Area Forecast Discussion
Issued by NWS Boston, MA

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130 FXUS61 KBOX 230232 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 932 PM EST Sun Jan 22 2017 .SYNOPSIS...
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Moist onshore winds will result in spotty light rain/drizzle tonight. Subfreezing temperatures are expected across the high terrain of Massachusetts. This will result in the risk for light snow showers...sleet and even freezing drizzle. A significant storm will reach the East Coast late Monday and move up the coast Monday night and Tuesday. This will bring a mix of rain and/or snow along with a period of icing well inland. The storm may also bring damaging winds to the coast and the coastal waters. High pressure then brings dry and mild weather Wednesday into Thursday. A cold front will cross New England late this week, followed by seasonably cold temperatures and blustery winds next weekend.
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&& .NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
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930 pm update... Moist low level northeast flow was resulting in areas of light rain showers and drizzle along the coastal plain this evening. Ptype was a mixture of sleet and light snow showers across the higher terrain of MA. Continued the special weather statement for the high terrain of the Worcester Hills and along the east slopes of the Berks, where there may even be some pockets of light freezing drizzle. Any amounts of light snow/ice will be very light, but slippery travel is possible on untreated roads overnight across the high terrain. The rest of the region will remain above freezing, so just pockets of light showers and drizzle but it will be raw with temps dropping into the 30s. Moisture and lift is shallow, so not expecting any significant precipitation overnight. Northeast wind gusts will be on the increase to between 25 and 35 mph along the coastal plain by daybreak.
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&& .SHORT TERM /6 AM MONDAY MORNING THROUGH MONDAY NIGHT/... updated 440 pm... Monday... Models have slowed down onset of steady/heavier precip until late in the day Monday and possibly not until early Mon evening. This is due to northern stream short wave moving across Quebec into New Brunswick. This keeps deep layer ridge over MA/RI and CT much of the day. This will result in dry air in the mid and upper levels of the atmosphere overspreading the area. This includes the snow growth region. This combined with increasing moist onshore flow will result in an abundance of low level moisture and will likely yield spotty light rain/drizzle much of the day. Therefore not expecting much if any snow and/or sleet until possibly late in the day. Much cooler day tomorrow with highs only in the 30s (seasonably cold) to around 40 along the immediate coast. Concern again is across the high terrain where temps may not get much above freezing. Thus low risk for spotty light freezing drizzle across the high terrain of MA. Winds will be on the increase as low level jet approaches. By sunset should see NE winds gusting up to 40 to 50 mph along the south coast including Cape Cod and islands. Monday night... Deep layer moisture and lift overspread the area Mon night as southern stream closed low comes rolling up the eastern Seaboard. 70+kt low level easterly jet off the Atlantic acts as a firehose into the region Mon night. This combined with the likelihood of a coastal front developing over eastern MA will result in a period of very heavy rainfall. Good model agreement on a period of very heavy rain over RI and eastern MA 06z-12z Tue with global guidance offering qpf up to 1.5 inches over this region. As you expect the high res guid is even heavier. Thus risk of significant street/highway/urban flooding along with small rivers and streams across RI and eastern MA. This supports our current Flood Watch area and reasoning. Farther inland very impressive warm nose above 850 mb with temps in this layer warming to +5C or so depending on model of choice. This will limit snowfall potential and have lowered snowfall forecast after collaborating with WPC. However below 850 mb there is a cold layer with temps as cold as -6C to -8C across northwest MA depending on model of choice. This will likely result in a period of freezing rain across the high terrain of MA especially western MA. Somewhat concerned that there is a low risk of ice accretion reaching or exceeding 0.50 inch. However surrounding offices including WPC were not as confident and thus have opted to go with a winter weather advisory for Monday night into Tue morning. Although will have to watch this area for potential upgrade to a warning if forecast confidence increases with time. Until then expect a winter mix of snow and sleet Monday evening changing to freezing rain across the high terrain with greatest risk of significant icing over northwest MA. Very strong winds remain in the forecast for Cape Cod and the islands where sustained winds of 40+ mph and gusts near 60 mph are likely. This is in response to a powerful 70+ kt low level easterly jet moving across this region. More marginal for these high wind speeds along the eastern MA coastline thus have continued the High Wind Watch for those areas. May eventually need a wind advisory for slightly lower wind speeds all areas including the high terrain as model soundings show wind gust potential up to 50 mph. Mild along the coastal plain as low level jet floods this region with maritime airmass and temps in the 40s. Low level cold air will remain entrenched northwest of I95 given CAD signature with high pressure over Quebec and pressure falls to our south and east. && .LONG TERM /TUESDAY THROUGH SUNDAY/... Overview... Cutoff H5 low pressure and its nearby surface low will slowly work up the eastern seaboard during Tuesday. The low will shift E-NE during Tue, with high pressure ridge building across Wed with dry and mild conditions as winds back to SW. 12Z models have some differences with approaching cold front, along with NE-SW oriented long wave H5 trough that will slowly cross the region late Thu or Fri. Noting differences in how individual models handle any possibility of light precip moving across as the front approaches. Cutoff H5 low sits across Hudson Bay into northern Quebec, which will help drive the upper system through late this week as H5 heights lower while another short wave begins to shift SE out of Manitoba late week. Once the upper system moves offshore, seasonably cold air pushes in, but it will be blustery late Fri through next weekend as another reinforcing shot of colder air works in on the NW flow at the surface and aloft. Details... Tuesday... Strong E winds will continue through midday Tuesday, highest across coastal areas. The strongest winds will occur Tue morning. Then, the low level jet associated with the strong winds will shift E during the day, so the surface winds will diminish. With the surface system trapped in the upper level cutoff flow, will see more steady rainfall across eastern Mass, with another 0.50" or so of rainfall across Cape Ann likely during Tue. Will also see mixed RN, IP and patchy FZRA across the higher inland terrain linger early Tue, but should change over to RN by around midday. May see a mix back to FZRA/IP Tue evening as the precip tapers off Tue evening over the higher terrain. The steady rain and patchy fog should exit E coastal areas after 07Z Wed or so. The low should cross Nantucket and the outer Cape Tue evening, while the cutoff upper system moves in tandem, passing near or just S of the 40N/70W benchmark. Winds will back around to N by Tue evening, with gusty winds across Cape Ann lingering. Quite a wide range in temps forecast for Tue. Highs from 35-40 across the N central and W Mass and the higher inland terrain to around 50 on the S coast. Readings then will fall back Tue night as colder air starts to work in. Temps bottom out in the mid 20s to around 30 across central and western areas, ranging to the mid and upper 30s along E coastal Mass. Wednesday-Thursday... High pressure ridge at the surface and aloft crosses the region Wed. As the ridge shifts E Wed night and Thu, may see scattered rain and/or snow showers move in across the E slopes of the Berkshires ahead of approaching weak cold front. Some timing differences on both the frontal passage as well as how far S any precip reaches Wed night or Thu. Temps will top off in the 40s both days, though they will start to fall to more seasonal levels Thu night as colder air starts to work in from Canada. Friday through Sunday... NW winds at the surface and aloft will usher in seasonable temperatures for late January. Should remain mainly dry, but may see some widely scattered snow showers over the E slopes of the Berkshires. Otherwise, should see mainly dry but blustery conditions during this timeframe. Some question about whether there may be a quick passing weak low and trough late in the weekend that might bring some light precip, but held off on this feature for now. && .AVIATION /02Z MONDAY THROUGH FRIDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Monday Night/...Moderate confidence thru this period. Tonight... Mainly IFR-LIFR CIGS across central and E areas, and around 1000 ft across CT valley. VSBYS locally IFR-LIFR across Worcester hills, otherwise mainly VFR with some low end MVFR in patchy fog. Patches of -RA/-DZ across E Mass/RI. May improve to MVFR all areas by around 12Z. May see light -FZRN/ -FZDZ across higher inland terrain as temps may fall to near or just below freezing mainly after 04Z- 05Z. Elsewhere in the lower elevations temps remain above freezing. NE winds increase overnight, gusting up to 35 kt along the south coast of MA and RI. Monday... MVFR with spotty light rain/drizzle...possibly freezing rain/drizzle high terrain. Steady precip holds off until 21z-24z from south to north. Strong northeast winds with gusts up to 50 kt by late in the day across Cape Cod and islands. Monday night... MVFR/IFR in periods of heavy rain and strong winds across much of the region with snow/sleet and freezing rain likely high terrain especially northwest MA. KBOS TAF...Moderate confidence in TAF. Greatest uncertainty concerns areal coverage of light precip. KBDL TAF...Moderate confidence in TAF. Greatest uncertainty concerns areal coverage of light precip. Outlook /Tuesday through Friday/...Moderate confidence. Tuesday... MVFR-IFR CIGS/VSBYS, with lowest conditions across central and E Mass into RI. Expect mainly -RA across the coastal plain and lower CT valley, with mixed -RA/-IP across N central and W Mass into far NW CT early Tue, then should change over to all -RA from S-N as temps increase to above freezing. Areas of fog. May see a mix with FZRA across highest terrain of N Worcester county and the E slopes of the Berkshires toward Tue evening as temps fall below freezing again. Expect CIGS/VSBYS to improve to VFR across central and western areas from 06Z-09Z. Precip should taper off across S and W areas by around midnight, though spotty RA/FZRA/fog may linger briefly after midnight. Remainder of the -RA across eastern areas should end by 07Z-09Z. E-NE winds gusting up to 25-30 kt inland, then up to 45-50 kt across Cape Ann and outer Cape Cod Tue morning which will slowly diminish by around midday. Gusts linger at around 25 kt through the afternoon around Cape Ann and moving across Cape Cod Tuesday night as winds back to N. Wednesday... VFR. An approaching cold front may bring a period of MVFR CIGS/VSBYS in -SHRA/-SHSN late Wed/Wed night across the E slopes of the Berkshires. Low chance for isold -SHRA and patchy fog across the Route 2 corridor of N Mass. NW winds gusting up to 25 kt on outer Cape Cod and the islands early Wed, then diminishing. Thursday-Friday... Mainly VFR. Patchy MVFR CIGS linger across N Central and W Mass Thu, along with sct SHRA/SHSN with local MVFR VSBYS Thu. SW winds gusting up to 20 kt on outer Cape Cod and the islands Thu, then up to 25 kt Thu night into Friday. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. *** Storm Force Wind Gusts Monday Night into Early Tue *** 710 PM Update...Only a few adjustments to bring conditions current. Tonight... Increasing NE winds overnight with gusts reaching 25-30 kt by morning except up to 35 kt along the south coast. Vsby reduced at times in areas of fog and drizzle. Monday... NE winds continue to increase to near 50 kt by sunset along the southern waters...35 to 40 kt elsewhere. Vsby reduced at times by drizzle and fog. Steady rain holds off until sunset across southern waters...later elsewhere. Monday night... The brunt of the storm is Mon night with ENE winds up to 55 kt. Strongest winds over eastern MA waters including Cape Cod and Nantucket waters. Very heavy rain with isolated thunder possible. Vsby reduced to less than a mile at times in fog and heavy rain. Outlook /Tuesday through Friday/...Moderate confidence. Tuesday... Low pressure moves across Cape Cod Tuesday morning. E wind gusts up to 50-55 kt on the eastern open waters early Tue, diminishing Tue afternoon as winds back to N. Seas up to 25 to 30 ft early, subsiding to 12-18 kt Tue evening. Rough seas will continue Tue night as NW winds continue to diminish. Visibility restrictions in rain and patchy fog into Tue night, then improving from S-N. Storm Warnings continue from Monday afternoon through Tuesday morning. This covers all waters except Boston Harbor and Narragansett Bay, where Gale Warnings remain in effect. Wednesday through Friday... NW winds on Wed shift to W-SW Thu and continue on Fri. Gusts up to 25-30 kt continue across most of the waters with seas remaining at or above 5 ft. Seas may linger around 5 ft during Wed at the outer entrance to Boston Harbor, otherwise winds and seas diminish below small craft criteria on Boston Harbor and Narragansett Bay. && .TIDES/COASTAL FLOODING... 440 PM Update... Still looking at potential minor with pockets of moderate coastal flooding along E MA coast during high tide Tue morning. Despite low astronomical tides (8.2 ft at Boston Mon evening and 9.4 ft Tue morning), combination of persistent and very strong E winds and high seas up to 20-30 ft offshore may lead to minor to perhaps pockets of moderate coastal flooding with Tue morning high tide. Surge guidance has values around 2.5 ft at the time of high tide along with very strong wave action with seas 20-30 ft across the eastern MA waters. This wave action ontop of the elevated storm tide will likely be sufficient for a risk of minor coastal flooding with a lower risk of moderate flooding. Still 36-48 hrs away and a difference in 6 hrs of timing the strongest winds can have a big impact on flood potential. Thus most likely scenario is a coastal flood advisory event. Thus will hold off on any watches at the moment. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...Winter Weather Advisory from 4 PM Monday to 7 AM EST Tuesday for CTZ002. MA...High Wind Warning from 9 AM Monday to 6 AM EST Tuesday for MAZ022>024. Flood Watch from Monday afternoon through Tuesday morning for MAZ005>007-013>021. High Wind Watch from Monday afternoon through late Monday night for MAZ007-018-019-021. Winter Weather Advisory from 4 PM Monday to 7 AM EST Tuesday for MAZ002>004-008-009. RI...High Wind Warning from 9 AM Monday to 6 AM EST Tuesday for RIZ008. Flood Watch from Monday afternoon through Tuesday morning for RIZ001>007. MARINE...Storm Warning from 1 PM Monday to 4 AM EST Tuesday for ANZ231- 232. Storm Warning from 1 PM Monday to 3 AM EST Tuesday for ANZ233- 234. Gale Warning from 7 AM Monday to 7 AM EST Tuesday for ANZ230. Gale Warning from 3 AM Monday to 4 AM EST Tuesday for ANZ236. Storm Warning from 7 AM Monday to 2 AM EST Tuesday for ANZ235- 237. Storm Warning from 11 AM Monday to 7 AM EST Tuesday for ANZ250- 254. Storm Warning from 2 PM Monday to 6 AM EST Tuesday for ANZ251. Storm Warning from 4 AM Monday to 4 AM EST Tuesday for ANZ255- 256. && $$ SYNOPSIS...Nocera/EVT NEAR TERM...Frank SHORT TERM...Nocera LONG TERM...EVT AVIATION...Nocera/EVT MARINE...Nocera/EVT TIDES/COASTAL FLOODING...Nocera

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