Area Forecast Discussion
Issued by NWS Boston, MA

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296 FXUS61 KBOX 210201 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1001 PM EDT Tue Jun 20 2017 .SYNOPSIS... Another cold front sweeps S New England Wednesday with some isolated to scattered shower / thunderstorm activity. Thereafter high pressure builds over the region Thursday. Warmer and more humid air will return by Friday. A cold front will cross the region Friday night. && .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/... 10 pm update... Overall quiet and comfortable with SW winds diminishing becoming light with the loss of daytime heating. At last check, dewpoints were roughly 10 to 15 degrees lower than 24 hours ago hovering around the mid to upper 50s. Less sultry than what we`ve been dealing with as of late. Remaining mostly clear with the last of the low cloud decks pushed out to sea by the earlier cold front as discerned from GOES-16, lots of dry air within the low to mid levels where sinking air motions are greatest. But a bit of a change overnight. Cyclonic flow within the mid- levels through which individual impulse energy is invoking ascent, already seeing some low to mid level cloud decks out across Upstate NY with some accompanying light shower activity. Anticipating such cloud decks to increase in areal coverage and thicken across N/W portions of MA and CT towards morning with the slight chance of a shower. In addition, the lowering heights over the region may will likely draw the offshore lows clouds back N over SE New England. So in summary, while mostly clear condition prevail presently, anticipating thickening cloud decks across a good chunk of S New England ahead of forecast wet-weather chances for Wednesday ahead of mid to upper level synoptics pushing a reinforcing cold front across the region tomorrow. && .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/... Wednesday... Mid level trough and a weak second cold front move across the area. This allows for some cooling aloft and steepening of low level lapse rates. With precipitable water values around an inch, moisture is limited, with CAPE up to a few hundred J/kg and TT in the low 50s. Mainly a low chance/slight chance for showers Wednesday, with a low risk for an isolated thunderstorm or two during the afternoon. If a thunderstorm were to develop, could have some gusty winds. Some lingering fog possible during the morning along Cape and Islands, should lift/dissipate by late morning. Highs will be mainly in the upper 70s to low 80s, except 70s Cape and Islands and east slopes of the Berkshires. Wednesday night... High pressure builds into the area, allowing for mainly clear skies and dry conditions. With good radiational cooling, temps will fall back into the mid 50s to low 60s most locales, with some low 50s possible in sheltered areas of the far interior. && .LONG TERM /THURSDAY THROUGH TUESDAY/... Highlights... * Showers and isolated thunderstorms possible Thursday night ahead of warm front * Approaching cold front will trigger scattered showers and thunderstorms Friday afternoon and night. Overview... The westerlies depress south sufficiently to place southern New England in a position vulnerable to passing short wave troughs over the next week. The timing and strength of short wave energy becomes increasingly uncertain during the second half of the forecast period. Details... Thursday...Looks to be dry during the day. Most interior locations should reach the lower 80s, although upper 70s over the higher elevations. South coast temperatures should max in the lower to mid 70s given an onshore component to the surface wind. Thursday night...Models suggest an elevated instability burst Thursday night with rapidly increasing K index values as warmer and more moist air returns to the region. This looks to be beefy to warrant a chance of showers and isolated elevated thunder. Friday and Friday night...Warm and humid air make a return to the forecast area with high temperatures in the mid to upper 80s back from the south coast and dewpoints in the mid to upper 60s. An approaching cold will likely trigger scattered thunderstorms over the western half of the area during the afternoon and across most of the area during the evening. Precipitable waters approach 2 inches and so there could be heavy downpours with a few of the thunderstorms. It is also possible that the short wave energy could entrain some tropical moisture from the Gulf states as well. There also looks to be a sufficient combination of instability and deep layer shear to support isolated strong thunderstorms with isolated severe thunderstorms with damaging wind gusts not out of the question. Saturday...There may be lingering clouds and showers in the morning until the cold front clears the coast. Temperatures should be near to a little above normal in most locations with highs in the lower 80s across the interior. Sunday through Tuesday...There is enough variance in individual model runs to have us keeping to a model consensus forecast through this period. Most of this period features slight chance to low chance POPs due to uncertainty in timing of short wave energy. The forecast probably conveys more shower activity than really thinking, since we need to account for the uncertainty. Also, there is a possibility that at least a portion of our area, especially southeast zones, may be the recipient of some moisture from the tropical system now in the Gulf of Mexico. && .AVIATION /02Z WEDNESDAY THROUGH SUNDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Wednesday Night/... Tonight...High confidence for all but southeast coastal areas where confidence is only moderate. Mainly VFR but some IFR fog and low clouds may linger or redevelop overnight at or near ACK. Chance for MVFR vsbys in fog Cape/coastal MA/RI. Wind gusts should diminish quickly after 00Z. Wednesday...High confidence, except moderate confidence ACK. VFR with sct-bkn cigs 5-7k ft developing. Exception is ACK with IFR to start, then improving to VFR in the morning. An isold t-storm possible in the afternoon across the area. W/SW gusts to 20 kt. Wednesday night...High confidence VFR. KBOS Terminal...High confidence. VFR. SW/WSW winds with gusts around 25 kts this afternoon. KBDL Terminal...High confidence. VFR. Outlook /Thursday through Sunday/... Confidence...High. Thursday...VFR. Thursday night...Areas of MVFR possible in showers and isolated thunderstorms especially across northern MA. Friday...Areas of MVFR cigs/vsbys in scattered afternoon and night showers and thunderstorms. Saturday through Sunday...Generally VFR. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Wednesday Night/... Late this afternoon...High confidence. SW gusts around 25 kt expected over nearshore waters until sunset, especially Boston Harbor and Narragansett Bay where best mixing was occurring. Less wind outer waters but hazardous seas up to 7 ft expected. SCA continues thru 8 pm for all waters for combination of wind and seas. Areas of reduced visibility linger near E/SE Cape/Islands, with some improvements. Tonight...High confidence. Diminishing SW winds becoming light overnight. Seas slowly subsiding but remaining above 5 ft over outer waters. SCA continues overnight for outer coastal waters and for this evening BI/RI Sounds. Areas of reduced visibility in fog may linger or redevelop overnight near and E/SE of Cape/Islands and south coastal MA/RI. Wednesday...High confidence. SW winds mainly below SCA although a few gusts to 25 kt can`t be ruled out. Seas 4-6 feet along the southern outer coastal waters and east of Cape Cod, subsiding Wed night. Outlook /Thursday through Sunday/... Confidence...High Fairly quiet marine conditions are anticipated through the forecast period at this time, although it is possible enough sw fetch could be generated on Friday for seas in the outer south coastal waters to approach 5 feet. There could be brief gusty winds in vicinity of any thunderstorms across the waters Friday evening. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 6 AM EDT Wednesday for ANZ235-237. Small Craft Advisory until 4 AM EDT Wednesday for ANZ250. Small Craft Advisory until 7 PM EDT Wednesday for ANZ254>256. && $$ SYNOPSIS...NMB/Thompson NEAR TERM...Sipprell/NMB/Thompson SHORT TERM...NMB LONG TERM...Thompson AVIATION...NMB/Thompson MARINE...NMB/Thompson is the U.S. government's official web portal to all federal, state and local government web resources and services.