Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
304 PM EDT Wed Apr 12 2023

Valid 00Z Thu Apr 13 2023 - 00Z Sun Apr 16 2023


...Central and Northern Rockies into the the Northern High
Plains...
Days 1-3...

A closed mid-level low over WA/OR will slowly fill through
Thursday as it pivots eastward into the Northern Rockies. Although
the amplitude of the core of this feature will weaken during D1,
the associated longwave trough in which it is embedded will deepen
across the entire Intermountain West/Great Basin by Friday morning
before gradually weakening and shifting east into the Plains by
Saturday morning. Within this longwave trough, lobes of vorticity
will repeatedly transit the trough, driving rounds of ascent
across the region, and aided by weakly coupled jet streaks aloft
providing enhance upper diffluence for ascent. The result of this
synoptic evolution will be surface low pressure development across
eastern WY or far northern CO, in the lee of the terrain, and this
low is likely to then advect northeast along a low-level
baroclinic boundary, lifting into the Upper Midwest on Friday.
This low is not expected to feature rapid deepening, and may end
up as a multi-centered elongated low pressure due to the multiple
shortwaves moving through the flow to help drive ascent.

However, moist advection downstream of this trough axis is likely
to become robust as noted by PW anomalies of +1 to +2 standard
deviations surging into the Northern Plains Thursday, driven by
strong 295-300K isentropic ascent with mixing ratios of around 4
g/kg. The relatively slow evolution of the pattern should allow
for a long duration of moist/warm advection into the region, which
will manifest as wintry precipitation where the column is cold
enough NW of the 850mb center and the associated dry slot.
Although guidance is not suggesting much in the way of TROWAL
development, some elevated instability noted by mid-level theta-e
lapse rates falling to < 0C/km across eastern MT will pivot
northwestward, leading to enhanced ascent potential in a region of
overlapped deformation and 700-600mb fgen. This should drive
impressive omega into the DGZ, supporting snowfall rates of 1"/hr
or more at times as reflected by the WPC prototype snow band tool.
Despite relatively low SLRs due to April sun, marginal thermal
structure, and antecedent soils warmed by 70+ degree temperatures,
rapidly accumulating snow is likely D1 into D2, especially in the
terrain from eastern ID through northern MT above 6000 ft. In the
lower elevations, snowfall will likely be less due to warmer
temperatures, but the banding potential should still be able to
overcome any hostile antecedent conditions and lead to moderate
accumulations. WPC probabilities D1 are high for more than 6
inches from the NW WY ranges northward through the Absarokas,
Little Belts, Big Snowy Mountains, and into the Northern Rockies.
Where upslope flow is most favorable into some of the higher N-S
oriented ranges, more than 12 inches of snow is likely. During D2,
low WPC probabilities for more than 6 inches continue across the
NW WY ranges, including the Tetons, with high probabilities for 2+
inches expanding into the High Plains of MT.

From D2 into D3, an impulse rotating through the longwave trough
will lift across the Central Rockies in tandem with the low-level
front dropping into KS/OK. This will likely lead to secondary
surface low development across eastern CO, while the overlap of
height falls/PVA with increasing upslope flow behind the front and
around the developing low pressure will yield significant ascent
beginning early on Friday and persisting into Saturday morning.
Once again, the thermal structure is marginal for heavy snowfall
outside of the higher terrain and below snow levels that will
gradually fall from around 8000 ft to 4000 ft into D3, but WPC
probabilities for more than 4 inches are high across the San
Juans, CO Rockies, and Front Range, primarily D2 into D3. Some
light snowfall is also possible across the Palmer Divide during D3.


The probability for icing over 0.1" is less than 5% Days 1-3.


Weiss