Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
447 PM EDT Thu Apr 11 2019

Valid 00Z Fri Apr 12 2019 - 00Z Mon Apr 15 2019


...Northern Plains and Upper Midwest...
...Day 1...

A powerful spring storm will continue to produce heavy snows with
strong gusty winds as it drifts northeast across eastern Nebraska
into far western Iowa today.  Supported by strong upper forcing
and low to mid level frontogenesis, heaviest snow accumulations
are expected to center from eastern South Dakota and southeastern
North Dakota into west-central Minnesota. As the low lifts across
southern Minnesota toward north Wisconsin Friday, anticipate an
areas of heavier snow across interior central/northern Minnesota
with lake enhancement/upslope along the northern shore of Lake
Superior.  For the 24-hr period ending 00Z Saturday, WPC
probabilities indicate a Moderate Risk for accumulations for
8-inches or more across this region with Moderate to High Risk
adjacent to Grand Marais toward the eastern Boundary Waters. East
of the heavier snow axis and the apparent surface low, a wintry
mix is expected, with significant sleet and ice accumulations
possible across portions of Wisconsin and northern Michigan. For
the Day 1 period (ending 00Z Saturday), WPC probabilities show a
Slight to Moderate Risk for ice accumulations of 0.25-inch or more
from the southern Wisconsin-Minnesota border northeastward into
the U.P. of Michigan. By late Friday, models show the system
beginning to accelerate northeast into Ontario with precipitation
winding down through the overnight hours. 

...Central and Southern Rockies into the High Plains...
...Day 2...

Energy currently over the Gulf of Alaska is expected to dive
southeast, supporting an amplifying trough over the Southwest-Four
Corners region Friday evening.  As it shifts farther east Friday
night, cold air aloft along with a period of upslope flow is
expected to support accumulating snows along the Colorado Front
Range into the Sangre de Cristo Mountains Friday into Saturday. 
WPC probabilities for the Day 2 period (ending 00Z Sunday) show a
Slight to Moderate Risk for snow accumulations of 8-inches or more
across this region.

Models show the system continuing to amplify as it moves farther
to the east, with a closed upper center over northern Texas and
strong surface cyclone developing across central-southern Texas on
Saturday.  While confidence is limited, both the NAM and ECMWF
support the idea of rain changing to accumulating snow across
portions of the High Plains along the back edge of the associated
comma-head. It should be noted that the 12Z deterministic ECMWF
did reduce snowfall amounts (as compared to the 00Z solution)
likely tied to the quicker/more robust upper level trough that
would result in higher snowfall totals. While this model may be an
outlier to the general model suite, EC ensembles/mean and GEFS
would support at least the potential for light accumulations with
locally higher amounts possible. WPC Day 2 probabilities (ending
00Z Sunday) show a Slight to Moderate Risk for accumulations of
4-inches or more extending from southeastern Colorado and
northeastern New Mexico into western Kansas and the Oklahoma and
Texas Panhandle regions.

...Middle Mississippi Valley into Upper Great Lakes...
...Day 3...

As the surface low lifts northeast from the Southern Plains into
the Middle Mississippi Valley/Upper Great Lakes, expect rain to
transition to snow along the deformation axis. High pressure
anchored across the upper mid-west will help wrap drier cold air
into the backside of the surface low with strong cold air
advection aloft. Ample mid-level forcing for ascent combined with
the aforementioned ingredients should result in a narrow area of
snow on the northwest side of the low track. A lot of model
uncertainty in terms of track, timing and intensity of the cold
air behind the system has led to large variations in snowfall
amounts.  WPC probabilities for Day 3 (ending at 00Z Monday) show
a ribbon of Low to Moderate Risk of 4 inches or more of
accumulating snow from northern Missouri into north Illinois,
southern Wisconsin and LP of Michigan. We will continue to monitor
the synoptic pattern and track of the low. 

...Pacific Northwest...
...Day 3...

Models show an upper level shortwave and surface low moving into
British Columbia on Saturday.  Decreasing snow levels behind the
trailing cold front moving across the Northwest Saturday night may
support significant snow accumulations across portions of the
northern Cascades.  For the Day 3 period, WPC probabilities
indicate a Slight Risk for accumulations of 8-inches or more for
portions of the western slopes above 3000 ft.

Pereira/Pagano