Area Forecast Discussion
Issued by NWS Burlington, VT

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000 FXUS61 KBTV 182056 AFDBTV Area Forecast Discussion National Weather Service Burlington VT 356 PM EST Mon Dec 18 2017 .SYNOPSIS... A warm front extending across northern New York and Vermont will maintain overcast skies with occasional periods of snow through this evening, transitioning to intermittent snow showers or patchy freezing drizzle overnight. Stronger south to southwest winds develop on Tuesday as the warm front lifts northward into southern Quebec. This will allow temperatures to warm into the upper 30s to lower 40s in most locations. A few additional rain and snow showers is expected, mainly late Tuesday into Tuesday night with passage of a cold front. The next chance for significant precipitation will arrive Friday afternoon into Saturday. A low pressure system expected to track northeastward across the St. Lawrence Valley is expected to bring moderate mixed wintry precipitation, likely transitioning to plain rain as warmer air moves in on south winds. && .NEAR TERM /THROUGH TUESDAY NIGHT/... As of 200 PM EST Monday...Warm frontal zone in place across the North Country with low overcast and ongoing periods of intermittent light stratiform snow. Surface warm front extends from Lake Ontario ewd across e-central NY, and will lift slowly nwd during the overnight hours with SW 850mb flow increasing from 25kt to 35kt. Appears most of the light accumulating snowfall will occur thru 03Z or so, with a weak 700-500mb shortwave transversing the frontal zone late this afternoon thru this evening. Snowfall amts haven`t changed much from previous forecasts...generally 1-2", except locally around 3" for the St. Lawrence Valley and nrn Adirondacks, as well as the spine of the Green Mtns. Some travel slowdowns are expected for the evening commute with light snowcover on roadways. Considerable mid-level drying occurs after 03Z this evening per NAM/GFS BUFKIT profiles, but expect low stratus to remain in place. Absence of deeper saturation suggests just intermittent light precipitation overnight. Will see temperatures rise near to just above freezing across wrn VT and the Adirondacks/srn St. Lawrence County, with lower temps in the mid 20s across the St. Lawrence Valley and east of the Greens with antecedent cold air in place. Given thermodynamic profiles, could see some patchy drizzle/freezing drizzle at times as well, and included mention in the forecast for the overnight/pre-dawn hrs. On Tuesday, mid-level trough crosses Ontario/swrn Quebec, with increasing SW gradient flow across nrn NY and VT. Should see sustained winds 10-20 mph, and wind gusts 20-30 mph during the late morning thru late afternoon hrs Tuesday, and temperatures rising to the upr 30s to lower 40s. Best large-scale forcing remains north of the intl border, but a few rain/snow showers are possible thru the day. Weak frontal passage and mid-level height falls will bring about a better chance for snow shower activity 21Z Tue-03Z Wed, with some light accumulation possible across the nrn mtns. Modest low-level CAA Tuesday night with lows falling into the low-mid 20s across nrn NY, and mid-upr 20s for VT. && .SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/...
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As of 348 PM EST Monday...Orographically forced snow showers will taper off throughout the day Wednesday as a surface ridge noses in from the northwest. Daytime highs in the 20s to low 30s will occur in the morning hours, followed by cooling throughout the early afternoon through overnight hours as cold air advection persists. The majority of the snow showers will come to an end by Wednesday night. There will still be enough cloud cover around to moderate overnight lows slightly, but areas with less cloud cover will radiate out the most effectively and see the lowest nighttime temperatures. Wednesday night will be around 20 degrees F colder than Tuesday night with lows in the single digits to low teens throughout the forecast area. Drier and colder weather will continue Thursday as building high pressure brings widespread subsidence to the area. Thursday night will feature increasing cloud cover as the next weather maker approaches from the southwest.
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As of 348 PM EST Monday....The next system to impact the forecast area poses a variety of forecast challenges for the Friday/Saturday forecast. A shortwave trough/surface low quickly lifting northeastward out of the Plains will move to the Great Lakes Region by Friday afternoon and continue northeastward into southern Quebec by Saturday morning. The low will spread precipitation over the forecast area Friday through Saturday, however models continue to struggle to come to a consensus on the exact track and timing of the low. The 06Z and the 12Z runs of the GFS and the 12Z ECMWF have increasingly shifted the track of the low further east. This shift in placement moves the low through southeastern Ontario and then tracks it northeastward nearer to the Saint Lawrence Valley. For reference, yesterday`s runs were tracking the low further west over Michigan and into Southcentral Ontario. The main impact of this eastward shift would be to place the forecast area less solidly in the warm sector of the cyclone (with widespread rain and little to no mixed precipitation) and closer to the center of the low (with more of a prolonged mixed precipitation transition). The weaker warm air advection associated with this shift would be slower to scour out the cold air at the surface. Model soundings are keying in on to this possibility, indicating the potential for a warm nose moving in with colder air holding on at the surface. This thermal profile would support some areas of freezing rain Friday, especially in the Saint Lawrence Valley where pressure gradient driven northeasterly surface winds will keep surface temperatures below freezing a little longer. Valley areas to the east of the Green Mountains in Vermont will also be slower to warm at the surface, and could also see some freezing rain out of the system. Will continue to monitor trends in the cyclone location over the coming days and will be adjusting precipitation types accordingly. In addition to precipitation type challenges, QPF amounts and precipitation timing going into the weekend are also fairly uncertain. We will likely see some light warm air advection snow Friday morning before the warmest air moves in, however the amount of snow on the front end of the system before the precipitation type transition remains uncertain. By Saturday morning however, most of the remaining cold air should be scoured out and the potential for most areas to see some rain is looking increasingly likely. It`s too early to peg down specific QPF amounts, but fairly strong southerly moisture advection feeding into the system along the warm conveyor belt could result in some moderate rainfall. Saturday night, the attendant cold front will push through and temperatures will return back down to near seasonal norms by Sunday. Some light snow showers are possible after the cold front passes Sunday. Looking ahead to early next week, much uncertainty remains. There is the potential for another low to track up the East Coast Monday, so after our rain on Saturday there still may be some hope on the horizon for a white Christmas. The deterministic GFS and ECMWF are both keying in on this feature, but the GFS ensembles are showing a fairly high level of uncertainty with the development and evolution of this low. Of greater confidence in the long term however is a much colder airmass moving in Sunday, leading to a return to seasonal or below seasonal temperatures to start the week.
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&& .AVIATION /21Z MONDAY THROUGH SATURDAY/... Through 18Z Tuesday...Abundant MVFR to IFR conditions expected during the next 24 hrs. Will see a combination of factors...including periods of 1-2SM -SN through 00Z. During the late evening and overnight period, will see cloud ceilings lowering with occasional periods of very light mixed precipitation, including a slight chance of freezing drizzle. Ceilings will generally range from 1-2kft through 12Z, IFR ceilings likely at KSLK in prevailing swly low-level upslope flow conditions. HIR TRRN OBSCD through the period. Will see some improvement back to MVFR-VFR ceilings mid-late morning Tuesday as winds increase from the south and southwest to 10-15kts, with gusts to 25kt expected at MSS/BTV with valley channeled flow regime 15-18Z Tuesday. Some minor impacts to aviation ground ops expected with 1-2" snow accumulation through 00Z, and outside chance for some freezing drizzle overnight. Outlook... Tuesday Night: VFR. Chance SHSN, Chance SHRA. Wednesday: VFR. Slight chance SHSN. Wednesday Night: Mainly VFR, with local MVFR possible. NO SIG WX. Thursday: VFR. NO SIG WX. Thursday Night: VFR. Slight chance SHSN. Friday: Mainly MVFR, with local IFR possible. Chance SHSN. Friday Night: Mainly MVFR, with areas IFR possible. Likely SHSN, Likely SHRA. Saturday: Mainly IFR, with local MVFR possible. Definite SHRA. && .BTV WATCHES/WARNINGS/ADVISORIES... VT...None. NY...None. && $$ SYNOPSIS...Banacos NEAR TERM...Banacos SHORT TERM...RSD LONG TERM...RSD AVIATION...Banacos is the U.S. government's official web portal to all federal, state and local government web resources and services.