Area Forecast Discussion
Issued by NWS Boston, MA

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543 FXUS61 KBOX 280558 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 158 AM EDT Wed Jun 28 2017 .SYNOPSIS... A frontal passage overnight has ushered in a drier airmass across the region early this morning. Other than an isolated shower or thunderstorm across northern Massachusetts this afternoon, high pressure south of New England will provide mainly dry weather along with temperatures slightly cooler than normal. Thursday will be our transition day as a warm front moves across the area with the risk of showers and thunderstorms Thu night. Summer heat and humidity arrives Fri and continues into the weekend. A cold front likely breaks the heat and humidity later Sunday and will be accompanied by thunderstorms. Not as warm or humid early next week including the fourth of July holiday. && .NEAR TERM /UNTIL 5 AM THIS MORNING/... 2 AM Update... Surface trough has moved offshore with light westerly winds ushering in a drier and cooler airmass across the region early this morning. Dew pts now down into the 40s at PSF/CEF & ORH! Thus an unseaonably cool morning ahead with some spots across western MA falling into the 40s before sunrise. Elsewhere temps will fall into the low and mid 50s. More like a fall-like airmass than early summer. Where winds drop off (interior valleys, low lying spots, etc) patchy fog will develop. However this will quickly erode after sunrise. && .SHORT TERM /5 AM THIS MORNING THROUGH 6 PM THURSDAY/... The upper trough is overhead along with its cold pool. So continued support for lift. But the airmass will be much drier with a moist layer limited to the 750-850 mb layer. Expect diurnal clouds but precip is unlikely. Temps aloft will be equivalent to 8-10C at 850 mb, supporting max temps 75-80. High pressure maintains mostly clear skies and light wind should allow overnight temps to reach the 50s, with lower 60s in the urban areas. && .LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/... Highlights... * Scattered showers/thunderstorms from Thursday night through Sunday night * Very warm, humid conditions return Friday, lasting through the weekend Overview... 12Z medium range models and ensembles continue same theme for summer conditions. Active northern stream in place with a broad mid level fast flow that looks to continue through the upcoming weekend at least. H5 short wave/cutoff low moves out of Saskatchewan and Manitoba on Thursday, which will push across southern Canada into the northern Great Lakes through this weekend, then into Quebec Monday. Weak short waves will move along in the general W-SW steering flow around the base of the cutoff low through the broad trough to its S and SW. Will see rounds of scattered showers/thunderstorms as each short wave moves across. Very sultry airmass in place thanks to Bermuda high sitting off the Carolina coast. Will see dewpts rising to the mid-upper 60s late this week through the weekend. The offshore high and western Atlantic mid level ridging will cause an approaching cold front to stall west of the region. Question will be when the H5 trough will move across the region early next week. Model solution spread leading to lower confidence on timing of cold front that may approach around Monday or the 4th of July. Details... Thursday-Thursday night... High pressure off the mid Atlantic/SE U.S. coast will keep dry conditions across the region through about midday Thursday. A warm front will start to approach, with some showers that may develop across N and W Mass, mainly N of the Mass Pike around or after 21Z. Will see some more instability start to work into W Mass/N central CT Thu night as the warm front moves across, so have included chance thunder from W-E. Highs Thursday will mainly be in the lower-mid 80s away from the S coast, only in the 70s along the S coast. Overnight lows will be in the mid-upper 60s. Friday through Sunday... With the ridge in place off the coast, continued SW flow through this period. Typical summertime airmass in place, with dewpts up to the mid and upper 60s with a few spots that may touch 70, along with H85 temps rising to +16C to +18C Fri-Sat. With the soupy airmass, will likely see nighttime fog develop mainly around or after midnight as temps fall back close to the higher dewpts. Will see scattered showers and thunderstorms from the midday through evening hours each day, especially across the interior. Looks like best shot for convection will occur Friday afternoon/evening, then again Saturday especially over western areas. The ridge offshore may allow for somewhat drier air and subsidence to move into coastal areas on Saturday, so could be drier during the daylight hours. Approaching cold front on Sunday will trigger another round of convection as well. Will likely see some brief heavy downpours in some activity thanks to PWATs increasing to 1.5 to 1.8 inches, especially Friday afternoon/evening and again Sat night-Sunday. Some question as to whether some stronger storms will develop during this timeframe. Will continue to monitor this aspect. One other issue is possible heat headlines on Fri and Sat, due to combination of high temps (lower 90s) and dewpts (up to 70 degs). Could reach high indices close to 95, which would trigger possible advisories. Current forecast suggests lower 90s. Will monitor this as well. Monday-Tuesday... Low confidence on this portion of the forecast due to model solution spread, mainly with frontal passage. Some question on frontal passage late Sun night or Mon morning which will wash out rather quickly as winds shift back to SW during Mon afternoon. Another front may approach during Tue which may trigger more convection. && .AVIATION /06Z WEDNESDAY THROUGH SUNDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Thursday/...High confidence. Updated 2 AM... Thru 12z... VFR, dry weather and light west winds. Patchy MVFR in areas of fog across the interior valleys. After 12z... VFR, dry conditions along with a modest west wind. Isolated -SHRA/-TSRA possible 18z-00z northern MA including Cape Ann area. Tonight... VFR, dry weather and light winds. Thursday... VFR with low risk of light rain across northern MA as warm front moves across the area. KBOS Terminal...High confidence in TAF. WNW wind 15-20 kt today will preclude seabreeze from moving onshore. KBDL Terminal...High confidence in TAF. Outlook /Thursday through Sunday/... Thursday...High confidence. Mainly VFR. Dry through midday, then scattered afternoon/ evening showers across N and W Mass mainly N of the Mass Pike. Showers/ thunderstorms possible entire region Thu night. Gusty SW winds to 20- 30 kt developing along the S coast late Thu/Thu night. Friday through Sunday...Moderate confidence. Patchy early morning fog each day with local MVFR-IFR conditions. Otherwise mainly VFR. Local MVFR-IFR in possible strong thunderstorms, mainly during the midday through evening hours. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Thursday/...High confidence. Tonight... Updated 750 PM... Showers and thunderstorms over land will move over parts of the waters this evening. Best chance will be along the Massachusetts North Shore. Have ended the small craft for the nearshore waters as S-SW winds have diminished. Expect gusts up to 20 kt through around 04Z or so. Seas generally 3 feet or less. Wednesday and Wednesday night... Winds become WNW at modest speeds. Seas 3 feet or less. Outlook /Thursday Night through Sunday/... Thursday...Moderate confidence. Small craft advisories will likely be needed. SW winds will increase, with gusts to 25-30 kt highest across the southern outer waters. Seas build to 6 to 8 ft over southern waters Thu night. Friday...Moderate confidence. SW winds continue, gusting to around 25 kt, diminishing after midnight Fri night. Seas up to 5 to 7 ft. Scattered thunderstorms, some with reduced visibilities and strong gusty winds. Patchy late night fog. Saturday-Sunday...Moderate confidence. SW winds continue, though diminishing. Seas around 5 ft on the outer waters Sat, then slowly subside. Patchy late night fog each night with reduced visibilities. Chance of showers/thunderstorms. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...WTB/EVT NEAR TERM...WTB/Nocera/EVT SHORT TERM...WTB LONG TERM...EVT AVIATION...WTB/Nocera/EVT MARINE...WTB/EVT is the U.S. government's official web portal to all federal, state and local government web resources and services.