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

Valid 12Z Fri Oct 12 2018 - 12Z Mon Oct 15 2018


Day 1...

...Northern Rockies...
A shortwave and accompanying jet maximum will dig into the
Northern Rockies from British Columbia on Friday. This feature
will push southward quickly, bringing enhanced moisture to the
region while driving a cold front towards the Northern Plains. Low
to mid level frontogenesis and upslope enhancement along the
Continental Divide will aid the synoptic lift to produce snowfall
across the terrain of the Northern Rockies. WPC probabilities show
an enhanced risk for more than 4 inches of snow near Glacier
National Park, with lower amounts elsewhere. As the system
progresses quickly to the south, some snowfall is possible within
the Absaroka range as well, but limited moisture and and weaker
forcing will limit heavier snowfall accumulations there.

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

Day 2...

...Central Rockies and High Plains...
Shortwave diving towards the Great Basin on Day 2 will sharpen a
positively tilted upper trough near the Four-Corners region by
12Z/14. A strong cold front will be driven southward beneath this
feature as high pressure settles in from the north. Mid-level flow
will remain from the southwest driving moist advection from the
Pacific. Strong lift due to increasing frontogenesis across the
mountains of Wyoming and Colorado combined with upslope
enhancement on increasing NE flow will support heavy snow across
the Colorado Front Range as well as the high terrain of the Sangre
De Cristos, Big Horn, and Wind River Mountains. WPC probabilities
show a high risk for 8 inches of snow or more in the highest
terrain, with 4 inches or more likely above 7000 feet.

As the upper trough sharpens late on Day 2 into Sunday morning,
and cold air depth increases behind the cold front, precipitation
will mix with and change to snow across the western High Plains of
Nebraska and Kansas. Isentropic lift will begin to increase and
WPC probabilities show a slight risk for 4 inches of snow across
this area by 12Z/14.

...Northern Plains...
Surface low pressure beneath an upper trough digging into southern
Manitoba will move towards the Great Lakes and occlude. 700mb
deformation NW of the mid-level low will enhance lift along the
Canada border, but the environment is rather moisture deprived.
Light snowfall is likely across far northern ND and northwest MN,
but WPC probabilities are low more than 1 inch of snow.

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

Day 3...

...Southern Rockies and Central Plains...
Shortwave continuing to dig into the Great Basin will reinforce
the positively tilted upper trough, while a second impulse lifts
northeast from Baja and into the southern Plains. Confluent flow
between these two features will drive strong moist advection
supporting heavy precipitation. Robust isentropic ascent in
response to southerly flow atop the low level cold front and
surface high will combine with strengthening mid-level
frontogenesis to produce strong omega, which area soundings
suggest will be forced into a saturated dendritic growth zone.
Strong northeast winds will provide upslope enhancement into the
Sangre De Cristo and southern Colorado Rockies, where WPC
probabilities show a high risk for 8 inches of snow or more.
Elsewhere across the Plains from western Kansas southward into the
Texas Panhandle, a stripe of enhanced snowfall is likely although
there remains some question into how efficiently snow will
accumulate due to low snow-liquid ratios. Ensemble plumes show a
wide spread of accumulations and models diverge as to the
placement of the strongest forcing and heaviest snowfall, so WPC
probabilities are moderate for more than 4 inches of accumulation,
with this uncertainty limiting higher confidence.

...Northern Plains...
Weakening mid-level deformation associated with an occluded low
pressure north of the Great Lakes will gradually shift eastward
through Day 3. Some light snow associated with the forced ascent
within the upper trough is likely, but WPC probabilities are low
for more than 2 inches of snow, primarily across northern
Minnesota.

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

Weiss