Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 151727 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 127 PM EDT Tue Aug 15 2017 .SYNOPSIS... Showers lingers today, with a stray thunderstorm possible before improvement arrives again tonight. High pressure brings fair weather Wednesday and Thursday. A weather system from the Plains will bring showers and scattered thunder Friday and Saturday. High pressure returns with fair weather Sunday and Monday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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10 am update... Am noting a few breaks in the clouds between the offshore convection colocated with a warm frontal boundary that is slowly advancing northward and the leftover upper mid-upper lvl moisture associated with an exiting shortwave. This break is likely the result of the modest height increases and implied meso-ridging across S New England this morning, leading to enough subsidence to erode the multiple cloud decks in spots. However, with another shortwave on approach will need to monitor for a secondary increase in SHRA activity along the S coast, within the axis of highest PWATs and for SHRA/TSRA mainly W, where some destabilization could occur. We do echo previous forecast concerns about the ability to destabilize both from a mid lvl dry air standpoint and from a somewhat dense BKN-OVC limiting low lvl warming. Therefore, will continue to highlight only chance POPs or less for this development, but will need to monitor for a spot storm nonetheless late. Otherwise, dropped high temps a few degrees today outside of where the clearing has concerned given the slower diurnal increase this morning. Previous discussion follows... No significant changes to the forecast. Warm front stalled to our south. Upper shortwave moving across New England brought two areas of showers, one in Northern Mass and the other in RI and SE Mass. Showers will become more scattered in nature by mid- morning but with noticeable humidity as dew points linger in the mid 60s. Another shortwave will move east from the Great Lakes this afternoon. Guidance is hinting that these will be more widespread and clip the south coast resulting in likely precip across the Cape and the Islands. Def not turning into a nice beach day today. Because of the focus on precip across the south, the potential for a few strong thunderstorms seems to be unlikely at this time. That potent shortwave will also bring a few strong storms across northern New England. Because of the precip potential across the south coast, cloud cover will be dense today. This will limit heating and thus anticipate highs only in the mid 70s to around 80. Mid-level moisture as indicated by the K values will also drop this afternoon away from the south coast. Thus limiting any strong thunderstorm potential. Can`t rule out one across NW MA closer to the northern shortwave and steeper lapse rates, but overall threat is low. Lastly, QPF amounts will generally be light today, but with PWAT values reaching above 1.5 to 2 inches south of the Pike, may need to watch for nuisance flooding.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM WEDNESDAY/... Tonight into Tomorrow... High confidence. Upper level trough will swing through the region tonight as High pressure to the west begins to build. Still plenty of low level moisture around so another night of stratus and fog will be on deck. This is shown in BUFKIT cross sections, especially across the south coast and CT valley. Conditions will improve once surface cold front swings through the area and winds switch to a more westerly direction bringing in drier air. High pressure will build on Wednesday resulting in mostly sunny skies and temperatures warming back into the 80s. Beach Forecast: One thing to watch is the high surf and rip current threat from Hurricane GERT. Swells up to 7 feet are expected to approach the region late tonight into tomorrow morning. This combined with a period of 14 sec and waves above 5-8 feet will result in a life- threatening conditions for all people who enter the surf from dangerous rip currents and high surf. Because of this, went ahead and issued a high surf adv for Wednesday. Use caution if heading to the beach. && .LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/...
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Big Picture... Subtropical high remains in control across the Southern USA while a general zonal flow continues across Canada and the Northern USA. A series of shortwave ridges and troughs will move through this flow. A closed low over Quebec and cyclonic flow over New England all shift east Wednesday night. Multiple shortwaves move through New England Friday through Sunday. Model mass and thermal fields from the 00Z suite are in general agreement through Friday. They are then similar in general through the weekend but show differences in details. Run-to-run consistency has had problems the past few days, which tempers confidence especially for the end of week system. Details... Wednesday night and Thursday... High confidence. Shortwave ridge moves over the region with dry air through most of the column. Light gradient means light surface wind Wednesday night, which should allow a period of radiational cooling. Mid and high level moisture increases Thursday, especially afternoon/evening. Overall, a fair weather period. With radiational cooling expected, we nudged min temps down a couple of degrees Wednesday night. With temps aloft 12-14C, expect Thursday max temps in the upper 70s to mid 80s. Thursday night to Saturday... Moderate confidence. Next group of shortwaves moves through late in the week. First will be with the approaching warm front, second with the cold front, and a third trailing the second. Moisture builds in the column with PW values building to 2 inches Friday, then diminishing late Saturday as the cold front moves through. Stability parameters are mixed, with the Totals marginal, but the LI unstable. Looks like showers with some potential for thunder especially on Saturday. High dew points will contribute to areas of fog each overnight/morning. Sunday-Monday... Lingering instability Sunday as the third shortwave moves through. But moisture fields dry out Saturday night, which makes it hard to support any forecast of showers. High pressure builds later Sunday into Monday. Overall, a dry forecast with near seasonal temperatures.
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&& .AVIATION /17Z TUESDAY THROUGH SATURDAY/...
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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 Wednesday night/... Through tonight...moderate confidence. VFR through early evening, then after 20-21Z another round of MVFR (occasional IFR on Cape/Islands) will spill across SE MA and RI through about 04Z. Some patchy ground fog thereafter, although with winds shifting to the west, improvement to VFR also possible. Ground fog possible at typically prone airports in the morning if winds go calm. Tomorrow and tomorrow night...high confidence. Any fog burns off within a couple hours after sunrise. Mainly VFR with NW winds. Gusts to around 15 kt at times through the daylight hours. KBOS Terminal...High confidence in TAF. KBDL Terminal...Moderate confidence in TAF. Issue at hand is whether a period of MVFR fog will impact the terminal during the overnight hours. Outlook /Wednesday Night through Saturday/... Moderate confidence. Wednesday night-Thursday... High confidence. VFR with light wind. Patches of fog are possible overnight and Thursday morning, especially in the usual fog spots. Thursday night through Saturday... Moderate confidence. VFR with areas of MVFR cigs/vsbys in showers and scattered thunderstorms. Lowest conditions are expected Friday night in areas of fog.
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&& .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Wednesday/...High confidence. Today... S winds develop with gusts to around 20 kts on the E waters during the day, but brief. Otherwise seas will begin to build as swells from GERT begin to approach the waters. Should remain below SCA until this evening. Otherwise widespread showers will impact the southern waters today. This evening and into Wednesday... A few showers will lingering but conditions will be improving. Still anticipate another round of fog tonight. Waves begin to reach above 5 feet across the southern waters as swells from GERT increase. SCA has been issued for tonight and lasts into tomorrow as waves will reach between 5-8 feet with a southerly swell near 7 feet. Outlook /Wednesday Night through Saturday/... Moderate confidence. Winds less than 20 knots through the period, although gusty at times especially on Narragansett Bay Friday and Saturday. Lingering 5-6 foot seas on the southern waters Wednesday night, subsiding to less than 4 feet Thursday through Saturday. Areas of fog will bring reduced visibility most mornings. Showers and scattered thunder Friday and Saturday will also bring reduced visibility. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...High Surf Advisory from 9 AM to 8 PM EDT Wednesday for MAZ020- 022>024. RI...High Surf Advisory from 9 AM to 8 PM EDT Wednesday for RIZ006>008. MARINE...Small Craft Advisory for hazardous seas from 11 PM this evening to 8 PM EDT Wednesday for ANZ235-237. Small Craft Advisory for hazardous seas from 7 PM this evening to 8 PM EDT Wednesday for ANZ255-256. && $$ SYNOPSIS...WTB/Dunten NEAR TERM...WTB/Doody SHORT TERM...Dunten LONG TERM...WTB AVIATION...WTB/Doody MARINE...WTB/Dunten

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