Area Forecast Discussion
Issued by NWS Burlington, VT

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000 FXUS61 KBTV 241800 AFDBTV Area Forecast Discussion National Weather Service Burlington VT 100 PM EST Fri Feb 24 2017 .SYNOPSIS... A brief cool down today before abnormally warm temperatures ranging 15 to 30 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. More seasonable weather returns Sunday with some light snow in the higher elevations before a quieter period of weather to start next week. && .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/... As of 923 AM EST Friday...Going forecast is in excellent shape this morning with only some minor tweaks needed to sky cover and hourly temps at this time. Regional radar shows some light echoes working into the St. Lawrence Valley and should increase in areal coverage across the rest of northern New York through the morning and into central/northern Vermont from around the noon hour onward. Previous forecast has all this covered very well, so no changes there. Previous Discussion...Friday sees cooler temperatures compared to yesterday, but temps are still ranging 10-20 degrees above normal. This will continue to lead to ice breakup in the rivers, especially northeast New York and areas along and north of Route 2 in Vermont where river ice still prevails. Adding to this, a warm front associated with a low pressure system over the Central US will push up into the region during the day before lifting north of the border by early morning Saturday. Models continue to show instability both surface based and aloft which to lead to some thunderstorm activity to go along with showers. QPF will generally be around two tenths to a quarter of an inch, but some areas that see convection could see some localized higher amounts. This extra water along with the continued warm temps will lead to further rises for rivers through Friday night, prepping things for the second round of precipitation on Saturday. High temperatures for today will range in the 40s up north to upper 50s to 60 in the southern counties. && .SHORT TERM /6 AM SATURDAY MORNING THROUGH SUNDAY/... As of 428 AM EST Friday...Very active and dynamic period of weather expected with near record to record warmth, gusty south winds, and potential river flooding concerns Sat/Sat Night. We`ll actually see post-frontal precipitation end as snow showers with some light accumulations expected across the higher terrain Saturday night into Sunday morning. The most critical aspect of the weekend weather though continues to be related to potential flooding (please refer to hydrology section below for full details relating to flooding potential). Flood watch remains in effect 00Z Saturday thru 00Z Monday. Deepening low pressure system near Georgian Bay at 12Z Saturday will lift NNEWD into Ontario well to the west of the North Country. Strengthening low-level southerly gradient flow east of the low will allow surface warm front to lift north of the intl border early Saturday morning, with very warm 850mb temperatures of +8 to +10C moving into the region. Good low-level turbulent mixing should allow low-clouds to scour out of the region, especially as 925mb winds increase to 40-50kt. Should result in period of filtered sunshine (thru high cloudiness) with sufficient heating to bring valley temps to record levels in the low-mid 60s (record high for Saturday at BTV is 56F). Even by 12Z Saturday, should see temps already into the lower 50s across the Champlain and St. Lawrence Valleys as low-level sly winds increase. Thru 18Z or so, looking at a partly sunny and windy day, with south winds 15-25mph with gusts 35-45 mph, likely strongest in the Champlain Valley during Saturday aftn with valley channeled flow. Strong low-level convergence expected along cold front approaching from the west...reaching St. Lawrence county around 18Z Saturday, and across the Champlain Valley by 22-00Z, and generally east of VT by 03Z Sunday. High resolution models, including the BTV4kmWRF, and NCEP 4km NAM suggest a reflectivity fine line associated with low CAPE and strong dynamical/frontogenetic forcing right along the bndry. With PW values near 1" along the front, looking for a brief period of moderate to heavy precip, with possible embedded convective elements. This rainfall will enhance runoff and bring main threat of minor to locally moderate flooding late Saturday into Saturday night. Overall rainfall amts generally 0.50 to 0.75", with locally up to 1" with orographic enhancement across the nrn Adirondacks. Negative tilt mid-level trough and sfc bndry shift east of our region after 03Z Sunday, with strong low-level CAA and a wly wind shift following FROPA. Should see rain end as a period of wet snow or snow showers, especially across the higher terrain during Saturday night, with temperatures falling into the upr 20s to lower 30s by daybreak Sunday. Some anafrontal character to the frontal zone per NWP time-height cross sections, and as such, should see 1- 3" snowfall across the higher summits late Saturday night as vertical temperature profiles cool sufficiently to support snow as the p-type. Even in valley locations, may see precipitation end as isold/sct snow showers, but with little or no accumulation expected. Will see sfc high pressure cresting into the nern CONUS by Sunday afternoon. Prevailing W-NW low-level flow on Sunday will result in much cooler conditions with afternoon highs generally in the low-mid 30s, except close to 40F in the CT River Valley with local downslope effects. Should see gradual clearing as the day progresses on Sunday. && .LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/... As of 428 AM EST Friday...A relatively fast and active WSWLY mid-level flow regime will persist through the extended forecast period, though the 00Z ECMWF and 00Z GFS differ on timing and track of embedded shortwave troughs potentially affecting the North Country. It appears that surface high pressure will remain in control Sunday night through 18Z Tuesday, providing a brief period of mainly dry weather conditions across the North Country. Should see lows in the 20s Sunday night, followed by highs in the low-mid 40s for Monday, and lows again in the 20s Monday night. Developing low pressure in SWLY 500mb flow takes shape across the lower OH river valley and srn Great Lakes by 00Z Wednesday. Will see ewd extending warm front bringing initial surge of isentropic ascent and precipitation later Tuesday afternoon through Tuesday night based on 00Z GFS. It appears primary low track is to our west, with late development of a secondary low off the Maine coast later Wednesday. Overall synoptic evolution favors initial mixed wintry precipitation, changing to rain, and perhaps ending as snow showers toward Wednesday evening into Thursday morning with deep layer NW flow as secondary low departs toward the Gulf of St. Lawrence. Given potential track/timing uncertainty, current forecast calls for a rain/snow mix, though a period of sleet/freezing rain is possible in advance of the warm front as precipitation commences. Maintained idea of likely PoPs (60-70%) Wednesday into the first half of Wednesday night. Overall, temperatures during the long-term period will remain above seasonal averages as we head into the beginning of March. && .AVIATION /18Z Friday THROUGH Wednesday/... Through 18Z Saturday...Light rain showers continue to affect the North Country this afternoon with VFR/MVFR conditions. Winds generally light, less than 8kts into this evening. MVFR to IFR ceilings are expected for most terminals tonight before a brief break to VFR possible 06Z-12Z. Heading into late morning/early afternoon, strong cold front will bring rain...heavy at times...to the North Country. Expect this rain to move west to east...affecting KMSS and KSLK starting around 16z-19z Saturday. North to northwest winds today will shift south overnight with an increasing low level jet moving over the area early Saturday morning. Have included LLWS for KPBG/KSLK/KRUT before winds mix to the ground, creating gusts of 20-30kts by mid morning. Outlook 18Z Saturday through Wednesday... 18Z 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 - 12Z Tuesday...Mainly VFR under high pressure. 12Z Tuesday - 00Z Thursday...VFR with chance MVFR/IFR snow showers. && .HYDROLOGY... As of 330 AM EST Friday...The flood watch remains in place and is effective from this 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 Thursday, 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 (approaching 50F), this seems quite reasonable. Add to that a period of warm frontal precipitation Friday aftn, and the 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 forecast changes are possible as we move closer to the main event on Saturday. && .BTV WATCHES/WARNINGS/ADVISORIES... VT...Flood Watch from 7 PM EST this evening through Sunday evening for VTZ001>012-016>019. NY...Flood Watch from 7 PM EST this evening through Sunday evening for NYZ026>031-034-035-087. && $$ SYNOPSIS...MV NEAR TERM...Lahiff/MV SHORT TERM...Banacos LONG TERM...Banacos AVIATION...KGM HYDROLOGY...Banacos

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