Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
426 PM EDT Fri Mar 13 2020

Valid 00Z Sat Mar 14 2020 - 00Z Tue Mar 17 2020


...Pacific Northwest to California...
Days 1-3...

...Heavy Snow is Likely in the Mountains of northern and central
CA this weekend into early next week, with several feet in the
northern Sierra Nevada likely...

A mid-upper level cyclone developing off the OR coast tonight will
drift south to be off the northern CA coast Sunday and central CA
coast Monday.  This allows Pacific moisture to rotate around the
low, onshore and inland across the WA/OR/CA Cascades on Day 1,
with slightly higher amounts due to longer duration low-mid level
convergence in the northern Sierra Nevada and Klamath/Siskiyous. 
Day 1 probabilities are moderate to high for 8 or more inches for
the WA/OR Cascades and Olympics.

Prolonged synoptic ascent through upper divergence, jet streak
diffluence, low level convergence, with aid from upslope
conditions in windward terrain will produce widespread heavy snow
in the ranges of northern to central CA on Day 2.  
Confidence continues to increase in prolonged heavy snow.  In
particular,  probabilities for 24 or more inches on Days 2/3 are
high between Lassen Peak and the central Sierra Nevada a little
south of Lake Tahoe with 3 to 5 feet likely through this time for
the northern High Sierra.

Amounts are expected to taper with the low weakening a bit on
Monday, but the slow movement still supports a long duration snow
event in the CA Sierra Nevada Mountains.  An additional foot to 2
feet are possible.  Differences start to develop with timing of
the low, as a few models, like the 12z NAM and ECMWF, sped up
forward motion to the south.


...Northern to central Rockies and Northern Plains...
Days 1-3...

Increasing diffluence in the vicinity of the upper jet streak
crossing ID and MT will produce large scale ascent across the 
Northern Rockies and high Plains. 700-500 RH will increase quickly
on moist southerly flow, with the combined warm/moisture advection
and pre-frontal convergence combining to produce ascent.  The
near-surface cold air sinking southward interacting with warm
advection aloft will lead to intensifying 850-700mb frontogenesis
which is likely to persist and become intense across parts of
Montana. The combination of synoptic ascent with this intense
frontogenesis and low-level upslope flow will produce an extended
period of moderate to heavy snow. The greatest snowfall is likely
across north-central MT between 110W and the eastern slopes of the
Rockies/Glacier National Park. Day 1 WPC probabilities are
moderate for 12 inches or more from the crest of the Lewis Range
in Glacier NP onto the adjacent High Plains. 

As the upper jet drifts east into eastern to south central MT and
southeast ID by Sun morning, upper divergence maxima and mid level
moisture advection focuses snow to shift into northeast MT to
south central MT and south into adjacent northwest WY. Day 2
probabilities for 6 or more inches are moderate to high in a swath
from north-central MT to the Tetons.

A shortwave impulse moves east from MT across ND Sunday, allowing
generally light snow to develop in ND on low level convergence and
ascent. The decreasing amplitude of the wave and weakening 700 mb
convergence leads to lighter snow amounts the further east you go
across the northern Plains. Day 2 probabilities feature low
probabilities for 4 or more inches centered over ND.


...Central Plains to the Midwest...
Day 1...

An upper trough will eject northeast across the central Rockies
tonight. The progression of the upper jet across CO places
favorable upper divergence maxima across the ranges of CO this
evening, but this tapers as the upper tough departs to the east. 
Snow has developed in the downstream warm/moist mid-level
advection and 700mb frontogenesis in southwestern NE this
afternoon. The area of snow is expected to continue moving across
NE tonight and into southwest SD. Several inches are likely in
these areas. Precipitation type in the primary uncertainty in
northern KS and the Missouri Rover Valley of NE/western Iowa, as
initial temps well above freezing lead to a possible mixture of
precip types.

The upper level shortwave trough shifts quickly east from the
central Plains into the Midwest tonight through Saturday with
moisture and warmth advection aloft getting lifted as the jet
streak crosses the region.
Joint uncertainties involve a short duration of snow plus possible
changes in precip type, keep probabilities for heavy snow low
across IA and IL into IN. The passage of the upper shortwave
brings the event to a close Sat.

For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Petersen