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

Valid 00Z Tue Oct 29 2019 - 00Z Fri Nov 01 2019


Days 1-3...

...Northern and Central Rockies/High Plains...

Snows will shift south from western and central Montana tonight to
Wyoming and then northern CO by early Tuesday.
The enhanced deep layer moisture and low level frontogenesis
maxima produce snow across WY and adjacent ID/UT/western
SD/western NE/northern Co into Tuesday and continuing Tuesday
night in Colorado.  

Upslope flow and low to mid level frontogenesis will raise the
threat for moderate to heavy accumulations into the foothills and
mountains of southeastern Wyoming and northern to central
Colorado.  Models are coming into better agreement as the GFS is
slowing down the progression of the mid level circulation.  Low
probabilities for amounts exceeding 6-inches along the I-25
corridor from the Cheyenne to the Denver metros are dependent on
the timing of the axis of the frontogenesis.  The higher amounts
in the nam appear to be the result of the frontogenesis band
stalling Tue evening in northeast CO, which was not shown in other
models.

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 accelerates northeastward from the central
Plains this evening to the northern Great Lakes by Tuesday
morning.  The wave of low pressure moves quickly across the
region, with the area left of the low track changing from rain to
snow.  This is expected to produce a stripe of mainly light
accumulations from northern Missouri and eastern Iowa to the U.P.
of Michigan. The quick movement of the wave limits precipitation
totals and thus snow amounts.

On Wed night and Thu., the next system moves northeast, with a
deepening 700 mb wave and possible closed low forming over the 
Midwest or Great Lakes.  As this system develops, there is
potential for precip to expend in coverage in the cold sector and
a period of accumulating snow from the mid MS Valley to the upper
Great Lakes.  The 12z GFS was on the high end of forecasts, with
heavy snow in eastern Iowa to eastern WI.  Other guidance was
lighter with QPF and potential snow west of the sfc low in IA/WI.
     
For Days 1-3, the probability of significant icing is less than 10
percent.

Petersen