Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 222310 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 710 PM EDT Sat Jul 22 2017 .SYNOPSIS... Showers and thunderstorms will pass to our south and west tonight, with nothing more than a brief shower or two across the Cape and Islands. It will be cooler across eastern New England Sunday, but still warm in the Connecticut Valley. Cool, damp and unsettled weather follows for Monday and Tuesday with improvement Wednesday. Some showers and thundershowers possible again Thursday and Friday. && .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
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7 PM UPDATE... Western Mass and Connecticut may see light showers this evening but these will likely dissipate over the next few hours. With high dewpoints and a southerly wind, we`re looking at the potential for fog tonight across the Islands. Fog will likely move in around 3Z and remain until late morning. Visibility could be as low as 3 SM on the Islands overnight, but is not expected to be lower. Otherwise, no significant changes have been made to the forecast. PREVIOUS DISCUSSION FOLLOWS... Southern New England will be in between a sprawling high pressure over southeast Canada and northernmost New England, and a low pressure somewhere along the Mid Atlantic coast from NJ to the Delmarva Peninsula. This low pressure to our south will likely be the remnants of a mesoscale convective system affecting that area tonight. For us, winds will turn to a northeasterly direction. Skies will be cloudy, and there is a chance of showers, mainly over northern CT, RI, and southeast MA, closer to the low pressure. Lows in the 60s.
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&& .SHORT TERM /6 AM SUNDAY MORNING THROUGH SUNDAY NIGHT/... Sunday... Low pressure will continue passing southeast of the region. It may cause scattered showers, mainly along the south coast in the morning. Then we should see a period of increasing sunshine through mid afternoon. Light easterly wind flow will keep eastern MA temperatures cool, in the low to mid 70s near the coast, but the partial sunshine could allow the CT River Valley to reach into the 80s. There is very little consensus on max temperatures for Boston tomorrow across the models. Given the easterly wind, it is likely that Boston will remain in the lower 70s for most of the day. The GFS seems far too warm for the eastern coast, but could be correct for the Connecticut River Valley where things will be able to heat up a lot faster. Sunday night... Skies will attempt to clear ahead of an approaching low pressure, allowing for minimal radiational cooling. However, temperatures will drop down into the low 60s, and even 50s in northern Massachusetts. Chances of showers increase again after midnight, mainly across northern CT and western MA, ahead of advancing low pressure. The main area of rain will likely not move into the region until after 12Z Monday. The NAM suggests that most of the area will remain dry throughout the morning. However, guidance from the GFS and ECMWF both agree on a late morning arrival. Opted to go more with the GFS/ECMWF timing, which was more in line with the previous forecast. Areas of patchy fog are possible along the southern coast of New England before midnight, and will likely persist and move northward throughout the early morning hours. && .LONG TERM /MONDAY THROUGH SATURDAY/... Highlights... * Mon: Below normal temps with some showers likely at times * Tue: A few leftover showers possible w/below normal temps * Temps moderate mid to late next week but still below average * Another round of showers possible Thu/Thu night but not a washout Overview and model preferences... Synoptically there is little significant disagreement with the mean jet remaining along the N CONUS, generally preventing subtropical warmth/moisture from reaching into New England through the period. This suggests the continued thinking of temperatures mainly at or below normal. Will be following this trend with this update regardless of the preferred model guidance. Initially weakening trof Mon-Tue will give way to brief shortwave ridging as a second robust wave slides through the mean jet from the PACNW later in the week. Therefore, an unsettled start will give way to a brief dry period followed by another short period of unsettled wx, pretty typical New England summertime pattern when the mean jet rests this far S. Not seeing significant enough differences in the overall pattern to favor any one model. Therefore, will use a general consensus blend for this update. Details... Mon and Tue... Early Mon, will need to monitor the track of remnant convection/MCS which will moving ENE from the OH Valley into NY/PA. This convection looks to generate a weak cold pool low pres which will slide near or just S of New England into early Tue. Given much of the initial convection will be driven by mesoscale processes, confidence on how quickly any precip/storms associated will make it into New England. However, with weakening shortwave moving in, the development of weak low pres, as well as an increase in column moisture (PWATS reach 1.25-1.50 inches), do expect to see on and off SHRA activity much of Mon into early Tue. E flow suggests sfc based convection somewhat unlikely although as previous forecaster noted there is modest instability aloft. Heights will be rising as the trof weakens, which also may mitigate convective development. Therefore, although unsettled, coverage will be showery with breaks, with only a low risk for an isolated thunderstorm given the mitigating factors. Otherwise, E flow and clouds will keep highs mainly at or below normal, while overnight mins near normal thanks to raised dwpts in the 60s. Wed... Ridging will follow the exit of the weakened trof. As of now, high pres associated with this ridging will have it`s strongest influence on Wed, as it could give way enough to yield some unsettled wx by Thu. Subsidence and generally dry wx is expected. Ridgeline is generally SW to NE oriented, suggest a slight wedge of cooler air from the Canadian Maritimes. Therefore in spite of more sunshine, highs will likely still fall at or below normal. Thu... Timing uncertainties for Thu. As mentioned above, the weak ridge and attendant sfc high pres will be sliding ENE out of the region, however the speed with which it does so will be strongly dependent on how much deepening the approaching trof undergoes. ECMWF is faster than the GFS, with ensembles on either side. The faster solution would bring an afternoon cold front through, with the possibility of showers and thunderstorms while the slower solution favors a mainly dry forecast, keeping the stronger forcing well upstream. Will feature some lower-end POPs for the possibility, but will need to monitor model shifts temporally. Otherwise, a slight uptick in mid lvl temps, closer to +13C or +14C at H85, suggest that with at least some sun ahead of the approaching front, highs will reach near to possibly even above normal in some spots. Fri and Sat... Cold front may linger Fri, with the potential for showers/thundershowers remaining, however general consensus is that it should be offshore by Fri night with a drier start to next weekend. Temperatures near normal based on current forecast mid lvl temp spread. && .AVIATION /00Z SUNDAY THROUGH THURSDAY/...
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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 Sunday Night/...High confidence. Tonight...Moderate confidence. VFR inland, Mainly mid-high clouds. Band of MVFR CIGS and visibilities mainly over the islands. Risk for IFR over Nantucket. Showers should stay further south, but may potentially clip the Islands late. Sunday...High confidence. VFR, MVFR/IFR dissipate over islands. Scattered showers should remain further south, but may clip the islands early. Sunday Night...Moderate confidence. VFR conditions should deteriorate to MVFR-IFR late. KBOS Terminal...High confidence. VFR. Sea-breezes forecast to move out around 00Z-02Z tonight. KBDL Terminal...High confidence. VFR, few showers 00Z-03Z with minimal changes in category. Outlook /Monday through Thursday/...Moderate confidence. Mon into early Tue...Moderate confidence. Mix of IFR/MVFR conditions in a mix of low CIGS/occasional SHRA and fog. E winds, gusting to 25 kt near shore but weaker inland. Winds shift to the N early Tue and begin to subside. Can`t rule out a few breaks to VFR, especially inland, but lower categories likely dominate. Late Tue into Wed...High confidence. Improvement to VFR with drier weather to follow, weakened W-NE winds. Thu...Moderate confidence. Mainly VFR. Afternoon showers/thunderstorms possible. Winds S, may gust to around 20 kt.
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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 Sunday Night/...High confidence. Tonight... Winds shifting to the northeast as high pressure builds to our north and low pressure passes to our south. Winds gust to 20 kt, especially from Cape Ann to Mass Bay and Provincetown. Waves 1-3 ft. Sunday... Northeast winds continue at 15-20 kts with seas building to 3-4 ft. Small chance of showers along the Cape and Islands. Sunday night... Northeast winds will turn due east late, with sustained winds at 10 kts. Winds will gust up to 20 kts across the waters. Waves 3-4 ft. Outlook /Monday through Thursday/...Moderate confidence. Mon and Tue...Moderate confidence. Low pres will be passing along the S waters late Mon into Mon night. Winds will shift from E, to NE then finally to N by Tue. Wind gusts during this time may gust to around 25 kt. Also, seas mainly across the SE and E waters will reach about 5 ft as the low pres passes. Therefore, small craft advisories may be necessary. Showers and fog possible through the period. Wed...High confidence. Seas will subside, winds remain below 20 kt. Quiet boating weather expected. Thu...Moderate confidence. S winds may gust to around 25 kt during the afternoon, especially along the E coast. Small craft advisories may be needed.
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&& .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...Belk/Doody/Correia NEAR TERM...Belk/Doody/Correia SHORT TERM...Belk/Correia LONG TERM...Doody AVIATION...Belk/Doody/Correia MARINE...Belk/Doody/Correia

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