Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
441 AM EDT Tue Apr 09 2019

Valid 12Z Tue Apr 09 2019 - 12Z Fri Apr 12 2019

...The West...
Days 1-3...

A trough currently pushing onshore over the Pac NW digs and
amplifies as the southern end reaches the Desert SW tonight with
the mid level closed low center over UT before the system shifts
east across the northern/central Rockies Wednesday. Residual
moisture exists over the west from the atmospheric river pushing
onshore prior to landfall which had also raised snow elevations.
Snow elevations decrease with the trough as does the influx of
Pacific moisture, so heavy snow is across much of the high terrain
from the Cascades to the northern Rockies and mountains of the
Great Basin on Day 1. Rapid cyclogenesis in the lee of the CO
Rockies Wednesday focuses heavy snow across WY and CO as well as
the mountains of UT on Day 2. A blizzard develops on the Great
Plains which is discussed below.

The next trough paired with Pacific jet energy will shift
southeast onto the Pac NW coast Wednesday through Tuesday day 3,
bringing renewed snow to the elevations above 3000 to 4000 ft from
the WA/OR Cascades eastward across the Northern Rockies.


...Northern Plains to the Great Lakes...
Days 1-3...

Rapid cyclogenesis will begin tonight in the lee of the CO Rockies
as an amplifying trough pushes across the Great Basin. This low
will stall over eastern CO until Wednesday afternoon as the parent
upper low closes and deepens as it moves over the CO Rockies. A
strong southerly LLJ will provide ample moisture into an already
saturated environment characterized by Pacific moisture and PWAT
anomalies of +1.5 standard deviations. While the airmass east of
the surface low will be quite warm, north and west of the low cold
air will be in place, and will intensify as sprawling high
pressure centered over Hudson Bay and the deepening surface low
combine to drag cold air southward into the Plains, and then over
the Great Lakes on day 3. As the low deepens, the parent upper
trough will close off, and a stripe of potent deformation and
intensifying 850-600 mb frontogenesis will drive mesoscale ascent,
and heavy snow is likely north and west of the low track.

Precipitation breaks out over SD Tuesday afternoon and dynamic
cooling is expected to cause a rapid changeover to snow Tuesday
night as the TROWAL sets up north of the developing low. This
TROWAL axis merely pivots as the low shifts east across KS
Wednesday afternoon and night and then into western IA Thursday.
Therefore a prolonged heavy snow threat is expected and two feet
of snow are possible over eastern SD. Another note about this
system is the positive tilt of the parent trough that allows Gulf
moisture to spread up the Great Plains, but not the typical
convective thunderstorm complexes so commonly south of the system
that reduces the Gulf moisture before it wraps around the system
as snow. Heavy rain is expected before it changes over, but a
great percentage of the incoming moisture will be precipitated as
snow which could maximize the snowfall, even with this being the
second week of April. There are a moderate probabilities for 18
inches in 24 hours in the Day 2.5 time frame.

In the high plains/Black Hills WPC probabilities are moderate to
high for 12 inches for Days 1.5 and 2 from eastern WY into central
NE and southern SD. It is across this area where blizzard
conditions are possible as this intense lift occurs atop a deep
isothermal layer in conjunction with -EPV near the DGZ to produce
the potential for thunder-snow and 1-2"/hr snowfall rates, along
with increasing winds. North of the low track, theta-e advection
wrapping cyclonically around the low within a robust TROWAL will
produce a prolonged period of heavy snowfall, and WPC
probabilities are high for 12 inches, and even moderate for 18
inches, from northeast NE across southeast SD and into southwest
MN. Despite relatively lowered SLRs due to high liquid-water
content snow, tremendous QPF and heavy snowfall rates suggest the
potential for more than 20 inches in some locations. There will
likely be a very sharp gradient between heavy wet snow and rain,
and some uncertainty remains in where this axis will settle over
the northern Plains.

The low lifts over the upper Midwest for Day 3 with the heavy snow
axis in the MN/WI area. The 00Z ECMWF shifted farther west with
the low tracking into south-central MN by early Friday, so this
track will need to be monitored. The low will be occluded/filling
at this point which can lead to broad dry slots, so this will all
need to be taken into account. As of now there are moderately high
probabilities for 8 inches from eastern SD to western Lake
Superior on Day 3.

Along the precip type changeover line across SD and near the MN,
IA border freezing rain over MN/IA is likely where intense
frontogenesis drives lift and surface temperatures cool beneath
the southerly LLJ induced warm nose. At this time a lighter
freezing rain event is preferred as isallobaric wind should cool
the column quickly in this region to change precip from rain to
snow more so than freezing rain. However, moderate probabilities
for 0.1 inches persist for Day 2. By the time the system lifts
over the upper Midwest a cold front will have set up to the south
allowing more intense moisture advection ahead of the front.
Overrunning, ahead of the cold front increases the risk for
freezing rain with low probabilities for a quarter inch over both
northern WI and northern MI. over Farther east over the Great
Lakes Further east into central MN and WI.


...Northeast...
Day 1...

A secondary low in the wake of the low passage last night crosses
central New England this afternoon and evening. Mainly continental
air in the wake of the previous system limits the potential of
this system though moderate probabilities for six inches jut into
western Maine and the probability for one inch reaches into
southern VT/NH. A light icing is also possible near the low track
in sheltered valleys of northern VT/NH.


Weiss