Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 201809 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 209 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 PM THIS EVENING/...
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2 pm update... Cold front continues to push thru the area early this afternoon, has moved east of ASH/FIT/IJD. Model sounding show some very limited CAPE and support potential for very isolated low-topped showers over the next couple of hours, mainly across the far interior. Otherwise a continuation of partly to mostly sunny skies this afternoon with W/SW wind gusts 25 to 30 mph. Low clouds and fog lingering along the Cape and Islands will take thru this afternoon to erode. Early morning discussion follows... Scattered showers lifting north across SNE early this morning. Low level jet and theta-e plume crosses SE New Eng toward 12z where main focus for showers and an isold t-storm this morning. Otherwise, models indicate good drying in the column through the day with theta-e ridge axis pushing offshore which is the main effect of weak fropa this morning. This will result in stratus and south coast fog giving way to partly to mostly sunny skies mid/late morning, but stratus and fog may linger over the Islands until early afternoon. Low risk of a brief shower for western MA this afternoon as shortwave lifts to the north. Deep mixing developing today with near dry adiabatic 1000-850 mb lapse rates will result in breezy conditions with SW gusts up to 30 mph. 850 mb temps 12-14C this afternoon support highs in the low/mid 80s away from the south coast with some upper 80s possible Merrimack valley. Cooler 70s along the immediate south coast with SW flow. Dewpoints will be dropping today, into the 50s this afternoon in the interior, but will remain in the 60s near the coast. Surf conditions...Gusty SW winds will result in seas up to 7 feet just offshore which will lead to high surf and increased risk for dangerous rip currents for ocean exposed beaches along the south coast. High surf advisory has been issued.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM WEDNESDAY/... Tonight... Quiet night with mostly clear to partly cloudy skies. Drier airmass will allow temps to fall back into the 50s in the interior and outlying locations with lower 60s near the coast and urban centers. Wednesday... Mid level trof moves into New Eng with cooling temps aloft and steepening low and mid level lapse rates. Moisture is limited but models indicate marginal instability with sfc CAPES up to 1000 J/kg in the coastal plain and TT increasing into the lower 50s. Low risk for an isold t-storm Wed afternoon with weak sfc trof possibly providing a focus for low level convergence. Any storms will have the potential for gusty winds given steep low level lapse rates and inverted V profile. Highs will be mostly in the lower 80s, except upper 70s higher terrain and south coast. && .LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/... */ 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 /18Z TUESDAY THROUGH SUNDAY/...
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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/... This afternoon...High confidence except moderate confidence Cape and Islands. Away from Cape and Islands, VFR. For Cape/Islands IFR, improving to VFR between 18Z and 22Z. SW/W wind gusts to 25kt. Brief wind shift to NW possible with cold front passage. Tonight...High confidence. Mainly VFR but some IFR fog and low clouds may redevelop overnight at ACK. 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. KBOS Terminal...High confidence. VFR. SW/WSW winds with gusts around 25 kts this afternoon. KBDL Terminal...High confidence. VFR. Outlook /Thursday through Saturday/... 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.
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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/... This afternoon...High confidence. A period of SW gusts to 25 kt expected over nearshore waters, especially Boston Harbor and Narragansett Bay where best mixing will occur. Less wind outer waters but hazardous seas up to 7 ft expected. SCA posted for all waters for combination of wind and seas. Improving vsbys later this morning and afternoon for south coastal waters. Tonight...High confidence. Diminishing SW winds becoming light overnight. Seas slowly subsiding but remaining above 5 ft over outer waters. Wednesday...High confidence. SW winds mainly below SCA although a few gusts to 25 kt can`t be ruled out nearshore waters. Outlook /Wednesday Night through Saturday/... 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 8 AM EDT Wednesday for ANZ254>256. && $$ SYNOPSIS...KJC/Sipprell NEAR TERM...KJC/Sipprell/NMB SHORT TERM...KJC LONG TERM...Sipprell AVIATION...KJC/Sipprell/NMB MARINE...KJC/Sipprell

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