318 FXUS61 KBOX 060237 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 937 PM EST Fri Feb 5 2021 .SYNOPSIS... Dry and blustery weather follows tonight into Saturday. A coastal low passing southeast is likely to bring accumulating snow Sunday into Sunday night. While exact accumulations will depend on the storm's track which is still uncertain, the greatest accumulations are expected in southeast New England with less across the interior. Drier weather returns on Monday. Active weather pattern for mid week and arctic air possibly moving in late in the week or next weekend. && .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/... 935 PM Update... Breaks in the clouds coupled with light winds and leftover moisture was resulting in the development of patchy ground fog this evening. That coupled with temperatures falling below freezing will result in patches of black ice developing on untreated roads and walkways. We did issue a special weather statement to highlight these concerns. The winds will pick up a bit after midnight behind the passage of a cold front. This should limit the concern of additional black ice formation, but previous icy spots will persist as temperatures bottom out in the 20s. Sunday Snowstorm Update... As for the Sunday snowstorm threat we will not be making any changes to the forecast until we evaluate the entire 00z suite of model data. That being said there is some concern that we may to expand higher snowfall amounts and Winter Storm Watches further to the northwest. Stay tuned. && .SHORT TERM /6 AM SATURDAY MORNING THROUGH SATURDAY NIGHT/... Rather quiet day in store for Saturday and most of Saturday night as high pressure passes by. Gusty winds expected Saturday as cold air advection continues. Winds diminish Saturday night. with winds switching directions to become easterly through north as a coastal low pressure approaches late at night. Near normal temperatures expected through this portion of the forecast. && .LONG TERM /SUNDAY THROUGH FRIDAY/... Highlights... * Coastal low passing near 40N/70W to bring accumulating snow Sunday into Sunday night to Southern New England, greatest accumulations south and east of I-95 and less north and west. Exact snow amounts will hinge on coastal low's track * Brief drier weather Mon. * Clipper system brings mainly light snow for most, possible mix with rain south coast/Cape Tues and/or Wed. * Dry with below normal temps Wed nite into Thurs. Details: Sunday into Sunday Night: ** Winter Storm Watch for southeastern MA into much of RI Sunday into Sunday Night ** 500 mb shortwave disturbance aloft is expected to round the southern periphery of the polar vortex across the Carolinas early Sunday morning, with a coastal low pressure expected to deepen as it moves NE to a position near 40N/70W by mid-day Sunday, continuing NE into waters south of Nova Scotia by late-evening Sunday. This coastal system is expected to bring a period of accumulating snow to Southern New England. Such a progged storm track tends to favor the Boston/Providence I-95 corridor or just east when it comes to the most snowfall, with less across interior locales. While exact amounts will ultimately hinge on the eventual track of the surface cyclone, most of the suite of 12z models have increased their storm- total liquid QPF substantially enough, and with precip-type as all- snow, confidence has increased to hoist Winter Storm Watches for much of RI eastward into southeast MA, Cape Cod and the Islands. Further increases in snow totals north and west of I-95 remain possible to the point where Watches may need to be considered in later shifts for the Boston-Worcester/northern Providence County area and into adjacent portions of CT; but was not as confident in hoisting Watches into these locales as yet. There still remains a considerable deal of uncertainty specific to the track of the cyclone, with a number of moving parts at play that may greatly shape outcomes. This includes the strength of the 500 mb shortwave, how the vortex/longwave trough itself interacts with the shortwave rounding its southern periphery, and how robust any pre- storm ridging may develop (after passage of trough energy affecting our area later tonight). Current impressions are such that snow may begin as early as Sunday morning across the South Coast into Cape Cod and the Islands, and shortly thereafter across CT, northern RI into most of the remainder of MA. Accumulating snow looks to fall the heaviest mainly south and east of I-95, and particularly in the mid-morning to mid-evening hrs. The heaviest of the snow will be accompanied by increasing winds particularly across Cape Cod into portions of southeast MA with gusts 40-45 mph possible at times. With less storm-total precip across northwest MA, there may be a sharp gradient/cut-off from NW to SE which renders confidence in the snow accumulations north and west of I-95 fairly low. Where that gradient evolves isn't clear, but the potential exists for further upward or downward increases and/or adjustments to the Watch area pending trends. The fairly persistent northwest shift in cyclone track and QPF shield does have me concerned that expansion further north and west could still be possible. Updated snowfall totals are around a couple inches from western MA into the Merrimack Valley, increasing to around 3 to as much as 5 inches southeast from there to include northern CT, Hartford, Worcester, Boston and North Shore areas; and around 4 to 8 inches for most of RI into southeastern New England. Temps in the mid 20s to mid 30s on Sunday, with lows around the teens to lower 20s. Monday into Thursday Night: Really put less emphasis on this forecast period with most of the focus being tied to the anticipated snow event for Sunday into Sunday Night. Stuck mainly with NBM guidance for this fcst period. Resuming quiet wx for Monday under high pressure, and zonal flow dominated by the polar vortex/longwave over Canada. A Clipper system may bring rain or snow around the Tuesday or Wednesday timeframe; while fairly progressive, this midweek system looks to be the next system to watch. Behind this Clipper system comes a strong shot of colder air for Wed nite/Thurs. && .AVIATION /03Z SATURDAY THROUGH WEDNESDAY/... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent 03z TAF Update: Tonight: High confidence. Lingering MVFR ceilings should depart most of the region by 6z or so. The other issue is patchy ground fog that has developed especially in the CT River Valley. We do expect some light winds to develop after 6z behind a cold front, which should allow low visibilities to improve. That means most locales should improve to VFR toward daybreak. Saturday...High Confidence. VFR with SW wind gusts of 20 to 30 knots and around 35 kt in a few spots. A few gusts up to 40 kt possible on the Cape and Nantucket. Saturday Night...High Confidence. VFR. Low risk for developing MVFR towards the south coast late. KBOS TAF...High Confidence in TAF. KBDL TAF...High Confidence in TAF. Outlook /Sunday through Wednesday/... Sunday: Mainly VFR, with areas MVFR possible. Breezy. Chance SN. Sunday Night through Monday: VFR. Breezy. Monday Night: Mainly VFR, with areas MVFR possible. Chance SN. Tuesday: Mainly MVFR, with local VFR possible. Breezy. Chance SN, chance RA. && .MARINE... High confidence in the trends across the waters. A cold front passes through the waters tonight, which a high pressure to follow late Saturday and Saturday night. Colder air being pushed over the waters Saturday will lead to gusty winds, especially across the southern coastal waters, where frequent gale force gusts are possible. Winds diminish Saturday night, with rough seas also gradually subsiding. Outlook /Sunday through Wednesday/... Sunday: Moderate risk for Small Craft Advisory winds with gusts up to 25 kt. Seas up to 5 ft. Chance of rain, chance of snow. Sunday Night: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of rough seas. Monday: Winds less than 25 kt. Seas up to 5 ft. Monday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Chance of rain, slight chance of snow. Tuesday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Seas up to 5 ft. Chance of rain, patchy fog. Local visibility 1 to 3 nm. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...Winter Storm Watch from Sunday morning through late Sunday night for MAZ017>024. RI...Winter Storm Watch from Sunday morning through late Sunday night for RIZ002>008. MARINE...Gale Watch from 7 AM EST Saturday through Saturday evening for ANZ231>235-237-250-251-254>256. Small Craft Advisory until 7 AM EST Saturday for ANZ231>235- 237-250-251-254>256. Small Craft Advisory from 7 AM to 7 PM EST Saturday for ANZ230- 236. && $$ SYNOPSIS...Belk/Loconto NEAR TERM...Frank SHORT TERM...Belk LONG TERM...Loconto AVIATION...Frank/Belk/Loconto MARINE...Belk/Loconto