Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 241959 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 359 PM EDT Wed May 24 2017 .SYNOPSIS...
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This evening will bring dry conditions with some late day sunshine. Then later tonight an approaching low pressure center delivers increasing clouds, followed by a widespread rain Thu into Fri. 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.
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&& .NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...
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355 PM update... This evening...Offshore low that affected our area earlier today continues to move east, with weak high pressure and a brief period of upper level ridging. This will allow for dry conditions thru midnight. The low pressure system centered over the lower Ohio River Valley will begin to send mid and upper level clouds our way later this evening. Overnight... Our weather after midnight will have more influence from the low pressure system approaching from the southwest, moving into Ohio. A secondary low may begin to form over the triple point closer in the Mid Atlantic region. Southern New England will see increasing clouds, with a low chance for measurable rainfall before daybreak associated with the warm front. Have slight chance to low chance pops towards daybreak, mainly south of the Mass Pike, and dry to the north. Overnight lows on the mild side, mainly low to mid 50s, but some locales may manage to dip into the upper 40s.
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&& .SHORT TERM /6 AM THURSDAY MORNING THROUGH THURSDAY NIGHT/...
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Thursday... Anomalously large and deep upper trough makes its way eastward to the eastern Great Lakes and Mid Atlantic Region. Surface low is initially in the vicinity of Ohio early Thu, however developing surface low over the triple point develops closer to the Mid Atlantic Coastline during the day and eventually becomes the primary surface low. For our area, precipitation ahead of and associated with the warm front will move in during the morning. Low level moisture convergence sets up in our vicinity. Most robust moisture looks to remain offshore, with PWATs increasing to 1 to 1.5 inches over southern New England. Could see QPF ranging from few tenths of an inch to near an inch during the day. GFS and NAM indicate dry slot reaches our area during the afternoon, which could allow for a break in more widespread precipitation until evening. Between the clouds, the rain, and an onshore steady east wind, temps are not expected to climb very much. Daytime highs mainly in the 50s. A few sites may hit 60 but Thursday will have the feel of a raw springtime day. Some areas of drizzle and fog may develop in the afternoon. Thu night... Continued good agreement in models that a potent short wave rounding the base of the high amplitude east coast trough, prompting a negative tilt. Surface low makes its way into southern New England, and lingers in our vicinity for much of the night. While periods of rain are anticipated in southern New England, models indicate the plume of deeper moisture is further offshore. GFS and NAM showing PWATs of 1 to 1.5 inches in our area for Thu night. Accompanied by decent lift, anticipating a night of soaking rainfall for our area. Thinking QPF for Thu night would be around an inch or so. NAM model soundings supportive of elevated instability, so embedded thunder is possible. That could produce allow for some localized higher precip totals. Low level inversion supports formation of low clouds and areas of fog.
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&& .LONG TERM /FRIDAY THROUGH WEDNESDAY/...
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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... 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.
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&& .AVIATION /20Z WEDNESDAY THROUGH MONDAY/...
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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 Thursday Night/...High confidence. This evening...High confidence VFR most terminals. Moderate confidence mainly VFR for Cape/Islands. Overnight...High confidence mainly VFR with easterly winds. Fog could develop across low lying terminals. Thursday...Deteriorating to MVFR/IFR cigs and vsbys in rain and fog with east wind gusting to 20 knots along the shoreline. Thursday night... IFR cigs and vsbys in rain and fog with east wind gusting to 20 knots along the shoreline. Rain moderate to heavy at times. KBOS TAF...High confidence in TAF. Easterly flow through the period. KBDL TAF...High confidence in TAF. Outlook /Friday through Monday/...moderate to high forecast confidence. 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.
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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 Thursday Night/...High confidence. Tonight...Steady easterly flow, departure of offshore low and approach of next system will allow for building seas. SCA headlines for tonight primarily for seas, on southern outer coastal waters, BI and RI Sounds. Thursday...Approaching system from the west will help increase winds and seas through the day. SCA continues where confidence is highest for seas above 5 feet. Good inversion through the day will limit gusts but anticipating some gusts around 25 kts where SCA headlines are hoisted. Thursday night... Poor vsbys in rain and fog Thursday night. Easterly winds around 25 knots eastern coastal waters with strongest winds during the evening. These winds then lift north of the waters. Seas building up to 10 feet across the Cape Ann waters, less elsewhere. SCA headlines have been extended thru Thu night for outer coastal waters. Outlook /Friday through Monday/... 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.
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&& .TIDES/COASTAL FLOODING...
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Combination of a very high astronomical tide and a .7 to 1.0 foot storm surge will likely result in minor coastal flooding about the time of the late Thursday night high tide along much of the MA east coast. The Salisbury to Cape Ann stretch of coastline is most at risk, since the low level ENE jet would have just moved north of this area just a couple of hours before high tide with a moderately significant NE wave fetch in place at the time of the high tide. However, the expected total water level looks to be high enough to support areas of minor coastal flooding along the coastline south of Gloucester to Plymouth as well as Provincetown. Thus, it may become necessary to issue a Coastal Advisory for much of the MA east coast including Cape Cod for the late Thursday night high tide. Waves will not be a major factor but looks like enough low level wind gradient to produce 6 to 9 foot waves in the near shore waters in Ipswich Bay, about Cape Ann, and Massachusetts Bay. Some erosion is likely along the Salisbury and Plum Island shorelines where wave action will be somewhat more significant. Elsewhere along the MA and RI coasts, the combination of the high astronomical tide and a tidal departure near a half foot may be enough to cause pockets of minor coastal flooding as has become more common during such king tide cycles. A statement may be needed for the Islands, and both the MA and RI south coasts, including Narragansett and Buzzards Bay shorelines, for the Thursday evening/night high tides. Given that we are expecting no impacts beyond minor coastal flooding, we will hold off on headlines for now and let the next shift have the opportunity to refine the coastal impact forecast and issue any advisories/statements deemed necessary. Little to no storm surge is expected at the time of the late Friday night or very early Saturday AM high tide, but that astronomical tide by itself may be sufficiently high to produce isolated pockets of minor coastal flooding and possibly worthy of a statement as that time draws nearer.
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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from 8 PM this evening to 6 PM EDT Friday for ANZ235-237. Small Craft Advisory from 6 AM Thursday to 6 PM EDT Friday for ANZ250. Small Craft Advisory from noon Thursday to 6 PM EDT Friday for ANZ251. Small Craft Advisory until 6 PM EDT Friday for ANZ254>256.
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&& $$ SYNOPSIS...Nocera/NMB NEAR TERM...NMB SHORT TERM...NMB LONG TERM...Nocera AVIATION...Nocera/NMB MARINE...Nocera/NMB TIDES/COASTAL FLOODING...RMT

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