Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 211120 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 720 AM EDT Sat Apr 21 2018 .SYNOPSIS... High pressure building over the Great Lakes today moves east over New England for Sunday and Monday, then off to the east Tuesday. This will bring our area dry weather and gradually rising temperatures. Wet and breezy weather forecast midweek. Perhaps a late week lull before becoming dreary again next weekend. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 730am update... SKC conditions are likely to give way to FEW-SCT CU this afternoon as a weak shortwave rotates from the N in the cyclonic flow. Otherwise, pleasant spring day on tap. Coastal areas should not experience sea breezes today thanks to winds around 10-15 mph out of the NW. Previous discussion follows... High pressure over the Great Lakes with a diminishing pressure gradient over Southern New England. Sunshine will generate some mixing, reaching to about 850 mb. Also today the models show a weak shortwave moving through the Northeast and supported by a 120 knot upper jet. Morning temps at or below freezing will quickly recover this morning. Winds in the mixed layer will support developing gusts around 20 knots midday and afternoon. There is enough of a gradient to maintain a west to northwest wind. Temperatures at 850 mb reach -2C to -4C, and when mixed to the surface this would support max temps in the 50s. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SUNDAY/... As the high pressure moves overhead, winds will become light and subsidence should make our skies clear. With dew points in the 20s, except 30-35 Cape and Islands, expect some areas away from the coast will dip below freezing, the rest will be in the 30s. The high maintains clear skies and light winds Sunday. This will allow sea breezes along the coast late morning through afternoon. Mixing will support max temps in the mid 50s to around 60. && .LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/... */ 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 /11Z SATURDAY THROUGH WEDNESDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Sunday/...High confidence. No significant changes with 12Z TAF update... Today... VFR. Patchy daytime clouds this afternoon, but with bases above 4500 feet. Northwest winds with gusts around 20 knots midday and afternoon, diminishing by evening. Tonight and Sunday... VFR. Mainly clear skies and winds less than 20 knots. Developing sea breezes Sunday late morning and afternoon. KBOS Terminal...High confidence in TAF. Sea breezes unlikely today. KBDL Terminal...High confidence in TAF. Outlook /Sunday Night through Wednesday/...High confidence. Sunday Night: VFR. Monday through Tuesday: VFR. Tuesday Night: Mainly VFR. Chance RA late. Wednesday: Mainly MVFR, with local IFR possible. Breezy. RA likely. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Sunday/...High confidence. Northwest winds gusting to 20 knots today, then lighter winds tonight and Sunday. Seas remain below 5 feet through the weekend. Outlook /Sunday Night through Wednesday/...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. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...WTB/Doody NEAR TERM...WTB/Doody SHORT TERM...WTB LONG TERM...Doody AVIATION...WTB/Doody MARINE...WTB/Doody

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