Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
422 AM EDT Sat Apr 24 2021

Valid 12Z Sat Apr 24 2021 - 12Z Tue Apr 27 2021

...Pacific Northwest, Northern Rockies, and California...
Days 1-3..

A closed 500mb low dropping along the coast of British Columbia
will advect onshore the OR coast Sunday morning while gradually
filling. This trough will then continue to move eastward into the
Great Basin and eventually the Four Corners by the end of the
forecast period. Despite a slow filling of the primary low, the
overall trough will deepen through time becoming a full latitude
trough across the West by Tuesday. As this trough shifts eastward,
a downstream 250mb jet streak is progged to intensify near the
base of the trough. This will act to both increase moisture into
the region as Pacific air floods eastward within the trough, while
also strengthening deep layer ascent through LFQ upper diffluence
and associated divergence maxima. W/SW mid-level flow will also
promote warm and moist advection across the West, and IVT is
forecast to reach 250 kg/m/s according to GEFS and ECENS members.

The combination of increasing column moisture, deep layer ascent,
both of which will be of long duration, is likely to spread rounds
of precipitation across much of the region through Tuesday. While
snow levels will initially be around 6000 ft on the warm
advection, they are expected to lower to around 4000 ft as an
associated cold front works eastward beginning late Sunday. For
D1, the heaviest snow is likely in the higher terrain from the OR
Cascades into the WA Cascades, and eastward to the Northern
Rockies and Absarokas where the best overlap of high mid-level RH
and mid-level divergence align. WPC probabilities on D1 are high
for 6 inches in these regions. By D2, precipitation snowfall
becomes more widespread but sinks slightly SE in response to the
shifting mid-level low. WPC probabilities for 6 inches are high
for the ranges of SW MT, NW WY, central ID, and into OR/northern
CA, with snow amounts in excess of 12 inches likely in the Sierra
where upslope enhancement will occur. By D3, forcing for ascent
shifts eastward, leaving residual high probabilities for 6 inches
confined to the Uintas, Big Horns, and areas around Yellowstone NP.


...Northern Plains...
Days 1-2...

A wave of low pressure moving eastward across the High Plains of
WY will drape a warm front eastward into the Dakotas. As this
front lifts northward Sunday morning, it will be accompanied by
increasing moist advection as low-level southerly flow taps the
Gulf of Mexico, but impressive warm advection along the frontal
boundary. Robust WAA will sharpen the baroclinic gradient
producing a stripe of intense mid-level fgen, aided by an upper
divergence maxima on the LFQ of a modest jet streak. These
features together will produce a narrow band of precipitation
which will likely being as rain, but transition to snow as it
lifts northeast, especially early on D2. A cooling column, both
through wet-bulb and dynamic effects will cause rain to change to
snow, and there is potential for snowfall rates to reach 0.5 to 1
inch per hour as shown by the HREF and WPC snowband probabilities.
These intense rates are likely to cause modest accumulations as
the band shifts northeast. While WPC probabilities for 4" at any
location are less than 5%, briefly heavy snow rates and gusty
winds could produce a few inches of accumulations, including
roadways, early Sunday morning across parts of central ND, eastern
SD, and southern MN. Lesser accumulations are likely further
northeast as the band weakens and April sun increases through the
afternoon.


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

Weiss