Area Forecast Discussion
Issued by NWS Boston, MA

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344 FXUS61 KBOX 182105 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 405 PM EST Thu Jan 18 2018 .SYNOPSIS... Dry and seasonably cold conditions into Friday, then a warming trend this weekend with continued dry weather. A storm system will affect the region sometime late Monday into Tuesday with mostly rain and coastal wind, but there is a risk of some wintry mix/ice across the interior. Warm weather Tuesday is replaced by blustery, dry and seasonably cold weather Wednesday and Thursday. && .NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
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4 pm update... Impressive offshore low discerned from latest satellite. Under- going pressure falls of 2 mb/hr down towards 990 mb later tonight as its nudged E by building high pressure over the SE CONUS, an approaching kicker through the N-stream. Near our next of the woods, still dealing with nuisance low-level stratus beneath a stout dry inversion as discerned from 12z upper air guidance. Fair amount of moisture slipping in from W but down- sloping over the high terrain, eroding. Meanwhile marine stratus immediately offshore. MET guidance doing the best with present winds with an onshore NE flow over the E/SE coast of MA. NW cold advection over the warmer waters, subsequent marine stratus, bit of an indication of a thermal boundary hanging along the coast extending from roughly Portland ME given NW / NNW convergence down the waters and out across the Cape Cod Canal. Expect going into this evening that low level moisture / marine stratus will traverse and we`ll see low clouds linger over the Cape and Islands likely through the evening and overnight as mid to high clouds increase from the W (more on that below in the TONIGHT section). Keeping it perhaps milder as a result along with the onshore flow. Tonight... Deamplifying H5 impulse sweeping across the region overnight. Accompanying ascent with differential vorticity advection and parent jet streak. Over-running response but atmosphere fairly void of moisture given precipitable waters up around 0.2 inches. Green`s and White`s likely to see some snow potentially extending as far S as the Berkshires as flurries. Otherwise scattered to broken cloud decks while monitoring low-level stratus lingering along the high terrain and along the Outer Cape. With clouds anticipated, leaning away from coldest of guidance, that being MET/MAV.
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&& .SHORT TERM /6 AM FRIDAY MORNING THROUGH FRIDAY NIGHT/... Friday... Departing H5 vortmax with accompanying enhancement to the low- level wind profile. Mixing out to around H9 through the day, there`s the possibility of some breezy W winds during the morning, otherwise relaxing through the remainder of the day beneath a follow-up brief, weak ridge of high pressure. Boundary layer lapse rates remaining well mixed, beneath a dry inversion, and anticipating temperatures to warm into the mid and upper 30s, with snow melt, can`t rule out some few to scattered pancake cumulus. Friday night... Continued energy and moisture accelerating through the pseudo- zonal flow, sagging S into the Great Lakes region. Up against high pressure over the SE CONUS, an amplified gradient wind response with accompanying speed max at H925. Warmer, dry air pushing out of the SW, warming within H9-7 apparent within model forecast soundings, limitations on mixing down to the surface. Thus expecting a gradient wind response with strongest winds potentially in excess of 30 mph along the S-coast, across the Cape and Islands. Keep it dry but also mild with the winds. Lows down in the 20s for most locations. && .LONG TERM /SATURDAY THROUGH THURSDAY/... 340 PM update... Highlights... * Dry and mild this weekend, possibly low 50s Saturday! * Another storm will bring mostly rain and gusty coastal winds late Mon into Tue with a risk of some interior mixed precip/ice This Weekend... Confluent flow aloft across New England provides dry weather to the region this weekend. Low pres tracking across southeast Canada combined with high pres over the southeast states yields a tight westerly pgrad over the area. Warm air just above the surface with 925 mb temps +2C to +4C. However model soundings indicate a stout subsidence inversion limiting blyr mixing. Thus how much warm air aloft mixes down to the surface? Given the tight westerly pgradient and associated downsloping winds/adiabatic warming looks to be an overachieving temp setup. Model 2-meter temps and MOS guid continue to trend warmer with each run. ECMWF MOS and GFS MOS are the warmest guidance sources and will lean in this direction. Thus will go warmer than guid Sat with highs in the upper 40s to lower 50s, warmest in the coastal plain of RI and southeast MA. Although cooler along the immediate south coast and islands given WSW winds coming off the chilly near shore waters. Regarding winds Sat, GFS model soundings suggest G35kt possible. Pgrad supports a windy so could see a few G40mph but gusts 15 to 30 mph more common. Another nice day Sunday however winds will be much less along with temps not quite as mild, 45-50. Still 10-15 degs above normal. Turning colder Sunday night as a backdoor front slips across the area as 1037 mb high builds southeast into Ontario and Quebec. Monday/Tuesday... Pacific short wave marches across the country with an influx of Gulf and Atlantic moisture as it approaches from the southwest with PWATs surging up to +2 SD Tue morning across southern New England. Fairly robust wind anomalies within this moisture plume with GEFS ensembles offering +2 SD 850 mb and 925 mb winds. ECMWF more amplified than the GFS but the GEFS lends some support toward the stronger ECMWF. Thus will base this portion of the forecast toward a ECMWF/GEFS blend. Meanwhile, shallow cold air bleeds southward into southern New England Monday behind the backdoor cold front and 1038 mb high pressure builds from Quebec into Maine and New Brunswick. This will provide a cold air damming (CAD) setup for especially northern MA later Monday and possibly lingering into early Tue morning. Thus potential for a period of mixed precip/ice across this region. Elsewhere expecting a chilly windswept rain Monday night into Tue morning with increasing ESE winds off the cool near shore waters ahead of a strong cold front and possible triple pt low. ECMWF and GFS both have 925 mb southerly jet up to 65-70 kt crossing southern New England Tue. In addition, it`s aways out but some of the deterministic guid has mid level lapse rates up to 6-7C/KM along and ahead of the frontal boundary. Thus could be some elevated convection which would enhance the threats of strong winds aloft reaching the surface and locally heavy rain. Both ensembles and deterministic guid have 1+ of rainfall possible. As for temps, possibly another 50+ day especially in the coastal plain where warm sector has best chance of advancing but also temps may jump into the 50s behind the front as post frontal winds scour out any leftover shallow cold air from Mon night/Tue morning. Wednesday/Thursday... Post frontal airmass is colder with 850 mb temps around -8C on both GEFS and ECENS ensembles, which is seasonably cool for this time of year. So expecting highs at or slightly above normal. Although it will probably feel a bit colder given gusty WNW post frontal winds. Dry weather expected Wed and Thu. && .AVIATION /21Z THURSDAY THROUGH TUESDAY/...
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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 Friday Night/...High confidence. Rest of today into tonight... VFR. BKN MVFR likely to hang just offshore of E-coastal MA but impact the Cape and Islands with a more N/NE onshore flow, eroding and becoming N towards Friday morning. Otherwise, increasing SCT-BKN high clouds towards evening, eroding into Friday morning. W/NW winds prevailing. Friday... VFR. Brief period of breezy W winds as SCT-BKN high clouds erode. FEW-SCT 035 possible during the day. Friday night... VFR. W winds on the increase towards Saturday morning. Mainly a gradient wind with the potential of sustained values up to 25 kts, gusts up to 30 kts. KBOS Terminal...High confidence in TAF. KBDL Terminal...High confidence in TAF. Outlook /Saturday through Tuesday/... Saturday: VFR. Windy with gusts to 30 kt. Saturday Night: VFR. Breezy. Sunday through Sunday Night: VFR. Monday: MVFR/IFR conditions possible. Chance RA, chance SN, FZRA likely. Monday Night: Mainly MVFR, with areas IFR possible. Breezy. RA, FZRA likely, SN likely. Tuesday: Mainly MVFR, with local IFR possible. Breezy. SHRA.
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&& .MARINE...
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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 Friday Night/...High confidence. SMALL CRAFT ADVISORIES trimmed as W winds remain around 10 kts which have allowed seas to diminish. Expect lower sea trends and continued W winds through Friday. Greater concern is on the Friday night into Saturday morning period with increasing sustained winds potentially up to 25 kts, gusts up to 30 kts. Headlines may need to be renewed and there is a low risk of gales. Outlook /Saturday through Tuesday/... Saturday: Low risk for gale force winds with gusts up to 35 kt. Areas of rough seas. Saturday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Local rough seas. Sunday: Winds less than 25 kt. Seas locally approaching 5 ft. Sunday Night: Winds less than 25 kt. Monday: Winds less than 25 kt. Seas locally approaching 5 ft. Chance of rain. Monday Night: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Local rough seas. Rain likely. Tuesday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Rough seas up to 9 ft. Rain showers.
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&& .HYDROLOGY...
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A Flood Warning remains in effect for the Connecticut River at Middle Haddam, where ice jams are causing river fluctuations. For details on specific area rivers, including observed and forecast river stages, please visit the Advanced Hydrologic Prediction Service (AHPS) graphs on our website.
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&& .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory for hazardous seas until 6 AM EST Friday for ANZ254>256. && $$ SYNOPSIS...Nocera/Sipprell NEAR TERM...Sipprell SHORT TERM...Sipprell LONG TERM...Nocera AVIATION...Nocera/Sipprell MARINE...Nocera/Sipprell HYDROLOGY...WFO BOX Staff

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