Area Forecast Discussion
Issued by NWS Boston, MA

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518 FXUS61 KBOX 261955 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 355 PM EDT Fri May 26 2017 .SYNOPSIS...
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Low pressure moves offshore tonight followed by weak high pressure this weekend which will bring mild days and cool nights. Low pressure in the Great Lakes generates a coastal low late Monday that passes off Nantucket on Tuesday. This will bring wet weather on Memorial Day. Daytime clouds and scattered showers will pop up each day Tuesday through Friday.
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&& .NEAR TERM /THROUGH TONIGHT/...
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140 PM update... Comma head showers continue to rotate south into SNE, focused mainly across central/W MA into CT/RI within defined trowal signature. These showers will decrease toward evening as the low pulls away but a few showers will likely linger into the early evening. Flow gradually becomes anticyclonic tonight as weak high pres builds in from the west so expecting a mainly dry night. Plenty of clouds will persist given presence of low level moisture which will limit temp drop. Mins will range from the upper 40s to lower 50s.
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&& .SHORT TERM /SATURDAY THROUGH SATURDAY NIGHT/...
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Saturday... Weak high pres builds across the region but low level moisture remains abundant below the inversion which indicates clouds will dominate with bkn-ovc cloud cover. Mid level shortwave approaches from the west by late in the day which will also bring increasing mid level clouds and can`t rule out a brief shower toward evening across CT. Soundings show mixing to about 925 mb with highs mostly in the 60s with a few lower 70s possible lower CT valley. Seabreezes expected in the afternoon which may hold temps in the upper 50s along east coastal MA. Saturday night... A brief shower possible from CT to southern RI in the evening as shortwave passes to the south, otherwise dry weather as weak high pres remains in control. Mostly cloudy skies persisting. Mins mid 40s to lower 50s.
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&& .LONG TERM /SUNDAY THROUGH FRIDAY/...
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Big Picture... Longwave scale maintains a ridge west/trough east pattern for much of the long term period. Shortwave scale shows a closed low slowly moving from Canada across the Great Lakes, eventually reaching New England late in the week. Two jets in place for much of the period. The northern jet sweeps around the closed low from Western Canada across the Northeast USA. The southern jet swings from Baja into the Southern Plains and Southeast USA. The two streams interact over the Ohio Valley and Mid Atlantic early next week. Model mass fields are similar through Tuesday...the GFS and ECMWF are similar through Thursday. GFS and ECMWF moisture and thermal fields are similar during the same period while showing differences in detail. This brings increased confidence in the overall pattern through much of next week, although lingering uncertainty regarding timing of individual shortwaves in the flow. Details... Sunday... High pressure is in control Sunday with subsidence and light wind flow. Moisture fields show an area of higher RH below 800 mb and suggests a cloud/sun mix with the fair weather. Temperatures in the mixed layer are equiv to 5-8C, suggesting max temps upper 60s to mid 70s. Sea breezes along the coast will hold temps there in the 60s. Fair skies linger early in the night, but the next approaching shortwave supports increasing clouds...expect this overnight. Dew points will be roughly 45-50 which with light flow and initial fair skies should allow temps to reach the lower 50s and possibly the mid 40s. Monday-Tuesday... Interaction between the northern and southern streams leads to a 120- knot jet over the Ohio Valley which races east ahead of northern stream shortwave during Monday. This crosses the surface cold front and generates a triple-point low as it crosses the Mid Atlantic coast. This in turn spins up a weak southerly low level jet that aims for Srn New England during the afternoon/evening. Precipitable water values reach 1.0-1.2 inches which is above normal but not excessive. Model consensus slows some slowing of the onset of measurable pcpn in Southern New England. The GFS holds off on measurable until after 12Z Monday while the GGEM and ECMWF bring it to the CT Valley by 12Z and possibly a little farther. We opted to slow pcpn onset to a compromise time with chance pops to the CT Valley around 09z and to Worcester and the RI Hills 12Z...then to most of Eastern MA by 14Z. We will bring likely pops to the western sections for late morning/early afternoon and for eastern sections for the afternoon. Stability parameters are somewhat favorable for convection, with totals around 50 while surface LI values will be near zero. We will include scattered to isolated tstms Monday afternoon/evening. The coastal low passes offshore Tuesday morning. Expect leftover showers diminishing Tuesday morning, then expanding again as another shortwave moves through. Wednesday through Friday... Upper low slowly sweeps across Eastern Canada during the period, with shortwaves moving through the flow across New England. Cold pool advection aloft will destabilize the airmass and support scattered daytime showers, especially in areas north of the Mass Pike. Temperatures in the mixed layer will support max temps upper 60s to mid 70s each day. Fair skies and dew points in the 50s will support mins in the 50s each night...possibly cooling to mid 40s to low 50s late in the week.
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&& .AVIATION /19Z FRIDAY 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/...Moderate Confidence. Through 00z...Mix of VFR/MVFR cigs will trend back to mostly MVFR with pockets of IFR Cape/Islands. Showers continue across central/W MA into CT/RI before diminishing toward evening. NW gusts to 20-25 kt Cape/Islands. Tonight...Mostly MVFR cigs, improving to VFR in central and western New Eng. Diminishing NW winds. Saturday and Saturday night...Mix of VFR/MVFR cigs. Seabreezes developing late morning into the afternoon. Low risk for a brief shower late Sat/Sat evening south of the Mass Pike. KBOS TAF...Moderate confidence. KBDL TAF...Moderate confidence. Outlook /Sunday through Wednesday/...Moderate confidence. Sunday... VFR. Light south wind becoming onshore on the Mass East Coast by midday. Monday... VFR at first, lowering to MVFR in the afternoon and IFR/LIFR at night. Southeast winds less than 20 knots. Rain/showers developing during the morning and midday, with potential scattered tstms in the afternoon/evening. Areas of fog at night. Tuesday-Wednesday... IFR Tuesday morning, improving to VFR Tuesday afternoon through Wednesday. Scattered showers, mostly during the daytime. Light east wind Tuesday becoming south in the afternoon, then west-southwest Wednesday. Speeds less than 15 knots.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 Saturday Night/...High confidence. Through early evening...N/NW gusts to 25 kt possible across eastern MA waters through this afternoon. Tonight...Winds will diminish but southerly swell will maintain 5-7 ft seas over the south coastal waters with seas subsiding below 5 ft over eastern waters. Saturday...Winds becoming E/SE with speeds below 15 kt. Seas will subside below 5 ft over southern waters. Saturday night...Quiet weather with light winds and seas below SCA. Outlook /Sunday through Wednesday/...Moderate confidence. Winds less than 20 knots and seas less than 5 feet through the period. Showers and isolated tstms Monday, showers and fog Monday night and early Tuesday.
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&& .TIDES/COASTAL FLOODING...
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Still observing a 0.5 to 1 ft surge along both coasts which ESTOFS and ETSS guidance is underestimating. Surge expected to diminish to around 0.2 to 0.5 ft at the time of high tide later tonight as winds and swell diminish. High tides this evening are quite high (Boston 12.41 feet around 1230 AM, Providence 6.45 feet around 930 pm), Even with minimal wave action, total water level above 12.5 ft in Boston typically produces very minor inundation for the notorious low-lying locations (i.e, Morrissey Boulevard in Boston). In addition, minor inundation possible along the south coast if surge still around 0.5 ft. We will issue a coastal flood statement to address these concerns.
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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 7 PM EDT this evening for ANZ231- 232. Small Craft Advisory until 10 AM EDT Saturday for ANZ235-237. Small Craft Advisory until 6 AM EDT Saturday for ANZ250. Small Craft Advisory until 8 PM EDT this evening for ANZ251. Small Craft Advisory until noon EDT Saturday for ANZ254>256.
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&& $$ SYNOPSIS...WTB/KJC NEAR TERM...KJC SHORT TERM...KJC LONG TERM...WTB AVIATION...WTB/KJC MARINE...WTB/KJC TIDES/COASTAL FLOODING...

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