There is still a high degree of model spread concerning snow amounts
for Saturday afternoon and night, especially across Western NY. The
12Z NAM, and all the high resolution NAM based guidance
significantly increased snowfall across Western NY, with much higher
QPF developing in the cold air later Saturday afternoon and evening.
The GFS and Canadian GEM are a little slower with the change to
snow, and also have less QPF following the change to snow. The past
few runs of the ECMWF are a compromise between the NAM and GFS based
guidance, and seems most reasonable. Cold advection to the west of
the deepening cyclone will only initially cool the column to a near
isothermal layer at or just below freezing Saturday afternoon and
evening. Surface temperatures will fall to near or just above the
freezing mark during this time. Given this scenario, snow:water
ratios will be low, likely less than 10:1 through early Saturday
evening before improving overnight as the column and surface grow
colder.
Given the above expectations, we have increased the speed of
changeover to snow in the new forecast, and also increased snow
amounts, but not to the extent of the NAM and high-res NAM based
guidance. The latest forecast is for 4-8 inches across the higher
terrain of the Chautauqua Ridge, Boston Hills, and Wyoming County
with the highest amounts along the Chautauqua Ridge with upslope
enhancement later in the event. We will start with a high end
advisory for now. It is possible some of this area may reach low end
warning criteria, especially if the forecast trends towards the more
aggressive NAM solutions. Went with 2-4 inches across the lower
elevations of Western NY, and 1-3 inches for points southeast of
Lake Ontario with a later change to snow. Again, if the more
aggressive NAM solution is correct these amounts will be higher.
East of Lake Ontario, the higher terrain of the Tug Hill plateau may
see 4-7 inches, with 2-4 inches across lower elevations. Most of
this will occur later Saturday night into Sunday morning once
westerly upslope and lake enhancement develop. This area may need an
advisory as well, but given ongoing uncertainties with this system
will hold off on the headline for now in this area.