Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
335 AM EST Sat Jan 26 2019

Valid 12Z Sat Jan 26 2019 - 12Z Tue Jan 29 2019


...Northern Plains to Upper MS Valley and Great Lakes...
...Day 1 through Day 3...

A low- mid level shortwave will drop southeast through the
Canadian Prairies into the northern Plains and then the upper MS
Valley and Great Lakes. Strong 700 mb warm/moisture advection and
convergence will occur in the vicinity of a focused baroclinic
zone, resulting in a swath of moderate to heavy snowfall from the
Dakotas southeastward into the Upper MS River Valley and Great
Lakes. As the system deepens and ingests increased moisture,
higher QPF is expected across portions of southeast MN, southern
WI, northern IL, and lower MI.  Consequently, these areas are
where the snow amount probabilities are highest. In this model
cycle, there were still some north-south differences in the axis
of QPF, which reflected north-south differences in storm track, as
well as a few differences in intensity.
The wave which will cause the low pressure to develop has't moved
onshore from the Pacific into Canada yet, so additional changes in
wave amplitude and phasing are possible once the system enters
North America. Nonetheless, good clustering exists of the low
track among the 00z UKMET, Canadian global, 00z GEFS Mean, 21z
SREF mean, and 00z NAM/gfs. The 00z ECMWF trended north towards
this primary group.

...Ranges of central to southern MT/WY days 1/2 and CO day 3...

Confluent flow through a  deep layer drives the persistence of a
deep layer north-northwest jet across the northern Plains today
into early Sun.  Upper divergence maxima cross MT near a jet
streak and into WY and also the Black Hills of adjacent SD,
targeting the Bighorn Mountains/Black Hills on day 1 with several
inches of snow. 
On Day 2, Sun, the next in the series of 300 mb jet maxima drops
south from Canada across MT and then downstream into WY, with
embedded upper divergence maxima triggering lift and snow focused
on the ranges impacted in MT and WY, extending into the mountains
of north central CO. Lighter amounts are expected for Day 3,
focused on the front range of CO into the Sangre de Cristo
mountains of southeast CO/northeast NM as the upper jet builds
south across these areas. 

...Northern New England...
...Day 2...

The models forecast a persistent mid-upper level vortex over
Ontario.  Downstream from the circulation, a 700 mb broad 50 kt
jet crosses northern VT/NH across ME, with warm/moisture advection
occurring and a modest 700 mb convergence maxima.  A period of
snow is expected as a result, with 3-5 inches of snow possible in
northern ME. Abundant cold air across the region allows for all
snow in interior ME. 

...Ohio Valley to the TN Valley and lower MS Valley Day 3...
The models indicate the potential for post-frontal cold advection
that allows the precip to change from rain to snow before ending. 
The NAM and ECMWF are more assertive in allowing accumulating snow
with slightly slower forward progression of the front and waves
along the front.  Given the lack of low development along the
front and a progressive frontal movement, potential is low for
heavy snow, with the more likely outcome a change over to snow
before ending, with a short duration of all snow.  A closed low
pressure along the front is needed for a longer duration
post-frontal snow event. 

The probability for significant icing is less than 10 percent
nationwide days 1-3.

Petersen