Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
425 AM EDT Sat Oct 13 2018

Valid 12Z Sat Oct 13 2018 - 12Z Tue Oct 16 2018


Day 1...

...Northern and Central Rockies into the Western High Plains...
A shortwave and associated jet maximum will drop south from
British Columbia to reinforce the positively tilted longwave
trough into the Great Basin. In the low levels, a cold front will
push southward through the Northern Plains. Synoptic lift within
the right entrance region to a departing upper jet max will
combine with increasing isentropic ascent to produce widespread
snow. Despite the progressive nature of the upper feature and
associated dynamics, a swath of moderate to heavy snowfall is
possible across the Nebraska Panhandle where omega is maximized
beneath the upper diffluence. Here, WPC probabilities indicate a
moderate risk for 4 inches or more of snow. In the high terrain of
Wyoming and northern Colorado, upslope enhancement is expected due
to low-level E/NE winds creating a high risk for more than 8
inches of snow in the Laramie and Big Horn ranges, with a moderate
risk in the Wind Rivers and northern Colorado Rockies.

The probability of significant icing (0.25 inch or more) is less
than 10 percent.

Day 2...

...Central and southern Rockies...
Confluent mid-level flow between shortwave energy ejecting
northeast into the Plains and a secondary impulse dropping through
the Great Basin to reinforce the upper trough will drive
increasing moisture into the Rockies and Central Plains. A
low-level cold front pushing southward into Texas will undercut
mid-level warm advection to produce increasing isentropic lift,
aided by upslope enhancement into the terrain of Colorado and New
Mexico. Heavy snowfall is likely across the high terrain, with WPC
probabilities indicating a high risk for 8 inches or more across
the Sangre De Cristo and San Juan mountains.

...Central Plains...
Warm advection ahead of a 700mb trough diving southeast through
the plains will top a surface cold front digging into Texas. This
creates robust isentropic lift within the 300-310K layer while
sharpening the mid-level baroclinic gradient to enhance
frontogenesis across the Central Plains. Heavy precipitation is
likely, with rain changing to snow as the column cools through the
morning. A swath of heavy snow is likely from southeast Colorado,
through Kansas, and into Nebraska where frontogenesis is maximized
and periods of intense lift occur where theta-e surfaces fold into
the saturated dendritic growth zone. This has the potential to
produce CSI banding which will dynamically cool the column through
heavy snowfall rates to overcome otherwise marginal temperatures.
Although some uncertainty still exists into the location of this
heaviest band of snowfall, ensemble plumes and recent model
guidance continue an upward trend in accumulations, and WPC
probabilities indicate a high risk for 4 or more inches of snow,
with moderate probabilities for 8 inches.

The probability of significant icing (0.25 inch or more) is less
than 10 percent.

Day 3...

...Southern Rockies...
Residual snows are expected to continue across the high terrain of
New Mexico early into Monday. The best forcing and moisture shift
east through the afternoon causing the precipitation to wane after
additional light accumulations.

The probability of significant icing (0.25 inch or more) is less
than 10 percent.

Weiss