textproduct: Burlington

WHAT HAS CHANGED

As of 206 PM EST Tuesday...No significant changes were made to the forecast.

KEY MESSAGES

As of 206 PM EST Tuesday...

1. Light to localized moderate snowfall associated with rounds of snow showers with possible snow squalls, primarily late Wednesday into Thursday and again on Friday, will produce some hazardous travel.

2. 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 144 PM EST Tuesday...

.KEY MESSAGE 1: A series of fronts, including a warm front tomorrow evening, a cold front on Thursday, and an Arctic boundary on Friday, will support snow showers through the rest of the week.

This afternoon, satellite imagery shows cloud streets along with a corridor of cooler cloud tops where light snow showers have been more numerous, downwind of an intense lake band in the Tug Hill plateau and also in far northwestern Vermont which also is seeing Lake Champlain and valley convergence effects. To some degree diurnally-driven instability is driving this activity so there will be some diminishing trends tonight, especially as the lake band retracts and shifts northward, such that snow returns to at least southern portions of St. Lawrence County overnight. Otherwise, the next round of light snowfall, which will be more widespread given the large scale forcing features significant 850 millibar warm air advection, will be in the form of up to a few hours of steady snow. Snowfall rates are most likely to be only up to 0.5"/hour, with greatest chances to approach 1"/hour rates and potentially impacting the Wednesday evening commute, in northern New York especially western and southern areas. Strong cloud layer winds (in the 40 to 60 MPH range) would point to fracturing of snow dendrites and reducing chances for seeing particularly high snow ratios despite otherwise favorable conditions. Current forecast in the 16 to 18:1 range might be a little high, but with uncertainties and generally low QPF/precipitation amounts think they are reasonable. Total snowfall through Wednesday night is mainly 0.5" to 2", except 2" to locally 4" in the aforementioned portions of northern New York where the heavier snowfall is favored.

For Thursday, model guidance remains consistent depicting the snow squall parameter increasing into the moderately high range due to instability driven by height falls ahead of an approaching cold front. Compared to the Monday event, the system looks a bit weaker in terms of winds/pressure rises and falls; earlier timing helps achieve greater surface instability which may compensate, leading to more snow shower activity but perhaps less intense. Since there will not a sharp low level thermal boundary or convergence (note winds will remain southwesterly for most locations Thursday night), it still looks like scattered activity, so it is hard to provide specific timing/location information. Generally A potent southwesterly jet, does reduce chances for appreciable snow shower activity in portions of the Champlain Valley downwind of the Adirondack high peaks, but this shadowing effect will be fairly localized and not as pronounced as during the Wednesday evening snow. Farther south, west-southwest flow will support more lake effect/hybrid snow shower activity that will probably necessitate another round of Winter Weather Advisories as heavier snow amounts in the 4 to 7 inch range currently are expected in total for Thursday through Thursday night. An intense band off of Lake Ontario is once again expected to develop behind this cold front Thursday night with favorable fetch and cold air advection, and for at least several hours will waffle into southern St. Lawrence County before sinking southward per model consensus and climatology.

Arctic cold frnt drops acrs our cwa on Friday with scattered snow showers and embedded snow squalls possible. The combination of lake moisture/instability and digging 5h s/w wl help to enhance snow shower activity along the boundary on Friday. Given multiple boundaries and cold air bleeding into our region, several rounds of snow showers are possible on Friday, which could have local impacts on travel. Soundings indicate the depth of moisture is rather limited, so areal coverage of snow showers wl be scattered.

.KEY MESSAGE 2: Forecaster confidence continues to increase for a significant and dangerous arctic outbreak this weekend. Extreme cold warnings and cold weather advisories will be needed for the forecast area late Friday into Sunday. 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.

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 -32C, while 925mb temps are in the -25C to -30C, 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. A progged 1040mb sfc high pres is expected to be centered over SLK at 06z Sunday, which should result in clear skies and light winds, with lows ranging from -20F to -30F SLK/NEK to -10F to -20F elsewhere. As always, the challenging part for Sat night-Sunday morning lows will be potential for high clouds in the westerly flow aloft and impacts on temps. For now have trended toward the NBM 50th percentile which has a low of -25F at SLK/Island Pond, -23F at MSS, and around -10F in the BTV area.

Have noted a slight shift northward for light snow late Sunday into early next week associated with southern stream system. I have my doubts if moisture can overcome 1040mb high pres anchored over northern New England to saturate the low levels enough to produce light snow, given progged track of sfc low pres well south of our cwa. For now I have increased pops into the schc/chc range, with plenty of time to shift northward if needed. Thinking most precip is virga, as low levels wl be very dry given prevailing north/northeast flow.

AVIATION /18Z TUESDAY THROUGH SUNDAY/

Through 18Z Wednesday...Localized lake effect snow continues to affect KSLK at this hour with reductions in visibility down to IFR and ceilings to MVFR, but the expectation is that improving conditions will begin developing by 20Z with VFR expected thereafter. Otherwise, all other terminals are VFR and will remain so through the period. Only other operationally impactful weather will be winds with surface gusts from the WNW up to 20kts this afternoon, and again from the SSW tomorrow. Additionally areas of LLWS are likely across northern New York during the overnight hours.

Outlook...

Wednesday Night: Mainly VFR, with areas MVFR possible. Windy with gusts to 30 kt. Chance SHSN. Thursday: VFR. Chance SHSN. Thursday Night: VFR. Slight chance SHSN. Friday: Mainly VFR, with local MVFR possible. Slight chance SHSN. Friday Night: VFR. NO SIG WX. Saturday: VFR. NO SIG WX. Saturday Night: VFR. NO SIG WX. Sunday: VFR. Slight chance SN.

CLIMATE

Record low maximum temperatures are possible on Saturday; below are the current records:

KBTV (Burlington area): -4 in 1907 KPBG (Plattsburgh area): 0 in 1976 KSLK (Saranac Lake area): -10 in 1936 KMSS (Massena): -5 in 2004 KMPV (Montpelier): 0 in 2004 SJBV1 (St. Johnsbury): -3 in 1907

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...None.