Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 251938 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 338 PM EDT Tue Jul 25 2017 .SYNOPSIS... Skies will clear tonight. High pressure will bring a return to dry and milder weather for Wednesday and into Thursday. Cold front with a surface low pressure wave impacts southern New England Thursday night and exits the area late Friday. Dry weather and seasonable temperatures expected next weekend. && .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/... Rest of the afternoon...A few showers may persist across northern MA until sunset as a short wave trough moves through the northeast. A few breaks in the cloud cover will take place across northeast MA. Elsewhere, skies will remain mostly cloudy but with some lifting and thinning. Tonight...Upper trough heads east of New England, and drier air filters into the region. Anticipate gradual clearing this evening. This should allow enough radiational cooling to allow interior temperatures to drop to the lower to mid 50s. There could be isolated pockets in the interior NW valleys that dip into the upper 40s, rather notable for late July. Winds will be light with a drift from the north. && .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/... Wednesday... Surface high pressure and upper ridging will dominate across a dry air mass. Anticipate plenty of sunshine with just some afternoon cumulus and temperatures generally in the upper 70s and perhaps near 80 in the lower portions of the Merrimack and Connecticut Rivers. Temperatures may be capped in the mid 70s along coastal areas due to developing seabreezes. Wednesday night...Expect dry conditions to prevail but will probably experience increasing high clouds late ahead of a shortwave trough dropping southeast from central Canada. A weak SW gradient will become established. && .LONG TERM /THURSDAY THROUGH MONDAY/... Highlights... * Dry most of the day Thu. * Showers and possible thunderstorms late Thu with showers likely lingering into Fri * Dry weather and seasonable temperatures likely for the weekend Overview and model preferences... Although weak ridging will begin this long-term period, it will be followed by an unseasonably strong shortwave which will begin to carve out long-wave trof across the northeast CONUS and much of the E seaboard late in the week, and into the weekend. Although the initial wave is likely to cutoff per latest ensemble and operational model runs, the damage will have largely been done, leaving the longwave trof in its wake as it shifts E as early as the late weekend. Given this, periods of unsettled wx can be expected, along with generally cooler than normal temperatures through much of the latter half of the week. Generally, will use a consensus blend of guidance for this update. However, will opt to lean away from 24.12Z ECMWF which generates an anomalously deep low pres crossing the region Thu night night into Fri. This seems a bit overdone and is a bit of an outlier. Details... Wed night into Thu... Weakening ridge will shift E through the period, but remains the dominant player for sensible wx across New England through most of the day on Thu. Cold front/low pres development associated with a robust wave will slow the overall progression of any precip/lift these features can generate. Therefore, will continue to suggest mainly NIL pops with only slight chance in the W/NW by 00Z Fri. Wed night mins comfortably cool for sleeping, mainly in the 50s to low 60s in the heat islands. Daytime highs will be dictated by speed of incoming cloud cover but with H85 temps rebounding to near +14C, could see a few spots in the upper 70s and low 80s. Thu night into Fri... Potentially most unsettled period of the forecast. Very robust shortwave, especially by July standards, will be carving out a trof across the NE. Exactly how this plays out at the sfc remains somewhat uncertain, as both ensembles and operational guidance suggest anywhere from a standard cold frontal passage, to as deep as a 992mb low pres passage directly over New England. The latter may be a result of convective feedback, however if true it would certainly suggest a period of moderate to heavy rainfall, as lift would be much more significant, and able to fully realize PWATS nearing 2.00 inches. In any case, destabilized soundings and modest shear will be in place, allowing for some convective elements to accompany the widespread SHRA/RA. Will continue to suggest some TS and continue the likely POPs put forth from the previous forecaster. Will need to watch this period, because if the more robust low pres is the ultimate answer, heavy rain could yield some localized urban/street flooding potentially more widespread than more localized convection. The increased cloud cover and cool air associated with the digging trof will keep highs mainly below normal, suggesting highs may once again struggle to reach the low-mid 70s, but may be cooler if the more amplified solution. Sat and Sun... Although continued digging of the longwave trof occurs, a cutoff forms in the Canadian Maritimes which allows for the more anticyclonic sheared (W half) of the trof to slide across New England. This should allow for drier NW flow, limiting any additional SHRA/RA and drying the column enough for mainly dry wx as well. Still a few clouds, particularly diurnally driven can`t be ruled out given the trof base is just downstream. Temps remain cooler than seasonal normals. H85 +10C to +11C would yield highs mainly in the mid 70s, potentially 5-8 degrees colder than normal. Early next week... Although longwave trof continues to dominate across the E, the weakening cutoff/vortex in the Maritimes will give way to brief ridging as secondary wave moves out of the Canadian prairies. Therefore, although cooler temperatures are likely to continue, exactly how this complex interaction of trof plays out could allow for some unsettled wet wx at times, but definitely not a washout, as it keeps the deep layer moisture well to the S. && .AVIATION /19Z TUESDAY 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/... Rest of this afternoon to 00Z...Moderate confidence. MVFR ceilings will gradually lift into the VFR range. Tonight...High confidence. VFR with clearing skies. Patchy late night fog may occur at some interior airports for a few hours. Wednesday...High confidence. VFR. Any patchy interior valley fog should burn off quickly. Wednesday night...High confidence. VFR. KBOS Terminal...High confidence except moderate confidence in precise timing of late afternoon/early evening transition. MVFR ceilings lifting to VFR 21Z to 00Z. VFR for the rest of the period. Sea breeze expected to become established by mid to late Wednesday morning. KBDL Terminal...High confidence. MVFR cigs lifting to VFR by around 22Z. VFR for rest of the period. Outlook /Thursday through Saturday/... Wednesday night through Thursday...High confidence. VFR. Mainly S to SW winds. Thursday evening through Friday...Moderate confidence. Mix of MVFR/IFR conditions with widespread SHRA and risk for occasional TSRA mixing with fog. Winds will be shifting through the period, but cannot rule out some gusts to around 20 kt as these shifts occur. Saturday...Moderate confidence. Mainly VFR especially during the day on Sat. && .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. Tonight...Seas will gradually subside overnight. Dropped the SCA for the outer coastal waters south of Block Island but extended the SCA for hazardous seas for Mass Bay and Ipswich Bay into this evening as even a light NE to NNE flow was keeping seas elevated to 5 or 6 feet there. Wednesday...High pres over the waters will result in light winds with near shore seabreezes. Seas will be below SCA thresholds. Outlook /Thursday through Saturday/... Conditions during most of the period will likely remain below Small Craft Advisory thresholds. However, this is a risk that winds and seas may approach 25 knots and 5 feet, respectively, in an increasing SW flow Thursday afternoon and a northerly flow Friday afternoon and night, depending upon the strength of a low pressure area that passes along the southern waters Friday. Rain and fog will lead to visibility restrictions especially Thu night into Fri. && .CLIMATE... Record low maximum temp of 66 degrees set at PVD yesterday. Previous record was 67 set in 1997. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory for hazardous seas until 4 AM EDT Wednesday for ANZ250-254-255. Small Craft Advisory for hazardous seas until 11 PM EDT this evening for ANZ251. && $$ SYNOPSIS...Doody/Thompson NEAR TERM...Thompson SHORT TERM...Thompson LONG TERM...Doody AVIATION...Doody/Thompson MARINE...Doody/Thompson CLIMATE... is the U.S. government's official web portal to all federal, state and local government web resources and services.