Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 242323 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 723 PM EDT Wed May 24 2017 .SYNOPSIS... Dry weather will continue most of the night. 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. Weak high pressure will bring a dry start to the holiday weekend Saturday and early Sunday. Another coastal low brings more wet weather late Sunday into early Monday. Leftover showers are possible through the first half of the week. && .NEAR TERM /UNTIL 6 AM THURSDAY MORNING/... High pressure over Newfoundland will maintain dry weather for much of the night. Meanwhile the right entrance region of the upper jet is moving over a portion of the Midwest/Midsouth cold front near its triple point, where the pressure pattern is baggy. Expect a secondary low to develop in this area as it swings east. This will bring increasing clouds, with a low chance for measurable rainfall toward daybreak associated with the warm front. We have slowed the onset of pcpn a couple of hours toward daybreak, with slight chance to low chance pops mainly south of the Mass Pike while areas to the north remain dry. Overnight lows on the mild side, mainly low to mid 50s, but some locales may manage to dip into the upper 40s. && .SHORT TERM /6 AM THURSDAY MORNING THROUGH THURSDAY NIGHT/... 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. && .LONG TERM /FRIDAY THROUGH WEDNESDAY/... Big Picture... The longwave scale retains cyclonic flow over the Eastern USA much of the period while a ridge builds over the Western USA. Vertically stacked closed low over Southern New England moves northeast toward the Maritimes Friday. Upper flow evolves northern and southern jets over the weekend. Closed low moved along by the northern flow will dip south from Canada into the Great Lakes over the weekend. Shortwave in the southern flow ejects northeast from the Southwest USA. Eventually the southern jet forms a coastal low off the Mid Atlantic states Sunday night that moves up the coast past our area Monday. The northern stream closed low remains stalled over the Great Lakes through the early week, with signs of it breaking down around Wednesday. Upper mass fields remain similar through Saturday. The models agree on the concept of a closed low moving from Canada Sunday to the Great Lakes Monday and Tuesday, but differ on placement. This in turn brings differences with the interaction with the southern jet and shortwave. All of this leads to higher confidence for the late week and lower confidence for early next week. Details... Friday... Low pressure over our area, surface and aloft. Forecast details will depend on the small scale placement of the surface low, but expect lingering wet weather leading the low, but diminishing as the low lifts northeast into the Gulf of Maine. Cold conveyor moisture will wrap around the low, and the cyclonic curvature will support lift. We will keep showers in the forecast much of the day, although less coverage than at the start of the day. The low moves off Friday night. Expect any remaining showers to taper off and partial clearing. Saturday-Sunday... High pressure brings a day of rest from the wet weather. Temps of 8C to 9C at 850 mb suggests max temps in the low to mid 70s. A light southeast surface flow will occur along the east Mass coast and hold max temps there in the 60s. Weak high pressure lingers through Sunday morning, but moves offshore during the day. Expect increased mid and high clouds Saturday night and Sunday. Dew points Saturday night will be around 50...so expect min temps near 50. Temperatures aloft warm a degree by Sunday, but also expect more clouds than on Saturday. Will forecast max temps similar to Saturday. Sunday night through Wednesday... Low pressure in the Great Lakes. Southern stream shortwave reaches the Mid Atlantic coast and generates surface low. Models show a 30- 35 knot south low level jet feeding up into our area Sunday night, then sweeping east on Monday. Expect a period of rain showers Sunday night and Monday. Low pressure lifting into Canada swings a cold front through the region Monday night or early Tuesday, maintaining a chance of showers until passage. Upper trough swings across about Wednesday bringing a chance of showers, mainly in Northwest Mass. && .AVIATION /23Z WEDNESDAY THROUGH MONDAY/... 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...High confidence mainly VFR with easterly winds. We are monitoring Nantucket for potential low ceilings in stratus overnight. Fog patches 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... Initial conditions IFR, but improve to mixed VFR/MVFR in showers as winds shift from the northwest. Saturday and Sunday daytime... VFR. Diminishing northwest wind Saturday. Light wind Sunday. Sunday night-Monday... Cigs/vsbys trending to MVFR/IFR in showers and fog. Winds from the southeast, trending from the south Monday. && .MARINE... 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... Low pressure center crosses coastal Massachusetts Friday morning and then exits northeast through the Gulf of Maine. Easterly winds around 25 knots leading the low center will shift from the north behind the low with gusts near 25 knots possible. Seas will maintain at 5-7 feet on the outer waters and the exposed nearshore waters such as Mass Bay and RI Sound. Small Craft Advisories will continue in these areas. Winds and seas diminish Friday night. Saturday-Sunday... Weak high pressure moves over the waters. Winds will be 20 knots or less both days. Seas on the outer waters will be around 5 feet on Saturday, otherwise seas should be less than 5 feet both days. Sunday night-Monday... Increasing southeast winds Sunday night become east on Monday. Speeds should remain less than 20 knots. Seas on the outer waters may approach 5 feet but otherwise expect seas to remain less than 5 feet. An area of rain moves across the waters during this time with locally poor vsbys in rain and fog. && .TIDES/COASTAL FLOODING...
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725 PM Update... Noted that we have a tide departure of about three quarters of a foot at most tide gages. This will result in the total water level topping out just below the still water flood stage at most points along both our east and south coasts. This is close enough to probably result in isolated minor coastal flooding of most vulnerable shore roads. For example the tide could be high enough to impact a portion of Morrisey Boulevard in Boston. Inundation depths should be generally under a half foot. Prior Discussion... 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... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 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. && $$ SYNOPSIS...WTB/NMB NEAR TERM...WTB/NMB SHORT TERM...NMB LONG TERM...WTB AVIATION...WTB/NMB MARINE...WTB/NMB TIDES/COASTAL FLOODING...Thompson

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