Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 291058 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 658 AM EDT Mon May 29 2017 .SYNOPSIS...
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A fast moving low pressure system will bring a period of showers to the region today, however a washout is not expected. Onshore winds, clouds and showers will result in unseasonably cool temperatures today. A couple of weak cold fronts may trigger scattered showers and possibly thunderstorms for the remainder of the week from to time to time. Otherwise fair weather with near seasonable temperatures are expected into next weekend.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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7 AM update... Two areas of rain this morning, first area is across NY state and is associated with parent low over the eastern Great Lakes. Second area of rain moving onshore to the CT/RI coastline including Marthas Vineyard and Nantucket. This area of rain is associated with a coastal low off the DE shore. SPC mesoanalysis shows lot of dry air at 925 mb and 850 mb over New England. Therefore the northern edge of the rain shield along the south coast will erode somewhat as it tracks northeast. Updated hourly pops to reflect this thinking. Expecting rain shield to become more fragmented this afternoon as short wave trough deamplifies as it climbs up and over the east coast ridge. Otherwise previous forecast verifying nicely. Temp not rising much today from current morning values as clouds, onshore winds and showers will keep temps only in the 50s. Normal high temp for today should be around 70. Thus much cooler than normal. Earlier discussion below. ====================================================================== A chilly and damp Memorial Day will be in store for southern New England today. Coastal low off the Mid-Atlantic coast will continue to develop and move eastward for today. This low will strengthen as it passes south of Long Island and SNE. Meanwhile, an upper level trough will pass to the west. The combination of these two systems will bring scattered to widespread showers for today. Latest guidance has trended downward in qpf amounts as coastal low has trended farther south than previous guidance. This may actually cut off some of the moisture for the western system. Thus, locations across eastern Massachusetts may experience just showers than heavy rainfall. The chances for thunder also seems slim as the region remains on the cool side of guidance. The bulk of the rainfall will move through the region late this morning into the afternoon hours. 700 mb dry slot will swing through during the later half of the day resulting in more drizzle than rainfall this evening. Temperatures for the region will remain well below average as Maritime airmass spread westward. This easterly flow combined with cloud cover will result in temperatures in the mid 50s.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM TUESDAY/... Tonight... Shortwave and deeper moisture will have exited the region by this evening. However, moist northeast low level flow will probably allow scattered light showers/drizzle to persist into the overnight hours. Highest confidence is across eastern MA but cannot rule out for points west due to upslope flow. Aside from drizzle, anticipate areas of low clouds and patchy fog to persist as low level moisture may remain trapped below the inversion. Low temps will mainly be in the upper 40s to the lower 50s. Tuesday... Weak ridging aloft on Tuesday will keep the region in southwesterly flow as weak surface high pressure moves south of the region. The area will remain mostly dry but cannot rule out a few isolated showers out west. A quick moving shortwave will pass through the flow during the day which could trigger showers and perhaps a thunderstorm or two. Highest confidence is out west where better instability resides. Some hi-res guidance supports the storms that develop in update NY will moves into the region by late afternoon/evening. This is in conjunction with SPC marginal risk. Tricky temperature forecast for Tuesday as 850 mb temps support warmer conditions then what guidance is indicating. However 925 mb temps show that the Maritime airmass will remain locked in over SNE until the later half of the day. Despite some heating in a mix of sun and clouds, temperatures will remain in the upper 60s to low 70s. && .LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/... Highlights... * Temperatures near normal this period with summer heat and humidity remaining suppressed well south of New England * Several episodes of showers but also many hours of dry weather too Synoptic Overview... Both GFS and EC ensembles support a slow moving omega block pattern moving across Canada this week into next weekend, resulting in cyclonic flow and below normal heights over New England. This suppresses the subtropical ridge across the southern states, precluding summer heat and humidity from advecting northward into New England. Thus temps here close will be near normal this period for southern New England. As for precip, cyclonic flow into the northeast will yield several short wave troughs and attending cold fronts sweeping across the region resulting in a few chances for showers and thunderstorms, but also many hours of dry weather too. Above average forecast confidence on the overall pattern/theme but high uncertainty on timing each individual short waves/fronts and associated timing of shower threat. Daily details... Tue night... Short wave trough accompanied by modest forcing for ascent coupled with sufficient deep layer moisture to support numerous showers, especially late evening and overnight. South to Southeast winds will result in onshore winds and weak low level warm advection. Thus temps will not be as cool as previous nights. Wednesday... Warmer than today and Tue with highs well into the 70s as maritime airmass moves offshore. Deep layer moisture and forcing for ascent over the area in the morning resulting in a risk for showers. However by late morning and especially the afternoon K indices fall rapidly indicating mid level dry beginning to stream across the region and dry the column from the top down. This drier air aloft does steepen mid level lapse rates however it appears the dry air becomes too deep to support convection. Meanwhile at the surface dew pts remain somewhat elevated in the MU50s to near 60. This yields MUcapes of 1000-1500 j/kg. Although deepening mid level dry air will likely preclude convection from developing. However still 2+ days away so will have to monitor later trends. Otherwise models in good agreement on a fairly nice day with the trend being morning showers giving way to partial afternoon sunshine. Thursday... Good model agreement that short wave trough exits east Wed night with slightly cooler and drier air post frontal airmass for Thu with dew pts falling from the 50s Wed to the 40s Thu. Seasonably warm with highs 70-75. Could be the pick of the week. Friday... WAA pattern ahead of next short wave trough with a risk of showers during the day and/or at night. Obviously model timing differences at this time range. Weekend... Surprisingly both deterministic and ensemble guidance in good agreement for the start of next weekend with mean mid level trough axis over or east of the region. This would suggest dry weather to start the weekend. However models differ on how quickly the northeast trough reloads with upstream jet energy. Thus Sunday`s forecast becomes highly uncertain. Temperatures look to be near normal next weekend...highs in the 70s and lows in the 50s. && .AVIATION /12Z MONDAY THROUGH FRIDAY/...
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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 Tuesday/... 7 AM update... IFR cigs over eastern MA and RI will continue to move westward into CT and western-central MA today. Rain overspreads the region from the south and west today. Not much change from previous TAFs. Earlier discussion below. =============================================================== Today and tonight...Moderate confidence. MVFR to IFR conditions overspread the region through the morning and into the early afternoon in a period of rain. Easterly wind gusts around 20 knots are expected on the coast, with up to 25 knots across portions of the Cape/Islands. The rain will taper off to light showers/drizzle from west to east late this afternoon/early evening. However, MVFR-IFR conditions may persist for much of Mon night with the lowest conditions most likely on the coast. MVFR on Tuesday with a few sites reaching VFR. Isolated shower or two is possible, with a better chance for showers/thunder across western terminals by the evening. KBOS TAF...Moderate confidence in TAF. KBDL TAF...Moderate confidence in TAF. Outlook /Tuesday Night through Friday/...Moderate confidence. Generally VFR. Potential for MVFR cigs/vsbys in afternoon/evening showers and scattered t-storms. Best chance for the latter will be in Western/Central Mass and Northern Connecticut Tue night into Wed morning. Thursday looks to be mainly dry with another risk for showers and isolated thunder Fri/Fri night. South winds Tuesday night, becoming southwest Wednesday, west Thursday, and southwest Friday. All winds should be less than 20 knots.
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&& .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tuesday/... Today...Moderate to high confidence. High pressure building across the Canadian Maritimes coupled with a wave of low pressure passing to our south will generate easterly wind gusts around 25 knots for the waters. Thus will continue with SCA. Easterly fetch will also build seas between 3 and 6 feet across our open waters. The strongest wind gusts/seas will be across our southern waters. Rain and patchy fog is also expected to develop and an isolated t-storm or two can not be ruled out across our southern waters. Tonight...Moderate to high confidence. Winds and seas will begin to diminish this evening and especially after midnight as disturbance moves away from the region and pressure gradient weakens. Areas of fog may persist into early Tue am reducing vsbys for mariners. Tuesday...High confidence. Improving conditions as weak high pressure moves over the southern waters. Could see areas of fog or drizzle but seas and winds will remain below SCA. Outlook /Tuesday Night through Friday/...Moderate confidence. Tuesday night through Friday... Winds less than 25 kt most if not the entire forecast period. Vsby may be restricted Tue night into Wed with scattered showers and isolated thunder possible. Dry/tranquil likely beginning Wed afternoon into Thu then another round of showers and isolated thunder possible Fri/Fri night. && .TIDES/COASTAL FLOODING... Latest observations continue to show a storm surge just over a half foot along the eastern MA coast for this current high tide. For tonight, another high astro tide of 11.5 feet occurs just after 3 am in Boston. While that is lower that previous nights, the onshore component will remain with a storm surge of a half foot or even a bit more. While no significant problems are expected, very minor nuisance coastal flooding is possible along the most vulnerable shore roads along the eastern MA coast. Will hold off on coastal flood statement at this time. && .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 8 PM EDT this evening for ANZ231>235-237. Small Craft Advisory from 11 AM this morning to 2 AM EDT Tuesday for ANZ250. Small Craft Advisory until 2 AM EDT Tuesday for ANZ254>256.
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&& $$ SYNOPSIS...Nocera/Dunten NEAR TERM...Nocera/Dunten SHORT TERM...Dunten LONG TERM...Nocera AVIATION...Nocera/Dunten MARINE...Nocera/Dunten TIDES/COASTAL FLOODING...

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