Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 211057 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 657 AM EDT Mon May 21 2018 .SYNOPSIS...
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Dry weather is in store through tonight. Cooler with passing showers Tuesday afternoon as a warm front approaches. Showers are still likely Tuesday night as weak low pressure moves across the region. Mainly dry weather Wednesday through Friday. Moisture increases through the weekend with the best chance of showers on Sunday. Much warmer Friday into the weekend.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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7 AM update... No changes to the current forecast. Previous discussion... High pressure over the Great Lakes this morning will build east into southern New England through the day. Dry weather will prevail with rather light winds and noticeably lower humidity. Plenty of sunshine will boost temperatures 5-10 degrees above normal across the interior. The light winds and higher temperatures set the stage for seabreeze development, so cooler conditions towards the immediate coast, Cape and islands.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM TUESDAY/... High pressure moves through our region tonight. Winds turn SW, starting the warm air advection process. Only impact should be increasing mid and high level clouds. Light winds to start should lead to modest radiational cooling conditions before the clouds arrive, especially with the lower dew points. Preferred the lower MOS-based minimum temperatures. For Tuesday, a passing mid level shortwave and a low pressure moving into the Great lakes should be enough to warrant at least a chance of showers. Low and mid level lapse rates expected to be rather poor to start the day. While low level lapse rates are unlikely to favor thunderstorms, mid level lapse rates may steepen just enough later in the afternoon and evening where a few thunderstorms may develop. Many more clouds in place, so expecting temperatures to be 5-10 degrees lower than Monday, still near to slightly below normal. && .LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/... Highlights... * Showers likely Tuesday night * Cool Thursday but much warmer into the weekend * Risk of showers increases by Sunday Overview... A northern stream trough will amplify to the north and east Wed into Thu which will push a cold front through the region. Rising heights and shortwave ridging follow Fri into Sat then the next northern stream trough approaches Sunday. Tuesday night...Low pressure moves across New England and off the coast late. Showers are likely in southeastern MA and RI with more scattered showers farther to the north and west. Although there is no surface instability, some marginal elevated instability could support an isolated thunderstorm. Wednesday...Early morning cloudiness gives way to mostly sunny skies. However, an approaching cold front could trigger widely scattered showers in eastern MA where there will be a weak trough, CAPE of 500-1000 J/kg, and a narrow axis of K Indices of 32. Highs 75-80 away from the coast. Thursday...Sunny skies but 850 mb temperatures will have cooled from +13C Wed to +8C on Thu. Winds will be light NW inland but NE at the coast. Expecting highs 70 to 75 inland but cool upper 50s to lower 60s along Cape Ann, Cape Cod, and the Islands. Friday...High pressure to our southeast and weak low pressure over northern New England will lead to a westerly downsloping flow of air across southern New England. That, coupled with ridging at 500 mb will lead to a much warmer day. Right now am calling for 80-85 away from the coast, but it is possible to see a few upper 80s readings. Saturday and Sunday...In general, moisture will be increasing through the weekend as flow aloft shifts to the southwest. Increasing precipitable waters as deep tropical plume that has been sitting along the southeast U.S. coast edges its way through the mid-Atlantic and we begin to get on the northern fringes of the moisture. K Indices increase to the mid 30s in southwestern CT by Sunday. Thus scattered showers on Saturday will become more numerous by Sunday. Cannot rule out scattered thunderstorms, too. In addition, a warm front will be in the vicinity. Its location will have major ramifications for the temperature forecast. If it moves completely northward through the region, then highs both Sat and Sunday could be well up into the 80s to near 90 in a few spots. But, as the GEM has been hinting, there is a chance that it only bisects the region and the northeast portions of the region could be stuck in the 60s. For now, have blended the models and gone a few degrees higher than the blend, which yields 80-85 inland Sat and 77-81 Sunday, but again, these are subject to much change. && .AVIATION /10Z MONDAY THROUGH FRIDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tuesday/...High confidence. Through tonight...VFR with winds less than 10 kt. Local seabreezes from mid morning into early evening. Tuesday...VFR. SHRA moving from west to east across southern New England during the day. Low risk for a few -TSRA, too. KBOS Terminal...High confidence in TAF. KBDL Terminal...High confidence in TAF. Outlook /Tuesday Night through Friday/...Moderate confidence. Tuesday Night: MVFR with areas of IFR. Showers likely. Areas of fog. Wednesday: VFR. Slight chance SHRA, mainly eastern MA. Wednesday Night through Friday: VFR. && .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 Tuesday/...High confidence. 7 AM update... Small Craft Advisory has expired for Rhode Island Sound as seas have subsided to below 5 ft. SCA remains posted for the southern outer coastal waters through 1 PM. Seas were still 5 to 7 ft in those waters but are expected to subside during the remainder of the morning. Winds will remain rather light across the coastal waters through Tuesday. Some showers arrive across the waters during Tuesday afternoon, and continue into the evening hours. Low risk for a few rumbles of thunder, too. Outlook /Tuesday Night through Friday/...Moderate confidence. Tuesday Night: Winds less than 25 kt. Showers likely. Areas of fog with visibility reduced to 3 nm or less. Wednesday: Winds less than 25 kt. Slight chance of a shower. Wednesday Night through Thursday night: Winds less than 25 kt. Friday: Winds less than 25 kt. Seas approaching 5 ft southern and eastern outer waters toward evening.
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&& .FIRE WEATHER...
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Spotty locations received as much as 0.5 inches of precipitation this past weekend, but several locations received less. Given fuels were dry, this may not have been enough to fully moisten them. RH values Monday will drop as low as 25 percent inland, but winds should remain light out of the NW, around 10 mph most of the day.
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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 1 PM EDT this afternoon for ANZ254>256.
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&& $$ SYNOPSIS...Belk/Field NEAR TERM...Belk/Field SHORT TERM...Belk LONG TERM...Field AVIATION...Belk/Field MARINE...Belk/Field FIRE WEATHER...Belk/Field

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