Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 181751 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 151 PM 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/... 200 PM Update... Upper level low continues to spin up in northern New England today. This keeps the region in a cyclonic flow aloft and with some mid-level moisture, has kept clouds around for much of the morning and early afternoon. Dry air is starting to make its way into the 500-700mb level which could result in more SCT conditions towards the afternoon with clearing skies this evening as we lose the mixing. Otherwise, mixing has topped off near 850mb which has kept wind gusts between 20-30 MPH. These gusts will also begin to slack over towards evening. Temperatures are still in track as a few sites have hit the 50F mark. Expect temps to warm another few degrees before we begin to fall back down as heating of the day is lost. && .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 /18Z WEDNESDAY THROUGH MONDAY/... 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... Before 00z... 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 to start the night, lowering to MVFR cigs/vsbys towards daybreak. This is due to widespread rain across much of the region and snow in the higher terrain. Tomorrow and Tomorrow night... Moderate confidence. MVFr falling to IFR across the region in snow for the higher terrain and rain across the coastal plain. Vsbys look to lift by the afternoon as precip turns more to showers. Cigs will still be MVFR/IFR lifting to VFR during the overnight. Lingering MVFR across the east coast due to ocean effect showers. KBOS Terminal...High confidence in TAF. Morning push looks to be impacted with heavy rainfall/MVFR conditions. KBDL Terminal...High confidence in TAF. Morning push looks to be impacted with heavy rainfall/MVFR conditions. 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. Extended the SCA for MA Bay into the early afternoon hours. Feel that gusts at or near 25 kts will linger until the early afternoon. 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 ANZ231>235-237-250. Small Craft Advisory until 2 PM EDT this afternoon for ANZ251. Small Craft Advisory until 6 AM EDT Thursday for ANZ254>256. && $$ SYNOPSIS...WTB/Doody NEAR TERM...Dunten SHORT TERM...WTB LONG TERM...Doody AVIATION...Doody/Dunten MARINE...Doody/Dunten

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