Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
354 AM EDT Sun Apr 28 2019

Valid 12Z Sun Apr 28 2019 - 12Z Wed May 01 2019

...Northern Rockies to Lake Superior...
Days 1-2...

Potent closed low and associated 120kt jet streak will drop ESE
through MT and ND today into Monday, pushing a strengthening
surface low beneath it. Deep layer ascent through height falls and
robust jet-level diffluence will combine with warm/moist advection
ahead of the surface low, and modest west-east oriented mid-level
frontogenesis to drive lift through a predominantly sub-freezing
and weakly unstable column. Guidance features a relatively small
envelope of surface low tracks, suggestive of reasonably high
confidence in a swath of heavy snowfall from eastern MT through
eastern ND. WPC super-ensemble plumes still depict a wide
variation in potential amounts due to the likelihood of mesoscale
banding, and moderate HREF probabilities for 1"/hr snowfall rates.
The highest accumulations are likely across the northern half of
ND, where WPC probabilities are moderate to high for 8 inches,
with lesser amounts likely surrounding this from eastern MT
towards the ND/MN line.

Further east and into Monday, the upper low begins to fill, with
subsequent weakening of the surface reflection expected into
Minnesota. Broad frontogenesis persists, as does WAA and
diffluence within a still potent LFQ of the upper jet streak.
However, the total intensity of the ascent will wane with time
into Monday, so other than some locally higher amounts within the
Arrowhead of MN Monday where SE wind upslopes into the Iron
Ranges, WPC probabilities are low for 4 inches across Minnesota.


...Central Rockies and Central High Plains...
Days 1-3...

Two significant snow events are likely through the forecast period
as multiple mid-level pieces of energy rotate through a longwave
western CONUS trough. The first is associated with a closed low
off Baja California which will open and eject northeast into the
Four Corners late on Monday. As this feature moves eastward, it
will spread anomalously high PWAT into the Rockies on deep SW flow
from the tropical Pacific. At the same time, ascent through the
RRQ of a strong northern jet streak centered over the Great Lakes
will combine with an increasing baroclinic gradient as a front
banks against the Front Range to induce low pressure and spread
snowfall across Utah, Colorado, and into Wyoming and western
Nebraska. Easterly flow along the front range will enhance
frontogenesis, and an extended period of heavy snow is likely in
the mountains of the San Juans and Colorado Rockies, as well as
into the mountains of SE WY. Further east, jet level dynamics will
spread the heavy snowfall into the High Plains, but lesser
accumulations are expected there. In the terrain above 5000 ft of
CO, WPC probabilities are high for 12 inches, with high
probabilities for 4 inches expanding into the High Plains. Also
into Monday, a weaker northern stream shortwave digging from
British Columbia will spread snow across the ranges of ID/MT/NW
WY, where WPC probabilities are moderate for 8 inches.

This first system will quickly move away Tuesday, but will be
replaced by another shortwave digging right on its heels. This
second system is weaker than the first, but is still accompanied
by high PWAT air on moist advection, and favorable diffluent jet
regions aloft for ascent. Another round of heavy snow is likely
Tuesday and Tuesday night across the same area, with the notable
exception being that heavy snow should be more confined to the
terrain, with just light snow possible in the High Plains of WY/CO
and into NE. WPC probabilities are moderate for 8 inches from the
Uintas into the ranges of CO, and northward through the Big Horn
and Absaroka ranges. A few inches of snow is also likely Tuesday
in the Black Hills of SD.


The probability for significant icing is less than 10 percent all
3 days.


Weiss