Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
340 AM EDT Tue Mar 12 2024

Valid 12Z Tue Mar 12 2024 - 12Z Fri Mar 15 2024


...Northwest...
Day 1...

The final in a series of shortwaves will push onshore this morning
over WA/OR and then dive southeast towards the Great Basin while
amplifying. This feature will rapidly intensify into a strong
closed low over the Great Basin on Thursday as anomalous ridging
bulges along the Pacific Coast. This evolution will bring an end,
finally, to the repeated rounds of precipitation across the
Northwest, especially to terrain above 2500-3500 ft. Ascent across
the region will be driven primarily via height falls and PVA
associated with this shortwave, but a modest 90kt Pacific jet
streak is also likely to orient WNW to ESE, driving favorable LFQ
diffluence into the area as well. This synoptic lift combined with
upslope flow in the W/NW oriented ranges will produce heavy
snowfall as moisture surges ahead of the best ascent on the
downstream confluent flow, although PW and IVT anomalies according
to NAEFS are generally near normal.  Still, there will be
plentiful moisture to be wrung out by the impinging ascent, and
WPC probabilities for more than 6 inches of snow are high (>70%)
in the Olympics, along the WA/OR Cascades, and eastward including
portions of the Northern Rockies, in the NW WY ranges, and south
to the Ruby Mountains and along the Wasatch of UT.


...Central Rockies, Southern Rockies, & Central High Plains...
Days 1-3...

***Significant and long lasting winter storm likely to produce
heavy snow across much of the terrain of the Central and Southern
Rockies, with impactful snowfall becoming more likely into the
lower elevations of the High Plains as well.***

A northern stream shortwave will dig out of British Columbia late
Tuesday, and then deepen rapidly as the mid-level flow becomes
increasingly amplified. A potent upstream ridge blossoming across
the eastern Pacific will become intense, characterized by
700-500mb height anomalies reaching as much as +4 sigma near
British Columbia by the end of the forecast period, driving
equally impressive downstream height falls as a 500mb closed low
sags S/SW into the Desert Southwest by Friday morning. The primary
deterministic global members have continued to trend deeper and SW
with this closed low as it cuts off beneath the omega block across
the Pacific. While this low then continues to spin slowly and
retrograde, it will produce an extended temporal duration of
impressive synoptic lift through divergence downstream across the
Four Corners and Central Rockies, with additional ascent occurring
in the post-frontal upslope regime as a cold front sags southward
through the Plains. The prolonged period of synoptic lift and
upslope NE flow will result in this long duration precipitation
event.

While it is almost certain that ascent will be prolonged and
impressive, there are still some questions marks about moisture
and position of greatest ascent. The guidance does appear to be
converging on a deeper more SW aligned system, creating confluent
flow emerging out of the Pacific and streaming northeast,
especially within the 700-500mb layer. This will increase column
moisture, but NAEFS ensemble tables suggest overall PW will be
near normal within this axis. However, at the same time, a leading
850-700mb wave will spin out of the primary gyre, causing a local
backing the lower level flow leading to the emergence of enhanced
moisture being drawn northward from the Gulf of Mexico. There are
still considerable model differences in this lead evolution, but
the trends have been for a slightly deeper but farther south low.
This is important because the downstream moist isentropic upglide
characteristic of the flow around this feature will likely lift
the theta-e ridge into an impressive TROWAL, pivoting SW around
the 850mb low and transporting additional moisture and instability
back into the High Plains. This is reflected by higher PW
anomalies in a NE to SW arc across the High Plains, and with
sfc-700mb flow likely strengthening out of the NE during this
time, it will transport significant moisture to prolong and
enhance snowfall, especially into the Front Range. During this
time as well, the setup appears to match the conceptual model for
a pivoting band of snow somewhere across eastern CO or into the
Central High Plains where a potent deformation axis and
overlapping fgen surge omega into the deepening DGZ. Confidence is
low in this evolution, and the column is marginally supportive for
heavy snow, but significant dynamic cooling could occur in this
band to allow for rapid snowfall accumulation. However, the higher
confidence part of this forecast is more about how the
intensifying NE flow around the 850mb low will help transport
additional moisture into the Rockies, which within the slow moving
synoptic ascent will produce heavy snowfall across much of the
terrain. Snow levels during the event will gradually cool,
starting around 5000-7000 ft, and dropping to 3500-5000 ft by
Friday. WPC probabilities across WY and CO D2-3 exceed 70% from
the Big Horns south into the Laramies, along the Front Range, into
the Palmer Divide and down through the Raton Mesa and Sangre de
Cristos. 2 or more feet of snow is likely in the higher terrain of
the Front Range, with locally up to 1 foot possible elsewhere.
Along the I-25 urban corridor and into the High Plains, amounts
will be more modest, but several inches of snow should create
impacts along this corridor as well.

The second phase of this event will begin during D3 as the closed
upper low over the Desert SW continues to amplify, resulting in
the prolonged mid-level divergence across the Four Corners. This
will lead to increasing 300-310K isentropic ascent surging NE from
AZ/NM into UT/CO, producing waves of heavy precipitation starting
late D3, primarily into the terrain north of the Mogollon Rim and
points northeast back into the San Juans and CO Rockies. WPC
probabilities D3 surge across the Four Corners, reaching above 70%
for 6+ inches along the Mogollon Rim, the White Mountains of AZ,
southern Wasatch and into the San Juans.


The probability of receiving at least 0.1" ice is less than 10%
across the CONUS through Day 3.


Weiss