Area Forecast Discussion
Issued by NWS Burlington, VT

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000 FXUS61 KBTV 251149 AFDBTV Area Forecast Discussion National Weather Service Burlington VT 649 AM EST Sat Feb 25 2017 .SYNOPSIS... Areas of dense fog will dissipate during the morning hours ahead of a strong cold front that traverse through the North Country this afternoon. Very warm temperatures ahead of this front combined with widespread rainfall will lead to increased snowmelt and runoff which in turn will increase the potential for ice jams and river flooding through the weekend. 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 /THROUGH TONIGHT/... As of 422 AM EST Saturday...Quite the 24 hours expected today as a multitude of forecast challenges exist with fog this morning, the potential for strong winds with a cold front, the threat of ice jam and snowmelt flooding, and snow in the higher elevation sites late this evening. To begin, areas of dense fog continue this evening with many sites reporting 1/2 mile or lower visibilities. A warm front is lifting north however the warm air is overrunning the colder surface air in the Champlain, Connecticut, and Saint Lawrence valleys. As such a strong low level inversion is playing out over the northern portion of the forecast area. Obs in immediate Champlain Valley and Connecticut river valleys are showing in the mid 30s, whereas at higher elevations above 1500 feet, temps are already in the upper 40s to low 50s. Based on the latest RAP and hires BTV wrf models, that warm front has slowed down significantly and isn`t expected to reach the Canadian border until shortly before daybreak. Thus the fog, dense at times, will likely continue through the rest of the overnight hours in the lower elevation sites, however we can expect gradual improvement over the next couple of hours. As the warm front lifts north, we will be solidly in the warm sector of the cyclone. The source region for our airmass is the same air that set records all across portion of central New York yesterday. With 925mb temps still projected to be +10-12C expect low to mid 60s for highs which also puts us in record territory for heat. So for the next installment, a well developed strong cold front will be tracking through the North Country this afternoon and evening. Guidance has been remarkably consistent dating back to Tues-Wed of the past week with the idea that a convective fine line will develop this afternoon and bring a narrow ribbon of moderate to heavy rainfall to the region as the anafront tracks across the North Country. Strong surface convergence and decent surface/elevated instability will combine to produce that aforementioned fine line which should begin to move into the Saint Lawrence valley between 17-19Z. The system will continue to propagate towards the east and should reach the Champlain Valley between 23z-01z. Expect timing the initial precip across eastern Vermont to be between 01-03z. With the convective nature of the rainfall, we are anticipating periods of heavy rainfall. That heavy rainfall, combined with the large amounts of snowmelt will cause some ice movement along the rivers. Thus the FLOOD WATCH remains in effect through Sunday with the full details in the hydrology section below for potential flooding. The pressure gradient with this frontal system will be quite strong and so ahead of the front expect strong and potentially gusty southerly winds. 850mb winds based on a local 2km wrf are as high as 50-60kts ahead of the front! This will be especially true on the east side of Lake Champlain as the winds become funneled with 15-25mph winds likely and gusts as high as 45mph. Then as the convective fine line develops and advances through Northern New York there is some potential for severe winds to be associated with the line. The Storm Prediction Center has put portions of southern Vermont and west in a slight risk for severe weather with a 15% prob for damaging winds. Most unstable CAPE is only in the 250 J/kg range but being winter and seeing how strong the low level convergence and lift are, its possible that a portion of the fine line will bow out bringing damaging winds. That threat will have to be monitored as we continue through the day. The dynamics fall apart though as the front moves east of the Champlain valley so the main threat for damaging winds will be for locations generally west of the Champlain valley. Behind the front, the cold air advection surges in and we will see 25-35 degree diurnal swings with lows in the mid 20s expected. Thus the precip lingering behind the anafront will likely end as end as a period of rain/snow in the valleys and heavy wet snow in the higher elevations where several inches of accumulation are possible. && .SHORT TERM /SUNDAY THROUGH MONDAY/... As of 348 AM EST Saturday...Upper trough axis is east of the area Sunday morning and flow aloft through Monday will generally be from the west. Cool down will be brief with highs in the 30s to around 40...which is a few degrees above normal...but we quickly warm right back into the 40s on Monday with a few 50 degree readings possible. Moisture and forcing will be limited Sunday through Monday. Could see some mountain showers on Sunday...with dry weather Sunday night...then a few mountain showers on Monday. && .LONG TERM /MONDAY NIGHT THROUGH FRIDAY/... As of 348 AM EST Saturday...No significant weather expected through the period. Flow aloft becomes southwest Tuesday through Thursday with above normal temperatures expected during this period...especially on Tuesday and Wednesday. Highs will be in the mid 40s to mid 50s. Dry weather is expected for the most part on Tuesday...but the chances for showers will increase Tuesday night and especially Wednesday and Wednesday night as upper trough axis approaches the region. Temperatures will be warm enough for the precipitation to be in the form of rain. A return to more seasonal temperatures will take place late Thursday into Friday with the passage of the upper trough and west to northwest flow aloft developing over the area. Cannot rule out the possibility for some showers Thursday night into Friday and temperatures would generally cold enough for the precipitation to be in the form of snow showers. && .AVIATION /12Z Saturday THROUGH Wednesday/... Through 12Z Sunday...Overall a mix of VFR and MVFR ceilings through 18z...but periods of LIFR/VLIFR ceilings at KMSS and KPBG where low clouds and fog will exist through about 14z. The fog will create LIFR/VLIFR visibilities at these two locations through 14z as well with VFR visibilities everywhere else through 18z. Lastly this morning...most locations will see some low level wind shear with south winds at 2000 feet in the 35 to 40 knot range. Winds at the surface will become rather gusty later this morning with much of the remaining period seeing south to southwest winds with gusts in the 20 to 30 knot range. A line of shower will move across the area between 18z and 02z. Rainfall will be heavy at times with this line and this will create MVFR/IFR ceilings and visibilities. After 02z... conditions will improve with VFR visibilities developing and ceilings in the VFR/MVFR categories. Outlook 12Z Sunday through Wednesday... 12Z Sunday - 12Z Tuesday...Mainly VFR under high pressure. 12Z Tuesday - 00Z Thursday...VFR with chance MVFR/IFR rain showers. && .HYDROLOGY... As of 430 AM EST Saturday...The flood watch remains in place and is effective through Sunday evening. The record setting warmth from Thursday through Saturday has already melted out a significant portion of our snowpack with more warm air this afternoon. The snow melt will combine with periods of heavy rainfall associate with a cold front and produce significant rises on nearly all rivers and streams across the region. 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 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. && .CLIMATE... Several more record high temperatures are expected to be broken tomorrow on February 25, 2017. Here are the current records for our long standing sites: Burlington, VT 55|1985 Montpelier, VT 59|1985 St. Johnsbury, VT 60|2016 Massena, NY 50|1956 Mount Mansfield, VT 49|1961 && .BTV WATCHES/WARNINGS/ADVISORIES... VT...Flood Watch through Sunday evening for VTZ001>012-016>019. NY...Flood Watch through Sunday evening for NYZ026>031-034-035-087. && $$ SYNOPSIS...Deal NEAR TERM...Deal SHORT TERM...Evenson LONG TERM...Evenson AVIATION...Evenson HYDROLOGY...Banacos CLIMATE...Lahiff

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