Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 211121 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 721 AM EDT SAT MAY 21 2016 .SYNOPSIS... High pressure moves offshore today bringing dry mild weather for much of the day. Low pressure along the Carolina coast moves up the coast and passes southeast of New England Sunday. Rain will convert to showers after the low moves past. Another round of showers and gray conditions is expected both Monday and Tuesday, but not a washout. Warming trend begins Wednesday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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7 am update... Early indications suggest models too moist in the lowest 10kft. Adjusted dwpts down with this thinking through the day. As a result also slowed timing of POPs this evening/overnight. This is also the trend of current mesoscale guidance as well, so feel it was warranted with this update. Previous Discussion Follows... High pressure is centered SE of Nantucket, but covers the Northeast USA. This shifts east out to sea later today, but with lingering surface ridging across Eastern Massachusetts this evening. Relative Humidity cross sections show limited moisture between 800 and 900 mb suggesting some cloud formation. Otherwise the moisture remains confined above 700 mb which means increasing and thickening mid and high clouds. Low level jet dynamics shows divergence at both low and high levels, which is not conductive to generating lift. Dry air and little/no lift does not speak to precipitation. The forecast backs off on earlier pops with 30-50 pct values south of the Mass Pike mid to late afternoon. Likely pops south of the South Coast through evening, which is when and where low level convergence/upper divergence comes together to support lift. Still concerned we are extending the low-end pops too far north too soon, but this is a good compromise. In the dry weather afternoon, mixing is forecast to reach 850 mb. Temps at that level support max temps around 70. ALB sounding last evening showed the mixing overachieved by 50 mb so this continues to be a concern, one that would support slightly warmer max sfc temps. We stayed with a range of upper 60s to mid 70s with the warmest values in Northern MA.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SUNDAY/... Most of the 00z model runs have shifted the track of the offshore low farther south, crossing 39N/70W rather than 40N/70W. The 00z NAM is the northern outlier of the group. Model convergence/divergence fields do show a coupled low level convergence/upper divergence couplet over Southern New England later tonight. RH cross sections show moisture aloft but drier air near the surface across Northern MA and Western MA/CT. Meanwhile the cross section for Nantucket shows moisture top-to-bottom and the one for Providence is close to that. Overall this would mean a chance of rain with the best chance along the South Coast and Islands. Also a reasonable chance over RI and SE Mass. Lower confidence farther north and west. Meanwhile the upper trough supporting this coastal low will move overhead on Sunday providing better support for showers especially over the interior. So everyone should get a potential for wet weather with the best chance south/east tonight and the best chance north/west on Sunday. && .LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/... Highlights... * Unstable/wet but milder Monday and Tuesday. * Warmer still and drier Wednesday * Warmer still late week, but potentially unstable. Overview and Model Preferences... In spite of what remains a relatively amplified flow pattern across the CONUS for much of next week, even operational guidance is in reasonably good agreement at least until the weekend. Cutoff remains the dominant player through Tuesday, followed by responsive ridging associated with srn stream warming. Therefore, will maintain the status quo for the most part, a seasonable, but unsettled and occasionally wet period for Mon-Tue followed by a warming trend late in the week, which could be accompanied by destabilization featuring shra/tstms. Feel fairly confident in using a GFS/ECMWF blend along with persistence as a starting point for this forecast update. Details... Sun night into Mon... Any remaining shra from either the instability to the W or the low pres exiting the Gulf Of Maine will whither Sun night giving way to mainly drizzle/damp conditions under continued NNE flow. Clouds/damp airmass yields a mild night with temps holding mainly in the 50s. By Tue expect some mixing and drying of the lower lvls as a secondary low pres moves up from the mid Atlantic states. Clouds/fog early still likely as Ely component to the flow continues. Even though unsettled conditions continue, a period with little in the way of measurable precip is likely through the day. The enhanced mixing should also lead to warmer highs than Sun, realizing some of the H85 temps near +8C, so Wrn areas /away from onshore flow/ should make it to near 70 while E areas remain cooler and in the 60s. Mon night into Tue... Secondary low pres associated with -2-sigma H5 cutoff moves up in a similar fashion to the Sat night-Sun feature, although it may actually make a closer pass or even just inside of the 40/70 benchmark. Pwats with this feature approach 2 std deviations above normal, so look for another round of widespread shra to develop early Tue morning with even the low risk of some isolated tstms thanks to H5 temps new -20C and TT approaching 50. Min/Max temps similar to Mon, even though some warming of the mid- lvls is expected, the increased clouds wet-bulb effect will maintain temps similar to those experienced the day prior. Wed... Brief ridge of high pres continues to be advertised as upstream mid and upper lvl ridging builds in response to warming srn stream flow. After early day showers/NE flow wains, expect some improvement through the latter portion of the day. Warming of H85 temps to near +14C and at least some observed sunshine should allow highs to rebound back into the mid 70s to even low 80s. Thu and Fri... Warm front shifts into Canada and Nrn New England with return flow and ridging. Looks like a good warm sector setup with potential destabilization possible. Therefore, diurnally driven shra/tstms are possible and will feature pops as such. Severity will be dependent on timing and mesoscale features such as LLJ. +16C Thu and +18C Fri are the average H85 temps, so with sunshine, highs in the 80s are quite likely. && .AVIATION /12Z SATURDAY THROUGH WEDNESDAY/...
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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 Sunday/... No major changes with this update except to delay timing of MVFR or lower conditions and rainfall. Today...High confidence. VFR conditions through the day, with a light south-southwest wind. Increasing sky cover through the day but bases should remain at or above 5000 feet. Tonight...Moderate Confidence. Developing MVFR/IFR in rain/fog/low clouds in RI and SE Mass as rain, fog and low clouds move ashore from the south. This may spread up the coastal plain to BOS/BED toward morning. Mainly VFR farther north and west...especially north of the Mass Pike. Sunday...Moderate confidence. MVFR/IFR in RI and Eastern MA in rain and low clouds. VFR in CT and Western/Central MA. KBOS TAF...Moderate to High confidence in TAF. KBDL TAF...High confidence in TAF. Outlook...Sunday night into Wednesday. Sun Night into Tue...Moderate Confidence. Although not a complete washout, periods of showers will continue on and off across the area with northeasterly flow. This also suggests low clouds. Therefore, periods of MVFR/IFR are likely, but it/s possible occasional VFR, especially in the west will occur. Wed...High Confidence. Mainly VFR with high pres.
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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. Today...High confidence. High pressure over the waters moves east out to sea but maintains dry weather most of the day. Winds and seas remain below Small Craft Advisory levels. Tonight...Moderate Confidence. Coastal low pressure moves northeast from the Carolina coast and passes well Southeast of Nantucket. Winds shift out of the East- Northeast with frequent gusts reaching 25-30 knots overnight. Seas build overnight with 5 to 7 feet on the Southern Outer waters by morning. The Gale Watch has been converted to a Small Craft Advisory starting after midnight on the Southern waters and after 5 AM on the Eastern Waters. Sunday... Moderate Confidence. Winds turn from the north Sunday while gusts 25-30 knots continue. Seas of 5 to 8 feet should spread to all of the exposed waters. The Small Craft Advisory will continue on most waters through the day. Outlook /Sunday Night through Wednesday/... Sun night into Mon...High confidence. Winds will diminish Sun night and remain below the 25 kt threshold for small crafts into Monday. However, with low pres just to the east a lingering swell reaching 5-8 ft will last well into Mon, maintaining the rough conditions and need for small crafts. Tue...Moderate Confidence. Winds still remain below 25 kt but a second approaching low pres system will allow for another building swell from the south with 5-6 ft seas possible. Small crafts may need to continue. Wed...High Confidence. High pres briefly builds over the waters, mainly quiet boating weather expected.
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&& .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from 4 AM to 8 PM EDT Sunday for ANZ232>234. Small Craft Advisory from 5 AM to 8 PM EDT Sunday for ANZ231- 250-251-254. Small Craft Advisory from 1 AM to 8 PM EDT Sunday for ANZ235- 237-255-256. && $$ SYNOPSIS...WTB/Doody NEAR TERM...WTB/Doody SHORT TERM...WTB LONG TERM...Doody AVIATION...WTB/Doody MARINE...WTB/Doody

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