Area Forecast Discussion
Issued by NWS Boston, MA

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297 FXUS61 KBOX 270210 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1010 PM EDT Fri May 26 2017 .SYNOPSIS... 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. && .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/... 10 pm update... Comma-head occlusion continues to peel off to the E round which there is some emphasis of boundary-layer focus of airmasses brought on by mid-level lobing energy round the backside of the low. Broken to overcast low cloud decks lingering along with scattered shower activity. This as cooler air surges rearward and onshore as discerned via latest radar imagery. Temperatures dropping towards their respective wet-bulb, it is behind the cool-frontal surge that patchy fog and drizzle development are possible, especially over E MA. But some improvement on the horizon. Heights rising as weak mid-level ridging builds in from the W. Building high pressure along with drier air, should see showers dissipate E as we go through the overnight hours. Winds should relax becoming more northerly with time. Lows bottoming around the upper 40s to low 50s. && .SHORT TERM /6 AM SATURDAY MORNING THROUGH SATURDAY NIGHT/... 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. && .LONG TERM /SUNDAY THROUGH FRIDAY/... 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. && .AVIATION /02Z SATURDAY THROUGH WEDNESDAY/... 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. 2z update... Tonight... MVFR CIGs mainly with low risk IFR, while an improving trend from the W with low-end VFR CIGs. Lowest conditions across E MA with the threat of DZ/FG as winds have turned NE onshore. Should see improving conditions W to E overnight, diminishing winds becoming northerly. SHRA activity concluding. 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...NE flow persisting as a cool front has pushed onshore. Should see NE winds diminish towards midnight becoming more N overnight. The brief period of IFR should be just that, the rest of the overnight period MVFR. KBDL TAF...BKN low-end VFR CIGs persisting. N flow. Outlook /Sunday through Wednesday/...Moderate confidence. Sunday... VFR. Light S 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. SE winds less than 20 kts. RA/SHRA developing during the morning and midday, with potential SCT TSRA in the afternoon into evening. Areas of fog at night. Tuesday-Wednesday... IFR Tuesday morning, improving to VFR Tuesday afternoon through Wednesday. SCT SHRA, mostly during the daytime. Light E wind Tuesday becoming S in the afternoon, then W/SW Wednesday. Speeds less than 15 kts. && .MARINE... 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. 10 pm update.... Tonight... Have modified headlines to account for hazardous seas which persist on the outer waters and S sounds of RI and Block Island. With high pressure building in from the W, should see mainly N winds which will be light allowing seas to diminish further into the Saturday morning timeframe. 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. && .TIDES/COASTAL FLOODING... 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. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory for hazardous seas until 10 AM EDT Saturday for ANZ235-237. Small Craft Advisory for hazardous seas until 6 AM EDT Saturday for ANZ250. Small Craft Advisory for hazardous seas until noon EDT Saturday for ANZ254>256. && $$ SYNOPSIS...WTB/KJC NEAR TERM...WTB/KJC/Sipprell SHORT TERM...KJC LONG TERM...WTB AVIATION...WTB/KJC/Sipprell MARINE...WTB/KJC/Sipprell TIDES/COASTAL FLOODING...WFO BOX Staff

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