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

Valid 00Z Mon Apr 29 2019 - 00Z Thu May 02 2019

...North Dakota to Lake Superior...
Day 1...

Low pressure ejects east from eastern MT across ND tonight and
across northern MN Monday. The system is sheared by the potent
120kt jet south of the trough and the currently closed mid-level
low opens this evening and weakens through its transect of
northern MN. Furthermore, temperatures are marginal, so low
elevations like the Red River of the North Valley should see less
snow. Day 1 probabilities are moderate for six inches on the
Pembina Escarpment with the highest probabilities for the
Arrowhead of MN where flow off Lake Superior and orographic lift
combine for moderate probabilities for eight inches despite much
of the snow falling during the day Monday.


...Intermountain West...Northern and Central Rockies onto the
Central High Plains...
Days 1-3...

Two significant snow events are expected through Wednesday from an
intensifying longwave western CONUS trough. First, a closed
southern stream low will cross San Diego late tonight and open as
it pushes east across the southern Rockies Monday night.
Anomalously high PWAT will stream ahead of this low into the
Rockies on deep SW flow from the tropical Pacific and an open Gulf
of Mexico. At the same time, ascent through the right entrance of
a strong northern jet streak centered over the Dakotas will
combine with an increasing baroclinic gradient as a front banks
against the Front Range to induce low pressure and spread snowfall
across UT, CO, and into WY/NE. Easterly flow along the front range
will enhance frontogenesis, and an extended period of heavy snow
is likely in the San Juans and CO Rockies, as well as into the
mountains of southeast WY. Farther 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 on Day 1.5, with high
probabilities for 4 inches extending to the Black Hills. Also into
Monday, a weaker northern stream shortwave trough digging from
British Columbia will spread snow across the ranges of ID/MT/NW
WY, where WPC probabilities are moderate for 8 inches on Day 2.

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 with the heaviest snow axis preferred by
the ECMWF for higher elevations from UT east-northeast across
northern CO/southern WY and on the central high plains of the NE
Panhandle and the Black Hills where probabilities for 8 inches are
low to moderate on Day 2.5.


The probability for icing above a tenth inch is less than 10
percent all 3 days.


Jackson