Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 211913 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 313 PM EDT Mon May 21 2018 .SYNOPSIS... Dry and tranquil weather continues tonight. Rather cloudy skies are anticipated Tuesday along with cooler temperatures. A warm front will bring some showers to the region...generally Tuesday afternoon and evening. 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. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 310 pm update... High pressure will continue to slowly move east and further away from the southern New England coast tonight. Nonetheless...it will maintain dry/tranquil weather tonight with light winds. The dry airmass in place along with light winds will allow overnight low temps to bottom out in the upper 40s to lower 50s in many locations...but upper 50s are anticipated in the urban heat island of downtown Boston. Mainly clear skies this evening should give way to an increase in clouds from west to east after midnight ahead of an approaching warm front. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM TUESDAY/... Tuesday and Tuesday night... A rather cloudy day is on tap for Tuesday ahead of a warm front. There appears to be enough forcing/moisture along with a modest low level jet for some showers Tuesday afternoon/evening. Not expecting a complete washout...but most locations should see a period of wet weather during this time. There is even a low risk for a rumble or two of thunder...mainly near the south coast with some marginal elevated instability. High temperatures will be held in the upper 60s to around 70 in most locations given the clouds along with the afternoon/evening showers. The bulk of the showers should come to an end Tuesday evening...but a few will remain possible after midnight as a weak cold front crosses the region. Clouds will probably hang tough for most of the night and may see some fog develop given ample low level moisture. This should hold overnight low temperatures in the lower to middle 50s. && .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 /19Z MONDAY THROUGH SATURDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tuesday night/... Tonight...High confidence in VFR conditions with light winds. Tuesday and Tuesday night...Moderate to high confidence. Mainly VFR conditions into Tuesday midday. It probably will take until later Tuesday afternoon or evening especially along the coast for MVFR to localized IFR ceilings and visibilities to arrive. These lower cigs/vsbys will likely persist Tuesday night. Some showers expected especially Tuesday afternoon and evening. A low risk for a rumble or two of thunder mainly near the south coast Tue evening. KBOS Terminal...High confidence in TAF. Still uncertainty though in wind direction through 22z. BOS sea breeze kicked offshore as of 19z...but still close enough where it needs to be watched for another few hours. KBDL Terminal...High confidence in TAF. Outlook /Wednesday through Friday/...Moderate confidence. Wednesday: VFR. Slight chance SHRA, mainly eastern MA. Wednesday Night through Friday: VFR. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tuesday night/... Tonight through Tuesday night...High confidence. Weak pressure gradient will keep winds and seas below small craft advisory thresholds through Tuesday night. Biggest concern for mariners will be the potential for some fog developing Tuesday night along with the low risk for a rumble or two of thunder across our southern waters. Outlook /Wednesday through Friday/...Moderate confidence. 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. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...Frank/GAF NEAR TERM...Frank SHORT TERM...Frank LONG TERM...GAF AVIATION...Frank/GAF MARINE...Frank/GAF

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