BTV hitting the uncertainty
Concerningly
the latest ECMWF came in very wet (snowy) with storm total QPF
in the 1.7" to 2.4" range across most of Vermont, and about half
of its ensemble members do have greater than 2" in south
central Vermont. This dramatically wetter idea than other global
models may be tied a quicker development of a strong upper jet
Friday night, and subsequent phasing of the northern stream and
southern stream systems, leads to a deeper and more westward
coastal low track on Saturday. Given these differences, there
unsurprisingly is a large and unfortunately growing spread in
our low end and high end snowfall amounts; as an example,
Newport, VT, currently has a mostly likely amount of 8" with low
end of 3" and high end of 20". Part of the challenge,
especially farther south, will be snow ratio forecasts.
The snow ratios are rather uncertain similar to snow amounts when
you look at the spread amongst model guidance, particularly during
the daytime on Saturday. The interquartile range in the NBM snow
ratios in a swath from the Adirondacks eastward across central
Vermont is about 5:1 to 12:1. Obviously, the lower snow ratio end,
if it is all in the form of snow, would be a very wet snow.
Currently we are indicating a mix of rain and snow during the day in
the valleys locations in this region, which is why snow ratios are
even lower at times. Looking at forecast soundings in this area,
while strong upward motion will likely be co-located with the snow
growth zone, it will be located more than 10,000 feet above ground
while a nearly isothermal and relatively warm layer exists close to
the surface. Depending on how warm the boundary layer gets, it is
reasonable to expect snow character to indeed transition from
average to wet by Saturday morning. However, there then will be a
period of time late in the event that snow character should trend
drier, and the model spread greatly decreases Saturday evening with
a 10:1 or higher snow ratio favored. The question is how much liquid
will be fall in the wetter snow. Hopefully we can answer that
question as we get closer to the event to help predict snow load
impacts.