Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
204 PM EST Wed Mar 5 2025

Valid 00Z Thu Mar 06 2025 - 00Z Sun Mar 09 2025

...Great Lakes and Central Appalachians...
Day 1...

Powerful low pressure will continue to produce blizzard conditions
across portions of the Upper Midwest and Great Lakes tonight, but
this system will gradually weaken through D1. The weakening of this
system will be in response to the opening of the upper closed this
evening, becoming negatively tilted as it shifts northeast into
Ontario. At the same time a secondary shortwave trough embedded
within the longwave cyclonic flow across the east will pivot into
the Mid-Atlantic states, and the interaction of these features will
push the primary surface low into Canada will driving a post-
frontal trough across the Appalachians. The overall weakening of
ascent and reduction in moisture will cause precipitation coverage
to fade and become more showery/snow-showery in nature, with the
primarily exception occurring across the U.P. of MI early D1, and
then across the Central Appalachians where pronounced upslope flow
will enhance snowfall Thursday afternoon. WPC probabilities for 4+
inches across these areas are generally 30-50%.


...California, Great Basin, Southwest, and Central Rockies...
Days 1-3...

A closed mid-level low dropping along the Pacific Coast Thursday
will amplify into a full-latitude trough centered across the Great
Basin Thursday night. This will manifest as height anomalies that
are progged via NAEFS to reach as low as -3 sigma with respect to
500-700mb heights late D1, with these anomalies persisting as the
trough slowly moves east with time into the weekend. As the core of
this mid-level low moves across the Four Corners, a downstream
subtropical jet streak will amplify, reaching 150+ kts as it
surges into the Southern Plains. This will place favorable LFQ
diffluence over the Four Corners and into the Central Rockies,
enhancing deep layer lift over the region.

Beneath this upper low, two distinct surface cyclones will also
move across the West. The first will be directly beneath the upper
low moving across the Great Basin and into the Central Rockies D1,
which will tap into increasing PW to expand precipitation across a
large portion of the area. The accompanying air mass if full of
Pacific air, so snow levels will be generally modest, around
4000-6000 ft, but will fall behind the low and accompanying cold
front to 2000-3000 ft. However, this also occurs with decreasing
precipitation coverage and intensity. While coverage of snow is
likely to be widespread D1, the heaviest is expected across
northern UT and into WY where the accompanying warm front will
result in enhanced fgen to drive stronger mesoscale ascent and more
prolonged snowfall rates of 1+"/hr. In this area, WPC probabilities
are high (>70%) for more than 12 inches on D1 across the terrain of
eastern NV, into the Wasatch, Uintas, and Wind Rivers, with
widespread high probabilities (>70%) for 4 inches across much of
NV, eastern ID, and WY. Additionally on D1, WPC probabilities are
high for 6+ inches of snow across the Sierra, and the Transverse
and Peninsular Ranges of CA. By D2, heavy snowfall persists across
the higher terrain of UT and WY while expanding into the CO
Rockies, where WPC probabilities for 6+ inches of additional snow
reach 70-90%.

SW of this secondary low, the pivoting upper low moving across the
Four Corners will drive another area of pronounced ascent leading
to periods of moderate to heavy snow across the Desert Southwest
and extending into the TX/OK Panhandles. The column across this
region is modestly cold enough to support snow, with snow levels
around 5000-6000 ft, but steep lapse rates aloft beneath the cold
core low will help cause strong omega to dynamically cool the
column. This suggests a mixture of rain and snow, possibly becoming
all snow near the surface at times of heavier precipitation rates.
At this time, WPC probabilities D1 are moderate (50-70%) for 6+
inches of snow across the Mogollon Rim, and moderate to high D3
(50-90%) in the Sangre de Cristos. Light snow may expand as far
east as the Panhandle of TX D3 as well.


...Central Rockies into the Central Plains...
Days 2-3...


Secondary lee cyclogenesis will occur east of the Rockies (beneath
the
aforementioned overlap of jet diffluence and height falls) D2,
leading to a rapidly deepening surface low pressure that will race
eastward in response to confluent/fast flow across the middle of
the country. The parent upper low will weaken with time as it
becomes embedded within this downstream confluent 500mb pattern,
causing the associated vorticity lobe to string out to the east.
Despite this weakening evolution, robust WAA and isentropic ascent,
especially along the 290-295K surfaces, will draw moisture
northward leading to enhanced precipitation shield. The guidance is
in good agreement that a swath of heavy snow, driven primarily by
the overlap of the LFQ of an upper jet streak and low-level
increasing frontogenesis, will drive a translating band of heavy
snow from eastern WY through Iowa D2 into D3. The most intense
fgen will drive ascent into a deepening DGZ (SREF probabilities for
50mb of DGZ depth above 70%) which will support a stripe of heavy
snowfall rates nearing 1"/hr. Although there remains some
latitudinal uncertainty into where this will track, current WPC
probabilities are high (>70%) for 4+ inches D2 from the Pine Ridge
eastward through much of northern NE, with low probabilities
(10-30%) extending into IA D3.


Weiss