Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
422 AM EDT Sun Apr 25 2021

Valid 12Z Sun Apr 25 2021 - 12Z Wed Apr 28 2021

...Ranges of California to the Great Basin and WY/CO Rockies...
Days 1-3..

A slow moving upper low drifting east into OR today and tonight
supports the trough reaching the CA coast and moving onshore
tonight.
As this trough shifts eastward, a downstream 250mb jet streak will
intensify near the base of the trough increasing Pacific moisture
and deep layer ascent from north to south over CA. The combination
of increasing column moisture and deep layer ascent of long
duration, is likely to spread snow across higher elevations of CA
through Monday. Snow levels in northern CA will initially be
around 5000 ft on the warm advection, but lower to around 4000 ft
tonight as an associated cold front pushes eastward. Day 1 snow
probabilities are moderate to high for 8 or more inches along the
CA Cascades and northern/central Sierra Nevada.

Tonight, as the upper level jet moves across NV into UT and
western WY, pre-frontal convergence picks up and induces ascent in
the ranges of central to northeast NV to western WY. As Monday
progresses, the upper jet continues an east drift across the
ranges of northern UT and western WY, with several additional
inches likely as 700 mb convergence maxima combine with the
difulent jet flow to produce ascent. Day 2 has moderate
probabilities for 8 or more inches for the Bighorn range of WY,
northern UT ranges and northwest WY ranges. 

As the upper low forms over the southwest Tuesday, the low level
front drifts across southeast Wyoming and CO. The ECMWF has a
slower frontal progression and longer period of return flow to
produce upslope conditions across the northeast CO Plains and
foothills/front range, resulting in heavier QPF and snow potential
than other models.
Given the ECMWF ensmebles were lighter than the operational run,
plus only modest 850 mb moisture fluxes and anomalies, the
operational ECMWF QPF and resultant snow potential appears
overdone. Given much better agreement among the 21z SREF Mean, 12z
ECMWF Ensemble Mean, and 0z GEFS Mean QPF and resultant snow,
these solutions were given more weight in the forecast. Day 3
probabilities for 8 or more inches of snow are low to moderate for
much of the CO Rockies front range.


...Northern Plains to Lake Superior...
Days 1-2...

As a low and associated front lifts northward Sunday morning, it
will be accompanied by increasing moist advection. Strong WAA will
sharpen the baroclinic gradient, producing a stripe of low to
mid-level frontogenesis that will produce a narrow band of
precipitation. Rain will transition to snow on the northern side
of the precip stripe through wet-bulb and dynamic cooling in
northeast SD and adjacent southwest MN.  There is potential for
snowfall rates to reach 0.5 to 1 inch per hour over northeastern
SD and southern MN as shown by the HREF probabilities.  Briefly
heavy snow rates could produce a few inches of accumulations on
roadways in these areas today with the warm frontal passage
possibly resulting in a change to rain. This keeps the probability
for 4 inches of snow as low.

As the warm front lifts northeast, a cross-lake Superior fetch
picks up moisture fluxes from the lake. Lee shore convergence in
the arrowhead of MN combined with an upslope component of flow
leads to a period of snow late tonight into Monday over northeast
MN. Day 1 snow probabilities are around 10 percent for 4 or more
inches in the Arrowhead of MN.
Day 2 snow probabilities are low, as the warm frontal passage may
allow precip to change over to rain, capping the event potential.

For Days 1-3, the probability of significant icing (0.25 inch or
more) is less than 10 percent.

Petersen