Area Forecast Discussion
Issued by NWS Boston, MA

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961 FXUS61 KBOX 222029 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 429 PM EDT Tue May 22 2018 .SYNOPSIS... Rain showers will focus themselves to the south of the Massachusetts turnpike this evening before moving offshore...but a few left over showers will remain possible overnight ahead of a cold front. Mainly dry weather with warm afternoons are anticipated Wednesday, Thursday and Friday. Another cold front will drop south and bring a chance for a period of unsettled weather during the holiday weekend...but not expecting a washout. && .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/... 310 pm update... An approaching surface warm front/modest low level jet will continue to generate some light rain showers into this evening. The bulk of these showers will begin to focus themselves south of the Pike and especially near the south coast...where stronger forcing resides. While most of the showers will be light...some briefly heavier showers may occur near the south coast with even the low risk for a rumble of thunder. Late this evening and into the overnight hours will feature mainly dry weather as deeper forcing will have exited the region. However...still expect a few showers overnight as a weak cold front crosses the region. The other concern will be for areas of fog developing overnight given light southerly flow and increasing low level moisture. Main focus for the more widespread fog will be along the south coast, Cape and Islands. Overnight low temps should only drop into the lower to middle 50s. && .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/... Wednesday... A beautiful day is in store for the region as drier air works in behind the weak cold front. Partly to mostly sunny skies should allow high temperatures to reach the lower 80s in many locations away from localized coastal sea breezes. While dry weather will dominate...a shortwave and a local sea breeze boundary may be enough to trigger a few brief late afternoon/early evening spot showers across eastern MA. Wednesday night... If a few brief spot showers are able to develop across eastern MA...they should dissipate after sunset. Otherwise, dry and tranquil weather anticipated as a weak ridge of high pressure noses in from the west. Overnight lows should bottom out in the upper 40s to middle 50s. && .LONG TERM /THURSDAY THROUGH TUESDAY/... Overview... Broad mid level ridging continues across the norther tier states into central Canada around 12Z Thursday as seen on 12Z model suite and ensembles, which an H5 cutoff low sits off the CA coast. The ridging does not last long, though, as H5 heights signal a NW steering flow that gradually flattens out by the end of the week. A mid level short wave and associated cold front moves along in the flattening flow from southern Ontario late this week into northern New England by the beginning of the holiday weekend. Meanwhile, the western periphery of the Bermuda high tries to extend toward the Carolina coast. Low pressure in the Gulf of Mexico shifts slowly N while subtropical moisture works N around the high across the southern Appalachians. Limited moisture from the approaching front will move south during Saturday, bringing scattered showers. May see enough instability late Saturday into Sat evening to even see a few thunderstorms. The front becomes parallel to the mid level steering flow, so it looks to stall near or just S of the region by Sunday. May see a few weak waves move along this front, which will keep the chance for showers through at least Saturday night. Some showers may linger into Memorial Day mainly along S coastal areas. Another short wave may approach sometime Tuesday with spotty showers. With H5 heights running in the 570-576 dm range late this week, will see temps running well above seasonal normals especially Friday and Saturday, then will fall back to near or a bit below normal late this weekend into early next week. Details... Thursday through Saturday... Large high pressure from the Great Lakes into New England will bring dry but somewhat cool temps with sea breezes across coastal areas. As the high moves offshore, winds shift to SW and, as mid level heights rise, temps will as well Friday into Saturday. Highs should run around 10 to 15 degrees above seasonal levels away from the coast. Clouds approach Friday night as cold front approaches from the north. Will see chance for showers move into the Route 2 area by late Sat morning. With the warm temps in place, along with pretty good instability ahead of the front (SLIs around zero to -1, K indices in the lower 30s and TQ values in the upper teens), could see isolated thunderstorms develop. Saturday night through Monday... Cold front crosses the region Saturday night, with scattered showers. Could also see isolated thunder Sat evening. Then, the front will slow as it reaches the S coast Sunday. Some showers will linger Sunday, then drier air moves in from Maine and N NH as high pressure passes from Quebec into northern Maine during Monday. Can not rule out some leftover showers especially along the S coast during the day as the front lingers. Kept low chance for showers mainly S of the Mass Pike. Tuesday... Still can`t rule out a few spotty showers at this point, but lower confidence during this timeframe with wide model solution spread. && .AVIATION /21Z TUESDAY THROUGH SUNDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Wednesday Night/... Tonight...Moderate to high confidence. Ceilings and visibilities should gradually lower this evening and especially after midnight. Timing uncertain but widespread MVFR conditions should develop with locally IFR/LIFR focused across the south coast, Cape and Islands where the greater risk is for lower clouds and fog. Some showers mainly this evening focusing south of the Pike with even the low risk for a rumble of thunder near the south coast. Wednesday...High confidence. Lingering lower cigs/vsbys should improve to VFR by late morning/early afternoon in all locations. These conditions will be slowest to improve across the Cape and Islands. Low risk for an isolated late afternoon/early evening shower or two across eastern MA. Wednesday night...High confidence in VFR conditions. KBOS Terminal...High confidence in TAF. Sea breeze should come to an end early this evening. KBDL Terminal...High confidence in TAF. Outlook /Thursday through Sunday/...Moderate confidence. Thursday: VFR. Thursday Night through Friday Night: VFR. Breezy. Saturday: Mainly VFR, with areas MVFR possible. Breezy. Chance SHRA, isolated TSRA. Saturday Night: Mainly MVFR, with local IFR possible. Chance SHRA, isolated TSRA. Sunday: Mainly MVFR, with local IFR possible. Chance SHRA. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Wednesday Night/... Tonight through Wednesday night...High confidence. Weak pressure gradient will keep winds and seas below small craft advisory thresholds through Wednesday night. Showers this evening with a low risk of an embedded t-storm across the southern waters. Main concern will be areas of fog overnight into part of Wednesday morning...which may be locally dense especially across our southern waters. Outlook /Thursday through Sunday/...Moderate confidence. Thursday: Winds less than 25 kt. Thursday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Friday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of seas approaching 5 ft. Friday Night: Moderate risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of seas approaching 5 ft. Saturday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of seas approaching 5 ft. Slight chance of rain showers. Saturday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Chance of rain showers, isolated thunderstorms, patchy fog. Areas of visibility 1 to 3 nm. Sunday: Winds less than 25 kt. Seas locally approaching 5 ft. Chance of rain showers. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...Frank/EVT NEAR TERM...Frank SHORT TERM...Frank LONG TERM...EVT AVIATION...Frank/EVT MARINE...Frank/EVT is the U.S. government's official web portal to all federal, state and local government web resources and services.