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

Valid 00Z Tue Mar 05 2019 - 00Z Fri Mar 08 2019

California/Pacific Northwest/Northern to central Rockies/northern
to central Great Basin...

Days 1-3...

Snow begins again in the higher elevations of the CA Sierra Nevada
on Tue as a cyclone off the coast starts drifting eastward, with
increasing layer relative humidity and ascent on Tuesday afternoon
leading to widespread precipitation and mountain snows.

On Tue night and Wed, an atmospheric river noted by PWAT anomalies
of more than +3 sigma above the mean will spread moisture onshore
into southern CA and well inland, with long duration snows
expected in the central to southern Sierra due to jet diffluence
and height falls. Intense orographic ascent on strong SW 850-700mb
winds will produce the heaviest snow in the Sierra.

After the primary low moves onshore, persistent 700 mb confluence
continues to provide a moisture influx from the Pacific across
much of CA, with additional snows Wed night-Thu in the Sierra
Nevada and ranges of northwest CA as well.
For the multi-day event, up to 5 feet may fall in the highest
terrain.


The persistence of the Pacific moisture fluxes Tue night-Wed night
combines with jet energy and orographic lift to produce locally
heavy snow across the ranges of central NV to UT and western CO.
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.

In the Pacific northwest, light snows are expected on Mon
night-early Tue as moisture moves inland downstream from the
closed low off the coast.  A lull appears to be in the offing late
Tue as the low weakens and moves northeast into Vancouver Island. 
After the lull, more snows are expected as the low off Ca advects
moisture into the OR Cascades/Blues late Tue night-Wed, extending
up into the Wa Cascades/ranges of northeast WA Wed. The moisture
extends downstream into
the northern Rockies Thu, with snow expected as shortwave energy
ejects east across the region from the Pacific northwest.

...Great Lakes...

Days 1-3...

Pieces of shortwave energy crossing the lakes will bring the
potential for Lake Effect snow as moisture and instability
persists with the core of the cold air in place.  The heaviest
snow through the 3 days is likely east of Lake Ontario where snow
may accumulate to 8 inches or more . The other area that may see
heavy snow is in the western L.P. of Michigan, where several
inches of snow are possible between now and mid week.

Lighter snows extend downstream from western lake Superior as
drier air aloft advects over the region Tue night. Snow tapers of
Lake MI Wed as the drier air moves downstream, followed by a a
resurgence of moisture Wed with a warm front approaching from the
west, and a period of light snow. Snow also tapers in the lower
lakes Thu as lapse rates and layer moisture become less favorable.

...Central Plains...

Day 3...

Shortwave energy is forecast to move steadily eastward out of the
Rockies Wed night-Thu from the WY/CO Rockies across the central
Plains. The gfs was more amplified than other solutions with the
700-500 mb wave, which allows the band of snow with the warm to
shift further north than other models indicate.  

Low-mid level warm air and moisture advection and frontogenesis
will produce a stripe of snow on Wednesday into Thu extending from
southeast WY and northeast CO across Nebraska into northern MO and
IA, possibly extending into parts of IL.  The rapid progression to
the east will limit total QPF and snowfall, but the strength of
the front plus moisture convergence along it is sufficient to
support a high risk of 4 inches of snow from southwest SD across
northwest Nebraska, with a few north-south band location
differences left to be resolved. 

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

Petersen