Area Forecast Discussion
Issued by NWS Buffalo, NY

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000 FXUS61 KBUF 210231 AFDBUF Area Forecast Discussion National Weather Service Buffalo NY 1031 PM EDT Tue Apr 20 2021 .SYNOPSIS... Low pressure will pass just south of the region late tonight through Wednesday, producing widespread accumulating snow across the region. The snow will likely have an impact on the Wednesday morning commute, with slushy and slippery travel. The colder air will bring some lake effect snow showers to areas southeast of Lake Erie and Lake Ontario Wednesday night into Thursday morning. Dry and warmer weather will return on Friday. && .NEAR TERM /THROUGH WEDNESDAY NIGHT/... A late season accumulating snow event is still on track through Wednesday. A cold front will remain stalled just to the south of the area across Pennsylvania. A mid level trough over the Upper Mid West will dig and sharpen overnight...reaching the central Great Lakes Wednesday. DPVA and flow adjustments downstream of the digging trough will force a strong baroclinic wave to develop along the stalled frontal zone over the Ohio Valley, with the resulting surface low tracking from the Ohio Valley to southern New England by Wednesday afternoon. A shield of widespread precipitation will be forced by strong frontogenesis and differential temperature advection to the north of the low track, with an added boost of strong DPVA and right entrance region jet dynamics ahead of the digging trough. Light precipitation has already developed across the Niagara Frontier and will spread northeast to the North Country by around midnight. Precipitation will be slow to develop at first from eastern Cattaraugus County ENE through the Finger Lakes to Oswego County, with stronger frontogenesis focused north and west of there. Widespread precipitation will hold off in these areas until late tonight and Wednesday morning. Colder air will still be in the process of filtering into the region on low level northerly flow this evening, so this initial light precipitation may be a rain/snow mix. Later tonight through Wednesday morning frontogenesis will intensify as the baroclinic wave reaches our longitude, resulting in a period of moderate to heavy precipitation. The combination of ongoing cold advection, cooling from melting processes, and wet bulbing of the column will cool the atmosphere sufficiently to change all the precipitation to snow. Forecast soundings continue to suggest there may be a narrow zone of freezing rain/sleet near the rain/snow transition line, and some of this may fall briefly across southern Cattaraugus and much of Allegany County late tonight through early Wednesday morning. The ground is warm in the second half of April, so that will initially inhibit snow accumulation. Snowfall rate is the key to accumulation at this time of year, if it snows hard enough the snow can overcome the warm surface and cover the ground. Once the ground is covered, the warm ground becomes irrelevant with an insulating layer of snow between the ground and new snow accumulation on top. Given the strong forcing in this case, we expect the warm ground to be overcome for a window from later tonight through Wednesday morning when snowfall rates are greatest. As far as accumulations go, expect 3-5" across much of Western NY and the lower Genesee Valley, including Buffalo and Rochester. The highest totals are likely to fall along the Chautauqua Ridge, Boston Hills, and Wyoming County where around 6" is expected. The North Country can also expect 3-5", with local amounts of up to 6" across the higher terrain of the Tug Hill. Snowfall rates in these areas may reach 1" per hour for several hours early Wednesday morning. 2- 4" is expected in Allegany County, along with the potential for a little freezing rain. The lowest accumulations will likely be found across Ontario, Wayne, N.Cayuga, Oswego counties where 2-3" is expected. Forecast snowfall amounts in this area are lower as a result of surface temperatures being slightly warmer, and the fact that most of the snow will fall during daylight hours. The widespread snow will taper off quickly from west to east Wednesday afternoon, to be replaced by a few snow showers as cold air pours into the eastern Great Lakes in the afternoon. Lake effect and upslope snow showers will become more widespread Wednesday night through Thursday morning as the pool of coldest air aloft crosses the region. Northwest flow will direct most of this into the western Southern Tier off Lake Erie, and from Orleans County eastward across Rochester to Oswego County and down into the Finger Lakes off Lake Ontario. Upslope flow will also keep more widespread snow showers going across the higher terrain east of Lake Ontario. All of this may produce localized additional accumulations of 1-2" Wednesday night. && .SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/... A cold, northwest flow will continue Thursday with 850mb temperatures between -10C and -8C. Lake effect snow showers will be ongoing Thursday morning with the most concentrated areas southeast of the Lakes. An organized band of lake effect may be present along the Lake Ontario shoreline from Orleans and Monroe counties to the Finger Lakes region Thursday morning. Snow showers will diminish through the day and snowfall accumulations will be light, less than an inch. There could be an isolated 1-2 inches across the higher terrain but chance is low due to the late April sun angle making it hard for snow to accumulate. Breezy conditions expected with northwest winds 15 to 25 mph. High temperatures will only reach the upper 30s to low 40s and with the winds, will feel like the low 30s. Snow showers will come to an end by Thursday night as warm air advection occurs across the region. Low temperatures will reach the low 30s across the higher terrain to the mid to upper 30s near the lake shores. Dry weather expected Friday and Friday night as an upper level ridge moves overhead. Warmer temperatures, however still below normal are expected with highs in the low to mid 50s and low temperatures in the low 30s across the higher terrain and mid to upper 30s towards the lakes. && .LONG TERM /SATURDAY THROUGH TUESDAY/... Two jets, polar and subtropical will produce their own troughs across the central United States Saturday. These two troughs and associated surface lows will eventually cross the area late Saturday afternoon through Sunday. The first of the two lows that will impact the region will be the southern low. This low will track northeast from the south into the Mid-Atlantic states Saturday before exiting off the East Coast Sunday. Meanwhile the low over the northern Mid-West will track northeast across eastern Canada just on the tails of the southern low. Therefore, expect increasing chances for showers late Saturday afternoon through Sunday with the passages of these two systems. Shower activity will decrease late Sunday afternoon as both features exit to the east. An omega block over the central United States will work its way east as the troughs exit the Northeast. Therefore, upper level ridging, supporting surface high pressure will push into the region beginning Sunday night and remain overhead through Tuesday. While under the influence of the surface high pressure, expect a couple of days of dry weather to start off the week. Otherwise, expect a range of 50s for the weekend with Sunday being the coolest day due to the passage of the frontal boundaries. Warm air advection associated with the upper level ridging will support a warming trend for the start of the new work week. Highs Monday will warm up into the mid to upper 50s, while Tuesday will be the warmest of the period with a range of 60s, and possibly a few 70s. && .AVIATION /03Z WEDNESDAY THROUGH SUNDAY/... Low pressure will move from the Ohio Valley to southern New England by Wednesday afternoon. A shield of widespread precipitation will develop to the north of the low track. An initial rain/snow mix this evening will quickly change to all snow overnight, with widespread snow lasting through Wednesday morning before tapering off from west to east Wednesday afternoon. The snow will produce widespread IFR conditions later tonight through Wednesday. The snow may become heavy for several hours early Wednesday morning, with VSBY possibly nearing airfield minimums at the TAF sites. Expect improvement to MVFR/VFR from west to east in the afternoon as the snow tapers off. Outlook... Wednesday night and Thursday...MVFR/local IFR with scattered snow showers and some lake effect snow showers southeast of the lakes. Windy. Friday and Saturday...Mainly VFR. Sunday...VFR/MVFR with a chance of showers. && .MARINE... Low pressure will move east across Pennsylvania late tonight night to New England by Wednesday afternoon. Winds will become northeast and increase tonight as this low approaches, then become northwest and increase further Wednesday afternoon and Wednesday night as cold air pours into the eastern Great Lakes behind the departing low. A long period of Small Craft Advisory conditions is expected on Lakes Erie and Ontario from Wednesday through Friday. The strongest winds will occur on Thursday, when sustained winds may approach 30 knots on Lake Ontario. && .BUF WATCHES/WARNINGS/ADVISORIES... NY...Winter Weather Advisory from 4 AM to 5 PM EDT Wednesday for NYZ007-008. Winter Weather Advisory from 2 AM to 2 PM EDT Wednesday for NYZ003-013-021. Winter Weather Advisory until 2 PM EDT Wednesday for NYZ001- 002-010>012-019-020-085. MARINE...None. && $$ SYNOPSIS...Hitchcock NEAR TERM...Hitchcock SHORT TERM...HSK LONG TERM...EAJ AVIATION...Hitchcock MARINE...Hitchcock

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