Area Forecast Discussion
Issued by NWS Burlington, VT

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000 FXUS61 KBTV 240022 AFDBTV Area Forecast Discussion National Weather Service Burlington VT 722 PM EST Thu Feb 23 2017 .SYNOPSIS... A weak cold front moving through the region this afternoon with bring cooler temperatures back to the area to end the week on Friday before abnormally warm temperatures ranging 15 to 25 degrees above normal return for Saturday. A stronger cold front moves through the North Country on Saturday with the combination of very warm temperatures and widespread rainfall around an inch leading to increased snowmelt and runoff which in turn will increase the potential for ice jams and river flooding. Cooler and drier weather returns for Sunday. && .NEAR TERM /THROUGH FRIDAY NIGHT/... As of 147 PM EST Thursday...What an unbelievable February afternoon with several high temp records broken with many locations in the mid/upper 50s and several in the lower 60s! As I`m writing this KBTV just broke the all time February high temp record sitting at 63F!!! Window for warming is closing rapidly though as a weak cold front is shifting through northern New York with some isolated rain showers and increased cloud cover associated with it. Front works its way through the area later this afternoon and early evening, stalling out over southern Vermont tonight with gusty southerly winds ahead of the boundary abating to nearly calm for most areas overnight. Low levels remain fairly saturated overnight and the combination of abating winds and warm temps over snowpack normally would lead to fog developing, but my confidence is low at this point based on mean mixed boundary layer winds in the 5-15kt range. Will allow the evening shift to assess that potential further. On Friday, the aforementioned stalled front lifts back northward as a warm front on the nose of a modest 30-40kt 850mb jet. Overrunning precipitation develops from southwest to northeast through the morning, and clearing the international border by the late afternoon. Latest NAM based guidance shows some minor instability along the boundary with slightly negative LI/SW`s so I can`t rule out a few rumbles of thunder. A dry Friday night is then expected in the warm sector ahead of a more impactful system on Saturday. Temps through the period remain mild for February standards with lows tonight ranging through the 30s and Friday night in the 40s. Fridays highs won`t be quite as warm as today, but still mid/upper 40s central/north and upper 40s to lower 50s central/south. && .SHORT TERM /SATURDAY THROUGH SATURDAY NIGHT/... As of 345 PM EST Thursday...very complex and dynamic situation on the way. 12z guidance not offering any significant variations on the prior forecasts, so confidence is fairly high in the overall scenario. Thought i`d break things down by components. Flooding/Hydrology -- see separate section in the AFD Temperatures -- looks like we will be solidly in the warm sector, with 925mb temperatures perhaps pushing 10-12C. Crazy for this time of year. Uncertainty in how much sun we`ll see, but even with more clouds than sun, all signs point to highs well into the upper 50s to lower 60s. With snow gone in the valleys, mid-60s is not out of the question. We may end up breaking some all-time Febraury temperature records that we just set today. Again crazy. Did shade toward the warmer side of guidance, but not the warmest. Potent front will roll across the region during the day. Not out of the question for a 20F degree drop in an hour. Tried my best to show this huge change by folding in some of the hi-res data from BTV 4km WRF, but uncertainty in specific frontal timing makes it difficult. Convection -- models all show instability that is surface based and aloft, at least enough to suggest isolated thunder is possible. However, strong jet, low level convergence and pressure rises associated with the front strongly suggest a line of convection will develop and accompany the front. Hi-res models show this orientation very well. Expect we will need to focus on radar and dust off some of our summer related procedures. Thinking as this line of primarily heavy rain showers moves through, we`ll have the potential for wind gusts 40-50 mph. Winds -- strong low/mid level southerly jet moves across the region. With enhanced channeling up the Champlain Valley and to an extent the St Lawrence Valley, frequent gusts in the 30-35 mph range are expected. As noted above, with the convective line there is the potential for localized stronger gusts. Behind the front, westerly winds will remain on the gusty side into Saturday night. Snowfall -- believe it or not, in a span of a few hours we`ll go from record heat to potential for some snow, especially the higher terrain. Tells you how strong this front is. At this point looks like any snow late Saturday/Saturday night will be at the higher elevations and be nothing more than 1-2". && .LONG TERM /SUNDAY THROUGH THURSDAY/... As of 345 PM EST Thursday...honestly, much of my focus was on Saturday into Sunday. Went with the model blend for the extended portion of the forecast as it looked like the 12z suite was in decent agreement with one another. Did note that mid-week it looks like another fairly potent system will push across the region. Could be a light wintry mix to rain situation per GFS, unless the 12z euro is correct. It`s got 925mb temperatures pushing 12-14C again, so if that pans out it`s another warm/wet system. The model blend I used, which also incorporates prior 00z runs indicated the cooler scenario to be more likely at this point, so I`ve got the snow/rain mix type of forecast. However don`t put too much stock in that right now. && .AVIATION /00Z Friday THROUGH Tuesday/...
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Through 00Z Saturday...Mainly VFR conditions are expected overnight as a weak ridge of high pressure will be over the region. A warm front will approach Vermont and northern New York on Friday. Expecting light rain showers to develop ahead of this warm front across the region between 15Z-18Z Friday, with areas of MVFR ceilings and visibilities developing across Vermont and northern New York by 18Z Friday and continuing through 00Z Saturday. 00Z Saturday - 15Z Saturday...Mainly VFR/MVFR. Light winds becoming gusty from the south 12-15Z Saturday. 15Z Saturday - 12Z Sunday...MVFR/IFR in rain with embedded heavy rain and possible thunder. Very strong southerly winds with localized areas of shear and turbulence likely. 12Z Sunday - 00Z Tuesday...Mainly VFR under high pressure. 00Z Tuesday - 00Z Wednesday...VFR with chance MVFR/IFR snow showers.
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&& .HYDROLOGY... As of 330 PM EST Thursday...The flood watch remains in place and effective from Friday evening all the way to Sunday evening. Still looking like a very warm stretch producing quite a bit of snow melt combined with some rain on top for good measure will be enough to produce significant rises on nearly all rivers and streams across the region. As of this morning, there was still a decent snowpack across the Adirondacks and most of Eastern VT, with depths averaging 12-30 inches. Latest guidance suggests we`ll melt enough snow to the equivalent of 3-4" of rainfall by Saturday. Given the all-time February temperature records set Thursday and perhaps again Saturday along with fairly high dewpoints, this seems quite reasonable. Add to that a period of moderate/heavy rain showers on Saturday that could produce another 1/2 to 1" of liquid, we are basically dealing with the equivalent of a 3-5" rainstorm. That`s a lot of liquid for our rivers to handle. And the ground is also frozen, so not much will soak in. Complicating matters is that some rivers, especially across northern/northeast VT and in the Adirondacks are still ice covered. Thus as the water levels rise and the ice weakens, we`ll start to see ice movement. Which could very well then get stuck in bends in the river or along bridges or where the rivers go from a steeper to more gentle slope. By their nature, ice jams are impossible to predict but can result in localized flooding occurring quickly. In general, it takes the river to rise 1.5 to 3 times the thickness of the ice to start the breakup process. Based upon NERFC forecasts as well as ensemble river simulations we continue to look at the Ausable @ Ausable Forks, Winooski @ Essex Jct., and the Mad River @ Moretown for the primary focus of more significant river flooding. Best river response will be late Saturday through Saturday night due to runoff from snowmelt and expected rainfall. Thickest river ice is in place across nern NY basins and nrn VT, including the Chazy, Ausable, Winooski, Lamoille, Missisquoi, and Passumpsic. Uncertainty in the river forecasts are due to unknown exactly how much snow melt we are getting and how much rainfall we will see on Saturday. Those uncertainties are pretty large, so expect forecast changes as we move closer to the event. && .BTV WATCHES/WARNINGS/ADVISORIES... VT...Flood Watch from Friday evening through Sunday evening for VTZ001>012-016>019. NY...Flood Watch from Friday evening through Sunday evening for NYZ026>031-034-035-087. && $$ SYNOPSIS...Lahiff NEAR TERM...Lahiff SHORT TERM...Nash LONG TERM...Nash AVIATION...WGH/Lahiff HYDROLOGY...Banacos

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