Area Forecast Discussion
Issued by NWS Boston, MA

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695 FXUS61 KBOX 182043 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 343 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/... 1 pm update... Storm digging off the Mid-Atlantic coast, trowaling back along 290-300K isentropic surfaces but running up against subsidence, drier air from the NW as discerned from latest satellite, the cloud shield eroding. But watching low level stratus off the lakes and over the Gulf of Maine. Mesoscale processes occurring as boundary layer mixes out, becoming trapped beneath the dry subsidence inversion as discerned from 12z soundings from Buffalo NY to Portland ME. So the biggest forecast nuisance as of present: stratus decks over the Berkshires, downsloping and eroding S/E with the NW flow, and marine stratus over the Gulf of Maine and Nantucket clearly seen via satellite. Expecting the marine stratus to push SSE with time while eroding, clipping the Outer Cape and Nantucket with low cloud decks. Otherwise high pressure in control well S/W over the SE CONUS as energy continues to stream to our N over S Canada. Subsequent westerly gradient lending to winds around 10 mph with gusts up to 20 mph especially over the high terrain. Temperatures are over-achieving where snow pack is less, warmer with highs in the low to mid 30s. Colder to the N/W with deeper snow pack as well as some scattered to broken cloud decks, highs upper 20s to low 30s. 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. && .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/... 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. Monitoring MVFR marine stratus over the Gulf of ME and across ACK. Anticipating low CIGs to remain immediately off- shore. W winds continue around 10 kts. SCT-BKN high clouds towards evening, eroding into Friday morning. 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/... 340 PM update... Saturday: VFR. Windy with gusts to 30 kt. Saturday Night: VFR. Breezy. Sunday through Sunday Night: VFR. Monday: MVFR/IFR conditions possible. Chance RA late except chance SN and/or sleet and FZRA northwest MA Monday Night: Mainly MVFR, with areas IFR possible. Breezy. RA except sleet, snow and FZRA possible northwest MA . Tuesday: Mainly MVFR, with local IFR possible. Breezy. SHRA. && .MARINE... 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/... 340 PM update... Saturday: Low risk for gale force winds with gusts up to 35 kt. Otherwise WSW 20-30 kt. Dry weather and good vsby. Areas of rough seas. Saturday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Local rough seas. Dry weather and good vsby. Sunday: Winds less than 25 kt. Seas locally approaching 5 ft. Dry weather and good vsby. Sunday Night: Winds less than 25 kt but shifting to N-NE accompanied by a backdoor cold front. 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. Vsby reduced in Rain and fog. Tuesday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Rough seas up to 9 ft. Vsby reduced in Rain and fog. && .HYDROLOGY... A Flood Warning remains in effect for the Connecticut River at Middle Haddam, where ice jams are causing river fluctuations. Cold conditions will persist through Thursday which will limit additional runoff. Continued ice jams will remain on some of the rivers. There will be some increase of snow pack across interior southern New England as well. For details on specific area rivers, including observed and forecast river stages, please visit the Advanced Hydrologic Prediction Service (AHPS) graphs on our website. && .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...staff

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