Marine Weather and Tide Forecast for South Burlington, VT

NEAR TERM /THROUGH SUNDAY NIGHT/
As of 635 PM EST Saturday...The main update was to add sections of central and northern Vermont into the Winter Weather Advisory. HRRR/HRRR outputs indicate a period of 1"/hr snowfall rates will be possible for a couple hours in northern Vermont and probabilistic guidance suggest 60-70 percent probabilities of exceeding 4" with the latest update. Otherwise, no other changes needed to the forecast with everything in great shape.

Previous Discussion...
Clipper low to track just north of the intl. border overnight into Sunday morning with widespread light snows affecting the region. General idea is a for a 3-6 inch event across the Dacks/SLV, 1-3" in the Champlain Valley, and 2 to 5" for the rest of VT with summit totals from 4-8 inches or so. Gusty south/southwest winds develop overnight on the immediate south side of said system with temperatures trending steady or slowly creeping upward over time into the upper 20s to lower 30s by sunrise Sunday. As precipitation tapers off Sunday mid to late morning, the dendritic snow growth zone trends unsaturated such that some patchy light freezing drizzle may occur in elevated terrain and portions of northern NY. Any ice accums. will be light and on the order of a few hundredths, but this thinking aligns well with latest WPC guidance. Late afternoon highs expected near seasonal early Dec. norms - mid 30s to around 40.
Then mainly cloudy tomorrow night with dry conditions expected as lows bottom out mainly in the 20s.

SHORT TERM /MONDAY THROUGH MONDAY NIGHT/
As of 341 PM EST Saturday...We remain in a very active pattern, with the next system on track to impact our area Monday into Monday night. A relatively weak low tracking northeastward north of the Great Lakes will send a warm front through the northeastern US. At the same time, a southern stream shortwave will help to develop a weak coastal low along the boundary near Long Island late Monday.
The end result for our forecast area will be a quick shot of precipitation with highly variable precipitation types. With sub- freezing air in place ahead of the system, expect precip to start predominantly as light snow with only marginal if any accumulations.
However, warm air advection from the southwest will allow for the development of a warm nose just off the surface, especially over northern NY and into NW VT, allowing for some mixed precipitation challenges. Some areas will see a brief period of freezing rain, mainly Monday evening over parts of the Saint Lawrence Valley and portions of the Adirondacks as warm air rides over a sub-freezing surface. There is also potential for some freezing rain over higher elevations of the Greens towards midnight, and then tapering off into freezing drizzle in eastern Vermont after midnight. Any showers that linger into Tuesday morning will change to all rain as surface temperatures rise. All said and done, icing amounts will range from 0.0 to 0.1 inch. Some surfaces may be slippery Monday evening into overnight.

LONG TERM /TUESDAY THROUGH SATURDAY/
As of 341 PM EST Saturday...Another impactful system is on the way for midweek, which will bring rainfall and snow to the area.

An amplified upper-level trough will become negatively tilted over the eastern US as a potent southern stream shortwave trough lifts northward. The negative tilt of the trough coupled with a 160kt + upper level jet and strong mid-level baroclinicity will allow a developing surface low to quickly strengthen as it tracks northeastward over the New England Coast. The amplified trough and a 75+ kt southerly 850 mb jet will result in impressive moisture transport ahead of the system, and precipitable water values will climb to 0.8 to 1.1 inch. This is 200 to 300% of climatological normals for this time of year. The high moisture air mass feeding into the sharp front with strong low level convergence and low/mid- level frontogenesis will result in some brief moderate to locally heavy precipitation rates, likely during the day on Wednesday or into Wednesday night, but this timing could change. Overall precipitation type will generally start as rain and transition to snow from west to east as the front moves through, but it's too early to get into specifics on timing of precipitation changeover. Given the strong dynamics and impressive moisture advection, the potential for moderate to locally heavy precipitation rates presents both flooding and/or heavy snow threats.

Given the placement of the warm air, higher hydro risk would be in Vermont, where the Weather Prediction Center has outlined a Marginal Excessive Rainfall Outlook. This means there is at least a 5% risk of rainfall exceeding flash flood guidance within 25 miles of any given point included in the outlined Marginal Risk area. With regards to the rivers, NAEFS and GEFS based MMEFS simulations available online from the Northeast River Forecast Center are suggesting around 0.5 to 0.75 inch of SWE loss for southern/central Vermont. The snowmelt, in combination with rainfall, will allow rivers to rise. At this point, however, the mean forecast stream levels from the ensembles are still well within bankfull. Something to watch at we head closer to the event, but nothing overly concerning at this point. The other concern will be the brief thump of heavy snow and potentially some slick surfaces that portions of the area will experience shortly after the changeover. The overall good news is the progressive nature of the system, which will limit residence time under moderate/heavy rain and snow, and thus overall limit impacts. Nonetheless, this system will be one to keep an eye on as we head into next week.

AVIATION /00Z SUNDAY THROUGH THURSDAY/
Through 00Z Monday...Flight conditions are beginning to gradually trend towards MVFR and IFR as light snow is beginning to overspread the region. Snow will be most prevalent between about 03z and 15z with 1-4SM visibility. Ceilings will trend towards 1200-2500 ft agl with pockets of IFR ceilings possible at times, but not widespread based on upstream observations. As snow arrives, south winds will begin to increase to 6 to 12 knots with gusts 18 to 24 knots possible. Fast flow aloft will be focused over KSLK and KMSS, where some LLWS is noted. As precipitation comes to a close between 12z-18z, there may be a mix of rain or drizzle as temperatures warm above freezing over the region. After 18z, any precipitation should taper towards the summits and ceilings will begin to steadily improves as the weather system departs. Winds should also make a relatively quick switch to the northwest trending towards 5 to 10 knots sustained.

Outlook...

Sunday Night: Mainly VFR, with local MVFR possible. NO SIG WX.
Monday: Mainly VFR, with areas MVFR possible. Likely RA, Chance SN, Chance FZRA.
Monday Night: Mainly MVFR, with local IFR possible. Definite FZRA, Chance RA, Chance SN.
Tuesday: Mainly MVFR, with areas IFR possible. Chance RA, Slight chance FZRA.
Tuesday Night: Mainly IFR, with areas MVFR possible. Definite RA.
Wednesday: Mainly IFR, with areas MVFR possible. Definite RA.
Wednesday Night: Mainly IFR, with local MVFR possible. Likely RA, Chance SN.
Thursday: Mainly VFR, with areas MVFR possible. Chance SHSN.

MARINE
As of 330 PM EST Saturday...A Lake Wind Advisory has been issued for all lake waters overnight through mid-morning on Sunday. Gusty south winds will develop across open lake waters later this evening into the overnight hours before tapering off and veering west to northwesterly by later Sunday morning into the afternoon. Mean sustained winds of 15 to 25 knots are expected with max gust speeds topping out in the 25 to 35 kt range along with significant wave heights of 2 to 5 feet.

BTV WATCHES/WARNINGS/ADVISORIES
VT...Winter Weather Advisory until 1 PM EST Sunday for VTZ003-004- 006>008-010-016.
NY...Winter Weather Advisory until 1 PM EST Sunday for NYZ026-027- 029-030-087.