Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 212251 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 651 PM EDT Fri Jul 21 2017 .SYNOPSIS...
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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.
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&& .NEAR TERM /THROUGH SATURDAY/...
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650 pm update... No significant changes planned at this time. Observations and satellite images show some cirrus over the region along with a few lower clouds. Sky cover will fill in a little overnight, but otherwise a fair night. Previous discussion... Quiet. Scattered cumulus dissipates however mid to high level clouds continue to filter across the region in association with remnant convection upstream. A weak, shallow trough boundary pushing S with more N flow and drier air in association with a mid level vortex slightly dropping S round a deeper low over the E Hudson Bay Region of Canada, forecasting dry air advection with lower dewpoints. All in consideration, if winds become light then there is some opportunity for spot locations to drop out. As we saw last night, sheltered valleys in N/W MA were able to drop into the upper 50s. Similar thinking for tonight with average lows across the region around the mid 60s.
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&& .SHORT TERM /SATURDAY NIGHT/... 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 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 /00Z 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 Saturday Night/...High confidence. Tonight... VFR. Increasing sky cover at 25K ft, but otherwise fair with diminishing westerly wind. Low risk for MVFR or IFR cigs/vsbys in fog at Nantucket. 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 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 Saturday Night/... Tonight... Winds diminishing, becoming light out of the NW. Seas below 4 feet. A risk of lower visibility with fog over the waters S/E of the immediate Nantucket shore. 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 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...Frank/Sipprell NEAR TERM...WTB/Frank/Sipprell SHORT TERM...Sipprell LONG TERM...Frank AVIATION...WTB/Frank/Sipprell MARINE...Frank/Sipprell

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