Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 192345 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 745 PM EDT Sat Aug 19 2017 .SYNOPSIS... 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. && .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
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740 PM update... Light northeast winds have been persistent with some fog in Boston Harbor. Forecasting winds to become light then southwest, with fog dissipating there. With high dewpoints, starting to see fog developing on Nantucket and parts of the south shore. Will need to watch this closely for a possible Dense Fog Advisory. Otherwise, strong thunderstorms stretched from northeast PA through Binghamton and to just west of Albany. The NAM and HRRR are handling this area quite well. The 18Z NAM has a coherent band of 38K, 50 Total Totals, and -3 Showalter Indices in that area now and progressing steadily across western MA around 03Z, central MA around 06Z and exiting northeast MA around 09Z. Somewhat unstable in northern CT as well. The NAM and HRRR have 60-80 percent PoPs in these areas tonight. As a result, have updated the grids to include a 50 percent chance of showers and thunderstorms in western sections tapering to a 30 percent chance in eastern and northeastern sections overnight. Previous discussion... 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.
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&& .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/... 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. && .AVIATION /00Z SUNDAY 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/... 2340Z 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. 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. However, could see development of IFR in fog along the south coast, including the islands due to high dewpoints. There is a chance of showers and thunderstorms spreading into western sections around 03Z, central sections by 06Z, and weakening and exiting northeastern sections by 09Z. Could have MVFR CIGS and VSBYS in any showers/thunderstorms. Sunday and Sunday night... VFR, dry weather and modest west wind. KBOS Terminal...Moderate confidence. Low clouds and fog will dissipate around 02-03Z as winds become light southwest. KBDL Terminal...High confidence in TAF. Chance of a shower or thunderstorm for a few hours from roughly 04-07Z. 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 areas of fog possibly redeveloping along the south coast. Chance of a shower late tonight, especially along the north coast. 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...Field/Nocera SHORT TERM...Nocera LONG TERM...Field AVIATION...Field/Nocera MARINE...Nocera/Field

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