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

Valid 12Z Thu Mar 12 2020 - 12Z Sun Mar 15 2020

...Northern Great Lakes to Northern New England...
Days 1-2...

A northern stream shortwave trough swings east from the northern
Great Plains today and becomes negatively tilted over the Great
Lakes tonight before closing off/occluding and lifting into Quebec
through Friday. Surface low pressure develops ahead of the trough
and lifts north from WI across the U.P. this evening before
shifting into Ontario with a secondary low developing at the end
of the occlusion over upstate NY Friday. Lake enhanced wrap around
snow has low chances for reaching 4 or more inches over the
Keweenaw Peninsula of the U.P. on Day 1. The surface low
development Friday aides higher elevation snow in the Adirondacks
and White Mtns where there are moderate probabilities for 4 or
more inches in the Day 1.5 (00Z Friday to 00Z Saturday) outlook.
Southerly flow ahead of the approaching system promotes pockets of
freezing rain with low probabilities for a tenth inch of ice in
the Green and White Mtns on Day 1.5.


...Pacific Northwest to central California and across 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 merely 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/MT. In this pattern,
700-500 RH will increase quickly on moist SW flow, which will
begin to overrun the low-levels as a surface cold front digs
southward from Canada. The 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 fgen 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 the Northern Rockies near Glacier NP
east to the Saskatchewan border. Day 2 WPC probabilities are high
for 8 inches or more in the higher terrain around Glacier NP and
moderate across the rest of the plains in this area. 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.
Farther south across southeast MT, a dry slot is forecast to wrap
into the system creating much lower or no snowfall.

To the west of this heaviest snow axis, prolonged synoptic ascent
through mid-level divergence, height falls, and jet streak
diffluence will produce widespread precipitation across the
terrain from the Olympics, southward through the Northern
California mountains, and east into Idaho. Snow levels will drop
as well as the upper low sinks southward, and light snow is likely
even in the valleys as far south as northern Oregon. The highest
snowfall is likely in the Olympics and WA Cascades where 2-day
totals may exceed 12 inches. Day 2 WPC probabilities are high for
8 or more inches in the WA and northern OR Cascades. High day 3
probabilities for 8 or more inches spread south through the rest
of the OR Cascades, the Siskiyous/Klamath, northern CA Cascades
and northern Sierra Nevada. Particularly heavy snow on Day 3 is
likely over northern CA as the slow moving system continues to
direct Pacific moisture into the terrain. There are moderate to
high Day 3 probabilities for 18 or more inches between Mt. Shasta
and Lake Tahoe.


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

A large closed low off the northern Baja this morning will shift
inland over San Diego tonight open as it ejects northeast over the
south-central Rockies late Friday ahead of the next system over
the Pacific Northwest. As this feature lifts northeast, it will
spread precipitation northeast from the southern Sierra through
the Four Corners and into the Central Plains. Precipitation will
expand in coverage through warm/moist mid-level advection and
height falls/PVA as the upper feature shifts northeast. Snow
levels of 7000 to 10000ft over the Desert SW will keep snow
limited to the highest terrain of southern CA/southern NV/AZ on
Day 1 with snow levels of 6000 to 8000ft over UT/CO promoting snow
over the southern Wasatch and San Juans of CO where Day 1.5
probabilities for 6 or more inches are moderate to high.

A secondary jet streak is expected to develop and shift towards
the Great Lakes placing favorable diffluence for ascent across the
central Plains. This will all be ongoing atop a surface cold front
dropping southward creating a band of enhanced frontogenesis as
well. Guidance has come in better alignment for precip and thermal
profiles over the central Plains with west-central Nebraska
favored for banded snowfall. Day 2 probabilities for 6 or more
inches have risen to 50 to 70 percent over the southern Sandhills
to south-central NE. Remnant energy shifts east across the Midwest
Saturday with cyclonic forcing keeping overrunning to a minimum
and rather low Day 3 probabilities for 4 or more inches over IA/IL
into IN.


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

Jackson