Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 181122 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 722 AM EDT Wed Apr 18 2018 .SYNOPSIS... An upper level system moving into the Maritimes will bring dry but cool weather today. A low pressure area from the Midwest will produce a period of chilly rain potentially mixing with wet snow during Thursday . Lingering snow showers Thursday night. The weekend looks to be dry but with more below normal temperatures with milder weather early next week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 7AM update... W flow generating a very defined cutoff between low clouds across W MA/CT and nearly SKC conditions across E MA/RI. However, soundings suggest enough trapped moisture for another round of diurnal SC development through the day. Not as widespread as yesterday, but some SCT-BKN conditions expected at times. With plenty of sunshine to start the morning, combined with the downsloping, will increase potential highs toward the mid 50s today. Otherwise, forecast is on track this AM. Previous discussion follows... Upper low pressure moves from Maine into the Maritimes, but with several shortwaves moving through the cyclonic flow. The most significant shortwave moves off through the Gulf of Maine this morning, with drier air moving in behind the system. Deep moisture from overnight diminishes, and skies did clear over much of Southern New England during the early morning. Sunshine and lingering cold air aloft will allow clouds to redevelop later this morning, but expect more sunshine than Tuesday. Can`t rule out a stray shower, but the diminished moisture aloft will work against that. Mixing extends through 850 mb, which will bring gusts of 20-25 knots to the surface. Temperatures at that level -3C to -5C support max sfc temps in the low to mid 50s...except mid to upper 40s in the Worcester Hills and Berkshire East Slopes. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM THURSDAY/... Tonight and Thursday... Another shortwave moves across the northern tier of the USA, crossing the Great Lakes Wednesday and approaching New England early Thursday. This system redevelops south of New England Thursday morning, and with a progressive axis to the upper system we expect the surface system to steadily move past. The upper system will be supported by a 100 knot jet with upper divergence reaching New England between 06Z and 12Z. This will enhance lift over our region, supporting precip through the day. Strongest lift should focus around midday and early afternoon. As to precip type: thermal profiles mostly suggest rain in our area. Profiles are more marginal in northern Mass and over the Berkshires, so a wet snow or rain/snow mix is more likely in these areas. Once the surface system moves past late Thursday and winds turn more from the northwest, colder air may move in and change the rain to snow especially in Western/Central Mass and Northern CT. Snow accumulations would mainly be in the Berkshire East slopes where 1-3 inches is possible in spots near the Berkshire County line. The northern Worcester Hills would be another area of concern. && .LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/... */ Highlights... - Potential N/W snows Thursday night - Lingering showers for Friday - Gradual warm-up weekend into early next week, dry weather Overview and model preferences... New England finally gets a lobe of the S stream through early next week. This is thanks to a merger of cutoffs between a stalled cutoff near Labrador and the wave/cutoff associated with low pres for Thu. While this final phased cutoff shifts E, New England remains under cyclonic/cold airmass through most of the weekend. However, reinforced ridging, fed in part by warm advection from a cutoff across the SW CONUS continues to build, forcing the cutoff E and allowing the ridge to take control into the middle of next week. While the airmass is warmer, it has been moderated in part by the N stream as well, given a strong vortex near Baffin Island. Therefore, while temps through early next week will warm, they are also likely to remain near normal only going slightly above. Synoptic patterns are well agreed upon, so a blend of deterministic guidance will be used as a baseline for this forecast. Details... Thu night... Low pres continues to deepen through the Gulf of Maine as it shifts toward Nova Scotia. By Thu evening the strongest low-mid lvl forcing will be shifting E, along with the peak moisture (PWATS drop back below 0.50in through the overnight hours). However, the merger of waves aloft continues into Fri morning and may provide enough forcing to translate into continued precip even as the column dries somewhat. Therefore, will likely see at least some light precip linger into the early morning hours especially across the E half of the region. Meanwhile, lower lvl temps continue to drop as drier air filters in, yielding wetbulbs below 0C. Rain will likely change to a period of wet snow, first in the higher terrain, then even possibly closer to the coastline. Accumulations will likely be relegated to grassy surfaces, and highest in the higher terrain where the change occurs earliest. Lingering QPF through early Fri is generally below 0.2in, accumulations of SN should generally remain around an inch or less. Fri and Sat... Cutoff continues to slowly shift E. Coldest air lags somewhat, with H92 temps remaining above 0C through the day. Lingering risk for clouds/showers with the cyclonic curvature aloft, limited mainly by a continually drying column. Low risk for some ocean enhancement mainly across SE MA. However, the clouds will likely limit mixing somewhat, yielding highs in the mid 40s to upper 40s. By Sat, a gradual transition to more anticyclonic flow is evident, which should limit additional cloud cover/shower risk. However, temps will still be limited as the colder air remains locked in aloft. Highs mainly in the upper 40s and low 50s, still below seasonal normal values. Sun into Wed... High pres exceeding 1030+ settles across the NE. Each day, temps continue do warm, mid to upper 50s Sun, followed by 60s Mon through Tue as subsidence inversion may continue to limit mixing. Also, with sfc high pres cresting at the sfc, weak flow will allow for the development of coastal sea breezes. && .AVIATION /12Z WEDNESDAY THROUGH SUNDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Thursday/... No significant changes with 12Z TAF update... Today... High confidence. VFR. West winds gusting to 25 knots, possibly near 30 knots around Nantucket. Tonight... High confidence through midnight, moderate confidence after midnight. VFR through 5 AM with increasing/lowering sky cover. MVFR cigs/vsbys develop in Western/Central Mass and Northern CT as the morning rush starts. Rain develops, with snow possible over the Berkshires. Thursday... Moderate confidence. MVFR cigs lowering to IFR in the morning. Vsbys lowering to IFR in rain and snow. Some freezing rain is possible in Northwest Mass early Thursday morning. KBOS Terminal...High confidence in TAF. KBDL Terminal...High confidence in TAF. Outlook /Thursday Night through Sunday/...Moderate confidence. Thursday Night: Mainly MVFR, with local IFR possible. Breezy. Chance RA, chance SN. Friday: Mainly VFR. Breezy. Friday Night through Saturday: VFR. Breezy. Saturday Night through Sunday: VFR. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Thursday/...High confidence. Today...West winds gusting 25-30 knots from mid morning through evening. Seas 5 to 8 feet, mainly over the outer waters and RI Sound. Small Craft Advisory remains in place for all except Boston Harbor, Narragansett Bay, and Cape Cod Bay. Tonight...Diminishing winds and seas. Lingering rough seas over the southern and southeast outer waters. Small Craft Advisory lingers in those areas. Thursday... A coastal weather system brings rain to the waters, especially the southern waters, with visibility of 1 to 3 miles. Winds remain less than 25 knots. Seas remain at 5 feet or higher along the southern outer waters, but lower values farther north. Outlook /Thursday Night through Sunday/...Moderate Thursday Night: Small Craft Advisory winds with gusts up to 25 kt. Areas of rough seas. Chance of rain. Friday: Winds less than 25 kt. Local rough seas. Small Craft Advisories linger. Friday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Seas locally approaching 5 ft. Saturday through Sunday: Winds less than 25 kt. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 10 PM EDT this evening for ANZ232>235-237-250. Small Craft Advisory until 11 AM EDT this morning for ANZ251. Small Craft Advisory until 6 AM EDT Thursday for ANZ254>256. && $$ SYNOPSIS...WTB/Doody NEAR TERM...WTB/Doody SHORT TERM...WTB LONG TERM...Doody AVIATION...WTB/Doody MARINE...WTB/Doody is the U.S. government's official web portal to all federal, state and local government web resources and services.