Area Forecast Discussion
Issued by NWS Boston, MA

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516 FXUS61 KBOX 202317 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 717 PM EDT Tue Jun 20 2017 .SYNOPSIS... A cold front moves across S New England this morning, followed by another weak front Wednesday. High pressure then 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/... 715 PM Update... A few light showers continue to be depicted on satellite and radar across northwest MA but should dissipate in a couple of hours with loss of heating. Other issue has been low clouds and fog across far southeast coastal areas. Drier air can be seen pushing a little further southeast and have experienced improvement over the past couple hours across the outer Cape and Nantucket. Some fog, however, will likely fill back in overnight over at least Nantucket. Gusty winds over land and near shore waters will diminish with sunset and less mixing. Tonight... Drier air has worked into the area as the cold front gradually pushes east. The front had made it to near a PSM/BVY/OWD/PVD line at 4 pm. The drier airmass will allow for mostly clear skies overnight for much of the area. May see some increase in clouds towards daybreak Wednesday across the far interior as a mid level trough approaches from the west. Challenge in forecast regarding south coastal areas especially the Cape and Islands, whether low level moisture fully scours out, or whether some low clouds and fog linger/redevelop overnight. Thinking we will have continued gradual improvement late this afternoon, though some redevelopment of fog is possible along the south coast and Cape/Islands, especially over Chatham to Nantucket which may not clear out. Temps to fall back into the mid 50s to low 60s across the interior, and mainly 60-65 near the coast. && .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 /23Z TUESDAY 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...High Surf Advisory until 8 PM EDT this evening for MAZ020-023- 024. RI...High Surf Advisory until 8 PM EDT this evening for RIZ006-007. MARINE...Small Craft Advisory until 8 PM EDT this evening for ANZ230>234-236-251. 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...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.