Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
510 PM EDT Fri Oct 12 2018

Valid 00Z Sat Oct 13 2018 - 00Z Tue Oct 16 2018


Day 1...

...Northern Rockies...
A strong ridge anchored over the eastern Pacific will allow
another well-defined shortwave trough to amplify and dig south
from western Canada into the Pacific Northwest and Northern
Rockies Friday night-Saturday morning.  Increasing low to mid
level frontogenesis and upslope flow ahead of the wave will
support developing snow showers along the northern Rockies -- with
WPC probabilities indicating a Slight to Moderate risk for amounts
of 4-inches or more across some of the higher peaks from northwest
Montana to western and central Wyoming.  Limited moisture and the
progressive nature of the system is expected to hamper the threat
for widespread heavier amounts.

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

Day 2...

...Central and southern Rockies and Plains...
Models show the previously noted shortwave continuing to amplify
and dig south into the central Rockies -- compromising the western
extent of a positively-tilted trough extending from the upper
Midwest back into the western U.S.  This will send a robust cold
front plunging south into the southern Plains and southwestern
U.S. on Sunday.  Snow showers will shift south along the Rockies
from the southeast Wyoming ranges late Saturday to the northeast
New Mexico mountains on Sunday.  Strong low northeasterly tapping
deeper moisture over the Plains along with low to mid level
frontogenesis will raise the threat for heavier precipitation
rates.  WPC probabilities continue to highlight the potential for
locally heavier totals across the high terrain -- with a Slight
Risk for amounts of 8-inches or more encircling the region. 
Meanwhile as the cold air deepens, rain changing to snow can be
expected further east out into the Plains, including portions of
Kansas and Nebraska and perhaps as far east as southern Iowa and
northern Missouri.  Models are still showing some uncertainty with
respect to how quickly the changeover occurs, however the general
trend of the day runs was to increase totals, suggesting the
heavier precipitation rates afforded in part by low to mid level
frontogenesis would support a faster changeover.

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

Day 3...

...Southern Rockies...
Ongoing snows are expected to continue along the southern Colorado
into the northeast New Mexico ranges into Sunday night, possibly
producing some additional heavy accumulations before precipitation
begins to wane.  As the upper trough and best low to mid level
forcing shifts further east, precipitation is expected to diminish
across the region on Monday.

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

Pereira