Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
454 AM EDT Mon Mar 11 2019

Valid 12Z Mon Mar 11 2019 - 12Z Thu Mar 14 2019


...Four Corners...

Day 1...
A strong southwesterly jet extending from the base of the
trough/closed low centered off southern CA will direct tropical
Pacific moisture across the Desert SW today. Snow elevations rise
in this warm flow, above 8000ft along the AZ Mogollon Rim to
southern CO with higher values east and lower west. Only the
highest Mogollon Rim peaks in the White Mountains in AZ and the
San Juan/Sangre de Christo mountains in CO/NM will see snow which
will be heavy at these high elevations. Day 1 WPC probabilities
are high in these high elevation locales for eight inches with 18
to 24 inches more likely for the highest San Juans.

Day 2...
The low ejects east along the Mexico and AZ/NM borders Tuesday as
a northern stream trough approaches from the Pacific NW. Snow
elevations lower to 6000 to 7000ft over the Four Corners region
through the day with more of the Mogollon Rim, the Sacramento
Mountains of southern NM and other Four Corners states ranges
having moderate to high probabilities for six inches with moderate
probabilities for 18 inches again in the White mountains of AZ and
the San Juan mountains in CO.

Day 3...
The closed low shifts away from CO Wednesday morning, but the
northern stream trough persists over the Four Corners with snow
elevations dropping to 4000 to 5000ft before precip ends as dry
continental air moves in. Still, moderate to high probabilities
for six more inches in the White/San Juan/Sangre de Christo
mountains. Three day snow totals in these three ranges are likely
2 to 4 ft.


...Pacific Northwest to the Northern Rockies...

Days 1-2...
Mountain snows are expected to return to the Northwest tonight as
an amplifying shortwave with a leading atmospheric river shift
into WA/OR after 00Z. Expect locally heavy accumulations above
about 2000ft to the northern Cascades, with generally lighter
amounts spreading across the Blue Mountains and northern Rockies
on Tuesday. This is progressive system, with drier weather
expected for the Northwest on Wednesday. Despite the short window
of precipitation, moderate to high probabilities for eight inches
are across the Olympics and WA Cascades on Day 1 and the OR
Cascades, Blue Mountains, and northern ID/western MT Rockies on
Day 2.


...Central Rockies...Central High Plains...to Northern Plains...

Days 2-3...
The closed low currently off southern CA interacts with a northern
stream trough currently entering the Gulf of Alaska across the
Four Corners region Tuesday night with surface low development in
the lee of the CO Rockies Tuesday night, rapidly developing nearly
in place Wednesday before ejecting northeast across the central
Great Plains Wednesday night.

Ongoing mountain snows in CO are expected in southern CO starting
today as moisture ahead of this low is pushed inland on a 130kt+
jet crossing the Sea of Cortez. However, snows do not get
appreciably north of the San Juans until late Tuesday when the low
reaching the NM/Mexico border. The low level cyclogenesis in the
lee of the Rockies brings upslope flow to northern CO/central WY
with plenty of Pacific and Gulf moisture wrapping in. A TROWAL
quickly develops in the NE Panhandle likely extending south into
northeast CO per the QPF preference of the 00Z GFS and 12Z ECMWF.
Particularly heavy snows are forecast to develop in this intense
TROWAL with surface low pressure centered near the CO/KS border
dropping into the 970s Wednesday morning as the northern stream
trough interacts with the low resulting in little motion of the
low for 12 to 18 hours. The low finally ejects northeast to the
NE/IA border Wednesday night. An axis of heavy snow in the TROWAL
is expected to develop from the NE Panhandle northeast to eastern
ND through Wednesday night. Moderate Day 3 probabilities for 18
inches are along this swath axis.

Placement of this axis (which will be accompanied by particularly
strong winds) is in decent agreement in the 00Z global suite. The
00Z ECMWF did shift a bit farther north/west with the axis due to
a flatter northern stream trough drawing the system farther in.
While exact placement has uncertainty the presence of a
particularly heavy swath of snow is likely with messaging
increasing for this threat.

As the system ejects northeast, it occludes with a dry slot
expected to wrap in from the east over the central Plains and
leading to precip placement uncertainty.

A swath of freezing rain on the east side of the TROWAL is likely
as cold/dry air advects in from a western CONUS ridge building in
behind the trough and extending up to Alberta. Moderate
probabilities for a tenth inch are along this swath from
north-central NE and northeast across eastern SD.

Jackson