NWS getting onboard:
Wednesday Night through Friday
At the start of the extended period, a mid/upper shortwave will be
diving out of the northern CONUS with surface high pressure receding
to the east. The passage of this shortwave will provide increasing
PoPs into the day on Thursday and Thursday night via increased
vertical motion and a more W/NW flow pattern near the 850mb level.
Recent model runs continue to suggest a more saturated column, even
in the lower elevations with 1,000-500mb thickness values falling
below 5,400 meters area-wide. As such, this will justify a continued
expansion of PoPs into the region with orographic considerations
suggesting increasing potential of accumulations in the higher
terrain. As of yet, low-level WAA during the daytime on Thursday
looks to keep the boundary layer largely too warm in the lower
elevations for much of the region to transition to snow before
moisture exits. Nevertheless, potential does exist for evaporative
cooling to help transition at the surface even in the lower
elevations. With continued N/NW flow and sufficient low-level
moisture into Friday, PoPs will be lingered during this timeframe.
Overall, the pattern does not suggest too ideal of a NW flow setup,
but light accumulations in the higher terrain at a minimum are
increasingly likely. Heading into the weekend, focus turns towards a
shortwave and developing system out of the Great Plains via
cyclogenesis.
Saturday through Tuesday
The aforementioned developing system will be the feature of focus
during the weekend with potential for significant impacts in
portions of the eastern CONUS. Much of the uncertainty lies with the
evolution of the upper pattern as well as the the track of the
surface low. Based on the latest GEFS mean, GFS, and ECMWF
deterministic runs, the upper-level pattern and surface low track do
support potential for widespread accumulating significant snowfall
across the region. It is important to note that, as we have seen
with recent snowfall events, a change in the track or evolution of
the system can make or break potential for snowfall in the area
based on downsloping and other factors. However, with the ensemble
and deterministic consensus with these model runs, confidence has
increased sufficiently for HWO wording to mention this potential. It
is also important to note that, depending on how the pattern evolves
850mb moisture and flow may be more favorable for northwest flow
snowfall in the mountains even beyond the day on Sunday. Beyond the
weekend, the pattern looks to be more settled, but below normal
temperatures could allow for any snowfall that does occur to linger.
