Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
358 PM EST Mon Jan 18 2021

Valid 00Z Tue Jan 19 2021 - 00Z Fri Jan 22 2021

...Great Lakes...
Days 1-3...

Cyclonic flow around a low-mid level circulation over southeast
Canada will continue to support lake effect snow showers off of
Lake Superior and Erie. Sustained west-southwest flow down the
major axis of Lake Erie supports continuing near shore
convergence, resulting in the likelihood of 6-10 inches for
southwest New York through Tuesday.  By late Tuesday, a warm front
crosses Lake Ontario into near shore areas and the Tug Hill. 
Frontal convergence triggers shows showers development, followed
by lake enhancement Tue night.  Snow will become locally heavy Tue
night as convergent westerly flow picks up latest heat and
moisture fluxes from the lake, spawning snow showers that come
into the Tug Hill.

Wed is a transitional day as a low level ridge builds upstream and
flow across the lower lakes veers, causing drying aloft and
leading to a likely end of the lake effect snow event.
The uncertainty related to event duration before conditions become
unfavorable for lake effect snow.

On Thu, the low level ridge retreats to the east and the next warm
front crosses from the upper to the lower Lakes.  Frontal
convergence again triggers snow shower development.  Probabilities
are low for heavy snow as the front progresses across the region,
limiting event duration.

Downstream from Lake Superior,a couple of different periods of
enhanced activity and break occur with a succession of secondary
low level boundaries. One passes through tonight with the eastern
UP of MI getting a period of enhanced moisture and boundary layer
convergence and lift lasting overnight into Tue before waning Tue
night.
Lake effect snow showers redevelop on Thu in the lee of Lake
Superior as a secondary cold front crosses the region.  Veering
low level flow results in northwest winds across the lakes late
Thu., with shoreline convergence and upslope flow resulting in
snow showers increasing in coverage/intensity.  Limited duration
of the snow showers keeps snow heavy probabilities low.

...Southern Rockies/Southwest...
Day 1...

A developing mid-to-upper level low dropping south through the
Great Basin into Southern California drives a 300 mb jet streak
across southern UT to central and southern CO tonight through
early Tue. Upper divergence maxima across northern NM and southern
CO combine with strong 700 mb convergence and moisture advection
to large scale ascent and increasing snowfall across
the San Juan Mountains and Sangre De Cristo Mountains of northern
NM and southern CO.  Heaviest accumulations are expected to center
across the San Juan and Sangre de Cristo mountains, where WPC PWPF
shows widespread probabilities for storm totals of 8 inches or
more and areas of 12-16 inches.

On Tuesday night, the low drifts south away from the southwest and
the strength of the low level flow weakens crossing CO/NM. This
weakens low level convergence and lift, so snow coverage/intensity
should wane, lessening snow rates and making day 2 snow light.  

For Days 1-3, the probability of significant icing (0.25 inch or
greater) is less than 10 percent.

Petersen