term forecast to bring conditions current and incorporated trends into overnight forecast. Previous Discussion... Weakening short wave trough is getting stretched as it runs into a quasi stationary upper ridge across New England. Areas of light rain may intrude into southwestern zones where temperatures are expected to remain above freezing. However, cannot rule out a few spotty areas of light freezing rain across Franklin, northern Worcester, and far northwest Middlesex County tonight, generally after 10 PM per a consensus of high resolution models. This is a fairly low probability event for any one spot but even a light amount of freezing rain on a bare surface can be quite treacherous. Special Weather Statement has been issued for this relatively limited area of northern Massachusetts where even under clouds may see temperatures slip a few degrees after dark. Evaporative cooling, if precipitation makes it into the region, could also help drop the temperature a degree or two tonight. QPF where any rain/freezing rain occurs should generally be under .05 inches. Have gone with the NERFC quantitative precipitation forecast.
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&& .SHORT TERM /6 AM SATURDAY MORNING THROUGH SATURDAY NIGHT/... Saturday...Main question is amount of lingering cloudiness during the day. For now have gone with partial clearing. The air mass is warmer than normal with 850 mb temperatures generally +2C to +4C across the area. With modest mixing and light west to southwest flow, anticipate high temperatures reaching the upper 40s to lower 50s across most of the area. Saturday night...With elevated dewpoints and light wind flow through a deep column, model guidance suggests areas of low clouds and fog forming. The air mass remains considerably milder than normal through the night, although colder air begins to sag into northern New England very late at night. Anticipate most areas will have low temperatures above freezing in the mid to upper 30s with just a few spots near freezing along the interior northern Massachusetts border. && .LONG TERM /SUNDAY THROUGH FRIDAY/...
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Details... Sunday... Weak front pushes S across the region early Sunday. Should be mainly dry across the region through midday as ridging builds down the coast. However, leading edge of precip working NE from strengthening low pres across the SE U.S. will start to approach during the afternoon. Light/variable winds will shift to N-NE, then eventually E during the day as the ridge over Quebec shifts E. E-NE winds will start to increase along the S coast late in the day as pres gradient increases between the exiting high to the north and the strong low pressure over the southern Appalachians. Gusts may approach 25-30 kt across S coastal RI and Block Island by 00Z Mon. Have brought low CHC POPs int S coastal areas between 21Z and 00Z as leading edge of precip slowly approaches. Highs will be mainly in the 40s. Sunday night through Tuesday... Impressive low pressure will continue to strengthen as it approaches the mid Atlc coast, along with its H5 cutoff low swinging across the SE U.S. Good low level moisture plume with this system, with PWATs around 1 to 1.25 inches crossing the region. Mid level system becomes negatively tilted as it rotates NE during the day Monday. This will bring good amounts of QPF across the region on the strengthening E wind flow. QPF forecast of 1.25" across NW Mass ranging to 2-2.25" across SE Mass/RI through 00Z Wed. Big question with this system will be how much colder air can filter down across the interior and, if this does occur, there will be PTYPE issues. At this point, looks like mainly rain across the coastal plain, with a wintry mix of RN/SN/IP mainly but could also see some FZRA mixed in mainly across N Mass along the Route 2 corridor to the MA/NH border for a time. Partial thickness patterns suggest mainly SN and IP at this point, though expect rather low rain/snow ratios as temps only fall back to the upper 20s to around 30. During Tuesday, with strong easterly flow off the ocean, will see precip change over to rain across all areas. Best shot for precip looks to occur from mid-late morning Monday through Monday night. This is also when heaviest rain is expected. Another aspect of this storm will be the winds. Noting a very strong low level jet, on order of 60-70 kt at 900 hPa, will come close to if not over S coastal Mass. Have noted excellent mixing from 900 hPa downward, so could see gusts up to 45-50 kt across Cape Cod and the islands, and possibly across more areas along E coastal Mass and possibly as far W as E RI late Monday and Monday night. High wind warnings will likely be needed, but too early to issue now. Some timing and track issues for the low`s exit, but looks like it should push across Cape Cod or Nantucket sometime Tue afternoon. Back of precip shield should exit late Tue or Tue evening. Clouds will linger through the night, though could break toward daybreak across the southern CT valley. Wednesday through Friday... Weak high pressure will build across the region Wednesday, bringing dry and mild conditions. Another fast moving front will cross the region during Thursday. This system looks rather dry as most of the moisture and energy moves into northern New England. However, can not rule out some patchy light rain and/or snow across the E slopes of the Berkshires, with light rain possible through the remainder of the CT valley. A persistent NW wind flow will be in place by Friday, along with the chance for some isolated light rain and/or snow showers as a digging upper level trough approaches during the day and into Friday night. Temperatures will be at more seasonal levels for late January.
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&& .AVIATION /00Z SATURDAY THROUGH WEDNESDAY/...
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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 Saturday Night/... Tonight... Moderate confidence. MVFR with spotty IFR conditions anticipated. Should be mainly dry through about midnight, then may see spotty -RA push NE across N CT. Not much QPF with this moisture, though. Depending upon temps and location of any precip, there is a low risk for some -FZRA across Franklin, northern Worcester and far northwest Middlesex County after midnight. Saturday... Moderate confidence. MVFR likely in the morning with spotty IFR possible. Anticipate gradual improvement to VFR by afternoon in most locations. Saturday night... Moderate confidence. Elevated dewpoints will help set the stage for areas of fog and low cloud development. Forecast guidance is fairly emphatic in lowering conditions to widespread MVFR ceilings and visibilities with areas of IFR conditions. Our forecast once again trended toward the more optimistic end of the guidance envelope. KBOS TAF...Moderate to high confidence in TAF. KBDL TAF...Moderate to high confidence in TAF. Outlook /Sunday through Wednesday/... Moderate confidence. Sunday... VFR to start Sunday morning, then lowering to MVFR from SW to NE in light rain during the day. Light/variable winds shift to E-NE during the afternoon. Gusts increase to 20-25 kt across outer Cape Cod and Nantucket toward evening. Sunday night through Monday night... MVFR-IFR CIGS/VSBYS across the region Sunday night in rain and/or snow, with some IP developing after midnight and continuing through Monday. May see LIFR conditions Monday afternoon into early Tuesday morning in patchy moderate rain and/or snow and patchy fog. E wind gusts up to 40-50 kt likely along portions of the immediate S coast, especially the outer Cape and Nantucket Mon night. LLWS likely as E winds at around 2000 ft increases to 50-70 kt Mon night. Tuesday... Areas of IFR CIGS/VSBYS in RA/SN/FZRA early, changing over to RA during the day. Precip mixes to FZRA/IP/SN over western areas early Tue night before ending. IFR-LIFR CIGS linger Tue night across eastern areas through midnight or so, then should improve toward daybreak Wed. Wednesday... May see local MVFR-IFR conditions across the E slopes of the Berkshires into the CT valley and central areas in SN/IP/FZRA as cold front approaches. Most of the energy remains N and W of the region, so looks like mainly VFR conditions elsewhere. NW winds around 10 kt shift to SW during the day. Conditions should improve Wed night.
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&& .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 through Saturday Night...High Confidence. Light winds and seas under 5 feet expected to persist through Saturday night. A 2 to 3 foot swell is expected to dominate during this period. Outlook /Sunday through Wednesday/...Moderate confidence. Sunday... Light N winds veer to E-NE during the day. Gusts will increase to 25-30 kt Sunday afternoon, with seas building to around 5 ft by late in the day. Patchy light rain may reduce visibilities during the afternoon along the S coast. Sunday night through Tuesday... Strengthening low pressure of the mid Atlantic coast Sunday night will push NE. Expect E winds to rapidly increase as strong winds will mix down from about 3000-5000 ft. Gusts will likely reach 50-60 kt across a good portion of the waters. Have already issued storm watches across most of the waters due to the potential threat of prolonged storm force winds. Seas will also build up to 15-20 ft. Expect the strongest winds and highest seas during Monday afternoon and night. This is a potentially life threatening storm for mariners. Visibility restrictions in rain and patchy fog likely through most of Tuesday. As the low passes across SE Mass toward the Gulf of Maine, leftover gale force winds will push out of the eastern open waters Tuesday morning. Winds will briefly diminish around midday and through the afternoon Tuesday, then NW gusts could reach 25-30 kt Tue night. Seas will remain at or above 5 ft over the open waters through Tuesday night. Some visibility restrictions continue early, then should improve from S-N. Wednesday... Winds will diminish as they back to W-SW during Wed. May see some gusts up to 25 kt on the southern waters. Seas remain at or above 5 ft over the open waters. May see some visibility restrictions on the waters S of RI in patchy light rain Wed afternoon and night. && .TIDES/COASTAL FLOODING... Given potent easterly LLJ and period of strong wind gusts, expect seas to probably build to 20 feet or higher across the eastern waters late Mon into Tue. Fortunately, astronomical high tides are about as low as they get, and the strongest winds Monday evening will coincide with an astro high tide of only 8.2 feet in Boston. Thus the risk of any significant coastal flooding is low. In fact, it is hard to imagine a scenario of much worse than some splashover for the Monday evening high tide, thanks to the lowness of the astro high tide. The Tuesday morning high tide is higher at 9.4 feet. If winds are still blowing strong out of the northeast along with 20+ foot seas just offshore, minor coastal flooding could occur along the eastern MA coast. It would take a storm surge of nearly 3.5 feet and waves greater than 20 feet to even approach a moderate level of coastal flooding. Unless this system progresses much more slowly than the current consensus of models indicate, it is unlikely that we will experience anything worse than minor coastal flooding and some beach erosion for the Tuesday morning high tide in spite of such dangerous marine conditions just offshore. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Storm Watch from Monday afternoon through late Monday night for ANZ232. Gale Watch from Sunday evening through late Monday night for ANZ233-234. Gale Watch from Monday morning through late Monday night for ANZ230. Storm Watch from Monday afternoon through Monday evening for ANZ231. Gale Watch from late Sunday night through late Monday night for ANZ236. Storm Watch from Monday morning through Monday evening for ANZ235-237. Storm Watch from Monday morning through late Monday night for ANZ250-254. Gale Watch from Sunday evening through Tuesday morning for ANZ251. Storm Watch from late Sunday night through Monday evening for ANZ255-256. && $$ SYNOPSIS...EVT/Thompson NEAR TERM...EVT/Thompson SHORT TERM...Thompson LONG TERM...EVT/JWD AVIATION...EVT/JWD/Thompson MARINE...EVT/JWD/Thompson TIDES/COASTAL FLOODING...

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