Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 241401 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1001 AM EDT Wed May 24 2017 .SYNOPSIS... A wave of low pressure tracks south of New England this morning bringing a period of light rain or sprinkles to the south coast. Behind the departing low drier air overspreads the area this afternoon and tonight along with late day sunshine. A more potent low pressure center delivers a widespread rain Thu and Fri along with cooler temperatures. The rain will likely be heavy at times Thu night into Friday morning as the low intensifies. The holiday weekend gets off to a dry start Saturday followed by the risk of showers late Sunday into Sunday night, then a drying trend possible Monday. Expect mild days and cool nights each of the three days. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 10 AM update... Weak surface low was heading northeastward, near the 40N/70W benchmark at 10 am. Continuing to see ample mid and high deck clouds over southern New England, with some light precip mainly over northeast CT, RI and southeast MA...and some patchy light rain in MA south of the MA Pike. This should be the northward extent of the light rain. After the surface wave passes near the 40N/70W benchmark, it will pull away from our area and out to sea. Subsidence behind this parting wave will prompt a drying trend this afternoon, with a mix of clouds and sun. The onshore east/northeast winds will keep temps cooler in the coastal plain, especially across Cape Cod and the Islands where ENE winds will pick up as the surface wave intensifies while it moves further offshore. Early morning discussion follows... ====================================================================== Overall trend in the forecast remains on track early this morning. Surface low off the coast of Delmarva will continue its northeasterly track towards the 40N/70W Benchmark. Still a spread in the guidance on precipitation shield for the near term. Currently believe that the 00Z NAM is an outlier and not accounting for the potential Fgen that is starting to set-up per SPC meso page. Thus anticipate the precip will move into the region early this morning esp across locations south and east of BDL to BOS. The Cape and the Islands have the best shot of steadier rainfall, with light showers/sprinkles elsewhere as the mid-levels are still quite dry. Precipitation chances will begin to wind down by the mid-morning hours with a few lingering showers across the Cape/Islands around 18z. Improving conditions will occur from west to east through the day. Consistent easterly flow will keep temperatures near or below seasonable especially across eastern MA/RI. If clouds break out this afternoon then temperatures could quickly warm into the mid 70s. Highest confidence is across the CT River Valley. However, if clouds remain steady then daytime highs will linger in the low 70s. Overall, a damp morning will lead to a dry afternoon with near seasonable conditions. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM THURSDAY/... Tonight... Weak mid to upper level ridging will pass through the area this evening. Dry weather to start, with precip chances increasing closer to dawn tomorrow. Mainly cloudy conditions will keep temperatures in the low to mid 50s. Thursday... Anomalously large and deep upper level trough and associated upper low will slide eastward into the Ohio/Tennessee River Valleys by morning. An upper level shortwave and associated mid-level warm front associated with this trough will begin to move through the area during the first half of the day. This frontal system will trigger widespread showers across the area., which could be heavy at times as PWAT values increase to near 1.5 inches thanks to the sub- tropical connection. Still a spread in the guidance on where the heaviest precip will occur. The GFS/RGEM keeps the bulk of the precip offshore while the Hi-res ARW and NAM pushes the heavy precip over the I-95 corridor. Despite on where the heaviest QPF will align, it appears a dreary morning will be on tap. During the later half of the day, better dynamics will be north of the region. Resulting in just showery weather as dry slot begins to push into the area. Latest soundings indicate that a good low level moist inversion will set-up during Thursday afternoon resulting in areas of fog and drizzle for the remainder of the day. Persistent easterly winds through the day will not only help increase the low level moisture, but also keep temps from warming. Blended the forecast with MAV/MET guidance which keeps temps in the mid to upper 50s. A few sites may hit 60 but it will be a damp, dreary day on Thursday. && .LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/... Highlights... * Widespread rain Thu night/Fri morning with locally heavy downpours * The holiday weekend starts dry Sat but then the risk of showers late Sunday into Monday, but a washout not expected * The holiday weekend will feature mild days and cool nights Details... Thu night... Good agreement among both deterministic and ensembles that potent short wave rounding the base of the high amplitude east coast trough takes on a negative tilt and captures a plume of tropical moisture from the lower latitudes northward up the eastern seaboard into New England. This also induces secondary low pres south of New England. Very strong jet dynamics with mid levels closing off combined with subtropical moisture will result in a period of heavy rain Thu night into Fri morning, especially as comma-head forms. The 12z EPS and GEFS both support modest probs of 1+ inches of rainfall with highest probs over northeast MA. Many of the global models have 1+ inches of rainfall in 9 hrs from about 06z-15z. It will be a wind swept rain for eastern MA as low level onshore jet sweeps across this region from south to north. Friday... A period of heavy rain is likely (esp in the morning) in response to trough amplification as negatively tilted closed low moves across the region. As mentioned above, very strong jet dynamics coupled with subtropical moisture will result in comma-head rains, heavy at times yielding nuisance poor drainage highway/street flooding, enough to impact/slow down the Fri morning commute. Temps will be cooler than normal given overcast conditions and periods of rain. Holiday Weekend... Occluded low exits into the Maritimes yielding rising heights and anticylonic flow across New England Saturday. This supports dry weather and seasonably warm conditions (70-75), although cooler across eastern MA with afternoon seabreezes. By late Sun next short wave trough approaches. This will result in clouds increasing Sun esp in the afternoon along with the chance of a few showers late Sun and Sun night, possibly lingering into Monday but by no means a washout. Temperatures will be near normal featuring mild days and cool nights. Tuesday... Much of the ensemble and deterministic guidance suggest deep southwest flow into New England with mid level trough over the Great lakes. This warm advection pattern should support temps warmer than normal and possibly dry weather given how far west position of mean trough axis. && .AVIATION /14Z WEDNESDAY THROUGH SUNDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Thursday/...High confidence. 7 AM update... Not much change from previous TAFs, light rain moves across CT/RI and southeast MA but cigs and vsbys remain in the VFR cateogory. Dry weather prevails across northern MA. ENE winds begin to increase across the Cape and islands later today. Earlier discussion below. =================================================================== Today...Showers south and east of BDL to BOS line will push eastward through the day. Any MVFR/IFR cigs will quickly improve to VFR by the afternoon. Tonight...VFR with easterly winds. Fog could develop across low lying terminals. Thursday...IFR cigs and vsbys in rain and fog with east wind gusting to 20 knots along the shoreline. KBOS TAF...High confidence in TAF. Easterly flow through the period. KBDL TAF...High confidence in TAF. Outlook /Thursday Night through Sunday/...moderate to high forecast confidence. Thursday night... IFR cigs and vsbys in rain and fog with east wind gusting to 20 knots along the shoreline. Rain heavy at times late. Friday...IFR/LIFR in heavy rain during the morning, then conditions improve to mixed VFR/MVFR in showers as winds shift from the northwest. Saturday... VFR. Diminishing northwest wind. Sunday... VFR early, trending to MVFR/IFR in showers and fog by Sunday night. Winds from the southeast. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Thursday/...High confidence. Today and tonight...Coastal low will move northeastward across the Benchmark this morning followed by high pressure during afternoon lasting into the overnight hours. This gradient between the high and low will increase wind gusts to 20 kts by the late morning hours. Seas will begin to build in response. Passing showers this morning will limit vsbys. Thursday...Approaching system from the west, will help increase winds and seas through the day. SCA have been extended where confidence is highest for seas above 5 feet. Good inversion through the day will limit gusts but can`t rule out a few reaching 25 kts. Outlook /Thursday Night through Sunday/... Thursday night - Friday... Poor vsbys in rain and fog Thursday night. Easterly winds around 25 knots with strongest winds during the evening. These winds then lift north of the waters tonight. Seas building up to 10 feet across the Cape Ann waters, less elsewhere. Low pressure moves past the waters Friday morning, at which point the winds shift from the NNW with speeds 20 knots or less. Expect seas 5-7 feet on the outer waters and RI Sound. Small Craft Advisory may be needed on some of the waters both days. Saturday-Sunday... Winds remain below 25 knots through the period. Seas subside below 5 feet early Saturday. Seas may return to 5 feet on the outer waters late Sunday night. Poor vsbys possible in showers and fog late Sunday and Sunday night. && .TIDES/COASTAL FLOODING... High astronomical tides will occur over several tide cycles along eastern Massachusetts coast from Thursday into Memorial Day, with tides around 12 feet in Boston and 4 feet on Nantucket. At the very least, minor nuisance flooding will occur in the most vulnerable locations such as Morrissey Blvd in Boston. Any surge on top of these tides would lead to more widespread, but minor, coastal flooding, including on Nantucket. Right now, the greatest risk for widespread minor coastal flooding is the Thu night/early Fri midnight high tide, which is 12/26 ft at Boston, due to expected onshore winds and surge. Model surge guidance (ESTOFS) shows a potential 0.7 ft surge, which would give a storm tide near 13 feet Thursday night in Boston and just under 5 ft on Nantucket. In addition waves up to 10 ft are possible just offshore with highest seas and greatest surge along the Cape Ann coastline to the NH border. This should be sufficient for widespread minor coastal flooding with a low risk of isolated areas of approaching low end moderate flooding across Cape Ann coastline to MA/NH border. Elsewhere minor coastal flooding is likely however the severity is expected to be much less. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from 8 PM this evening to 9 PM EDT Thursday for ANZ235-237. Small Craft Advisory from 6 AM to 9 PM EDT Thursday for ANZ250. Small Craft Advisory from 11 AM to 9 PM EDT Thursday for ANZ251. Small Craft Advisory from 2 PM this afternoon to 9 PM EDT Thursday for ANZ254>256. && $$ SYNOPSIS...Nocera/Dunten NEAR TERM...Nocera/Dunten/NMB SHORT TERM...Dunten LONG TERM...Nocera AVIATION...Nocera/Dunten MARINE...Nocera/Dunten TIDES/COASTAL FLOODING...

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