Area Forecast Discussion
Issued by NWS Boston, MA

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586 FXUS61 KBOX 161722 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 122 PM EDT Wed Aug 16 2017 .SYNOPSIS... Dry weather will prevail today and Thursday along with plenty of sunshine. A weather system from the Plains will bring showers and scattered thunder Friday and Saturday along with clouds and humid air. High pressure returns with fair seasonable weather Sunday through Tuesday. A drying trend likely follows for Sunday into early next week along with seasonable temperatures. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 10 am update... Little change needed this morning, noting the development of some diurnal CU but wedge of drier air will also begin to filter N-S across the region, so will see a gradual drop in dwpts through the late morning and afternoon. Tricky call on sea- breezes. If they begin it would be late, and not penetrate as deeply inland as usual, but this may mitigate downsloping at some E coastal locations. Previous discussion follows... Cold front will move offshore today as longwave trough axis moves overhead. Some mid to upper level moisture will help filter in clouds late this remaining into the afternoon, but otherwise a dry day is in store. Breezy conditions behind the front as winds turn to the NW. Gusts could reach between 15 to 20 MPH. Low confidence if a sea breeze will develop today. Guidance suggests that the winds slack off just enough to allow for the sea breeze to push onshore, however, if winds are stronger than models projected, then sea breeze will remain offshore. Despite cold front moving through, and weak CAA, 850 mb temps will still be near 12-14C. This combined with westerly downsloping will help warm temps into the low to mid 80s. BEACH Forecast: We still expect areas of high surf along with a high risk for rip currents along primarily south facing beaches which are open to the ocean. This extends from Watch Hill all the way across to the outer arm of Cape Cod and includes all Islands. A building swell this morning, from Gert, will reach a 14 second period and nearly 8 ft offshore. This will create a potentially life- threatening situation due to high seas and rip currents. Will maintain the current High Surf Advisory as it stands. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM THURSDAY/... Tonight into Tomorrow... High confidence. Upper level trough will push offshore tonight as mid-level ridge builds into the region. At the surface, high pressure will begin to slide eastward resulting in dry weather and light winds tonight. High level cirrus could filter into the region, but still feel clouds will be thin enough to allow for radiational cooling. This will allow for temps to drop into the mid to low 50s. Metro regions of PVD and BOS will stay a few degrees warmer into the low 60s. A secondary front will push into the region early Thursday morning. This is noticed as 850 mb dewpoint drops from 8C to -2C. This stout dry air will keep Thursday quite dry and clouds at bay. Weak CAA will keep temps a few degrees cooler than today but overall a pleasant day will be in store for southern New England. BEACH Forecast: While waves will begin to subside from GERT, may need to watch for lingering swell which could result in moderate risk of rip current. Nantucket and outer Cape would have the highest potential if this did occur. && .LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/... Big Picture... Subtropical high remains in control across the Southern USA while a general zonal flow continues across Canada and the Northern USA. Multiple shortwaves move through New England in two periods: Friday through Sunday and Tuesday night. Model mass and thermal fields from the 00Z suite are in general agreement through Monday. Differences in details emerge Monday and Tuesday. Details... Thursday night to Saturday... Moderate confidence. A group of shortwaves from the Great Lakes moves through during this time, supporting a surface cold front that moves through early Saturday. Zone of upper difluence moves over New England during Friday afternoon through Saturday morning. Deep layer of moisture arrives in Western CT during Friday morning, then reaches Eastern Mass toward midday. Increasing PW values, reaching 2 inches for Friday afternoon. Stability parameters are mixed, especially in Friday. No CAPE on Friday, but values of 500-1000 J/Kg on Saturday. Low level airmass will be equiv to an 850 mb temp of 8-10C, supporting max sfc temps in the 70s Friday. Airmass warms several degrees Saturday with support for max temps in the upper 70s to mid 80s. Looks like showers with some potential for thunder especially on Saturday. High dew points will contribute to areas of fog each overnight/morning. Sunday through Tuesday... Moisture fields dry out Saturday evening/night, becoming dry through the deep layer while hanging onto a layer of moisture based at 850 mb. High pressure builds later Sunday into Monday, moving offshore Monday night and Tuesday. This looks like a dry forecast most of the period, but with increasing moisture at all levels late Tuesday as another shortwave approaches. This would support showers Tuesday night. && .AVIATION /17Z 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 night/... Through 00Z...High confidence. N-NW winds continue with gusts around 15kt at times. These subside especially after 21Z. VFR. CU with bases around 5kft. Tonight into tomorrow...High confidence. VFR except some very patchy ground fog early AM mainly at typically prone terminals. Winds shift lightly W during the day except sea breezes along coast. Tomorrow night...Moderate confidence. Area of rainfall accompanied by a mix of MVFR/IFR conditions late, especially during the early Fri morning hours. VFR to start the overnight. Winds shift to the S. KBOS Terminal...High confidence in TAF. Sea breeze should generally remain offshore this afternoon. KBDL Terminal...High confidence in TAF. Outlook /Thursday Night through Sunday/... Thursday night through Saturday... Moderate confidence. VFR trending to MVFR/IFR in showers and scattered thunderstorms Friday and Saturday. Areas of fog also possible early morning Friday and early morning Saturday. Lowest conditions are expected Friday night. Sunday...IFR in morning fog/low clouds, then improving to VFR. && .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. Building southerly swell today. Reaching as high as 7-9 ft in the open ocean waters well south of Nantucket, meanwhile seas could reach 5-7 ft in nearer to the shore as this long period swell peaks toward the morning hours. Small Craft Advisories will continue for all southern waters. These seas will gradually subside tonight. Lingering swell on Thursday may keep seas from falling as fast as guidance is suggesting so SCA may need to be expanded. Outlook /Thursday Night through Sunday/... Thursday night-Friday... Southerly winds less than 20 knots and seas 3 feet or less. Increasing chance of showers west to east during Friday. Showers and scattered thunderstorms Friday night. Saturday... Lingering showers and scattered thunderstorms Saturday. Seas build to 6 feet on the outer waters, and possibly the exposed portions of the southern waters. Southerly winds Saturday turn from the west Saturday night. A Small Craft Advisory may be needed. Sunday... High pressure builds over the waters. Winds remain 20 knots or less, and seas 4 feet or less. Leftover showers early, but these move off to the east during the morning. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...High Surf Advisory until 8 PM EDT this evening for MAZ020- 022>024. RI...High Surf Advisory until 8 PM EDT this evening for RIZ006>008. MARINE...Small Craft Advisory for hazardous seas until 8 PM EDT this evening for ANZ232>234. Small Craft Advisory for hazardous seas until 3 AM EDT Thursday for ANZ235-237. Small Craft Advisory for hazardous seas until 8 AM EDT Thursday for ANZ254>256. && $$ SYNOPSIS...WTB/Dunten NEAR TERM...Doody/Dunten SHORT TERM...Dunten LONG TERM...WTB AVIATION...WTB/Doody MARINE...WTB/Dunten

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