Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 202020 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 420 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 temperatures return late in the week as the high moves off to the east. Another cold front crosses from the west over the weekend. && .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/... 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... - Comfortable and dry Thursday - Periods of wet weather for Friday and Saturday - Pleasant Sunday, scattered showers for Monday */ Overview... Dominant H5 ridge pattern over the W CONUS contributing to surface high pressure over the Central CONUS and the recent record-breaking heat over the desert SW. Until that breaks, as some of the ensemble mean guidance suggests by late June into early July, the NE CONUS finds itself oscillating between sultry, humid air brought about by S flow round NW Atlantic high pressure, and cooler, comfortable air in wake of cool fronts associated with mid-latitude disturbances sweeping through a preferred troughing pattern S of the Hudson Bay. Wet weather chances at the emphasis of airmass exchanges which are discussed in detail below. */ Details... Wednesday night through Thursday night... Comfortable, dry conditions with high pressure in control. A non-NAM consensus as the NAM struggles with the cool front stalling it along the S-coast. Prefer rather to clear it out Wednesday night as high pressure settle in. Light winds, an radiational cooling opportunity into Thursday morning. Keep lows close to lowest MOS guidance around the mid to upper 50s. High pressure and light winds Thursday, expect a comfortable, pleasant day with seasonable highs around the upper 70s to low 80s, cooler along the shore with sea-breezes. Clouds on the increase, lowering and thickening into Thursday night, may see some shower activity towards Friday morning as S/SW winds enhance, converging higher surface dewpoints / low-level theta-E air into the N lifting warm front. Friday through Saturday... Periods of wet weather, potentially heavy at times especially with any thunderstorms. Digging trough axis ahead of which sub-tropical moisture becomes stretched. Enhanced SW draw of high theta-E air contributing to a muggy, humid environment across S New England. Dewpoints back into the 70s and precipitable water values around 2 inches. Lift beneath favorable ascent of low-level convergent winds ahead of the cool front, a measure of instability and some decent shear, can not rule out updraft maintenance that contributes to the potential of 1-2 inch per hour rainfall rates. A low confidence forecast though siding with the slower 20.0z GFS. Sunday into Monday... Perhaps squeeze out a pleasant Sunday, slightly cooler and drier, prior to a consensus forecast of the main H5 trough axis and vort- max energy sweeping through the region Monday contributing ascent upon available moisture lending to scattered shower, possible thunderstorm activity. Go with chance PoPs and keep temperatures close to seasonable for this time of year, upper 70s to low 80s. Tuesday onward... Beneath the cold pool, lingering cyclonic flow, can`t rule out the possibility of scattered shower activity for Tuesday. By Wednesday, early hints of the H5 ridge breaking down across the W CONUS that contribute to higher mid-latitude heights downstream, enhancing the NW Atlantic high, reaffirming warm, muggy S flow over time. With any frontal boundaries through the remaining week, can`t rule out shower and thunderstorm activity. && .AVIATION /20Z 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/... Late this afternoon...High confidence except moderate confidence Cape and Islands. Away from Cape and Islands, VFR. For Cape/Islands IFR, improving to VFR thru 22Z. Exception may be ACK, there is a chance that low clouds/fog linger past 22Z. SW/W wind gusts to 25kt. Tonight...High confidence. Mainly VFR but some IFR fog and low clouds may linger or redevelop overnight at ACK. Chance for MVFR vsbys in fog Cape/coastal MA/RI. 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/... Thursday... Becoming SKC Wednesday night, yet CIGs return Thursday into Thursday night, lowering and thickening towards low-end VFR / MVFR. Light W winds overnight becoming S Thursday, sea-breezes possible along the E coastline. Friday and Saturday... Low-end VFR CIGs with MVFR/IFR along S/SE coastal terminals. SCT SHRA / TSRA. TEMPO IFR possible with any RA/+RA which is forecast. S/SW flow 10 to 15 kts with gusts to 25 kts. Winds diminish Friday night into Saturday while turning W. Sunday... Improving towards SKC, SCT-BKN low-end VFR CIGs during the day over mainly interior terminals. Light W/SW winds overall which may allow for sea-breezes along the coastline. && .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/... Thursday... Light W winds overnight becoming S Thursday, sea-breezes possible along the E coastline. Waves quickly diminishing on the S/SE outer waters thus allowing the conclusion of SMALL CRAFT ADVISORIES. Friday and Saturday... Scattered showers and thunderstorms at times, some of which may result in reduced visibility. S/SW flow 10 to 15 kts with gusts to 25 kts. Winds diminish Friday night into Saturday while turning W. Seas heighten to 5 feet on the outer waters briefly on Friday. Sunday... Improving towards SKC, SCT-BKN low-end VFR CIGs during the day over mainly interior terminals. Light W/SW winds overall which may allow for sea-breezes along the coastline. If any seas in excess of 5 feet they will quickly diminish. && .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...Sipprell/NMB NEAR TERM...NMB SHORT TERM...NMB LONG TERM...Sipprell AVIATION...Sipprell/NMB MARINE...Sipprell/NMB

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