Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 211727 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1227 PM EST Tue Nov 21 2017 .SYNOPSIS... Mild this afternoon as winds pick up out of the southwest. A cold front approaches the region tonight, and will combine with moisture working up the eastern seaboard to bring a period of rain through Wednesday morning, especially SE New England. Dry and colder conditions expected Wednesday night through the end of this week. Milder with a few showers possible Saturday as a cold front approaches, then blustery and colder weather follows Sunday into Monday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... Building southern stream ridge along the eastern seaboard, while northern stream trough digs over the Great Lakes. This points to an increasing southwest flow and warm advection. Cross sections show mixing to 950 mb on the 12Z NAM and 06Z GFS. Temperatures at that level are equiv to -2C to -4C at 850 mb, which would support max temps in the 50s. Morning soundings from Chatham and Upton NY suggest potential to mix as high as 900 mb, but resulting in a similar max temp potential. Winds in the mixed layer show 25 knots in observed data and 30-35 knots in forecast data. So no changes in the forecast...a sunny day with patchy cirrus. Max temps in the 50s, possibly 60 in spots. Southwest winds gusting to 30 knots/35 mph and possibly to 35 knots/40 mph. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM WEDNESDAY/... Dry weather to start tonight ahead of approaching cold front from the West. High pressure to the east of the region will help increase southerly low level moisture off the ocean. This flow will increase low level moisture and thus some isolated showers after 2 AM across the south coast. Upper level shortwave will help push the front closer to the region during the early morning hours. While moisture appears to be meager with this front, the focus is more on the coastal low which will develop off the coast of the Carolinas. Moisture from this low will stream into the region resulting in PWAT values increasing to 1 inch which is 1-2 STD above normal. Appears that the 2 systems will remain split from each other, however the interactions between the upper and low level jets will help increase lift across the region. In fact, some model guidance suggest strong lift around daybreak which could lead to heavy downpours which will impact the Wed morning commute. Still a spread in where the heavy axis will be located with the EC and RGEM on the western envelope compared to Hi- res guidance and NAM. The GFS continues to the eastern outlier and thus trended away from it. Right now appears the the axis of heavier rainfall will occur across the I-95 corridor and points eastward. However this axis could still shift if the two streams do meet up, of if coastal low trends farther eastward. Regardless, looks like a good slug of rain with QPF amounts around 0.5-1 inch. These amounts could also increase within any thunderstorms. Went ahead and inserted a slight chc of thunder as dewpoints reach into the 50s and models continue to show some surface Cape. Highest confidence will be across the Cape and the Islands. Conditions will quickly improve from west to east during the day on Wednesday. In fact, once the cold front moves through the region, dry air will quickly usher into the region resulting in clearing skies and dry weather. CAA will spill into the area resulting in temperatures to fall during the day. NW winds will also be on the increase as wind gusts increase to 25-30 MPH by the afternoon. && .LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/... Highlights... * Mainly dry and cool Thanksgiving Day and Friday * Milder with a chance of showers Saturday * Blustery and colder Sunday/Monday 21/00Z guidance suite remains in rather good agreement on the overall pattern through this portion of the forecast. In general, looking at a persistent mid level trough lingering near our region, although the amplitude and axis varies with time as various shortwaves move through the flow. This should mean temperatures trending below normal, especially late this weekend into early next week. Dry weather likely to persist into Friday, despite a cold front getting close to the northern MA border Friday. The most supportive moisture and dynamics for showers are expected to remain over northern New England. Our next chance for precipitation looks to be sometime this weekend. It`s not looking like a great risk, as humidity levels are somewhat lacking. However, there are a couple of fronts to move through, so this potential will need to be watched over the coming days. Colder air arriving Sunday into Monday could generate some ocean-effect showers toward the coast. && .AVIATION /17Z TUESDAY THROUGH SATURDAY/...
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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 Wednesday/...High confidence. Through tonight... VFR through at least early morning, with MVFR CIGS and RA arriving mainly after 09Z from the S. Areas W of a line from IJD-ORH-MHT may keep VFR CIGS through sunrise. Winds remain SW, with gusts 25-35 kt through this evening then gradually dissipating overnight. Tomorrow... Mixed MVFR/IFR CIGS through the day tomorrow with RA and VSBYS 1-3SM at times, especially E of the same line (IJD-ORH-MHT). Areas W may remain mixed MVFR/VFR with less RA. The rain clears, and rapid improvement to VFR occurs after 18Z from W-E as winds shift to the NW. Tomorrow night... VFR. NW flow around 10 kt. KBOS Terminal...High confidence in overall TAF trends, timing of lower categories early AM may be off by an hour or two. Clearing occurs mainly after 20Z tomorrow. KBDL Terminal...Moderate confidence in TAF. Uncertainty in MVFR CIGS as CIGS could remain low VFR and bulk of the RA remains E. Outlook /Thursday through Saturday/... Thanksgiving Day through Friday: VFR. Friday Night: VFR. Breezy. Saturday: VFR. Breezy. Slight chance SHRA.
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&& .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Wednesday/...High confidence. This afternoon...Building seas across the region as low level winds increase ahead of an approaching cold front. Southwest wind gusts 30 to 35 knots with potential for 40 knots...this will mean borderline gales possible. No changes to the existing gale headlines...winds increasing this afternoon. Tonight into Wednesday...Southwesterly gales will continue but subside across the near shore waters. SCA will be needed as seas will remain above 5 feet and gusts near 30kts. Approaching cold front will bring widespread rain for the waters and limit vsbys by Wed morning. Front will sweep through Wed afternoon switching winds to the NW with gusts near 30 kts by Wed evening. Outlook /Wednesday Night through Saturday/... Wednesday Night: Low risk for Small Craft Advisory winds with local gusts up to 30 kt. Seas up to 5 ft. Thanksgiving Day: Winds less than 25 kt. Local rough seas. Thursday Night through Friday Night: Winds less than 25 kt. Seas up to 5 ft. Saturday: Winds less than 25 kt. Seas up to 5 ft. Slight chance of rain showers. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Gale Warning until midnight EST tonight for ANZ231>235-237-250- 251-254>256. Gale Warning until 6 PM EST this evening for ANZ230-236. && $$ SYNOPSIS...Belk/Dunten NEAR TERM...WTB SHORT TERM...Dunten LONG TERM...Belk AVIATION...Belk/Doody MARINE...WTB/Belk/Dunten

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