Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 230546 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 146 AM EDT Wed May 23 2018 .SYNOPSIS...
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Isolated showers linger this morning, with a second shot late today into this evening. Otherwise mid to late week warm, dry weather prevails. Unsettled weather on tap for the holiday weekend ahead of another sweeping cold front, however not a washout.
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&& .NEAR TERM /THROUGH TODAY/...
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145 am update... Mid-level trof axis, attendant vortmax energy beneath which moisture is plentiful. Aside from some shower activity, main issue continues to be nighttime cooling lending to lower clouds and/or fog development. It`s been no picnic, rather a headache in anticipating when, where, and magnitude of fog development. Surface dewpoints higher S/W out in the CT River Valley where winds are also light, allowing for spotty lower visibility conditions. Anticipating lower conditions with fog to creep E along the S-coast towards morning as a weak cold front slides S/E. Locations 2 miles or less, impacts to motorists and mariners, low beam headlights recommended. Today... Pleasant, dry day on tap, highs upper 70s to low 80s. Mid level trof axis having swept through, deamplifying in advance with stout ridging W, weak cold frontal boundary becomes diffuse. Light winds, midday sea-breezes allowed prior to late day into evening N push associated with clipping stout N-stream vort-lobe across SE Canada / N New England. Forcing mechanism upon available moisture, daytime heating yielding boundary layer destabilization, perhaps a spot shower or two beneath cyclonic flow. However, got to get the right conditions. Fairly dry while a bit of a cap around H6-7. If anything pops, it`ll be brief.
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&& .SHORT TERM /TONIGHT THROUGH 6 PM THURSDAY/...
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Tonight... Isolated shower activity over N/E MA this evening, otherwise dry, turning cool. H3 jet streak amplified, diving S in response to enhanced upstream ridging, a N-stream vort-lobe and attendant colder H85 temperatures skirt by. N surface wind response, onshore along the coast. Drier airmass pushing across the region, lowering both dewpoints and temperatures. Push lows down into the upper 40s for many locations, potential impacts to sensitive vegetation. Thursday... High pressure in control, another pleasant day on tap. However beneath continued influence of skirting N-stream vort-lobe / cooler airmass. Light winds overall, allowance of sea-breezes along the coast. Mixing up to around H85, can`t rule out a few W wind gusts along the interior, especially over high terrain. Mixing drier air as well, dewpoints falling into the 30s for interior locations.
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&& .LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/... Overview... Broad mid level ridging continues across the northern 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 /05Z WEDNESDAY 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 Thursday/...High confidence. Through this morning... MVFR-LIFR CIGs/VSBYs continue to be a hassle through 12z. Lower conditions throughout the CT River Valley, also creeping E along the immediate S-coast ahead of a NW wind-shift. Improvement beginning 10-12z. Today... Quick interior VFR improvement, however slower for Cape and Islands. Light NW flow allowing sea-breezes along the coast. Late day N wind-shift ahead of which isolated SHRA activity is possible, mainly N/E MA and RI. Wednesday night... VFR. Thursday... VFR. Sea-breezes along the coast. KBOS Terminal... Potential sea-breeze around midday. NW winds becoming E for a brief period. Isolated SHRA possible around late afternoon into evening. N wind shift behind SHRA activity into Wednesday night. KBDL Terminal... Low CIGs overnight, likely to waver between MVFR-IFR through the Wednesday morning push, quickly improving around 12z. Outlook /Thursday Night through Sunday/...Moderate confidence. 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.
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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 Thursday/...High confidence. Weak pressure gradient will keep winds and seas below small craft advisory thresholds through Wednesday night. Showers this evening. 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 Night through Sunday/...Moderate confidence. 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.
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&& .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...Sipprell NEAR TERM...Sipprell SHORT TERM...Sipprell LONG TERM...EVT AVIATION...Sipprell/EVT MARINE...Frank/EVT

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