Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 271958 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 358 PM EDT Sat May 27 2017 .SYNOPSIS... A weak ridge of high pressure will bring pleasant temperatures and result in mainly dry weather on Sunday, other than perhaps a brief spot shower or two. 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. Upper low and cold pool linger most of the week while surface high pressure builds in. Expect daytime clouds and scattered showers will pop up each day Tuesday through Friday. Otherwise fair weather with near seasonable temperatures. && .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/... 320 pm update... Tranquil weather tonight with a weak ridge of high pressure in control. Partly cloudy skies this evening should give way to mostly cloudy skies after midnight. This a result of a cooling boundary layer allowing for more in the way of low clouds to develop. A decent shortwave will also drop down in northwest flow aloft, but with little in the way of low level forcing. Therefore, dry weather will dominate and do not expect anything more than perhaps a brief passing sprinkle or two. We also can not rule out a touch of patchy drizzle toward daybreak if cloud bases can get low enough, but again no real impact. Low temps will mainly be in the upper 40s to the lower 50s. && .SHORT TERM /6 AM SUNDAY MORNING THROUGH SUNDAY NIGHT/... Sunday... An abundance of clouds is one again expected with model cross sections indicating a fair amount of low level moisture. Nonetheless, do expect to see some peeks of sun at times with temporary upper level ridging building into the region. This should allow for temps to be a bit milder than today. Highs are expected to top out between 70 and 75 in most locations, but weak gradient will once again allow for sea breezes on the immediate coast holding high temps in these locations in the 60s. While the vast majority of Sunday will be dry, enough moisture combined with diurnal heating may result in a few brief spot showers. The best chance for this will be in western and northern MA, but even in these locations the vast majority of the day will be dry. Sunday night... Dry weather should prevail the majority of the night. While models differ on specific timing, an approaching shortwave/warm advection aloft will allow for the potential of showers to increase toward daybreak Monday across the interior. Low temps will once again be mainly in the upper 40s to the lower 50s. && .LONG TERM /MONDAY THROUGH SATURDAY/... 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 the Great Lakes into Eastern Canada, maintaining a cyclonic flow over the Northeast USA and especially New England through Saturday. Phasing northern and southern jets Monday-Tuesday generate a coastal low that passes southeast of Nantucket Tuesday. Mass fields are similar among the long-range models through Wednesday, with the GFS and ECMWF similar over New England until Friday and possibly Saturday. Thermal fields are similar through Thursday. Moisture fields are noisier, but show a general similarity through Thursday with some divergence late week. This brings increased confidence in the overall synoptic pattern through much of next week, although lingering uncertainty regarding timing of individual shortwaves in the flow. Details... Monday-Monday night... Wet weather moves through Southern New England Monday and Monday night. Surface low north of the Great Lakes swings an occluding front through Upstate NY and PA during the day. Phasing upper jet moves across this front and generates a triple point low over the Mid Atlantic states. This low then moves offshore and passes south/east of Nantucket on Tuesday. The developing coastal low forms a weak low-level southeast jet that feeds into Southern New England Monday afternoon and evening, then shifts up the Maine coast later Monday night. This brings precipitable water values of at least 1.2 inches into the region Monday. This value is above average for late May in our area, but short of the 90% threshold. These moisture values diminish Monday night as the low level jet moves off to the northeast. Model consensus shows measureable rain arriving in the Worcester/Central Hills 6 to 8 am and reaching the I-95 corridor in MA and RI between 7 and 9 am. Convective parameters show a destabilizing airmass at most layers. However the easterly low level flow may bring a stable layer near the surface. Even so, the potential remains for an elevated unstable layer sufficient to generate isolated thunderstorms. Rain/showers taper off Monday night as the low level jet moves off. Continued low clouds/fog/drizzle possible in the lingering east-northeast flow overnight. Tuesday through Friday... Upper low and associated cold pool remain to our west Tuesday but could be close enough to generate scattered showers. A shortwave moves through the cyclonic flow Wednesday advecting overhead with it. This brings a better chance of daytime showers Wednesday and possibly Thursday. More question marks and less confidence on Friday and Saturday as additional shortwaves move through the flow. Cross sections show adequate moisture between 600 and 850 mb Friday, enough for minimal chance pops. Less available moisture Saturday, so we kept it dry for now. Temps at that level support max temps in the mid to upper 70s inland Wednesday, with 80 possible, and low to mid 70s Thursday to Saturday. Dew points fall to 45-50 by Thursday, so expect min temps 45-55 during the period except a couple degrees milder Wednesday morning. && .AVIATION /20Z SATURDAY THROUGH THURSDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Sunday Night/... Tonight...Moderate confidence. Mainly VFR conditions into this evening. However, a cooling boundary layer and some trapped low level moisture will likely result in some MVFR conditions developing late with perhaps some localized IFR cigs/vsbys. Overall, areal extent of lower clouds and how quickly they develop are uncertain but do feel there will be some especially toward daybreak. Sunday...Moderate confidence. MVFR to perhaps some localized IFR conditions early should improve to mainly VFR by afternoon, despite a scattered to broken deck of clouds persisting and perhaps a brief spot shower or two. Sunday night...Moderate confidence. Mainly VFR conditions for much of the night. However, some lower conditions may arrive toward daybreak Monday especially across the interior depending on the timing of rain showers. KBOS TAF...Moderate confidence in TAF. E sea breeze should shift to SE by late afternoon/early evening and finally SSW by late evening. SE sea breezes should re-develop by late Sunday morning or early afternoon. Mainly VFR, but may see a period of some lower clouds between 7z and 15z Sunday. KBDL TAF...Moderate confidence in TAF. VFR into this evening, before MVFR to perhaps some IFR conditions developing overnight. Improvement to mainly VFR should occur by late Sun morning/early afternoon. Outlook /Monday through Thursday/...Moderate confidence. Monday... Showers move in during the day with isolated thunder. Cigs and vsbys lower from VFR early to MVFR and areas of IFR in the afternoon and night. Southeast winds less than 20 knots except gusts to 25 possible late in the day on the Cape and Islands. The wet weather diminishes Monday night, but expect lingering fog/low clouds/drizzle during the night...especially over Eastern/Central Mass and RI. Tuesday... Lingering IFR in fog/drizzle early in RI and Eastern Mass. Conditions trending to VFR all areas. Potential for brief MVFR vsbys in scattered showers in the afternoon especially over Western/Central Mass. South winds remain less than 20 knots. Wednesday-Thursday... VFR. Scattered afternoon showers with MVFR vsbys. Westerly winds with gusts less than 20 knots. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Sunday Night/... Tonight and Sunday...High confidence. Weak ridge of high pressure will keep winds/seas below small craft advisory thresholds. Sunday night...Moderate to high confidence. Winds and seas will generally remain below small craft thresholds most of the night. However, approaching low pressure will increase the pressure gradient toward daybreak and enough southeast fetch may develop across our southwest outer-waters for some marginal 5 foot seas near 12z Monday. It is still too early to hoist any marine headlines for a potential late 3rd period event. Outlook /Monday through Thursday/...Moderate confidence. Monday... Southeast winds gusting to 20 knots. Seas near 5 feet on the outer waters, but less than 5 feet closer to shore. Showers and isolated thunder. Drizzle/fog linger at night. Tuesday through Thursday... Winds less than 25 knots and seas less than 5 feet through the period. Scattered afternoon showers possible. && .TIDES/COASTAL FLOODING... We opted to issue a coastal flood statement roughly between midnight and 3 am Sunday along the eastern MA coast. Boston has another very high astronomical tide of 12.3 feet just before 130 am Sunday. Storm surge values are running a bit higher than guidance, and may still be close to a one half foot during this coming high tide. We did have a few reports very early this morning along the eastern MA coast, so felt it was worth a statement for some of the most vulnerable shore roads experiencing some very minor flooding near high tide. Overall though impact will be minimal, so just went with the statement. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...WTB/Frank NEAR TERM...Frank SHORT TERM...Frank LONG TERM...WTB AVIATION...WTB/Frank MARINE...WTB/Frank TIDES/COASTAL FLOODING...

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