Area Forecast Discussion
Issued by NWS Burlington, VT

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000 FXUS61 KBTV 250058 AFDBTV Area Forecast Discussion National Weather Service Burlington VT 758 PM EST Fri Feb 24 2017 .SYNOPSIS... Isolated to scattered rain showers associated with a warm front lifting northward into the region this afternoon will dissipate during the evening hours with a mainly dry night expected ahead of a strong cold front which will move through the North Country on Saturday. 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 SATURDAY NIGHT/... As of 609 PM EST Friday...Main change for 630 pm update is to add some mention of fog across the area, dense in some areas. Have also adjusted temps to reflect current obs. Previous discussion follows. Forecast is playing out well this afternoon with this morning`s stalled front near the VT/NH/MA border is providing isolated to scattered showers across central and northern portions of the North Country as the boundary slowly moves back northward as a warm front. Widely variable temperatures have been observed across the forecast area today with mid/upper 30s in the northern St. Lawrence Valley, to 50s in the Northeast Kingdom of Vermont, to lower 70s in far southern Vermont in the north Bennington area which broke out into the warm sector and some sun. As the boundary continues to move north tonight, in general precipitation will wane and surface temps will gradually rise to the upper 40s and low 50s by sunrise tomorrow. Exception will be the northern St. Lawrence and northern Champlain Valley`s very shallow north-northeasterly flow will keep temps locked in the upper 30s to lower 40s through much of the night. Saturday and Saturday night continue to look like a very active and potentially hazardous period of weather with continued record warmth, gusty winds, and widespread moderate to heavy rain. The most critical aspect of the next 36 hours continues to be the flood potential due snow melt and ice breakup/jams with a FLOOD WATCH remaining in effect from tonight through Sunday. Please refer to hydrology section below for full details relating to flooding potential. Synoptic setup hasn`t changed a bit from previous thinking with a deepening low pressure system near Georgian Bay at 12Z Saturday lifting northeastward to the eastern tip of James Bay by 00z Sunday. Strengthening low-level southerly flow east of the low and ahead of a strong cold front will allow the aforementioned surface warm front to lift north of the international border early Saturday morning, with very warm 925mb temperatures of +10-12C moving into the region. Already in the warm sector this afternoon we`re seeing sunshine and temperatures well into the 60s and even low 70s so there`s no doubt in my mind that the North Country will see some filtered sunshine tomorrow and sufficient heating to produce another round of record high temps area-wide in the low-mid 60s (see our climate section below for the current records). Through 16-17Z or so expect not much in the way of precipitation in the warm sector ahead of the approaching cold front, but it will be rather windy with southerly winds of 15-25 mph with gusts 35-45 mph, especially in the northern Champlain islands due to valley channeled flow. Main action comes after 16-17Z where strong low-level convergence is expected along cold front moving into the St. Lawrence Valley around 18Z, across the Champlain Valley from 21-00Z, and generally east of VT by 03Z Sunday. Just about all of the available high resolution models, including our locally run 2km and 4km WRF`s continue to suggest a very narrow reflectivity fine line on the leading edge of the front, so we`re looking for a brief period of moderate to heavy precip, with possible embedded convective elements which will enhance runoff and bring some main stem rivers to minor and possible moderate flood stages late Saturday into Saturday night. Overall rainfall amounts are expected in the 0.50 to 0.75" range, with locally up to 1" with orographic enhancement across the northern Adirondacks. Behind the front for Saturday night, temperatures fall rapidly with strong low-level cold air advection and wind shift developing where precipitation lingering across portions of central/northern Vermont will end as a period of rain/snow in the valleys and wet snow in the higher elevations where several inches of accumulation are likely. A dusting to perhaps and inch is possible at the valley floor. Lows return to more seasonal values ranging through the 20s by early Sunday morning. && .SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/... As of 350 PM EST Friday...As the low pressure system exits east, moisture will decrease across the North Country in west to northwest flow. The pressure gradient force will continue to produce gusty winds as lingering showers focus around the higher terrain. Temperatures will be closer to normal Sunday, but remain about 5 degrees above normal for this time of year. With clouds increasing from the north, due to approaching shortwave that may bring some light showers on Monday. Temperatures on Monday will range from the mid 30s to mid 40s. Monday night will be relatively quiet with temperatures falling below freezing. && .LONG TERM /TUESDAY THROUGH FRIDAY/... As of 350 PM EST Friday...Complex system or two follow each other Tuesday and Wednesday. Differences continue to exist in the model guidance. More significant trough possible Thursday, but again models continue to differ, with GFS having the system develop north and east of the forecast area and ECMWF having it along the New England coast. Overall expect above normal temperatures throughout the period with several chances for precipitation. && .AVIATION /01Z Saturday THROUGH Wednesday/... Through 00Z Sunday...A warm front lies just south of the forecast area as of 00Z Saturday, mainly over southern Vermont. Surface observations showing quite a bit of fog and low ceilings developing north of the warm front at this time, so expecting widespread IFR/LIFR in fog and low ceilings overnight. expecting ceilings and visibility to improve to MVFR/VFR between 12Z-15Z Saturday, as warm front lifts north into Canada. Surface pressure gradient to tighten across the region during the day on Saturday, with southerly surface wind gusts at or above 20 knots developing between 12Z-15Z Saturday and continuing at least through 22Z Saturday. Showers ahead of an approaching strong cold front Will move into northern New york between 18Z to 21Z Saturday, and then into Vermont between 21Z- 24Z Saturday. Guidance also indicating some isolated thunderstorms are possible ahead of the cold front Saturday afternoon. Outlook 00Z Sunday through Wednesday... 00Z Sunday - 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 600 pM EST Friday...The flood watch remains in place and is effective through 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 afternoon, 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. && .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...Lahiff NEAR TERM...Lahiff/Neiles SHORT TERM...KGM LONG TERM...KGM AVIATION...WGH/KGM HYDROLOGY...Banacos CLIMATE...Lahiff is the U.S. government's official web portal to all federal, state and local government web resources and services.