Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 222051 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 451 PM EDT Mon May 22 2017 .SYNOPSIS...
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Showers will cross through southern New England this evening, with the most numerous showers affecting RI, northern CT, and MA south of the Mass Pike. A weak but moisture rich area of low pressure will likely keep showers in the vicinity of RI and southeast MA overnight and even into Tuesday morning on the Cape and Islands. Low pressure will pass south of New England, bringing scattered showers Tuesday night into Wednesday. Weak high pressure brings a brief period of dry but seasonable conditions. Another stronger low moves northeast out of the Ohio Valley, bringing more showers Thursday and Friday. The low shifts to the Gulf of Maine Saturday, so will see mainly dry conditions. Another weather system may approach late Sunday or Memorial Day with another period of showers possible.
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&& .NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...
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Tonight...Radar at 430 pm showed scattered showers north of the MA Pike, with more numerous showers to the south. Showers will continue to overspread much of southern New England over the next few hours while a short wave traverses the region. Precipitation continues overnight due to the approach and passage of a cold front, and a wave of low pressure that develops to our southwest. Models showing the wave developing near the Mid Atlantic region. The low then tracks northeastward with the center passing southeast of Nantucket on Tuesday. Across interior southern New England, the passage of the cold front will bring a wind shift to the NW around and after midnight, with drier air moving in and diminishing chances for rain. However, it is a different story across the southeast portion of our area. Precipitable Water increases to the east of the cold front tonight, potentially placing southeast MA in the vicinity of 1.5+ inch PWATs late tonight into Tuesday morning. In addition, models are showing at least a modest low level jet affecting southeast MA, especially the Cape and Islands. NAM is the most robust with the LLJ peaking at 40-45 knots. Also K indices in the low 30s support the chance for thunder in the vicinity of RI and southeast MA as well. So in this area rain chances continue right thru daybreak. Cape and Islands could see an inch or so of rainfall. To the north and west of RI/southeast MA, while precip chances will be diminishing overnight, low level moisture trapped under a developing subsidence inversion will likely allow for areas of fog and possibly low clouds. Temps continue to look tricky thanks to the risk for clearing in the far NW interior. Some cooling there could allow temps to drop back into the 40s while areas further E remain nearly steady in the low to mid 50s.
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&& .SHORT TERM /6 AM TUESDAY MORNING THROUGH TUESDAY NIGHT/...
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Tue... Away from southeast MA, expect improving conditions with decreasing clouds and a north wind, primarily under the influence of weak high pressure. However models are showing the potential for the offshore low pressure to cross southeast of Nantucket during Tuesday, and showers associated with it may linger in portions of southeast MA during the day. With high PWATs near the Cape/Islands in the morning, could see a continuation of moderate or brief heavy rainfall early in the day. Forecast high temps reflect the anticipated difference in conditions along the Cape/Islands, and elsewhere across the region. Highs 70 to 75 should be common for much of the area, with somewhat cooler temps along east coastal MA and south coastal MA/RI where sea breezes would keep temps a bit lower. Across the Cape and Islands, thinking highs will be in the upper 50s to mid 60s, warmest near the Canal. Tuesday night... Models showing next chance for rain with another wave of low pressure. It appears the low center will pass well southeast of Nantucket, but cloudiness may stretch into interior southern New England. Have primarily chance pops for portions of northern CT, RI, and southeast MA Remainder of the region especially along and north of the MA pike likely remains dry. However there may still be some changes to the pops as we get closer to Tuesday night, depending on if this low tracks closer to our area or looks to stay offshore.
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&& .LONG TERM /WEDNESDAY THROUGH MONDAY/...
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Highlights... * Scattered showers Wed morning, then drying out * Next area of showers late Wed night, continuing through Fri * May see another weather system approach late this weekend or Memorial Day with more showers possible Details... Wednesday... 12Z model suite continues to push low pressure further off the S coast, but northern edge of precip shield looks to bring scattered showers mainly across south coastal areas, with a few spotty showers possible further inland through about midday Wednesday. As the low pushes further offshore, precip will taper off, though a few might linger across the E slopes of the Berkshires into Wed night. Some sun may break through Wed afternoon across portions of the interior, but a persistent onshore wind will keep clouds lingering near the coast. High temps will range from the 60-65 degree range along E coastal areas to the mid-upper 70s across the CT valley. Thursday and Friday... H5 cutoff low pres and associated long wave trough will shift NE during this timeframe. The low rotates across the region during Friday. Another area of precip will work up the eastern seaboard Thu, bringing another batch of rainfall. Big question will be whether another surface low develops off the mid Atlantic coast and shifts NE late Thu/Thu night, which could enhance the precip. Have noted some elevated instability with moderate lapse rates, but with stable conditions in the lower levels, do not expect convection at this point but something to keep an eye on. The precip shield should start to lift NE Fri night. Forecasted QPF amounts from 0.7 to 1.25 inches possible from Thu afternoon through Fri. Will also see steady easterly winds, which will keep cool and damp conditions in place, along with patchy fog developing each night. Holiday Weekend... Have lower confidence during this timeframe due to model solution spread in bringing H5 short wave eastward out of the Great Lakes, with a possible weak low and front developing across the mid Atlantic states. At this point, the low pushes eastward by Saturday, so will see some drying conditions though can not completely rule out a few spotty showers Sat afternoon across the interior. Big question will be possible development and movement of low pressure from the mid Atlantic states eastward for late this weekend into Memorial Day. Have carried chance POPs for both Sun and Mon, though temperatures look seasonal.
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&& .AVIATION /21Z MONDAY THROUGH SATURDAY/...
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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 Night/... 430 pm update... Late this afternoon and evening...Moderate confidence, mainly due to some uncertainty in timing. Cigs MFR or dropping to MVFR late afternoon with localized lower vsbys with numerous SHRA. Cigs dropping to IFR between 22Z and 03Z across much of the area, though these lower cigs may hold off across Cape/Islands and vicinity until later tonight. Winds SE/E. Overnight...Moderate confidence. IFR/MVFR with low CIGS/fog/SHRA mainly S and E of a line from HFD- ORH-PSM, meanwhile some improvement is possible N and W of this line especially during the early morning hours. Otherwise a damp night. Some S-SW winds gust 20-25 kt mainly Cape/Islands. Otherwise, winds shift to the NW by sunrise Tue. Tue...Moderate confidence. Improvement to VFR most locales during the morning. IFR conditions in SHRA may linger during Tue across Cape/Islands. Sea breezes possible on coastlines. KBOS TAF...High confidence in TAF trends, lower confidence in exact timing for the lower categories this afternoon and evening. Sct showers in KBOS vicinity this afternoon, with showers becoming more likely for this evening. KBDL TAF...High confidence in TAF trends, lower confidence in exact timing for the lower categories. Showers in vicinity of KBDL this afternoon (mainly to the S), becoming more numerous towards/after 21Z. Additional showers probable this evening. Tue night...Moderate Confidence. VFR except MVFR possible south coast in risk of showers. Outlook /Wednesday through Saturday/... Moderate confidence through the period. Wednesday...Mainly VFR. May see periods of MVFR-IFR VSBYS in patchy fog Wed night along the S coast, Cape Cod and the islands. MVFR-IFR CIGS after midnight moving from S-N in low stratus and scattered showers. Thursday...MVFR to local IFR conditions in showers. Areas of fog developing Thu night with IFR VSBYS. Friday...Mainly IFR VSBYS in patchy early morning fog, improving to VFR. MVFR CIGS linger in low clouds and scattered showers, ending from S-N late Fri and Fri night. Saturday...Patchy early morning fog along the coast and across N Central Mass, improving by mid-late morning. Otherwise VFR. May see isold showers across portions of interior central and N Mass around midday or early afternoon.
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&& .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 Night/... Tonight...Moderate confidence. During this evening, showers will become more numerous, combining with stronger southerly winds reaching around 25 kt this evening/tonight. While seas should generally remain at or below 5 ft, continuing a small craft advisory for these winds thru tonight until the winds shift to the W-NW tomorrow morning. These will mainly be for the open waters, rather than the sheltered bays/sounds. Fog/showers will reduce visibility mainly late this afternoon through the overnight. Tomorrow...High confidence. Gradual improvement across the waters expected, though showers may linger thru the day in the vicinity of Cape/Islands and east. Winds and seas will remain below small craft advisory thresholds. Winds may gust to 25 kt early on the eastern outer coastal waters. Tuesday night...Moderate confidence. Weak wave of low pres tracks just south of New England Tue night and Wed morning. Winds/seas expected to remain below SCA thresholds for this period. Outlook /Wednesday through Saturday/... Wednesday...Moderate confidence. Low pres passes across or just S of the southern waters. E-NE winds 15-20 kt on the outer waters with seas up to 5-6 ft. Visibility restrictions in patchy fog Wed night. Thursday...Moderate confidence. Persistent E-NE wind gusting up to 20 kt on the outer waters with seas up to 5-6 ft. Winds shift to SE- S Thu night and diminish but seas remain at or above 5 ft on the outer waters and the southern near shore waters. Visibility restrictions in morning and nighttime fog and areas of showers. Friday and Saturday...Low to moderate confidence. Winds shift to NW Fri afternoon/evening. Gusts up to 25 kt on the southern outer waters during the afternoon/evening. Seas remain at or above 5 ft, highest on the southern outer waters. Visibility restrictions in early morning and nighttime patchy fog. Scattered showers early Fri.
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&& .TIDES/COASTAL FLOODING...
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High astronomical tides will occur over several tide cycles along eastern Massachusetts coast from Thursday into Memorial Day, with tides around 12 feet in Boston and 4 feet on Nantucket. At the very least, minor nuisance flooding will occur in the most vulnerable locations such as Morrissey Blvd in Boston. Any surge on top of these tides would lead to more widespread, but minor, coastal flooding, including on Nantucket. Right now, this looks to be a possibility Thursday into Friday due to expected onshore winds. Model surge guidance (ESTOFS) shows a potential 0.6 ft surge, which would give a storm tide near 13 feet Thursday night in Boston and just under 5 ft on Nantucket.
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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from 8 PM this evening to 8 AM EDT Tuesday for ANZ231-232. Small Craft Advisory from 8 PM this evening to 4 AM EDT Tuesday for ANZ233-234. Small Craft Advisory until 4 AM EDT Tuesday for ANZ235-237-256. Small Craft Advisory until 8 AM EDT Tuesday for ANZ250-254-255.
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&& $$ SYNOPSIS...EVT/NMB NEAR TERM...NMB SHORT TERM...NMB LONG TERM...EVT AVIATION...EVT MARINE...EVT/NMB TIDES/COASTAL FLOODING...JWD

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