textproduct: Burlington

WHAT HAS CHANGED

As of 207 PM EST Monday...Winter Weather Advisory in northern New York now ends at 5 AM. Most of the snow will end soon after midnight.

KEY MESSAGES

As of 150 PM EST Monday...

- Periods of snow showers with brief embedded heavier snow squalls possible later today and again on Thursday, creating localized hazardous travel.

- A significant and dangerously cold arctic outbreak is expected late week into the upcoming weekend with wind chill values 20 to 40 degrees below zero possible.

DISCUSSION

As of 150 PM EST Monday...

.KEY MESSAGE 1: A couple of deepening low pressure systems passing well to our northwest, and their associated cold fronts, will bring us scattered snow showers with embedded snow squalls possible later today and tonight, and again on Thursday. Some areas of light snow also are expected for tomorrow and Wednesday but less significant.

For today, difficult travel will be primarily in western portions of northern New York due to a combination of lake- effect and non lake- effect snow showers organized by a pre- frontal trough. Hi-res model guidance continues to show bands of intense snow waffling across St. Lawrence and Franklin counties, with heaviest snowfall amounts still in southern St. Lawrence County. However, compared to previous forecasts, the duration of snowfall has trended shorter, leading to slightly lower snowfall amounts in the Advisory area, now mainly 2 to 5 inches.

That being said, impacts are unchanged related to rapid changes in conditions when driving with sharp reductions in visibility and snow covered roads. This system's low track will support efficient mixing of southwesterly winds such that gusts of 30 to 40 MPH in much of northern New York will resulting in blowing of falling snow, with greatest risk between 5 and 9 PM. An additional surge of gusty winds may also occur closer to midnight, although snowfall chances will be diminishing at that time. East of the Adirondacks, and including all of Vermont, scattered snow showers are also expected but with more localized impacts with very brief heavy snowfall rates in a couple of showers. We have pretty large dew point depressions out there with low level dry air likely inhibiting valley snowfall potential and moisture transport; higher elevation passes are more likely to see significant precipitation tonight. It appears greatest chances for this activity is between about 7 and 10 PM.

The bulk of this event will be over with in the early morning hours, hence the earlier end time for the Winter Weather Advisory, as a single, intense snow band develops off of Lake Ontario soon after midnight and the westerly flow shifts precipitation south of our forecast area in New York. Additional chances for lake-effect, or at least lake-enhanced, snow showers during the day Tuesday will be focused more in Adirondacks and in central Vermont and with more limited impact. Strong cloud layer west-southwest flow and ample instability with steep low level lapse rates as an upper level trough passes through should be enough for at least some snowfall. This snow shower activity, as well as yet snow showers later Wednesday into Wednesday night, does not look capable of producing widespread travel concerns, but note gusty winds and localized low visibilities will be possible during these periods.

Another round of snow showers with brief embedded heavier snow squalls are possible on Thursday as 1004mb clipper like system passes to our northwest. The locally developed snow squall parameter indicates values in the 2 to 5 range on Thursday aftn, especially acrs northern NY into the mtns of northern VT. Sfc based CAPE values off the NAM12 show readings in the 50 to 100 J/kg range with some localized higher values, while moderate 925mb fgen forcing is present with arctic boundary. However, similar to previous events this winter, setup is not ideal for organized snow squalls, as sfc convergence is limited as boundaries come in a series of waves. Also, 700-500mb vorticity is shearing apart in the fast west/southwest flow aloft, instead of digging with a deepening mid/upper lvl trof acrs our fa. These factors suggest scattered to numerous snow showers are likely with a few heavier embedded snow squalls possible, especially northern NY, where some lake enhanced moisture/instability is possible. Temps with southwest flow should warm into the mid 20s to lower 30s. Given strong 925mb to 850mb southwest flow ahead of approaching boundaries, anticipate the typical downslope precip shadow here in the CPV and lower CT River Valley.

.KEY MESSAGE 2: Forecaster confidence continues to increase for a significant and dangerous arctic outbreak late week into this upcoming weekend. Little change has been noted with regards to thermal profiles with this 12z guidance package. Friday night into Saturday will be an advection type cold with dangerously cold wind chill values in the 20 to 40 degrees below zero with radiational cold on Sat night into Sunday. Extreme cold warnings and cold weather advisories will likely be needed for the forecast area.

The core of the coldest 925mb to 850mb temps with sub 490 thickness values are directly overhead btwn 03z and 12z Saturday. Progged 850mb temps range btwn -28C and -34C, while 925mb temps are in the -24C to -32C, which supports lows -5F to -20F with wind chills on Sat morning between -20F and -40F, coldest in the mountains. Highs on Saturday will struggle to reach 0F in the CPV with -5F to -10F in the mountain valleys. For Sat night a 1036 to 1040mb high pres builds into our cwa with very cold 925mb temps prevailing. This should allow for clear skies and light winds with lows ranging from -25F to -30F SLK/NEK to -10F to -20F elsewhere. Its interesting to view the NBM percentiles for this outbreak, which shows the deterministic value of -3F for MSS, -4F for SLK and +3F for BTV, while 50th percentile is -7F for MSS/SLK and -2F for BTV, while 90th percentile is 0F, 2F, and 8F respectively for highs Sat. For lows on Sat night the NBM deterministic is -18F, -21F and -8F, while 50th percentile is -27F, -30F and -12F for MSS/SLK and BTV respectively. I did trend toward the 50th percentile for both highs and lows during this time period. Temps slowly warm a few degrees on Sunday into Monday, but still well below normal for mid/late January. No significant snow is anticipated for late week into the upcoming weekend.

AVIATION /00Z TUESDAY THROUGH SATURDAY/

Through 00Z Wednesday...Currently a mix of VFR and MVFR conditions, with earlier snow shower activity becoming more focused in a single snow band in the Adirondacks. This snow will occasionally impact SLK through the next six hours, and intermittently at EFK, with other sites including MSS, PBG and RUT with low probabilities (10-30%) of brief MVFR snow through this timeframe. Have continued use of PROB30 and TEMPO groups to highlight these intervals of MVFR/IFR conditions thru the overnight.

Brisk south to southwest winds 15 to 25 knots will create some areas of blowing/drifting snow, and localized LLWS over the next few hours, along with some reduced sfc vis at MSS and SLK. Conditions improve to VFR with MVFR cigs in the mountains by midnight with mostly VFR by sunrise on Tuesday. Winds shift to the west/northwest by morning, with gusts increasing again during the day mainly 15 to 20 knots.

Outlook...

Tuesday Night: VFR. NO SIG WX. Wednesday: VFR. Windy with gusts to 35 kt. Chance SN. Wednesday Night: VFR. Windy with gusts to 35 kt. Chance SN. Thursday: Mainly VFR, with local MVFR possible. Chance SHSN. Thursday Night: VFR. Slight chance SHSN. Friday: VFR. Slight chance SHSN. Friday Night: VFR. NO SIG WX. Saturday: VFR. NO SIG WX.

EQUIPMENT

NOAA Weather Radio station WXM-44, transmitting from Mt. Ascutney, Vermont, on frequency 162.475 MHz is non-operational at this time. NWS technicians have diagnosed the problem, but repairs will likely not be able to occur for quite some time due to circumstances beyond our control. Therefore, the time of return to service is currently unknown. The following NOAA Weather Radio transmitters may be able to provide service during this outage: WWG 50 from Burke Mtn, VT at 162.425 MHz and WNG 546 from Hanover, NH at 162.525 MHz.

The Colchester Reef meteorological station is out of service. This site is not serviced by the NWS and there isn't an estimated return to service at present. Use extra caution when navigating the broad waters of Lake Champlain. Please contact us if you observe winds significantly deviating from the recreational forecast.

BTV WATCHES/WARNINGS/ADVISORIES

VT...None. NY...Winter Weather Advisory until 5 AM EST Tuesday for NYZ029-087.