Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
445 PM EST Wed Mar 06 2019

Valid 00Z Thu Mar 07 2019 - 00Z Sun Mar 10 2019

...Western United States...
Days 1-3...

A closed low initially located just west of British Columbia will
start to propagate eastward later tonight or on Thursday as it
begins to open up.  On-shore flow on the southern side of the
system will continue to deliver deep moisture into the Sierra
Nevada range...while a separate on-shore flow will enhance
higher-elevations of the southern Sierras and the coastal ranges
of southern California. 

A modest Atmospheric River will lift northeast through CA and into
the Great Basin north of 40N on increasing SW 850-700mb flow and
within the LFQ of a 120kt upper jet. PWAT values over +2 standard
deviations above the climo mean will create an environment ripe
for heavy snow, with synoptic ascent through the aforementioned
jet level diffluence, mid-level height falls and orographic
enhanced lift to produce heavy snow. The highest accumulations
days 1 and 2 will be across the Sierra Nevada.  Elsewhere across
the West through Thursday morning, WPC probabilities are high for
12 inches in the mountains of Utah and Colorado, with generally
lesser amounts spread across the remaining western ranges.

In due time, the moisture and best dynamics will make their way
into the Intermountain Region.  Areas of moderate to heavy
snowfall will also spread deeper into the Intermountain region
with the heaviest amounts focused in regions of best upslope flow.


...Great Lakes...
Day 1...

Enhanced lake effect snow should gradually be tapering off later
tonight as the best dendritic growth zone shifts east and low/mid
level temperatures begin to warm. 


...Central Plains into Ohio Valley and parts of the Mid Atlantic...
Days 1-3...

Shortwave ejecting out of California will traverse near the Four
Corners and through Wyoming before cresting the ridge and shifting
southeast through the Plains and into the Ohio Valley...eventually
reaching the upslope region of the Appalachians on Day 2.

This will be accompanied by high column moisture and a weakening
Pacific Jet, through which diffluence embedded in the LFQ will
provide synoptic ascent across the region. A slowly developing
surface wave will be preceded by warm moist advection and a
sharpening 700mb temperature gradient /frontogenesis/ to produce
mesoscale lift coincident with the upper diffluence. Guidance is
in good agreement that a progressive band of snowfall will spread
from eastern WY through SD/NE and into IL/IN/OH and WV.

There remains some uncertainty among the models into how
significant QPF will be.  There had been another uptick in
amounts...especially across areas over the Ohio Valley.  While
some of the initial precipitation is expected to fall as snow,
increasing southwesterly flow will likely force a transition over
to liquid precipitation for a period which would limit amounts to
values that were forecast overnight. 

To the west of there, WPC probabilities for 4 inches remain along
the western NE/SD border, but fall off rapidly to the east except
in the terrain of WV where upslope flow may enhance accumulations,
and probabilities for 4 inches are near 30 percent.


...Lee of the Rockies to the Northern Plains...

Confidence is building that cyclogenesis to the lee of the Rockies
from late Friday evening into Saturday morning will lead to a late
winter storm across parts of the Northern Plains into the Great
Lakes region on Saturday...even though details are likely still in
a state of flux.

The greatest chance for heavy snowfall will extend from portions
of Nebraska and South Dakota into portions of Minnesota and
Wisconsin.  one of the main problem is where the transition from
liquid precipitation to solid precipitation occurs, especially on
the eastern side of the system and when cold air advection allows
a change from liquid to solid precipitation.  The area from
northern Iowa/southern Minnesota into the eastern Dakotas looks to
have the best chance for moderate to heavy precipitation with
lesser concern over precipitation-type. This led to highest
probability values in this area.  Of additional concern will be
the potential for strong surface winds to develop in the wake of
the low, resulting in snow/blowing snow capable of significantly
reducing visibility.

While some improvement will have begun from southwest to northeast
on Saturday, there will still be areas of snow and wind which
linger beyond the end of the Day 3 forecast.

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

Bann