Area Forecast Discussion
Issued by NWS Boston, MA

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683 FXUS61 KBOX 230201 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1001 PM EDT Sat Jul 22 2017 .SYNOPSIS... Scattered light rain the heaviest of which will be along the immediate S-coast into early Sunday morning. Cooler through the day. Cool, damp and unsettled weather follows for Monday and Tuesday with improvement Wednesday. Some showers and thunder- showers possible again Thursday and Friday. && .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/... 10 pm update... Scattered light rain gradually concluding. The outcome of low to mid level ascent N of the weak wave low along the quasi-stationary front beneath shortwave energy and the right rear quadrant of the upper level jet. But a fair amount of low level dry air discerned from the 23.0z soundings out of Albany NY and Gray ME. Battle of airmasses N to S along a roughly W to E quasi-stationary frontal boundary across PA and off the NJ coast. As energy pushes E, with heavier outcomes skirting the S-waters of New England, high pressure is allowed to build S with wake subsidence. Subsequent N flow, slightly cooler yet notably drier air with dewpoints in the 50s slides S. A drying trend towards morning, and perhaps some partial clearing N/W, will see lows drop into the 60s. As to fog, there is still the potential along the S-coast, especially Nantucket, given expected wet-weather along with the low temperature- dewpoint depressions. Thinking short-lived and eroding into the morning period as winds turn N ushering drier air S. Some reduction in visibility not out of the question as has already been observed on Nantucket. && .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 /02Z SUNDAY THROUGH THURSDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Sunday Night/... Tonight...Moderate confidence. Mostly VFR with SCT -RA, the heaviest along the immediate S-coast. Low risk MVFR-IFR for KACK. Clearing out towards Sunday morning. Sunday...High confidence. VFR. SCT SHRA linger along the islands early. Sunday Night...Moderate confidence. VFR conditions should deteriorate to MVFR-IFR late. KBOS Terminal...VFR. Sea-breezes forecast to move out around 00Z-02Z tonight. KBDL Terminal...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. && .MARINE... 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. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...Belk/Doody/Correia NEAR TERM...Belk/Doody/Sipprell/Correia SHORT TERM...Belk/Correia LONG TERM...Doody AVIATION...Belk/Doody/Sipprell/Correia MARINE...Belk/Doody/Sipprell/Correia

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