Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 240228 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 1028 PM EDT Wed May 23 2018 .SYNOPSIS...
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Widespread showers with embedded thunderstorms concluding, the potential for lower conditions into Friday morning. Thereafter, dry, warm and humid conditions return into Saturday ahead of an approaching cold front from northern New England, bringing scattered showers and isolated thunderstorms late Saturday into Saturday night. The front stalls south of the region, with patchy showers lingering through the remainder of the holiday weekend.
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&& .NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...
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1030 pm update... Widespread showers with embedded thunderstorms concludes over SE Massachusetts and adjacent waters. Recapping, a weak cold front swept S/E over S New England last night into this morning, becoming diffuse, held up along the coast, reinforced back W by sea-breeze motions given diurnal heating of a lingering humid airmass along said front, some locations earlier today saw dewpoints in the 60s. Meanwhile, vortlobe charging through the N-stream round the Baffin Bay low, clipping N New England with attendant H3 jet streak, a strong push of high pressure / cooler air surging the more prominent, less diffuse cold front over E ME back S/W as a back door cold front (BDCF) into S New England in the last hour. Convergence of airmasses / boundaries over E/SE MA where MLcape and MUcape values were on a couple hundred J/kg set the stage for forced ascent of the aforementioned warm, slightly humid airmass, and boom, convective initiation, thunderstorm development, which swept S/E with the mean wind round the cyclonic flow as discerned from H925-7 WSR-88D vertical wind profilers. With the BDCF pushing through, cooler, drier air is in tow along with drier weather. But along the leading edge of the airmass, nocturnal cooling, still a decent surface low-level moist, shallow boundary layer, there`s the possibility of low clouds and/or fog over E/SE MA overnight. GOES-16/E showing some lower clouds where present weather is not obscuring approaching from the W. Some indication lower visibility may be problematic for SE coastal New England going into Thursday morning prior to eroding out. Weather concerns ongoing, one thing for sure, turning overall cooler and drier as mentioned, good sleeping weather, and with the threat of rain over with, not a bad idea to open the windows.
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&& .SHORT TERM /6 AM THURSDAY MORNING THROUGH THURSDAY NIGHT/... Thursday... Large high pressure in control will result in plenty of sunshine and a beautiful day. Somewhat cooler mid level temps behind tonight/s cold front will keep high temps mainly in the 70s. However...weak pressure gradient will allow for sea breezes holding high temperatures in the 60s along the coast. Thursday night... High pressure will move off the coast Thu night allowing for a return south to southwest flow of milder air. Initially boundary layer may decouple enough to allow for overnight lows to drop into the upper 40s to the lower 50s near or just after midnight in some locations. However...some boundary layer mixing may occur resulting in rising temps toward daybreak in locations that are able to decouple for a time. && .LONG TERM /FRIDAY THROUGH WEDNESDAY/... Overview... 12Z model suite continues to signal the flattening of the northern stream H5 ridge across the Great Lakes into southern Ontario by this weekend. A broad, nearly zonal steering flow will take over through the holiday weekend into early next week. Short wave in the northern stream flow will shift E late Sat/Sat night, which looks to merge with another short wave working S-SW out of northern New England as a backdoor cold front moves across. SW winds ahead of this front will bring moisture up the eastern seaboard, along with the moisture associated with the southward bound cold front. The front should clear the S coast Sunday morning, bringing cooler temperatures, then stalls in the zonal flow off the S coast early next week. This may mean a continued unsettled pattern with patchy showers until later Tuesday as another front pushes SE out of central Canada. Medium range models continue to signal drier conditions with near normal temperatures by the middle of next week. Details... Friday and Friday night... High pressure off the mid Atlantic coast will bring a warm W-SW wind flow across most of the region. However, it will be cooler along the S coast with the onshore wind. Excellent mixing in place through H85 with lapse rates at 8C/km or higher, so expect temps to rise to the mid and upper 80s away from the S coast. May also see W-SW winds gusting up to 20-25 kt along the immediate S coast, Cape Cod and the islands. Dewpts slowly increase Fri night with the SW wind flow, so temps will only fall back to the upper 50s to mid 60s, coolest along the immediate S coast. Saturday and Sunday... Clouds increase from N-S as a backdoor cold front works S-SW out of northern New England. Noting dewpoint pooling, up to the lower-mid 60s by late Sat or Sat night, highest across N CT/RI/SE Mass. PWATs increase to around 1.5 to 1.7 inches Sat night, highest along the S coast. Best instability moves in as the front approaches, with total totals from 48-50, K indices up to 30-35 and TQ values in the upper teens. So, have mentioned slight chance for thunderstorms starting across NE Mass around 21Z or so Sat, then spreading S Sat night. Highs will again reach the 80s away from the S coast. The front should slowly push across the region through around 12Z Sun, then stall just off the S coast as it becomes parallel to the mid level steering flow. Spotty showers may linger through Sunday as moisture continues to feed up the coast and along the stalled front. Easterly winds set up, so cooler temps will prevail. Expect readings to only reach the lower 60s along the immediate E coast, ranging to the lower 70s across the CT valley. Monday through Wednesday... Expect continued unsettled conditions Monday into Tuesday with the stalled front remaining off the S coast. May see a weak wave move along the front during Monday, which may enhance some of the scattered showers. Then, another front pushes SE out of central Canada during Tuesday. This should push the moisture S of the region, though a few showers may still linger especially around the Route 2 area of N Mass. Medium range in fairly good agreement in pushing another large high pressure center out of southern Ontario into the northeast U.S. by next Wednesday. This should bring dry conditions and near seasonal temperatures. && .AVIATION /02Z THURSDAY THROUGH MONDAY/...
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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 Night/... Tonight... N/NE wind shift ongoing for E/SE terminals while prevailing NW over all else. With wind shift, brief gusts upwards of 15-20 kts. Some concern over E/SE terminals later of low clouds and/or fog, IFR- LIFR. Low confidence presently, area of concern especially for SE coastal terminals. Thursday... Perhaps low confidence IFR-LIFR conditions linger for S-coastal terminals. Otherwise VFR with N winds at first, NE along E/SE- coastal terminals, gradually turning S late. Wind gusts upwards of 20 kts possible for E/SE terminals. Thursday night... Hold VFR but concern there could be lingering issues for S-coastal terminals of low clouds, possible fog. S/SW winds with gusts up around 20 kts for E/SE terminals towards Friday morning. KBOS Terminal... E onshore flow persisting, turning S late Thursday. Some concern of lower CIG conditions overnight. Will address with later TAF updates should confidence increase. KBDL Terminal... VFR. N winds much of today, gradually turning S late. Outlook /Friday through Monday/...Moderate confidence. Saturday: Mainly VFR, with local MVFR possible. Breezy. Chance SHRA. Saturday Night: Mainly MVFR, with local IFR possible. Chance SHRA, slight chance TSRA. Sunday through Sunday Night: Mainly MVFR, with local IFR possible. Chance SHRA. Memorial Day: Chance SHRA. Monday Night through Tuesday: Slight 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 Night/...High confidence. Tonight and Thursday... Back door cold front sweeping across the waters as of 10p will keep flow mainly NE over the waters through late Thursday. Gusts up to 20 kts possible;. Thursday night... High pressure moving off the coast will result in SW winds developing. Enough of a low level jet may result in marginal small craft conditions with southwest wind gusts of 20 to 25 knots and marginal 3 to 5 foot seas across the outer-waters. Outlook /Friday through Monday/...Moderate confidence. Saturday: Winds less than 25 kt. Areas of seas approaching 5 ft. Saturday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Chance of rain showers, areas fog, slight chance of thunderstorms. Local visibility 1 to 3 nm. Sunday: Winds less than 25 kt. Seas locally approaching 5 ft. Chance of rain showers. Sunday Night: Winds less than 25 kt. Local rough seas. Chance of rain showers, areas fog. Local visibility 1 to 3 nm. Memorial Day: Winds less than 25 kt. Areas of seas approaching 5 ft. Chance of rain showers. Monday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Slight chance of rain showers, patchy fog. Local visibility 1 to 3 nm. Tuesday: Winds less than 25 kt. Seas locally approaching 5 ft. Slight chance of rain showers.
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&& .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...Frank/EVT NEAR TERM...Frank/Sipprell/EVT SHORT TERM...Frank LONG TERM...EVT AVIATION...Frank/Sipprell/EVT MARINE...Frank/EVT

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