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

Valid 00Z Sun Apr 25 2021 - 00Z Wed Apr 28 2021

...The West...
Days 1-3..

A closed 500mb low shifting southeast off the Pacific Northwest
this afternoon will reach the OR/CA coast Sunday morning and stall
over northern NV through Monday before a reinforcing shortwave
trough shifts the focus to southern CA Tuesday. 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, 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. 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 for 8 or more
inches along the CA Cascades and northern/central Sierra Nevada
(increasing to high for Day 1.5 on the Sierra Nevada) and low for
additional amounts over the northern Rockies of ID/MT/eastern WA.
The focus shifts inland over the Great Basin and north-central
Rockies on Day 2 with moderate probabilities for 8 or more inches
for the eastern NY ranges, northern UT ranges and northwest WY.
The southeast movement continues with Day 3 probabilities for 8 or
more inches for much of the CO Rockies.


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

Lee side cyclogenesis downstream of the low approaching the CA/OR
border forms in eastern WY tonight. As this low/front lifts
northward Sunday morning, it will be accompanied by increasing
moist advection as low-level southerly flow taps the Gulf of
Mexico. Strong WAA will sharpen the baroclinic gradient producing
a stripe of low to mid-level fgen 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 effects.
There is potential for snowfall rates to reach 0.5 to 1 inch per
hour over northeastern SD and into or through southern MN as shown
by the 12Z HREF and WPC snowband probabilities. These rates are
likely to cause modest accumulations as the band shifts northeast.
Day 1 WPC snow probabilities for 4 or more inches have risen to
30-50 percent along the eastern SD/ND border and over the Coteau
des Prairies in eastern SD down to the Buffalo Ridge in southwest
MN. Briefly heavy snow rates and gusty winds could produce a few
inches of accumulations on roadways in these areas Sunday morning
with lesser accumulations are likely farther east as the band
weakens and during peak heating from the strong April Sunday
afternoon.

A secondary impulse travels along this frontal boundary with
precip blooming again over northern MN late Sunday night. Day 2
snow probabilities are around 10 percent for 4 or more inches in
the Arrowhead.


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

Jackson