Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 192044 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 444 PM EDT Sat Aug 19 2017 .SYNOPSIS...
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A second cold front will cross the region overnight, accompanied by a few showers and possibly an isolated thunderstorm. Behind the front, warm but very pleasant late summer weather with lower humidity Sunday. High pressure brings dry and seasonable weather Monday. It will be hot and humid Tuesday. ahead of an approaching cold front, which will bring a chance of showers and thunderstorms from late Tuesday afternoon into early Wednesday afternoon. High pressure will bring dry and cooler weather late in the week into next weekend.
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&& .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/... 3 PM update... Thru sunset... Warm but pleasant weather this evening across CT/RI and western- central MA where temps at 330 pm were well into the 80s (except 91 at PVD, record 94 in 2002) with dew pts in the upper 50s and lower 60s, so somewhat tolerable. However eastward into eastern MA stalled boundary combined with seabreeze circulation has kept it humid with dew pts in the upper 60s to lower 70s. These higher dew pts combined with temps as warm as 91 at OWD has resulted in about 500 j/kg of SB cape. This instability combined with low level convergence has yielded a modest TCU field with an isolated shower near Plymouth and Duxbury. This shower threat will continue until about sunset but will be confined to eastern MA. In addition, deep mid level dry air aloft will limit areal coverage of showers to isolated. Thus much of the region remains dry. Other issue for eastern MA will be seabreeze circulation will continue to advect low clouds and fog into this region. However toward sunset onshore winds will veer to the south and will essentially cut off low level moisture from coming onshore. Otherwise warm, dry pleasant weather this evening. After sunset... Mid/upper trough swings thru the region overnight. This feature is already generating lots of convection across eastern OH and western PA/NY. Impressive trough amplitude for late Aug however trough is deamplifying as it moves into our region as wind fields aloft decrease. This results in weak synoptic scale lift over our area. In addition, despite a brief spike in K indices, mid level moisture is limited along and ahead of short wave trough. So given limited mid level lift and moisture combined with loss of daytime heating, most of the guidance including the 00z NCAR ensembles weakens or completely erodes convection as it approaches western New England from NY state. Thus other than the risk for a few showers (perhaps isolated T-storm) across western MA, expect mainly dry conditions overnight. Given the fropa is not until after 06z west and closer to sunrise for eastern MA, expect another warm and muggy night ahead although not as humid as last night. Given this could see some patchy fog tonight. && .SHORT TERM /6 AM SUNDAY MORNING THROUGH SUNDAY NIGHT/... Sunday... Spectacular late summer weather tomorrow with mid level mean trough axis pushing offshore resulting in height rises across the area. Warming temps aloft and flow becoming less cyclonic will yield less diurnal clouds/more sunshine than today. Model soundings show a deep boundary layer with drier air aloft mixing to the surface. This combined with downslope WNW winds will result in dew pts falling into the upper 50s. So despite temps climbing well into the MU80s to near 90, lowering dew pts will make it somewhat tolerable. Given the deep blyr and downsloping winds, increased temps a few degs warmer than guid. Sunday night... Any diurnal clouds erode with sunset giving way to mostly clear skies. This combined with high pres building into the area will promote radiational cooling and temps falling into the upper 50s outside of the urban areas. Used MOS temps to derive mins tomorrow night. && .LONG TERM /MONDAY THROUGH SATURDAY/...
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Big Picture... At upper levels, closed low pressure will be moving slowly across Canada, from west of James Bay Tuesday to eastern Quebec by Saturday. The trough axis extending southward from the low will remain to our west until Saturday. The cold pool of air will arrive over our region around Thursday and remain in place into next weekend. At the surface, high pressure will pass southeast of the region Monday. A cold front moves through Wednesday. Then strong high pressure builds over the Great Lakes states, with its influence extending eastward into New England. Details... Monday... High pressure off the DelMarVa coast will provide sunny skies for southern New England, warm temperatures, and relatively light southwest winds. Forecasting highs iin the mid to upper 80s over most of the interior, with comfortable humidity. Tuesday... Southwest breezes behind departing high pressure and well in advance of an approaching cold front will usher in very warm and more humid air. With 925 mb temperatures of about 25C, am expecting highs to reach the upper 80s and lower 90s. Winds could gust to 15-20 mph during the afternoon. Dewpoints will be rising to near 70 by late afternoon and heat indices will be in 90-94 range, just shy of heat advisory criteria. With the afternoon heating will come a chance of a few showers and thunderstorms late in the day, especially in western MA and northern CT, closer to the approaching front. Tuesday night-Wednesday... A cold front will slowly work its way across the region. Showers and thunderstorm probabilities increase to 40-60 percent Tuesday night and mainly in eastern sections on Wednesday. The ECMWF is slower with the frontal progression than the GFS. Wind fields are somewhat strong, with 35-40 kts at 925 mb and 850 mb Tuesday night. Cannot rule out a few strong thunderstorms Tuesday night, even though they would not be occurring during the maximum heating. Skies will be clearing from west to east during the day. West to northwest winds could gust to 20 to 25 mph behind the front. Highs mainly 80 to 85 Wednesday. Thursday-Saturday... Strong high pressure building into the Great Lakes states will exert its influence across New England. At the same time, an upper level trough will be moving overhead, along with it associated cold pool with 500 mb temperatures reaching -18C Thursday and continuing into Saturday. It also will be cooler in the lower levels. Forecasting high temperatures in the 70s each day, possibly near 80 in spots. It should be dry for the most part, with scattered diurnal clouds.
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&& .AVIATION /20Z SATURDAY THROUGH THURSDAY/...
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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 Night/... 1830Z update... Thru sunset... VFR most locations. The exception will be along the immediate eastern MA coastline with MVFR/IFR conditions in low clouds and fog. Isolated brief shower possible in eastern MA otherwise dry conditions prevail. After sunset... Onshore winds along the eastern MA coastline shift to S and then SW which will give way to improving conditions, likely VFR by 03z or so. Otherwise VFR prevails with just a low risk of brief MVFR in isolated shower overnight. Sunday and Sunday night... VFR, dry weather and modest west wind. KBOS Terminal...Moderate confidence. Low clouds and fog at 18z will play tag with the airport thru 00z or so. Thereafter winds shift from east to south and will promote improving conditions to VFR by 03z or so. KBDL Terminal...High confidence in TAF. Outlook /Monday through Thursday/... Monday through Tuesday...High confidence. VFR. Only exception will be IFR in patchy late night fog, mainly Monday night, then improving to VFR by mid morning Tuesday. Tuesday night-Wednesday...Moderate confidence. VFR Tuesday but areas of MVFR cigs/vsbys in showers and thunderstorms late Tuesday afternoon in western sections, across the region Tuesday night and Wednesday morning, and in eastern sections Wednesday afternoon. IFR cigs possible in early morning fog/low clouds especially in areas that have had rain. Clearing in west half Wed. afternoon. Clear Wed night. Thursday... High confidence. VFR.
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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 Night/...Moderate confidence. Tonight... Onshore winds across the eastern MA waters veer to the SW after sunset and then west toward daybreak. Patchy low clouds/fog will also move offshore with the wind shift this evening and especially toward daybreak. Elsewhere, SSW winds continue with low risk of an isolated shower until about sunset then again overnight. Sunday... modest WNW winds will promote dry weather and good vsby. Sunday night... WNW winds become very light as high pres build in from the west. Dry weather and good vsby prevail. Outlook /Monday through Thursday/... Monday through Tuesday...High confidence. High pressure south of the waters. Southwest winds remain less than 15 knots Monday but may increase to near 20 knots late Tuesday. Seas less than 3 ft, but increasing to 3 to 4 ft over the outer waters late Tuesday. Tuesday night-Wednesday...Moderate confidence. A cold front approaches from the Great Lakes, crossing the waters late Wednesday or Wednesday night. Southwest winds increase with gusts 20 to 25 knots during Wednesday. Winds shift to the northwest Wednesday night. Seas build Tuesday night and Wednesday reaching 5 to 6 ft on the outer waters. The southwest flow may also nudge those higher seas into RI Sound as well. A Small Craft Advisory may be needed on some of the waters during this time. Thursday... High confidence. As high pressure to our north and west builds over the northeast states, winds will turn more to the north and northeast. But they will be diminishing to less than 10-15 knots and seas will subside to 3 ft or less.
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&& .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...Nocera/Field NEAR TERM...Nocera SHORT TERM...Nocera LONG TERM...Field AVIATION...Nocera/Field MARINE...Nocera/Field

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