Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
350 PM EDT Tue May 07 2019

Valid 00Z Wed May 08 2019 - 00Z Sat May 11 2019


...The West...

Lead shortwave will eject northeast from the Four Corners tonight
into the Great Lakes by Thursday morning. Immediately following
this feature, a secondary northern stream trough will dig into the
west, re-asserting the positively tilted longwave trough as it
digs into a closed feature near Southern California Friday. The
first shortwave will be accompanied by a departing jet max leaving
right entrance region diffluence across the Rockies, while a front
banked against the Front Range both increases low-level
frontogenesis but also serves to drive easterly upslope flow into
WY and CO. These features together will produce heavy snow from
the Northern Rockies southward into the mountains of Utah and
Colorado on day 1. Snow levels will be generally high, 6000 ft in
the far north to 8000 ft in the UT/CO. Above these elevations,
heavy snow is likely however, and WPC probabilities are high for 4
inches across most of the terrain, with more than 12 inches
possible in the Wind Rivers, Big Horns, and Absarokas.

More significant snow is likely days 2-3 as the positively tilted
trough amplifies and drives significant moist advection atop the
region. Persistent SW flow aloft will push PWAT anomalies towards
+2 standard deviations above the climo mean. As this occurs, 700mb
flow will become increasingly southerly, driving mid-level
frontogenesis across the area. This enhanced mesoscale ascent
combined with upslope flow due to the orthogonal nature of the
low-level winds, will produce an environment favorable for
persistent heavy snow Thursday into Friday, especially in the
Colorado Rockies, Sangre De Cristos, and San Juans. Snow levels
will be quite high, 8000 ft or more, but above these levels WPC
probabilities are high for 12 inches in all of these ranges, and
some locations in the San Juans may approach 30 inches by Saturday
morning.

Additionally on Friday, snow will increase across the highest
terrain of the Sierra in California as heights fall beneath the
upper trough and moist advection intensifies. WPC probabilities
here reflect a moderate risk for 8 inches above 9000 ft on day 3.


...Upper Midwest...

Opening mid-level trough will propel a weak surface low on
Thursday from the eastern Plains into the L.P. of Michigan along a
baroclinic gradient. The guidance has trended slightly NW with
this feature, and hence is warmer than previous runs suggesting
the maximum snow swath will be displaced NW as well. Low-level
thermal profiles are marginal, but robust ascent through
isentropic lift, left exit region jet level diffluence, and
increasing 700-600mb frontogenesis will at times allow the column
to dynamically cool enough to produce heavy snowfall. The NAM and
CMC appear to be much too amplified with the driving shortwave,
pushing moisture too deeply into the cold air, and are thus likely
too far NW and too robust with snowfall. The other guidance,
including the higher res CONEST, are more in line with continuity
and the general ensemble envelope and are preferred for this
update. This has prompted a shift of the heaviest snow axis into
the western U.P. of MI, far NW WI, and the Arrowhead of MN, where
WPC probabilities feature a slight risk for 4 inches, highest
along the shore of Lake Superior both in the Arrowhead of MN, as
well as the elevated terrain of the lowlands of NW WI. However,
some locally higher amounts are possible if snow rates become
heavy enough, especially since this may occur overnight in
conjunction with a maturing TROWAL lifting northward into the
region, and some enhancement off Lake Superior is possible.



The probability of significant icing is less than 10 percent for
Days 1-3.

Weiss