Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
356 PM EDT Thu Mar 12 2020

Valid 00Z Fri Mar 13 2020 - 00Z Mon Mar 16 2020



... Northern Rockies to the Northern High Plains...
Days 1-3...

A potent, positively tilted shortwave trough will dig south along
the WA/OR coast Friday before closing off and drifting south to
off the OR/CA border through Saturday night. To the east of this
feature, mid-level divergence will intensify, combining with
increasing diffluence within the right entrance region of the
downstream jet streak to produce large scale ascent across the
Pacific Northwest and Northern Rockies. In this pattern, 700-500
RH will increase quickly on moist SW flow.  The near-sfc 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 heaviest snow is likely across north-central MT from
Glacier NP. Day 1+2 WPC probabilities are high for 8 inches or
more in the higher terrain around Glacier NP and moderate across
the adjacent high Plains. 

As the upper trough drifts southeast, the axis of snow on day 3
shifts into eastern MT to south central MT/adjacent northwest WY.
The GFS shows higher magnitude 700 mb convergence across the
ranges of southern ID to northwest WY/south central MT and weaker
in northeast MT, so higher QPF amounts and resultant snow are
favored in southern ID to northwest WY/south central MT. Day 3
probabilities for 8 or more inches are low over northeast MT with
high probabilities for the Tetons and Wind River Ranges in WY.
Light snow develops in ND as an inverted low level trough
approaches, inducing low level convergence and ascent. The 12z GFS
and GEFS Mean produce more snow than the ECMWF/UKMET/Canadian
global models, so differences remain to be resolved. 

...Pacific Northwest to central California...

A mid-upper level cyclone is forecast to drift slowly south across
the eastern pacific just ff the west coast. 
Prolonged synoptic ascent through upper divergence, jet streak
diffluence, low level convergence, with aid from upslope
conditions in windward terrain will produce widespread
precipitation across the mountains from the WA Olympics, and south
along the WA/OR Cascades southward through the Northern and
central California mountains. Snow levels will drop as the upper
low sinks southward, and light snow is likely even in the valleys
as far south as northern Oregon.

Heavy snowfall is likely in the Olympics and WA/OR Cascades, and
CA Shasta/Siskiyous, where 2-day totals are forecast to be 12-18
inches. Amounts pick up even more in the CA Sierra Nevada
mountains, as the cyclone nearly stalls off the northern CA coast,
producing prolonged moisture fluxes and ascent in the northern to
central CA Sierra. There are moderate to high Day 3 probabilities
for 24 or more inches between Mt. Shasta and Lake Tahoe. There is
potential for 3 feet of snow on Day 3.

...Southwest through the Central Plains to the Midwest...
Days 1-3...

A large closed low off the southern CA coast this evening is
forecast to eject northeast across the southwest and become an
open wave. On Day 1, the progression of the upper jet across AZ/NM
places favorable upper divergence maxima across northern AZ and
southern UT across northern NM and southern CO.  The heaviest snow
are expected where moderate to locally precipitation occurs where
it remains coldest for the event int the mountains of southern UT
and northern NM/southern CO.
Over the southern Wasatch and San Juans of CO/adjacent northern NM
Day 1 probabilities for 6 or more inches are high.

On Day 2, precipitation will expand in coverage through warm/moist
mid-level advection and height falls/PVA as the upper feature
shifts northeast out of the Rockies on to the southern and central
Plains.  Because of the warm advection, snow changes to mixed
types and rain across southwest to north central KS.
A secondary jet streak is expected to develop and move across NE,
so pockets of upper divergence combine with the low-mid level warm
advection to produce area with several inches of snow expected.
Day 2 probabilities for 6 or more inches have risen to 50 to 70
percent over the southern Sandhills to south-central NE.

...Upstate New York/ Northern New England...
Day 1...

A trough swings east from the Great Lakes tonight across southern
Ontario and then across Quebec/northern New England Fri.
Downstream from the trough, low-mid level warm/moisture advection
produce a swath of precipitation that progresses steadily across
northern New York and northern New England.  There is precip type
uncertainty with the warm air advection possible inducing a change
from snow to mixed sleet and freezing rain across the Adirondacks
of NY and across the Greens of VT/White mountains of NH, ranges of
western Maine. 

Several inches of snow are expected in portions of interior Maine,
where initial temps are cooler than most of NY/VT. 
An inversion 850-700 mb leads to a transitional sleet/freezing
rain in mountains of VT/NH/Maine.
Low probabilities for a tenth inch of ice in the Green and White
Mtns on Day 1.

The event ends as the system departs into the Canadian maritimes
Fri night, with lift ceasing and drying aloft developing. 

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

Petersen