Area Forecast Discussion
Issued by NWS Boston, MA

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202 FXUS61 KBOX 211928 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 328 PM EDT Sat Apr 21 2018 .SYNOPSIS...
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High pressure remains in control through Tuesday bringing dry weather with moderating temperatures. Wet weather returns Wednesday into Thursday as low pressure moves into the region. Mainly dry weather follows on Friday before becoming dreary again next weekend.
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&& .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
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High pres builds in from the west tonight. Diminishing wind, dry air and light winds will result in good radiational cooling conditions and a chilly night. Lows will be mostly in the low/mid 30s with some upper 20s possible in the colder spots in the CT valley and interior eastern MA.
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&& .SHORT TERM /6 AM SUNDAY MORNING THROUGH SUNDAY NIGHT/...
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High pres in control will result in clear skies and light winds which will allow seabreezes to develop in the afternoon. Highs will range from the upper 50s to lower 60s away from the immediate coast. Another chilly night Sunday night with good radiational cooling. Subfreezing temps likely in some of the colder locations.
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&& .LONG TERM /MONDAY THROUGH SATURDAY/... */ Highlights... - Warm up early week, dry, watching for seabreezes - Wet, breezy midweek - Perhaps a reprieve Friday, possibly Saturday - Return of wet, dreary weather for the following weekend Overview and model preferences... Defined ridging is expected to define the sensible wx early this coming week, with height anomalies finally becoming positive by early Mon. H85 temps respond as well, reaching above 0C and likewise, positive anomalies. However, will need to watch a developing trof just W of the continental divide, a response to enhanced ridging along the W coast of the PACNW and BC. This wave shifts rapidly E through the early portion of the week and looks to phase with a slowly meandering S stream cutoff. This phasing will bring about a return to longwave trof across the E, and yield a return to unsettled and wetter wx reinforced yet again by a secondary shortwave out of the NW territories late week. Recent runs are struggling somewhat with the sfc low pres development/track with the leading wave. This is likely due to the fact it will not be fully developed or sampled until Sun. Given this, will lean most heavily on ensemble means to account for differences in final outcomes. Details... Mon and Tue... As mentioned above, ridging remains in control. H85 temps rise above 0C, to an average near +3C Mon, then +5C on Tue. This should allow several locations to reach the 60s. Can`t rule out a stray 70F, but noting strong subsidence inversion which may limit mixing. Also, with weak flow under cresting high pres and SSTs still in the low-mid 40s, sea breezes are likely especially Mon, but also possible Tue. This will lead to strong coastal temp gradient. Cool overnights with radiational cooling. Wed... Low pres develops across out of the cold pool from remnant convection across the SE CONUS. Depending on how quickly the wave aloft deepens, the low will either track more N, or E with time. This is where the uncertainty in rainfall totals on Wed reside. The E track would usurp the available moisture and shift strongest LLJ to the S while a deeper trof would draw the parent low/moisture/dynamics N toward New England. Ensemble means still favor the former over the latter, so this update will feature this solution, but this will need to be watched. In either case, PWATs and LLJ nearly 2 std deviation above normal as well as a conditionally unstable profiles suggest at least a risk for periods of moderate to heavy rain unless the E track verifies. Low probs of QPF near 1.00 suggested. Thu and Fri... The depth of the wave will also define sensible wx details on Thu and Fri. If the trof deepens later (the low pres E track suggested above), then Thu could feature continued wet and unsettled wx. Otherwise a brief period of meso-ridging will follow. Whether this occurs on Thu and Fri or Fri only is still somewhat uncertain. With a developing longwave trof and some moisture clouds/SHRA possible either day, but POPs will be generally lower than Wed. Next weekend... As mentioned in the overview, a secondary trof will be moving across the CONUS and merging with the longwave trof, deepening it across the E. This may lead to another round of wetter and cooler than normal conditions, but this is of course toward the end of most deterministic/ensemble skill set. && .AVIATION /19Z SATURDAY THROUGH THURSDAY/...
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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 Night/...High confidence. VFR through the period. NW gusts to 20 kt diminishing by this evening. Lighter winds Sunday with coastal seabreezes developing Sunday afternoon. KBOS Terminal...High confidence in TAF. Moderate probability of a sea breeze Sunday afternoon. KBDL Terminal...High confidence in TAF. Outlook /Monday through Thursday/...High confidence. Monday through Tuesday: VFR. Tuesday Night: Mainly VFR. Chance RA late. Wednesday: Mainly MVFR, with local IFR possible. Breezy. RA likely.
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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 Sunday Night/...High confidence. Light winds and seas. Winds turning onshore near the coast Sunday afternoon as sea breezes develop. Outlook /Monday through Thursday/...High confidence. Sunday Night through Tuesday: Winds less than 25 kt and weak seas. Tuesday Night: Winds less than 25 kt and weak seas. Slight chance of rain. Wednesday: Winds less than 25 kt. Seas locally approaching 5 ft. Rain likely, rain showers likely.
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&& .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...KJC NEAR TERM...KJC SHORT TERM...KJC LONG TERM...Doody AVIATION...KJC/Doody MARINE...KJC/Doody

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