Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
352 AM EDT Thu Apr 25 2019

Valid 12Z Thu Apr 25 2019 - 12Z Sun Apr 28 2019


...Northern Plains/Upper Midwest...
Days 2-3...

A shortwave digging southeast from the Pac NW will amplify into a
fast but potent closed mid-level circulation on the nose of a
90-100 kt upper level jet. As this feature moves rapidly across
the Northern Plains Saturday, it will catch up to a more slowly
departing but strong jet max across S Central Canada, and the
coupled jet structure will lead to surface cyclogenesis diving
from South Dakota into Ohio by the end of the period. Guidance has
come into better agreement with a more southerly track of this
surface low, and although the NAM is a strong/cold outlier, an
ECMWF/GFS/CMC compromise is favored and spreads a swath of heavy
snow from the western Dakotas into Michigan.

Some uncertainty still remains in both the placement of the
heaviest snow and the accumulation. Temperature profiles will be
marginal, but intense synoptic ascent through the aforementioned
coupled jet structure, as well as increasing mesoscale lift due to
strongly sloped frontogenesis embedded beneath a maturing TROWAL
suggests intense snowfall rates are likely which should overcome
the marginal profiles. Working against heavy snow accumulation is
that it is almost May, and afternoon snowfall with a high sun
angle will be tough to accumulate. There is a large spread in the
ensemble plumes for snowfall, but many ensemble members suggest a
swath of more than 4 inches of snow, with the highest likelihood
occurring from SE MN through southern WI where the best
deformation axis pivots, and WPC probabilities show a moderate
risk for 4 inches. Significantly higher amounts are possible in
isolated locations, and small probabilities do exist for more than
8 inches in this same area. However, confidence is lower than
average at this time and accumulations will be highly dependent on
snowfall rates, time of day, and if the best overlap of forcing
can occur concurrently for a prolonged period of time. Elsewhere,
from the ND/SD/MT border eastward into lower Michigan there is a
moderate chance for 2 inches of snowfall.


...Northern Rockies...
Day 3...

A shortwave and robust 120kt Pacific Jet will rotate onshore
Washington State late Saturday driving a surface low and
associated cold front into the northwest. Robust lift within the
LFQ of this potent jet max will produce precipitation across this
area, but initially the column will be too warm for snow outside
of the highest terrain of the Washington Cascades and Northern
Rockies. As the cold front shifts inland, snow levels will drop to
3000-4000 ft in the ranges of ID and MT, while mesoscale ascent
increases as a deformation band sets up NW of the developing
surface low. This combined with modest mid-level fgen in the
vicinity of the cold front will produce heavy snow before the low
exits to the east late on day 3. WPC probabilities are moderate
for 8 inches across the Northern Rockies through Sunday morning.

Weiss