Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 271759 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 159 PM EDT Tue Sep 27 2016 .SYNOPSIS... A cold front will cross the region today with a period of showers. This front will stall near the coast late today and tonight keeping the threat for showers along the south coast, Cape Cod and the islands through tonight. A cool, dreary and drizzly pattern maintains into Saturday. Some partial improvement into Sunday prior to return of high pressure from the north with cooler air. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 2 PM update... Showers have shifted south of the coast and will affect mainly ACK through the afternoon. Modified PoPs for current radar trends. Previous discussion... Band of showers pushing across Cape/Islands and south coast as drying is moving in from the west. Model timing is reasonable and shows mid level drying reaching the south coast by early afternoon so expect partial sunshine developing from west to east, especially interior with clouds lingering SE New Eng. PWAT plume and deep moisture axis stalls just offshore and will keep threat of showers into the afternoon for Cape Cod and for much of the day over ACK. Adjusted PoPs accordingly. With developing sunshine, temps should reach the lower 70s for much of the region, except a bit cooler Cape/Islands. Winds diminishing over the Cape/Islands as low level jet moves east. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM WEDNESDAY/... Tonight... High pressure will build down the coast from the Gulf of Maine, bringing a wind shift to NE as well as gusty winds mainly after midnight from Cape Ann down to Plymouth. Scattered showers will linger across SE Mass/RI tonight, but may start to see some showers and patchy drizzle develop and move inland as the onshore winds develop. Some question as to how far inland the shower activity may push, which will be dependent upon a digging long wave trough across the Great Lakes into the Ohio Valley. Held CHC POPs across eastern areas, but can not rule out a few showers further inland overnight though precip amounts will be light. Will also see redevelopment of patchy fog. Overnight lows will range from the upper 40s across the higher inland terrain to around 60s along the immediate coast. Wednesday... Some questions as to how far the moisture plume off the ocean will push inland during the day. Lower confidence on timing and placement of the moisture, though looks like the best shot for any shower activity will continue along with coast. QPF amounts could be up to 0.25 inches across the Cape and islands. Gusty NE winds will make for a raw fall day. Highs will run around 5 degrees below seasonal normals for late September. && .LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/... */ Highlights... - A cool, dreary, drizzly pattern through Saturday - Possibly salvage a decent day on Sunday prior to a cold front - High pressure and cooler air for next week - Monitoring Invest 97L */ Discussion... Some interesting signals interpreted from the N Hemispheric pattern. Within the W Hemisphere, contributing negative signals from both the EPO and PNA are yielding a prevailing trough pattern over the W and Central CONUS. Meanwhile over the E Hemisphere, a subtle positive trend in the AO / NAO signals a S Greenland / Icelandic low. Between the two: higher heights and ridging prevail. Any energy that wobbles out of the progressive flow regime becomes cut off as is the case for much of the forecast period outlined below. There are also implications upon the N Atlantic High and subsequent steering flow of the tropics, pertinent when monitoring Invest 97L. A lot of unknowns in the forecast especially out through the longer term. A forecast consensus is warranted though greater weight is given towards ensemble probabilistics. Details below. */ Details... Wednesday Night through Early Saturday... Cut off low across the Ohio River Valley E of which expect roughly S to N low to mid level isentropic upslope along the 300-315K surfaces of sub-tropical air off the NW Atlantic with pwats up to 2 inches and decent theta-e content. This both N and aloft of a kinked warm frontal occlusion lying S and offshore of New England as shallow, surface cold air damming prevails along with a component of N flow from high pressure situated over SE Canada. An over-running event with some impetus of deep-layer forcing per individual waves of mid-level energy rotating N across the region round the broader cyclonic flow of the cut off low, beneath mid to upper level diffluence. But closer to the surface, up against cooler drier air associated with high pressure to the N. New England lying within the battleground of airmasses and thus forecast guidance having difficulty coming to consensus on rainfall outcomes. Welcome probabilistic datasets in this instance yielding confidence as to where wet weather is most likely. So in all, expect a cloudy, dreary, drizzly at times, cool pattern for the period with unseasonable cooler than average temperatures. Likely bouts of widespread showers with embedded heavier downpours with the focus more towards the S. Chance to likely showers N-S with little mention of thunder as the warm frontal occlusion lies S. If all goes well, some places over a 48 hour period could see upwards of +2 inches of rain, again greatest confidence S of the Mass Pike. Bulk of the heavier rain Friday into Saturday as mid level energy focuses convergence of deep layer moisture along the lifting warm frontal occlusion towards the S shore of New England. Later Saturday through Sunday... Drier air entraining as the cut off low is kicked back N into the progressive flow regime, deamplifying and transitioning into a positively-tilted trough axis. Offshore warm frontal occlusion kicks back N perhaps giving us a decently mild day for Sunday prior to the cold front sweeping the region late into Sunday evening with some light showery weather and blustery W winds during the transition process. Next week... A fair amount of weather features to evaluate. High pressure builds S out of Canada along with some cooler air aloft towards midweek. Coolest conditions around the early week period before the airmass moderates ahead of a deep trough axis emerging out of the W/Central CONUS. It`s prior to this as the mid to upper level low transition out that the combination of diurnal heating beneath cyclonic flow aloft yields to daytime shower activity. Will keep some PoP chances for the Monday and Tuesday daytime timeframe. Way too early to say much on Invest 97L. Not ignoring track guidance there still remains considerable spatial and temporal spread within the breadth of guidance, though good agreement that the system will strengthen into our next named tropical disturbance. Keeping an eye on the system, staying situationally aware and informed, but not jumping on any one particular solution. && .AVIATION /18Z 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/... High confidence on trends and overall theme. Lower confidence on exact details. Through tonight...MVFR/IFR conditions over the Cape/Islands will eventually expand inland from east to west across SNE later tonight with patchy fog developing as well. CT valley will be last to see lower clouds. Occasional showers will persist across ACK tonight with areas of drizzle developing elsewhere after midnight. Wednesday and Wednesday night...Widespread MVFR cigs likely with pockets of IFR possible. Areas of drizzle. NE wind gusts to 25 kt developing along the coast. KBOS TAF...Overall high confidence in TAF. Expect IFR cigs developing later tonight into Wed with slight improvement possible Wed afternoon. KBDL TAF...Moderate confidence in TAF. Mainly MVFR cigs developing late tonight into Wed. Outlook /Thursday through Saturday/...Moderate confidence. Overall a persistent N/NE flow with gusts up to 30 kts at times. Expect a mix of MVFR-IFR with likely drizzle and scattered to widespread SHRA activity, especially over S/SW New England terminals. Possible LLWS issues, more closer towards Saturday with winds at the surface remaining out of the N, but out of the S at 2 kft agl. && .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. 930 AM update... Winds diminishing from west to east through midday with speeds below 20 kt this afternoon. Small craft advisories will continue over outer southern waters due to seas. Showers gradually exiting with improving vsbys, but will persist over SE waters through the day. Previous discussion... Tonight...S-SW winds shift to NE from N-S as high pressure noses across the waters from the north. Will see gusts up to around 25 kt after midnight, mainly on the eastern waters. Seas around 5 ft on the southern waters, but beginning to build late on the eastern waters. Expect reduced vsbys scattered showers and patchy fog, especially SE waters. Small crafts will be needed again late tonight. Wednesday...NE winds gusting to 25-30 kt, highest on the eastern waters. Seas building up to 5-7 ft. Scattered showers with locally reduced vsbys, along with patchy fog early. Outlook /Wednesday Night through Saturday/...Moderate confidence. Persistent N/NE flow with gusts to near gale force at times will make for steady wave heights around 6 to 9 feet on the waters away from inner harbors and sounds. Strongest wave action is likely over the E/SE waters and over the far SW waters. Visibility restrictions more than likely given dreary, drizzly conditions forecast with embedded scattered to widespread showers. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 2 PM EDT this afternoon for ANZ232. Small Craft Advisory until 5 PM EDT Wednesday for ANZ254>256. && $$ SYNOPSIS...Sipprell/EVT NEAR TERM...KJC SHORT TERM...EVT LONG TERM...Sipprell AVIATION...KJC/Sipprell MARINE...KJC/Sipprell

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