Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 230104 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 904 PM EDT Tue Aug 22 2017 .SYNOPSIS... A cold front will bring the potential for scattered showers and thunderstorms late tonight, which may persist into Wednesday morning along the coastal plain. High pressure brings dry weather with very comfortable humidity levels Thursday through early next week. && .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
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***Severe Thunderstorm Watch Through At Least Midnight Across Western MA And Expanded Into Northern CT*** 9 pm update... Still a very tricky forecast late this evening into the overnight hours. Scattered strong to severe thunderstorms continue to move northeastward into Eastern NY and Northern New England this evening. Despite marginal mid level lapse rates, latest SPC Meso-Analysis shows 40 to 50 knots of effective bulk shear coupled with MLCapes still exceeding 1500 J/KG MLCape across western MA and northern CT. We still have a lot of uncertainty how well this convection will survive as it tracks east as much of the mesoscale guidance weakens the activity as it attempts to move into our region. However, given the amount of instability/high dewpoints and strong shear felt it was worth expanding the severe thunderstorm watch into northern CT and continue it across western MA through midnight. That may need to be extended to a later time. The main concern will be locally strong to damaging wind gusts depending on how well activity survives. Latest SPC meso analysis indicates 0 to 1 km helicity values between 250 and 300 across western MA and northern CT along with low LCL/s. Therefore, "IF" the activity holds together there is the risk for an isolated tornado. Brief torrential rainfall and localized street flooding is also possible with any of the storms. What ever happens to the activity across western New England, should certainly weaken as it enters a less favorable environment across eastern MA/RI after midnight. However, given LLJ will have to look for a bit of a resurgence near daybreak across the far southeast New England coast.
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&& .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/... The threat for scattered showers and thunderstorms may persist into Wednesday morning across the coastal plain, and perhaps even into early afternoon across the Cape and islands. Much drier air will arrive behind a cold front, which should move offshore by late afternoon. Near normal temperatures expected through this period. && .LONG TERM /THURSDAY THROUGH TUESDAY/... Overview and model preferences... 22.12Z model guidance suite continues the trend of a mainly dry forecast through early next week. Synoptically, there remains good agreement, and this is in spite of a fairly amplified pattern across the N CONUS and Canada, something long range guidance typically struggles with. Reinforcement of an Alaskan vortex is the catalyst for building a highly amplified ridge across central Canada, and although not a true block, leads to enough downstream trof digging to reinforce the trof upstream of New England. In spite of the cyclonic curvature, flow aloft is confluent predominantly, shifting the mean jet over New England rather than in the typical ageostrophic lift region of the mean jet. This coincides with an area of subsidence in development of reasonably strong high pres which will generally define the sensible wx across New England through early next week. Thermal profiles are supported by drier/cooler flow within the upstream trof out of central Canada, supporting the previous forecaster`s thinking that a break from the summer heat/humidity can be expected. Details... Thu... Front shifts further offshore allowing deeper column drying and cold advection aloft. Dewpoints drop into the 50s and H85 temps drop to +10C to +12C, which should keep highs mainly slightly below normal, or in the mid-upper 70s with a few spots around 80. Subsidence maintains a dry forecast outside of CU development. Fri... Vigorous shortwave will rotate through during the daytime Fri. Lapse rates are reasonably steep given the cold advection leading the wave aloft, nearing 6.0C/km across much of the region. A slight spike in moisture possible as well as PWATs shift closer to the 1.00 inch mark according to some soundings. Although some convective feedback QPF is noted, but likely not well progged, will feature diurnally timed increase in POPs to at least slight chance during the afternoon and early evening hours Fri with this wave passage. Shower, or even a very isolated thunderstorm not out of the question. Otherwise, highs once again slightly below normal, mainly in the mid-upper 70s. This weekend... High pres crests over the region through the period as the base of the longwave trof also shifts W-E across New England. Soundings continue to be mostly dry, suggesting the risk for even any showers remains quite low, especially given the lack of strong forcing. Will continue to feature a mainly dry forecast with NIL POPs. Highs in the 70s predominantly, with comfortable overnights dipping into the 50s thanks to dewpoints in the upper 40s and 50s as well. Early next week... Low lvl ridging is reinforced by the influence of the stronger synoptic ridge shifting E in New England as the longwave moves E of New England. In fact, already noting that high pres will approach 1030+ hPa. Dry weather looks likely to continue, along with comfortable dewpoints. && .AVIATION /01Z 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/... Tonight...Moderate confidence. SCT SHRA/TSRA should move east of the Berkshires between 01Z- 04Z. Storms are expected to weaken as they shift east with time, but a few stronger storms cannot be ruled out. This line weakens for a bit early in the morning, but then another round of showers and storms could impact mainly southeastern MA after 10Z. Mix of mainly VFR with occasional MVFR/IFR in showers/storms, low confidence in IFR/LIFR stratus mainly Cape Cod and the Islands. Winds gusting 20-30 kt at times, with LLWS given 40-45 kt at around 2kft. Wednesday...High confidence in trends, but lower confidence in overall timing. SCT SHRA/TSRA may persist near and especially southeast MA into early Wednesday afternoon. Improvement to VFR conditions expected from west to east, but timing somewhat uncertain. Wednesday Night...High Confidence. VFR. KBOS Terminal...High confidence in TAF trends. Moderate confidence in timing. KBDL Terminal...High confidence in TAF trends. Moderate confidence in timing. Outlook /Thursday through Sunday/... Mainly VFR. Mainly W-NW flow with sea breezes by day. && .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. Strong southwest low level jet moves across the waters tonight into Wednesday morning. Gusty southwest winds ahead of an approaching cold front. Some risk for thunderstorms, but showers more likely. Will continue Small Craft Advisory headlines, but did tweak times. Winds and seas diminish Wednesday afternoon, with more tranquil boating conditions starting Wednesday night. Outlook /Thursday through Sunday/... Generally quiet boating weather. NW winds dominate, with gusts peaking only around 15 kt at any given time. Seas/waves remain below 4 ft. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until noon EDT Wednesday for ANZ231>234- 250. Small Craft Advisory until 2 AM EDT Wednesday for ANZ230-236. Small Craft Advisory until 2 PM EDT Wednesday for ANZ235-237. Small Craft Advisory until 8 AM EDT Wednesday for ANZ251. Small Craft Advisory until 4 PM EDT Wednesday for ANZ254>256. && $$ SYNOPSIS...Belk/Doody NEAR TERM...Frank SHORT TERM...Belk LONG TERM...Doody AVIATION...Belk/Doody MARINE...Belk/Doody is the U.S. government's official web portal to all federal, state and local government web resources and services.