Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 221911 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 311 PM EDT Tue May 22 2018 .SYNOPSIS...
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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.
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&& .NEAR TERM /THROUGH TONIGHT/...
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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.
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&& .SHORT TERM /WEDNESDAY THROUGH WEDNESDAY NIGHT/...
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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.
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&& .LONG TERM /THURSDAY THROUGH MONDAY/...
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Big Picture... The flow over North America will continue to feature northern and southern streams. Trough in the southern stream will continue to linger over the Gulf of Mexico through the week. Meanwhile the northern stream will feature one shortwave moving east of New England Wednesday night and a second shortwave ejecting from the Western USA, then phasing with another shortwave from northern Canada over the weekend as it moves east toward New England. The solutions show agreement through Friday, then diverge in details over the weekend and early next week. So confidence is good through Friday, then diminishes over the weekend. Contour heights remain above normal through the period. Expect much of this time to feature near or above normal temperatures. Concerns... Thursday through Saturday... High pressure center moves overhead Thursday. It then settles to our south Friday and Saturday, bringing a westerly flow to our region. The weak flow Thursday will allow sea breezes. Mixing will reach 850 mb with temps aloft supporting max sfc temps in the low to mid 70s but cooler along the coast. The westerly flow Friday and Saturday will be an offshore flow most places and bring warm temperatures all the way to the coast. Mixing should reach to 800 mb Friday and Saturday, with temps at that height supporting max sfc temps in the 80s. A cold front drops south from Canada on Saturday, with a chance of showers in the afternoon and at night as it moves through Southern New England. Best chance of showers will be in Northern Mass. Sunday-Monday... Cold front moves south of the region Sunday, but stalls close enough to support showers during this time. Meanwhile, high pressure tries to nudge south over the Maritimes and bring drier weather to New England. Confusing? Consider the GFS which also feeds Gulf moisture up the coast while the ECMWF keeps it well to our south. Low confidence in the forecast for these two days. We will show a low- end chance of showers both days.
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&& .AVIATION /19Z TUESDAY THROUGH SUNDAY/...
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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 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 Saturday/...Moderate confidence. Wednesday Night through Thursday Night: VFR. Friday through Friday Night: VFR. Breezy. Saturday: Mainly VFR. Local MVFR possible in fog and afternoon showers. Breezy.
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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 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 Saturday/...Moderate confidence. Wednesday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Thursday through Thursday Night: Winds less than 25 kt. Friday: Low risk for Small Craft Advisory with seas approaching 5 feet. Wind gusts near 20 kt. Friday Night: Winds less than 25 kt. Local rough seas. Saturday: Winds less than 25 kt. Areas of seas approaching 5 ft. Slight chance of afternoon rain showers.
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&& .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...Frank NEAR TERM...Frank SHORT TERM...Frank LONG TERM...WTB AVIATION...Frank/WTB MARINE...Frank/WTB

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