Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
500 PM EDT Wed Apr 10 2019

Valid 00Z Thu Apr 11 2019 - 00Z Sun Apr 14 2019

 
...Northern/Central Plains to the Great Lakes... 
Days 1-2... 
 
Rapid surface cyclogenesis will continue as a deepening low shifts
east over KS through the overnight. A strong southerly LLJ across
the Great Plains will supply ample Gulf moisture into the system.
Sprawling surface high pressure centered over Hudson Bay persists
over the northern Plains, aiding frontogenesis on the north side
of the low. 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, most notably
across north-central NE, east SD into west-central MN. 
 
The TROWAL that has developed over western SD today will expand
and slowly pivot through northern NE and central/eastern SD as the
low shifts north and east toward the NE/IA border through
Thursday. Therefore a prolonged heavy snow threat is expected and
two feet of snow are likely over east-central SD. The positive
tilt of the mid-level trough allows Gulf moisture to spread up the
Great Plains and into this system, but the typical convective
thunderstorm complexes so commonly south of Plains systems (that
are usually neutrally or negatively tilted) will be largely absent
through Thursday. Heavy rain is expected near the front before it
changes over, but a great percentage of the incoming moisture will
be precipitated as snow which should maximize the snowfall, even
with this being the second week of April. There are moderate to
high 24hr probabilities for 12 inches from far northern Nebraska
to west-central MN; moderate probabilities continue across central
to northeastern MN. There are moderate to high Day 1 probabilities
for 18 inches in 24 hours in east-central SD and thirty percent
probabilities for 30 inches there in 48 hours.
 
Another area of concern is the ptype associated with frontogenetic
banding which is currently situated across southern MN/southwest
WI.  This line of precipitation will become enhanced as forcing
increases (frontogenesis, instability and TROWAL signatures) ahead
of the main surface low approaching.  Expect the tight gradient of
heavy snow/freezing rain to lift north overnight into early
Thursday with warm advection aiding in the transition to rain
across southeastern/east central MN into western WI later in the
day. There will be a brief period of time when the strong
frontogenesis and TROWAL signature aligns with snow before
transitioning. This would result in higher hourly snowfall rates,
but the overall snowfall totals may be limited. In addition,
mid-level dry air wrapping around the apparent low may lead to
atmospheric drying and the reduction of saturation within the DGZ
and aloft. This may lead to lesser snow amounts across this region
through Thursday. Given confidence is a bit low due to continued
uncertainty, only lowered snowfall slightly across this region.  A
stripe of low probabilities for quarter inch of ice are along the
MN/IA border for Day 1 and over northeastern WI/near the Straits
of Mackinac on Day 2.
 
The low lifts over the upper Midwest for Day 2 with the heavy snow
axis across northeastern MN and far north WI with the
aforementioned ptype issues occurring later on in the event.
Models have remained rather consistent with the low tracking into
south-central MN by early Friday with the model preference toward
the EC/GFS. The low will be occluded/filling at this point which
will continue to product more dry slots. Also, with convection
ongoing across IA this may aid in reducing the available moisture
thus diminishing snowfall rates/amounts as the low tracks
northeast.  Ahead of the main surface low, expect enhanced lake
effect and upslope snow along the northern shore of Lake Superior
leading to moderate probabilities for snowfall of 8 inches or
higher. There remains low to moderate for 8 inches from across
central MN to western Lake Superior on Day 2 as the low quickly
progresses northeast. 


...Pacific Northwest...
Days 1-2... 
 
A ridge into the Gulf of Alaska direct shortwave troughs down the
BC coast to the Pac NW today through Thursday night before a break
in precip as the ridge axis pushes ashore Friday night. Onshore
flow and snow elevations hovering around 3000 to 4000ft make for
moderate probabilities for 8 inches on the WA/OR cascades both
Days 1 and 2.

Day3...

The next shortwave trough will approach Saturday morning impacting
the Washington Cascades and Olympic Mountain ranges Saturday
afternoon through the overnight, which is getting into Day 4. 
Strong onshore flow with orographic effects should enhance lift
along the upslope regions leading to moderate to heavy snow across
elevations above 4000 feet.  


...Southern Plains...
Day 3...

Energy from the longwave trough across the western half of the
CONUS will swing south and east across the Desert Southwest Friday
night into Saturday.  As a result, surface low pressure will
develop along the southwest TX/Mexico border quickly lifting
northeast across TX as the upper level trough becomes negatively
tilted.  A coupled jet will lead to intense atmospheric ascent
with mid-level vorticity/southerly LLJ east of the surface low. 
With cold high pressure across the upper mid-west, this cold air
being fed into the intensifying low, precipitation may be in the
form of snow across the TX panhandle into western OK.  As the
system become vertically stacked later on Saturday, anticipate
frontogenesis to increase with the potential for heavy snow within
this region, thought confidence is below average at this time. 
Models spread is pretty significant, but coming into better
agreement on the overall trend for accumulating snowfall across
this region.  For now there is low probabilities of 4 inches or
more within this region. This system will be closely monitored. 

 
Jackson/Pagano