Area Forecast Discussion
Issued by NWS Burlington, VT

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000 FXUS61 KBTV 150255 AFDBTV Area Forecast Discussion National Weather Service Burlington VT 955 PM EST Thu Dec 14 2017 .SYNOPSIS...
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High pressure over the North Country today will continue to provide the region with clear skies and very cold temperatures tonight and Friday. An upper trough and surface low passing northwest of the region will bring a chance for snow showers for Friday night through Saturday, with the weekend ending cool and dry on Sunday.
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&& .NEAR TERM /THROUGH FRIDAY NIGHT/...
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As of 955 PM EST Thursday...With boundary layer decoupling temps in sheltered areas continue to fall as low as -13 at Bakersfield VT and Newark VT -9 already, 5 above to 5 below elsewhere except warmer near Lake Champlain with continued westerly winds. Continue to try to adjust sheltered valleys down cooler than the higher terrain with good noctural inversion but with much variation. Clear skies and fresh snow cover will provide a wide range of temps from -20 in the coldest locations to single digits near Lake Champlain. A bit more changeable weather expected Later Friday and Friday night as high pressure shifts east of the area by Friday afternoon, while an upper trough and associated weak surface low approach from the Great Lakes. Surface and mid- level flow backing to the south/southwest will usher in closer to normal air with temps rising into the teens and 20s, but also will increase the chances for lake effect snow showers up the St. Lawrence River during the afternoon and evening hours. As the trough swings over the forecast area Friday night, lake effect is replaced by more widespread scattered snow showers as the flow turns more west/northwest. Best chance at any accumulating snow is with the lake effect along the river shoreline Friday, and in the northwest upslope regions of the Adirondacks and Greens Friday night where a dusting the perhaps 2" is possible. Lows Friday night will range through the teens.
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&& .SHORT TERM /SATURDAY THROUGH SATURDAY NIGHT/... As of 332 PM EST Thursday...No big changes to the forecast after reviewing the latest guidance. Still looking for a cold front to trek across the region during the morning to mid-day hours. That does make the hourly temperature forecasts a little tricky, so did try to lean on some of the hi-res model timing. The front, and associated upper level trough, will trigger some snow showers during the morning, especially across higher terrain. As the flow turns west/northwest with the trough passage, some of what should be a potent Lake Ontario effect snow streamer will get caught up in that flow and transported into the south- central Green Mountains. Could be a situation where places like Killington pick up a few inches. BTV4km and even the NAM 12km model suggest this very scenario, so nudged the hourly PoPs with their output. Also supporting the idea that we`ll get snow showers into Vermont is the relatively high values of the Snow Squall Parameter -- meaning we`ll have some convective instability and enough moisture around to generate snow showers. Not a lot of moisture to work with, but with SLRs of at least 20:1, we`ll have some light fluffy snow accumulations. I came up with about 1" in the higher terrain of the `Dacks and northern Greens, with 2-4" in south- central Green Mountains on Saturday. All the snow showers fade away by late in the day. Good amount of subsidence aloft should result in fairly clear skies overnight. Could see good radiational cooling in spots, and I did trend toward the colder guidance for places like Saranac Lake and the northeast Kingdom where sub-zero readings are expected. && .LONG TERM /SUNDAY THROUGH THURSDAY/... As of 332 PM EST Thursday....Still looking like, in general, an unsettled week. Biggest weather maker still appears to scheduled for Tuesday into Tuesday night based upon the 12z guidance. It`s a progressive synoptic flow, and as a result, the primary models (GFS and ECMWF) differ in timing on the order of 12 hours or so especially Tuesday onward. That does lead to a rather significant amount of uncertainty (eg: is the precipitation coming in at night or during the day) to the forecast. For the most part, I stayed close to the model blend. That does result in precipitation chances being more generalized and spread over time than what will likely occur. However, that`s the nature of trying to forecast in higher uncertainty situations. A few notes for each day follow: Sunday: Quiet and chilly, but with more than typical amounts of December sunshine. Monday: GFS says dry, albeit increasing clouds as warm air advection starts. 12z Euro has a weak shortwave come across with a few snow showers. Given previous GFS runs did have at least some precipitation, that probably means a ECMWF scenario may be what we end up seeing. So maintained low chances of snow showers. Tuesday: Guidance continues to show burst of warm air aloft as strong southerly low/mid level winds advect the airmass in from the south. With trough moving into the region, we`ll also have precipitation breaking out. Still looking like there could be some wintry mix (eg: sleet perhaps a touch of freezing rain) before we turn over to rain in most areas. Those details of exactly when and where we could have some mix is still rather fuzzy. The ECMWF solution is a little cooler and would suggest a bit higher chance for mix. At this point, kept with a snow to rain situation without any additional wintry p-types in the gridded forecast. As we get closer, we`ll be able to get a better understanding of how the temperatures aloft will line up which will guide us to the wintry mix potential. Even so, the system doesn`t look like it will bring any heavy precipitation, so any wintry mix impacts would probably be minor in nature. Wednesday: As Tuesday weather maker passes by, colder air will move on in as the flow turns northwest. GFS is quicker with the frontal passage Tuesday night and first thing Wednesday, but has more low level moisture meaning scattered snow showers for much of the day. ECMWF dries things out quicker so by afternoon any snow showers would be ending. Again, not a big weather maker -- looks like run of the mill higher terrain snow shower situation with light accumulations. Kept with model blend, so chance level PoPs for the entire day at this point. Thursday: Seems generally quiet, perhaps a little on the cool side. ECMWF starts to bring in clouds and moisture (light snow) late in the day as a warm front to our south starts to move north ahead of the next low pressure. GFS is slower with this scenario. Split the difference with low PoPs for western areas. && .AVIATION /03Z FRIDAY THROUGH TUESDAY/... Through 00Z Saturday...VFR conditions will largely continue through the period for VT and the Champlain Valley of NY. MVFR possible for KSLK and KMSS aft 18z. Clear skies through midnight will give way to increasing mid/high clouds from the west/southwest with a gradual lowering aft 12z to 040-080 by 18z. Snow showers in the vicinity of Lake Ontario and St.Lawrence valley during Friday, gradually shifting into Adirondacks aft 00z Sat. WNW winds 5-8 kts this evening, becoming lgt/vrbl by midnight then eventually SSW around 5 kts by 12z. Outlook... Friday Night: Mainly VFR, with local MVFR possible. Chance SHSN. Saturday: Mainly VFR, with local MVFR possible. Chance SHSN. Saturday Night: VFR. NO SIG WX. Sunday: VFR. NO SIG WX. Sunday Night: VFR. Chance SHSN. Monday: VFR. Chance SHSN. Monday Night: Mainly MVFR, with local IFR possible. Chance SHSN. Tuesday: Mainly MVFR, with local IFR possible. Chance SHRA, Chance SHSN. && .BTV WATCHES/WARNINGS/ADVISORIES... VT...None. NY...None. && $$ SYNOPSIS...Lahiff/Sisson NEAR TERM...Lahiff/Sisson SHORT TERM...Nash LONG TERM...Nash AVIATION...SLW

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