Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
417 AM EDT Sun Mar 31 2024

Valid 12Z Sun Mar 31 2024 - 12Z Wed Apr 03 2024

...The West/Rockies...
Days 1-2...

Anomalous closed mid-level low positioned west of the CA coast
early this morning will pivot southeast towards Baja California by
tonight, with additional pieces of shedding vorticity moving into
the Great Basin. By Monday morning, the phasing of shortwaves will
result in a large, broad longwave trough centered over the Desert
Southwest, angled to the northeast into the Central Rockies. THe
southwesterly flow ahead of it will pull plentiful moisture into
the region, characterized by PW anomalies around +1 sigma. A
widespread precipitation event from AZ/NV to WY/southern MT is
expected.

An area of low pressure will pass through the Great Basin with its
accompanied cold front passing through the Four Corners region. The
low eventually becomes occluded over UT once it becomes vertically
stacked with an embedded 500 mb closed low. Heavy snow rates up to
1-2"/hr will be possible today from southern ID, the Uintas in UT,
and across the western WY ranges. A secondary area of 1"/hr snow
rates will be possible along the Mogollon Rim in AZ. Along/ahead of
the front, snow levels are expected to be between 5000-7000 ft, but
fall to around 4500 ft post frontal passage. The higher terrain
areas are expected to see the greatest accumulations.

For Day 1, the WPC probabilities for at least 6 inches are high
(>70%) along the Mogollon Rim in AZ, the San Juans in CO, the
higher peaks in central/northeast NV, and then across the Uintas
and western WY ranges. Some localized 12 inch totals are possible,
especially in AZ, San Juans, and western WY ranges.
By D2 the heavy snow continues across the Mogollon Rim and White
Mountains of AZ, but the greatest probabilities for 6 inches are
confined to the San Juans and Sangre de Cristos and are above 50-60
percent.


...Central High Plains into the Northern/Central Plains...
Days 1-2...

Large scale positively tilted upper trough oriented SW to NE across
the Intermountain West will gradually pivot eastward, amplifying
some as it moves over the Rockies and into the HIgh Plains. At the
same time, the subtropical jet streak will arc from the southwest
to the northeast ahead of it, providing additional forcing for
ascent. Meanwhile, the southwesterly flow will allow for modest
moisture to stream into the region. This overlap of forcing and
moisture along the existing stationary boundary will allow for lee
cyclogenesis late tonight/early Monday.

A longer duration frontogenetical band is expected to set up across
portions of South Dakota and northern Nebraska where guidance has
begun to cluster and show better agreement. The QPF signal has
increased for upwards of 0.5" QPF across southern/western SD into
WY. SLRs were increased some given the potential for
banding/mesoscale forcing where dynamic cooling may lead to some
overachieving snow bands. Mesoscale forcing both due to fgen in
the lead precip shield, and due to intensifying upslope into the
terrain, will likely result in at least periods of heavy snow rates
and dynamic cooling to cause p-type changeover from rain to snow
at times in otherwise marginal thermals. This will cause even
lower confidence than usual in snowfall amounts, but WPC
probabilities are high for the Black Hills at the 4 inch threshold
(80+ percent) while are lower to the 20-40 percent range to the
east over south-central SD.

...Northeast...
Day 3...

THe positively tilted trough advances toward the Mississippi and
Ohio Valleys Tuesday-Wednesday. Meanwhile, a potent northern stream
shortwave slips toward the Upper Midwest and Great Lakes and by
the latter portions of the Day 3 period, phasing of the two
features is increasingly likely with the latest ensemble and
deterministic guidance showing a closed 500 mb low forming in the
Midwest. The primary/parent low pressure will likely deepen as it
approaches the Great Lakes region while by the end of Day 3 period,
a secondary area of low pressure may form near the Mid- Atlantic
coast. Favorable forcing for ascent is provided by the impressive
upper level diffluence and moisture anomalies approach +2 sigma
with this system. Widespread precipitation is expected from the
Ohio Valley, Great Lakes and into the Mid- Atlantic/Northeast.

There is a lot of model uncertainty in the evolution of the major
synoptic features including the strength/deepening of the parent
low in the Great Lakes and the timing of the secondary low pressure
off the Mid-Atlantic coast. The influence of high pressure over the
Hudson Bay may play a factor in the dry air and blocking. Given the
early April timing, thermal profiles are also a factor, especially
for the lower elevations. Altogether, by Day 3 (but certainly Day
4-5), a complicated/messy weather system could bring wet, heavy
snow to parts of the Northeast, mainly interior and higher
elevation areas. There is low confidence but for now, the latest
WPC probabilities for 4 inches are 40 to 60 percent for the
interior terrain from the northern Adirondacks, the VT/NH
mountains, and southwest/western Maine. The latest Day 3 Winter
Storm Outlook shows 20 to 30 percent probabilities for warning
criteria snow for the same areas.

The probability of significant icing is less than 10 percent
across the CONUS.


Weiss