Area Forecast Discussion
Issued by NWS Boston, MA

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456 FXUS61 KBOX 190344 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1044 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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1020 PM Update... Still noting low clouds lingering along east coastal areas including Cape Cod and the islands as intensifying low pres continues to move offshore. Another area of low clouds will push across central and northern portions of the CT valley as seen on GOES-16 Nighttime Microphysics channel as well as hourly obs. This area is associated with quickly passing H5 short wave in the fast northern stream flow aloft, then should lift further into northern New England overnight. Will watch this closely, as short range models showing this feature pushing a bit further S than previous forecast runs. Noting NW wind gusts across the nearshore waters at 15-20 kt. Overall forecast in pretty good shape but did refresh to bring near term conditions current. Temps were a couple of degrees either side of forecast which were adjusted. Previous discussion... Deamplifying H5 impulse sweeping across the region overnight. Accompanying ascent with differential vorticity advection and parent jet streak. Overrunning response but atmosphere rather 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/...
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Friday... Departing H5 vort max 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.
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&& .LONG TERM /SATURDAY THROUGH THURSDAY/...
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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 G35 kt possible. Pgrad supports a windy so could see a few G40 mph 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.
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&& .AVIATION /04Z FRIDAY 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. Overnight...Mainly VFR. MVFR CIGS will likely hang close to E coastal MA, but impact the Cape and Islands with a more N/NE onshore flow. Expect winds to become more northerly towards daybreak Fri. BKN-OVC MVFR along the slopes of the Berkshires. Otherwise mostly clear skies. W-NW winds prevail. Friday... VFR. Brief period of breezy W winds as SCT-BKN high clouds erode. FEW-SCT around 3500 ft possible during at times around midday. 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. 1030 PM Update... Small Craft Advisories remain in effect over the open waters E and S of Cape Cod and the southern outer waters. Winds gusting to around 20 kt there with seas remaining around 5 ft. Seas will hold at around 5 ft through the night, then should subside briefly on Friday. 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. Small Craft headlines may need to be renewed. There is also a low risk of low end 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... 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. && .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...EVT/GAF SHORT TERM...Sipprell LONG TERM...Nocera AVIATION...Nocera/EVT/GAF MARINE...Nocera/EVT/GAF HYDROLOGY...Staff

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