Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
402 AM EST Tue Jan 05 2021

Valid 12Z Tue Jan 05 2021 - 12Z Fri Jan 08 2021

...Colorado Rockies...
Day 1...
A shortwave embedded within otherwise generally zonal flow will
cut across the Intermountain West and into the Plains this aftn.
Brief upper diffluence will occur in conjunction with this
shortwave as the LFQ of a modest jet streak arcs overhead. The
combination of the upper diffluence and modest height falls will
provide ascent sufficient for moderate snowfall, with moisture
being provided through brief backed SW flow. The duration and
intensity of forcing is modest, and models have shown a general
downward trend in QPF. However, enough ascent and moisture is
expected to overlap that WPC probabilities for 4 inches of snow
are as high as 80% in the highest terrain, with local maxima of 8"
possible.


...Pacific Northwest...
Days 1-3...
Yet another impulsed embedded within moist/confluent mid-level
flow will approach the WA/OR coast Wednesday morning, driving a
warm front, followed by a cold front, onshore. Mid-level flow
backing to the SW ahead of the trough axis will drive increased
moist advection into CA/OR/WA, which will be tapped by height
falls and modest upper diffluence to produce snowfall in the
terrain from the Olympics and Cascades of WA on D1, expanding
south and east on D2 as far as the Shastas/Siskiyous of CA and the
Northern Rockies of MT. By D3, much of the forcing will be exiting
the region and just lingering moderate snow is likely in N CA and
parts of MT. WPC probabilities for heavy snow are highest in WA
which will have the best overlap of intense ascent and most
significant column moisture. WPC probabilities on D1 are high for
8 inches in the Olympics and WA Cascades, with locally more than
12" likely. By D2, WPC probabilities for 8 inches wane, but expand
to include the Northern Rockies and OR Cascades, with
probabilities peaking around 40%. Lingering snowfall D3 may
accumulate to 4 or more inches in northern CA, associated with the
lingering forcing of the first impulse, aided by an approaching
jet streak from the west.


...Southern Appalachians...
Day 3...
A closed mid-level low moving across OK/AR on Thursday will drop
southeast to be positions over Georgia the end of D3. This closed
low will be accompanied by a modest jet streak, and the LFQ of
this streak will combine with height falls and mid-level
divergence to spread precipitation across the TN VLY and into the
Carolinas/Virginia. A wave of low pressure will also develop and
track eastward, staying far enough to the south that some of the
precipitation is likely to be snowfall. The antecedent airmass is
marginally cold, so attm the most significant snow accumulations
are likely to occur in the mountainous terrain of NC which will
benefit from colder temps and better isentropic ascent aided by
upslope enhancement. A few inches of snow are possible from N GA
into SW VA, and WPC probabilities for 4 inches are as high as 50%
near the Pisgah National Forecast of NC.


The probability of significant icing (0.25" or more) is less than
10 percent all 3 days.

Weiss