780
FOUS11 KWBC 251712
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
112 PM EDT Fri Apr 25 2025

Valid 00Z Sat Apr 26 2025 - 00Z Tue Apr 29 2025


...Sierra Nevada and Great Basin...
Days 1-3...

An impressive closed mid-level low (NAEFS 500mb height anomalies
falling below the 1st percentile) will move onshore southern CA
Saturday aftn before slowly filling while lifting northeast through
the Great Basin on Sunday. Downstream of these robust height
anomalies, significant synoptic lift will occur downstream through
height falls, mid-level divergence, and overlapping upper
diffluence as the LFQ of a strengthening subtropical jet streak
digs through the trough axis and ejects meridionally. The overlap
of this ascent into a region moistened by 700mb S/SW flow and an
accompanying ribbon of IVT above the 90th percentile (from NAEFS)
will result in widespread precipitation Saturday morning through
Sunday afternoon.

Snow levels during this time will be quite low in
response to the anomalous upper low, falling to around 4500 ft,
which is nearing the 10th percentile. This suggests that snow will
accumulate across much of the CA and Great Basin terrain above this
level through Sunday, but at least light accumulations or mixing
with snow is possible much lower due to steep lapse rates and
dynamic cooling. The heaviest accumulations, however, should remain
across the Sierra, where D1 and D2 probabilities for 6+ inches
exceed 70%, and 30%, respectively, and locally more than 12 inches
is possible before precipitation wanes late D2.

As this upper low continues to track northeast, the accompanying
moisture and ascent will move across the Great Basin, bringing some
light to moderate snow to the higher elevations of NV through
Monday morning. WPC probabilities D3 reach as high as 50% for 6+
inches across the Ruby Mountains.


...Northern Rockies...
Day 3...

The closed low ejecting from the Great Basin will fill and open
into a positively tilted trough Monday as it shifts east into the
Rockies. Despite this evolution, deep layer ascent will remain
impressive as periodic lobes of vorticity swing northeast
downstream of the trough axis, and work together with mid-level
divergence and upper diffluence in the LFQ of a jet streak arcing
into the Northern Plains. Together, this will cause pressure falls
over CO/WY leading to surface cyclogenesis, and this low will then
track northeast into the Dakotas and Minnesota by the end of D3.
While there is good model agreement in the general synoptic setup
and evolution, low-clusters from the various ensembles still
feature considerable spread in both timing and placement,
additionally reflected by modest differences in the primary 500mb
EOF on the D3 WPC clusters.

The primary driver of these variances appears to be the speed at
which this trough opens and ejects, with 2/3 of the ECENS
suggesting a lower ejection, while 50% of the GFS members make up
the faster end of the envelope, and by the end of D3 there are
height differences among the cluster means of more than 80m across
the Central High Plains. At this time, a solution somewhere between
the camps as a consensus is probably best.

While the exact placement of the low and its associated synoptic
forcings will be critical, a cold front digging south through the
Northern Rockies Sunday night into Monday will likely interact with
an inverted surface trough, leading to enhanced ascent across parts
of MT and WY D3. This is likely to occur regardless of the low
evolution, but a slower system may produce more enhanced ascent and
stronger accompanying deformation than a faster ejecting system.
Either way, impressively combined upslope with fgen should result
in heavy snow rates, with dynamic cooling response to the column
occurring even outside of the higher terrain. Snow levels will be
generally low anyway, (near the NBM 10th percentile after
coordination with the local WFOs), suggesting that while the
heaviest accumulation will remain above 5500 ft, significant
accumulations are possible to 4500 ft, and lighter accumulations
even lower than that. This is reflected by WPC probabilities that
are high (>70%) D3 across the Big Horns, Absarokas, and other
terrain around Yellowstone NP, but also much more widespread
moderate probabilities for 2" expanding across much of the ID/MT/WY
juncture.


The probability of at least 0.10" of freezing rain across CONUS is
less than 10%.


Weiss


$$