Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
342 AM EST Mon Mar 04 2019

Valid 12Z Mon Mar 04 2019 - 12Z Thu Mar 07 2019

...Maine...
Day 1...

Deepening low pressure will move rapidly to the northeast to be
east of Nova Scotia tonight. Some very heavy snow will persist
across Maine where the best combination of mid-level deformation
and low-level frontogenesis coincide to produce snowfall rates of
1-2"/hr at times. Despite the rapid transit of the low and its
associated forcing, high probabilities exist for 8" across
Downeast Maine where several hours of heavy snow will produce
these high accumulations. Elsewhere, points further west and south
into NH and NE MA, WPC probabilities are low for even an
additional 2", much of which will fall first thing this morning
before dry air wraps into the area behind the departing system.


...Great Lakes...
Days 1-3...

Broad but deep trough centered over Hudson Bay will eject spokes
of vorticity and cold advection across the Lakes through the
middle of the week. Each piece of energy will bring the potential
for Lake Effect snow as moisture increases and instability rises
over the Lakes. However, much of the surface of the Lakes are
ice-covered now except Ontario and Michigan, limiting snowfall
potential area-wide. However, there is a strong signal for modest
snowfall amounts beginning late on Day 1 and carrying into Day 2
/Tuesday/ as low-level lapse rates steepen and winds become
unidirectional westerly through 700mb. Despite a relatively low
EL, a saturated and low DGZ with modest forcing will produce bands
of snow into Michigan as well as upstate New York. The heaviest
snow through the 3 days is likely east of Lake Ontario where snow
may accumulate to 8 inches or more, but WPC probabilities are
modest for 4 inches on any given day. The other area that may see
heavy snow is in the western L.P. of Michigan, when tonight
forcing maximizes, and several inches of snow are possible.


...Central Plains...
Day 3...

A shortwave and associated Pacific Jet maximum will race eastward
from California on Wednesday. This feature and its associated
height falls will flatten the overall flow across the CONUS,
causing it to advect quickly into the Central Plains by the end of
day 3. Beneath this forcing, weak surface cyclogenesis will
develop with WAA ahead of it spreading moisture into NE/SD/IA, and
the combination of the synoptic ascent aided by this WAA and
intensifying mid-level frontogenesis will produce a stripe of snow
on Wednesday. Forcing is significant, but the rapid progression to
the east will limit total QPF and snowfall, and WPC probabilities
are moderate for 4 inches from eastern WY to along the NE/SD
border. With robust forcing in place there is potential for higher
amounts, and the evolution of this system will need to be
monitored with future updates.


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

Some lingering snow across the high terrain of Colorado today will
get cut off as ridging blossoms across California. This will be
short lived however, as yet another mid-level and surface low
approach California during Day 2 before moving onshore Wednesday.
As the trough and surface low shift inland, an atmospheric river
noted by PWAT anomalies of more than +3 sigma above the mean will
spread moisture well inland, reaching as far east as Colorado and
as far north as Montana. Synoptic ascent due to jet diffluence and
height falls will produce widespread snowfall, especially late on
Day 2 and Day 3. Snow levels will be quite high however, 6000 ft
or more in the Sierra and other points along 40N latitude, falling
to 2000-3000 ft across ID/WA/MT. Intense orographic ascent on
strong SW 850-700mb winds will produce the heaviest snow in the
Sierra, where WPC probabilities are high for 18 inches day 2 and
day 3, and up to 5 feet may fall in the highest terrain. The other
high terrain, including the mountains of ID, UT, and CO, all have
moderate probabilities for 12 inches during the 2 days. With snow
levels remaining high, no accumulation is expected in the valleys
with this system.


The risk for significant freezing rain of 0.25 inches or more is
less than 10 percent all 3 days.


Weiss