Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
414 AM EDT Mon Oct 28 2019

Valid 12Z Mon Oct 28 2019 - 12Z Thu Oct 31 2019


Days 1-3...

...Northern and central Rockies/High Plains...
The shortwave trough associated with ongoing snows across the
central Rockies and adjacent High Plains is forecast to swing east
through the Four Corners region and across the central/southern
Rockies into the High Plains on Monday.  Axis of better organized
snow will continue to shift south into southern Colorado through
the morning hours, with locally heavy accumulations possible along
the San Juan and Sangre de Cristo mountains.  Then by late Monday,
as the previously noted shortwave begins to accelerate northeast
through the Plains, expect snows to diminish from west to east
across the region.

Meanwhile, snows will begin to develop and shift south along the
northern Rockies, as the next shortwave trough drops south out of
Canada on Monday.  While widespread heavy amounts are not
expected, guidance continues to show the potential for locally
heavy accumulations along the western Montana to the western
Wyoming ranges on Monday into early Tuesday.

Models show this second system continuing to amplify on Tuesday,
with a closed upper low developing over Utah and western Colorado
by early Wednesday.  This will bring a second round of snow into
western Nebraska, southern Wyoming and Colorado beginning Tuesday
and continuing into Wednesday.   Upslope flow and low to mid level
frontogenesis will raise the threat for moderate to heavy
accumulations from the High Plains of the Nebraska Panhandle,
southeastern Wyoming and northeastern Colorado back into the
adjacent foothills and mountains of southeastern Wyoming and
northern to central Colorado.  Unfortunately, there remains a
significant amount of model spread with respect to the timing of
the upper trough/low, with the GFS becoming a notably faster
solution by late Wednesday.  Model solutions continue to diverge
through the remainder of the period.  While the GFS remains a
faster solution, the NAM becomes a significantly slow outlier. 
With respect to QPF/temperature profiles, WPC preferred a
compromise solution similar to the ECMWF and the European Ensemble
Mean.  In addition to heavier totals over the mountains, this
solution continued to support an axis of higher probabilities for
amounts exceeding 6-inches along the I-25 corridor from the
Cheyenne to the Denver metros.

As the upper low moves east from the Rockies into the central
Plains, WPC continued to follow a solution similar to the ECMWF
and European Ensemble Mean through the remainder of the period,
with snows ending across the central Rockies by late Wednesday, as
lighter amounts spread east across the Plains into the mid
Mississippi valley.

...Mid Mississippi valley to the upper Great Lakes...
The leading shortwave exiting the Rockies Monday morning is
forecast to accelerate northeastward from the central Plains
Monday evening to the northern Great Lakes by Tuesday morning. 
This is expected to produce a stripe of mainly light accumulations
from northern Missouri and Iowa to the U.P. of Michigan.
     
For Days 1-3, the probability of significant icing is less than 10
percent.

Pereira