Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
302 PM EDT Sun Apr 23 2023

Valid 00Z Mon Apr 24 2023 - 00Z Thu Apr 27 2023

...Pacific Northwest and northern/central Rockies...
Days 1-3...

A sharp upper trough or compact/small upper low with 500 mb
heights as low as -1 standard deviation below the climatological
mean is forecast to move onshore coastal Washington late tonight
and continue southeastward into southwestern ID by Monday
afternoon. Modest precipitable water values will combine with
progressive height falls and upslope flow to wring out several
inches of snow D1 over the Cascades and Olympics, but the system
will certainly continue east of the terrain. From the Blue
Mountains eastward across central ID to southwestern MT into D2,
snow will expand as the LFQ of the upper jet promotes broad
divergence which will combine with lower-level
convergence/frontogenesis along/east of the Divide where WPC
probabilities of at least 6 inches are moderate (>40%).

Into late D2 and much of D3, a preceding weak shortwave will
maintain lighter snow over northern UT into the CO Rockies, ahead
of a developing area of heavier QPF. Southerly to southeasterly
flow in the lower levels over the High Plains out of the Gulf will
supply the CO Rockies with sufficient moisture as the upper low
moves into southwestern CO late Tuesday. Column cooling should
increase past dark Tuesday night with snow levels lowering from
over 7000ft to under 6000ft as upslope into the Front Range will
support heavier rain below the freezing line and heavy snow into
the higher terrain. The position of the upper low over southwest
CO on Tuesday night will be in an ideal position for the central
CO Rockies to realize sufficient upper-level divergence combined
with strong upward vertical motion within a healthy DGZ for this
time of year. Much of the Front Range that resides above 8000 feet
could see up to 1-2+ feet of snow. Rain could briefly mix with
snow below 6000 ft Wednesday morning across more of the CO High
Plains as the upper low slides southeast, but any accumulations
are expected to be minimal. WPC probabilities of at least 8 inches
of snow D3 are high (>70%) above 8000ft, but are moderate (>40%)
for at least 4 inches down to the Foothills above 7000 ft,
including high chances across the Palmer Divide.


The probability of significant icing greater than 0.25" is less
than 10 percent.


Snell