Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 190006 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 806 PM EDT Tue Jul 18 2017 .SYNOPSIS... * Updates to near- and short-term forecast portions only... Bermuda high pressure will result in very warm and humid weather through Thursday. While dry weather will dominate for the remainder of the work week, a few diurnally driven showers and thunderstorms will occur at times. A cooler and unsettled weather pattern at times may be in store for the region this weekend into early next week. && .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/... Showers and thunderstorms continue to develop along the northern edge of an instability axis. Latest SPC mesoanalysis still has CAPE values near 1,000 J/kg. Not expecting severe weather, but locally torrential rainfall is a given within the heavier showers and thunderstorms. Expecting showers and thunderstorms to slowly diminish between now and midnight. Nantucket has already shown indications of marine stratus and fog returning. Issued a Dense Fog Advisory for south coastal MA, where confidence is highest in dense fog. Issued a Special Weather Statement for southern RI. A Dense Fog Advisory may need to be expanded there later tonight. Tried to update timing of rainfall chances based on latest trends. Also tweaked temperatures to reflect latest observations. Previous Discussion... Return of low clouds and/or fog. Highest confidence over Southeast MA including Cape Cod and the Islands given light SW winds, continued high dewpoint air around the upper 60s to low 70s over the slightly cooler waters. Beneath weak mid to upper level ridging aloft acting to maintain the dry inversion trapping low level moisture, temperatures cooling, should see low clouds and fog return. This may require a DENSE FOG ADVISORY. Will highlight the threat of visibility down to a quarter mile or less in the HAZARDOUS WEATHER OUTLOOK. Across the interior, considering showers dissipating and antecedent rains during the day combined with light winds and the expectation of clearing in areas, in addition to high dewpoint air, could see the threat of fog, especially within low-lying river valleys. Lows around the upper 60s to low 70s given the maintenance of the humid airmass across the region overnight. && .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/... Wednesday... Low clouds and fog dissipate S/E as heat and humidity builds across the region. Some spot locations seeing their second day of 90 degree temperatures, namely Hartford. Parent with a weak mid-level trough disturbance across the region, along with daytime heating and boundary layer mixing up around H85 where temperatures range +16-18C (the NAM/EC on the lower end of the envelope over the GFS/Canadian camp), a local enhancement to SW winds aloft should mix down to the surface scouring out low clouds and fog rather quickly during the morning hours. SW gusts up to 20 mph as temperatures warm up around the upper 80s to low 90s. The mix-down of drier air as well, areas N/W across the interior have a better shot of lower dewpoints, however those areas ranging around the upper 60s to low 70s will consequentially see heat indices around the mid 90s. A thermally-driven axis along with potential sea-breezes, can`t rule out a spot shower over areas around the MA-pike that would be forced by colliding boundaries, especially with the sea-breeze and likely higher surface dewpoints associated, otherwise the environment aloft is fairly dry and not conducive in promoting wet- weather. Isolated showers with no thunder given low K-indices just near 30. Looking like a mostly dry day. Wednesday night... Clearing out and the beginning of a drying trend. Mid-level wave pushes through overnight with a surface cool-frontal reflection. Mainly a dry passage, maybe a spot shower, but limited given the nocturnal timeframe and lack of daytime heating / boundary-layer mixing. Winds reverting W/NW ushering drier air into the region. Surface dewpoints dropping NW to SE lessening the mugginess of the air. Should see any fog and/or low clouds limited to SE New England, eroding out to sea during the early morning hours. Lows dropping down to around the mid 60s, the warmest spots in urban centers. && .LONG TERM /THURSDAY THROUGH TUESDAY/... Highlights... * Very warm and Thursday * Less humid on Friday but still very warm * Unsettled weather possible at times this weekend into early next week with high temps near or below normal Details... Thursday... Heat and humidity continue ahead of a weak cold front moving southeast through southern New England. Instability is marginal, so only expecting showers at times. Should not be a washout for the entire day. Will need to monitor the heat indices closely. A few locations could reach 95 degree two consecutive days, which would require a Heat Advisory. Friday... Somewhat less humid air will work into the region from the west behind a weak cold front, but it still will be very warm. Still thinking weather will be mainly dry given lack of synoptic forcing. However, will need to monitor progress of the weak front. If it lingers closer to coast, it may be enough to trigger a few showers. Its tough to find as it is not very sharp. Saturday through next Tuesday... Strong nearly zonal mid level flow develops this weekend into early next week. Lots of differences in model outcomes with the location and timing of a developing mid level trough Monday into Tuesday. it is not uncommon to have these differences when strong nearly zonal flow is involved, as it leads to significant timing differences of various shortwaves. As a result, plenty of uncertainty on when and where and showers will develop as this weekend into early next week. Still not expecting this entire period to be a washout. Will continue to forecast a chance for showers and thunderstorms, but have little confidence in the timing. && .AVIATION /00Z WEDNESDAY THROUGH SUNDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Wednesday Night/...Moderate confidence. Tonight... IFR/LIFR fills back in, especially across SE terminals, possibly into the interior. Otherwise, VFR, light winds. Scattered SHRA/TSRA diminish in coverage between 19/00Z and 19/04Z. Wednesday into Wednesday night... Any IFR/LIFR scours out during the morning as SW flow increases with the potential for gusts up to 20 kts, especially over SE MA and RI terminals. Low risk of a SHRA during the day as there is also the potential for sea-breezes developing across portions of E MA around midday. IFR/LIFR may return across SE MA and RI into the evening hours. However, winds turning W/NW overnight should scour low conditions towards Thursday morning. KBOS Terminal... Still thinking the terminal will flirt with IFR during the over- night hours. Will continue the mention into the early morning hours Wednesday while prevailing MVFR. SW flow during the day may be stronger S. There is the possibility of an onshore sea- breeze developing again on Wednesday shortly after 15z. KBDL Terminal... SHRA dissipating into evening. IFR possible overnight. Anticipating a repeat of MVFR-IFR that was observed Monday night into Tuesday morning looks to repeat again, scouring out into the early morning hours. Outlook /Thursday through Sunday/... Thursday through Friday...Moderate confidence. Mainly VFR outside of an isolated to widely SCT SHRA/TSRA threat. There may also be some late night/early morning fog patches in the typically prone locations and especially the south coast, Cape, and Islands where it may be more widespread and persistent at times. Saturday through Sunday...Low confidence. VFR probably dominates, but will have to watch for periods of showers/isolated thunderstorms moving across the region at times. && .MARINE... FORECASTER CONFIDENCE LEVELS... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Wednesday Night/...High confidence. While winds and seas will be light enough for good boating weather, there are two concerns for the forecast period: 1.) Overnight into early morning issues of dense fog with visibility down to a quarter mile or less in spots, highest confidence over the SE waters. Both tonight into Wednesday morning and again Wednesday night into early Thursday morning. 2.) SW winds on Wednesday gusting up to 20 kts, especially along the near-shore waters of S RI and SE MA, within Narragansett Bay. Outlook /Thursday through Sunday/... Wednesday through Saturday...Moderate confidence. Expecting winds and seas to remain below Small Craft Advisory thresholds. However, some nearshore mid to late afternoon southerly wind gusts around 20 knots at times may result in a few hours of choppy seas in the typically vulnerable harbors and bays. Areas of fog may impact the southern waters at times, especially during the overnight/morning hours. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...Dense Fog Advisory from 4 AM to 9 AM EDT Wednesday for MAZ020>023. Dense Fog Advisory until 9 AM EDT Wednesday for MAZ024. RI...None. MARINE...None. && $$ SYNOPSIS...Belk/Sipprell NEAR TERM...Belk/Sipprell SHORT TERM...Sipprell LONG TERM...Belk AVIATION...Belk/Sipprell MARINE...Belk/Sipprell is the U.S. government's official web portal to all federal, state and local government web resources and services.