Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 152019 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 419 PM EDT Tue Aug 15 2017 .SYNOPSIS...
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Light rain will linger mainly along south coastal areas through the evening, giving way to dry weather which will linger tomorrow and Thursday along with plenty of sunshine. A weather system from the Plains will bring showers and scattered thunder with locally heavy rainfall possible Friday and Saturday. A drying trend likely follows for Sunday into early next week along with seasonable temperatures.
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&& .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
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WV imagery shows modest mid lvl shortwave moving through central portions of the forecast area, with deep layer dry air already beginning to filter across NY state in its wake. Meanwhile along with this small vort-max, slight strengthening of LLJ is occurring within the deepens moisture plume, mainly 1.5-2.0 inches, along the immediate S coast. Will continue to focus POPs through this area through 00Z, but thereafter, a combination of AVA and dry air should bring this S coastal rain to an end. Meanwhile, areas of W and NW MA have shown some signs of destabilization with cooling/drying of the airmass aloft, enhancing mid lvl lapse rates. However, as forecast, the widespread cloud cover continues to inhibit CAPE increases, and LAPSE is barely showing 500j/kg in the CT valley, with much higher values across N New England and N NY state. While a few SHRA have popped up here, feel TS chances will remain limited through the evening as drier air, although continuing to steepen lapse rates will actually provide a cap. Will continue with slight to low end chance pops, but limit any TS to isolated through the evening. Any that do occur should dissipate with sunset. Do note some low lvl moisture trapped beneath the subsidence and dry air building aloft through the late evening and early overnight hours, this could allow for some evening and early overnight fog development, as well as low clouds building mainly in the same areas (SE MA/RI) as last night. However, with a weakening cold front swinging through around midnight or just after, the shift to a more W component to the wind may scour these out toward the morning except areas where winds remain mostly calm and radiational fog develops. These will be in typically prone locations. Overnight mins remain a bit mild, mainly in the low-mid 60s as sfc dwpts are likely to remain elevated until mixing can scour these out tomorrow by day.
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&& .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/...
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Tomorrow... Gert makes its closest pass during the early AM hours, just as it swings more to the E about 400sm S of Nantucket, captured by the weak front as it shifts offshore. The usurping of moisture and net gradient will yield NW flow across New England and, aside from some remnant wisps of CI. Although H85 temps will be dropping through the day, as well as mixing out of sfc dwpts, temps are likely to actually be above seasonal normals especially where the downsloping W component of the flow can be maximized. There for, looking at highs around 80 in the coolest spots to the upper 80s elsewhere. Leaned most heavily on the bias-corrected guidance which provided these warmer numbers. The main impacts from Gert will be felt on some area beaches... Beach Forecast: We still expect areas of high surf along with a high risk for rip currents along primarily south facing beaches which are open to the ocean. This extends from Watch Hill all the way across to the outer arm of Cape Cod and includes all Islands. A building swell into tomorrow morning, from Gert, will reach a 14 second period and nearly 8 ft offshore. This will create a potentially life-threatening situation due to high seas and rip currents. Will maintain the current High Surf Advisory as it stands. Tomorrow night... Pleasant sleeping weather as sfc dwpts will continue to drop even after mixing diminishes through the overnight hours. Weak gradient and mainly SKC conditions should promote some radiational cooling suggesting some min temps could dip into the low 50s. In fact, where radiational cooling is typically maximized, would not be surprised to see a few upper 40s for min temps, in the coldest valleys.
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&& .LONG TERM /THURSDAY THROUGH TUESDAY/...
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Highlights... * Very pleasant Thu with dry weather and low humidity * Scattered showers/T-storms with locally heavy rain Fri-Sat * Drying trend Sunday into early next week w/seasonable temps Synoptic Overview... Active northern stream jet along the Canadian/US border continues this period and will result in the risk for scattered showers/T- storms Fri and Sat. Although with the upper air pattern progressive there will also be dry weather days as well including Thu followed by a drying trend Sun into early next week. This active northern stream jet will also suppress the subtropical ridge and its excessive heat and humidity to the southern states this period. Temperatures... Residual post frontal airmass combined with 1018 mb surface high overhead will provide seasonable temperatures (U70s-L80s) and very pleasant humidity across the region Thu. Cooler Fri (70s) with overcast conditions and scattered showers/T-storms and warm sector airmass remaining west of New England. Saturday will be a warm and muggy day ahead of an approaching cold front with highs in the low 80s and dew pts in the low 70s. Behind the frontal passage, trending less humid Sunday into early next week but temperatures will be seasonably warm (highs low to mid 80s). Precipitation... Dry pleasant weather lingers into Thu. Then an airmass change Fri ahead of next short wave trough with PWATs jumping up to 2+ inches Friday. Warm sector remains west of New England so any instability will be elevated. Nonetheless scattered showers and T-storms with locally heavy rainfall possible Fri/Fri night. By Saturday cold front is over or entering the region. Cyclonic flow aloft, frontal convergence and at least marginal instability should be sufficient for another round of scattered showers/T-storms. Drier air begins to work in from the best so the highest risk for storms may be across RI and eastern MA. Instability and wind fields aloft appear marginal for severe weather so heavy rain/downpours may be the main concern. Trailing short wave trough arrives Sunday but less moisture and instability to work with per ensembles and deterministic guidance. Thus shower/T-storm coverage likely less than Sat. So not expecting a washout Sunday. Short wave ridging and associated dry weather appears to arrive early next as trough exits later Sunday.
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&& .AVIATION /19Z TUESDAY THROUGH SUNDAY/...
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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 Wednesday Night/... Through tonight...moderate confidence. VFR through early evening, then after 20-21Z another round of MVFR (occasional IFR on Cape/Islands) will spill across SE MA and RI through about 04Z. Some patchy ground fog thereafter, although with winds shifting to the west, improvement to VFR also possible. Ground fog possible at typically prone airports in the morning if winds go calm. Tomorrow and tomorrow night...high confidence. Any fog burns off within a couple hours after sunrise. Mainly VFR with NW winds. Gusts to around 15 kt at times through the daylight hours. KBOS Terminal...High confidence in TAF. KBDL Terminal...Moderate confidence in TAF. Issue at hand is whether a period of MVFR fog will impact the terminal during the overnight hours. Outlook /Thursday through Sunday/... Thursday...VFR, dry runways and light winds with seabreezes along the coast. High forecast confidence. Friday...VFR to start but trending toward MVFR/IFR as showers and isolated T-storms overspread the region from west to east. High confidence on trends but lower on details. Saturday...MVFR/IFR in morning fog/low clouds likely improves to VFR- MVFR during the day however scattered afternoon showers/T-storms possible esp eastern MA/RI. High confidence on trends but lower on details. Sunday...low risk of IFR/MVFR in morning fog/low clouds but then improving to VFR by midday. Spot shower/T-storm possible. Moderate confidence.
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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 Wednesday Night/...High confidence. Through tomorrow night... Building southerly swell overnight. Reaching as high as 7-9 ft in the open ocean waters well south of Nantucket, meanwhile seas could reach 5-7 ft in nearer to the shore as this long period swell peaks toward the morning hours Wed. Small Craft Advisories will continue, and have been hoisted even for some of the area bays and sounds (Buzzards Bay, Nantucket and Vineyard Sounds) as some near 5 ft seas are possible on the periphery of these zones, especially thanks to some tidal influences. These seas will gradually subside tomorrow night. Meanwhile, an area of rain/fog will linger through the overnight hours tonight mainly across the S waters, leading to visibility restrictions. Outlook /Thursday through Sunday/... Thursday...fine boating weather with high pres over the region. Southerly swells from Gert may linger across the ocean waters. Dry weather and good vsby. High forecast confidence. Friday...a warm front lifts across the area with SE winds becoming SW late. Showers and TSTMS limit vsby. Moderate confidence. Saturday...southwest winds ahead of an approaching cold front along with scattered showers/T-storms. Vsby will be restrict in fog in the morning and showers/T-storms much of the day. Moderate confidence. Sunday...main cold front pushes offshore providing an improving trend in vsby and drier conditions. However a secondary front may trigger an isolated shower/T-storm in the afternoon near shore. Moderate confidence.
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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...High Surf Advisory from 9 AM to 8 PM EDT Wednesday for MAZ020- 022>024. RI...High Surf Advisory from 9 AM to 8 PM EDT Wednesday for RIZ006>008. MARINE...Small Craft Advisory for hazardous seas from 5 AM to 8 PM EDT Wednesday for ANZ232>234. Small Craft Advisory for hazardous seas from 11 PM this evening to midnight EDT Wednesday night for ANZ235-237. Small Craft Advisory for hazardous seas from 11 PM this evening to 5 AM EDT Thursday for ANZ254. Small Craft Advisory for hazardous seas from 7 PM this evening to 5 AM EDT Thursday for ANZ255-256.
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&& $$ SYNOPSIS...Nocera/Doody NEAR TERM...Doody SHORT TERM...Doody LONG TERM...Nocera AVIATION...Nocera/Doody MARINE...Nocera/Doody

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