Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 220553 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 153 AM EDT Sat Jul 22 2017 .SYNOPSIS... Dry weather prevails through Saturday with lower humidity, but it will be hot. High pressure from Canada builds over the region Sunday, then moves off to the east. Low pressure from the Great Lakes approaches on Monday bringing showers and scattered thunderstorms. High pressure moves in during Tuesday and Wednesday with dry weather. Another area of low pressure moves through late in the week. && .NEAR TERM /THROUGH 6 AM TODAY/...
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145 AM update... High resolution models show that dewpoints should be falling in the CT River Valley over the next 3 hours, so our forecast of lows in the mid 60s may still work out. BDL dewpoint has dropped from 70 to 67 already. Otherwise, high cloudiness across the region. We are watching a large area of mid level cloudiness moving across the eastern Great Lakes and western NY, since it would move across southern New England during the day if it holds together. NAM shows 925 mb wind max over central MA of 25-30 kt, which accounts for why the winds still gusting to 18 kt at KBOS. This max heads southeast over southeast MA and RI by 09Z and off the coast by 12Z. Radar shows showers developing along the southeast CT coast, where have some CAPE and higher dewpoints. Latest HRRR shows showers blossoming over the next few hours from Block Island to the waters south of Nantucket. Will update near-term grids accordingly.
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&& .SHORT TERM /TODAY THROUGH 6 PM SUNDAY/... Saturday into Saturday night... Keeping it mostly dry. In brevity, the quasi-stationary boundary is bowled further S of S New England with the latest forecast suite, acting as the focus of warm-moist S inflow beneath modest W winds and shear lending to the development of thunderstorm complexes. With drier air maintained in place by N flow, have lowered PoPs. Think any interior activity initiated along the slopes of high terrain with whatever moisture is available will be well N/W across Upstate NY / VT / NH. Expect increasing clouds with time subsequent of cloud tops from convection S with the chance for some rain overnight into Sunday morning across CT/RI/SE MA, greatest chances further S. The air- mass of +17C air around H85 unchanged lending to highs around the upper 80s to low 90s, some relief along the shores given sea- breezes as winds will be light during the day, begins to filter out towards Sunday as cooler air slides slightly S with increasing N/NE winds per isallobaric response of a weak wave low along the quasi-stationary boundary. Lows once again around the mid 60s. && .LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/... Big Picture... The longwave/large scale pattern across the Northern USA remains as it has been... ridge west and trough east. If there is a variation in this theme, it is that the ridge should shift east to the Rockies/High Plains later next week and the eastern trough shifts east to the Maritimes. Two shortwaves in the flow will affect New England in the next seven days. One starting over Alberta Canada will cross our area late Monday or Tuesday. The other coming from the Gulf of Alaska will come ashore this weekend and reach New England late in the week. Model mass fields showed notable differences in handling the shortwaves last night, but the 12Z runs have come into better agreement, especially regarding the early-week shortwave. Confidence is moderate-high for the early-week shortwave and low-moderate for the late-week shortwave. Details... Sunday... Fair weather as high pressure from Hudsons Bay builds over Maine and the Maritimes. This brings an east flow to our area which should keep surface temperatures cooler than normal. All models show a low pressure wave passing south of our area. The NAM12 and GFS grids show no pops and no QPF, while the ECMWF shows lingering precip over CT-RI-SE Mass. Based on the position of the ridge, we favor the drier solution. Sunday night through Tuesday... The early-week shortwave moves a surface low from the Great Lakes toward New England with increasing lift ahead of the system. Expect clouds to fill in, with increasing chance of precip, especially after midnight. We have adjusted pops to show this trend. The shortwave moves across our area Tuesday morning, so we will expect showers through that time with the highest pops during Monday. Clouds, showers and the low level east flow should keep daytime temps no higher than the 70s. Wednesday... High pressure moves overhead, bringing dry weather and dew points in the 50s. Temps at 850 mb will be 8-10C, so expect max temps in the 70s. Thursday-Friday... The late-week shortwave moves across our area. Current model timing would bring the shortwave and surface cold front through during Thursday afternoon/evening, but timing has varied a little from one model run to the next, so potential exists for the system to move through a little earlier or a little later. Once it does move through, it will be followed by an area of drier weather...probably Friday. && .AVIATION /06Z SATURDAY THROUGH WEDNESDAY/...
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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/...High confidence. Overnight... VFR. Increasing sky cover at 25K ft, but otherwise fair with diminishing westerly wind. Low risk for localized MVFR or IFR cigs/vsbys in fog at Nantucket or in CT River Valley. Scattered showers along the immediate RI coast before daybreak. Saturday into Saturday night... VFR. SCT mid-high clouds initially, lowering towards low-end VFR overnight with a low risk of a SHRA. Light NW winds allowing for daytime sea-breezes. Increasing by evening with breezy N/NE flow. KBOS Terminal...Sea-breezes forecast around 16z Saturday. As the sea-breeze comes ashore, winds are likely to be a bit stronger around 6-10 kts. KBDL Terminal...VFR. Outlook /Sunday Night through Wednesday/... Moderate confidence. Sunday... VFR with an east wind. Ceilings/vsbys lower to MVFR Sunday night. Monday-Tuesday... MVFR ceilings/vsbys with areas of IFR in fog and heavier downpours. Scattered thunderstorms Monday afternoon. A cold front moves through during Monday night with winds shifting out of the north. Drier air moves in during Tuesday with conditions improving to VFR. Wednesday... VFR. Areas of IFR in early morning fog. Winds become variable with sea breezes along the coasts.
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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/... Overnight... W to NW winds 10-15 kt diminishing to 5 to 10 kt. Seas below 4 feet. A risk of lower visibility with fog over the waters S/E of the immediate Nantucket shore. Scattered showers over the waters south of RI and southeastern MA before daybreak. Saturday into Saturday night... Light NW winds allowing for daytime sea-breezes, increasing into evening becoming breezy out of the N/NE with gusts up to 20 kts as a weak wave low passes W to E well S of the S waters. As the low passes, there is a chance of showers across all S waters. Seas below 4 feet. Outlook /Sunday Night through Wednesday/... Moderate confidence. Low pressure south of the waters Sunday moves out to sea. Broader area of low pressure from the Great Lakes brings showers and scattered thunder to the waters from Sunday night through early Tuesday. East winds lead the system, then turn from the North as the weather system moves out to sea Tuesday. Winds will remain less than 25 knots through the period. Seas will be less than 5 feet on most waters through the period. Seas may reach 5 feet on the southern outer waters, where a small craft advisory may be needed.
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&& .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...WTB/Sipprell/GAF NEAR TERM...Sipprell/GAF SHORT TERM...Sipprell LONG TERM...WTB AVIATION...WTB/Sipprell/GAF MARINE...WTB/Sipprell/GAF

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