Heavy Snow in Washington, February 5 to 6, 2023

Area Forecast Discussion
National Weather Service Pendleton OR
313 AM PST Sun Feb 5 2023

.Corrected some wording errors

.SHORT TERM...Today through Tuesday night...A potent Pacific
weather system will move into the region, bringing moderate to
heavy amounts of snow to the mountains, and rain to the lower
elevations. The heaviest snow will fall between 4 AM this morning
through 4 AM Monday. The snow will decrease on Monday, and then
end Monday night. There will be a brief break in the weather
Monday night through Tuesday morning, except for lingering snow
over the WA Cascades and rain in northwest areas of the Yakima and
Kittitas Valleys. Then another weather system will move into the
region by Tuesday afternoon and continue through Tuesday night
into the extended period.

For the Blue Mountains, snow will increase early this morning,
with heavy amounts of snow above 3500-4000 feet. The NBM 4.1 WSUP
viewer shows snow amount probabilities (along with forecast QPF
amounts) that support Winter Storm Warnings above 3500 ft for the
northern Blue mountains and above 4000 ft for the southern Blues,
with high confidence (70-90 percent). There is a 70-90 percent
chance of more than 10 inches of snow above 3500 feet for the
northern Blue Mountains. For the southern Blue Mountains, the NBM
4.1 WSUP viewer shows a 50-70 percent chance for greater than 4
inches for a good portion of the area above 4000 feet. In addition,
the HREF 24 hr snowfall totals ending at 4 AM Monday range from 6
inches over the lower ridge tops to as much as 24 inches over the
highest peaks and ridges. These combined tools result in high
confidence (70-90 percent) of warning amounts of snow above 4000 ft.

In the Ochco-John Day Highlands, confidence is moderate to high of
receiving advisory amounts of snow (50-70 percent) above 4000 feet.
The NBM 4.1 WSUP shows about a 60-90 percent chance for greater than
2 inches, and a large range of 30-80 percent chance for greater than
4 inches of snow, with the greatest chances over the eastern ridges
and peaks in this zone. In addition, the HREF 24 hr snowfall totals,
ending at 4 AM Monday, range from 4 inches over the lower ridges up
to 12 inches over the highest peaks and ridges, with most areas
above 4000 feet in the 4 to 6 inch range.

For the east slopes of the OR Cascades, the NBM 4.1 WSUP viewer
shows a 50-90 percent chance of receiving greater than 6 inches of
snow. The WA Cascades looks to receive less than advisory amounts of
snow, except for the highest peaks along the crest, especially north
of Snoqualmie Pass. Snoqualmie Pass will likely receive about 3 to 5
inches of snow.

The lower elevations will receive rainfall amounts of less than a
tenth of an inch from central OR north to the eastern Columbia
Gorge, and also in the Yakima and Kittitas Valleys. The Lower
Columbia Basin and the Foothills of the southern Blue Mountains
are expected to receive 1-2 tenths of an inch. The Foothills of
the northern Blue Mountains are expected to receive 2-3 tenths of
an inch by early Monday morning.

It will become very windy over the OR Lower Columbia Basin and the
OR Blue Mountain Foothills. A Wind Advisory will be in effect for
these areas from 4 PM this afternoon until 7 AM Monday. The NBM 4.1
WSUP Viewer supports advisory level wind speeds and gusts, resulting
in high confidence (70-90 percent). Elsewhere, most of the forecast
area will become breezy to windy by late this afternoon and tonight,
but with wind speeds less than advisory criteria.

A transient high pressure ridge will bring a break in the weather
from Monday afternoon through Tuesday morning, and then another
weather system will move into the region, bringing more mountain
snow and low elevation rain. This second system will be weaker, and
highlight amounts of snow in the mountains are not expected at this
time. It will become breezy to windy again by Tuesday afternoon and
evening. However, at this time, advisory wind speeds are not
expected, though are possible in some areas, with low confidence (10-
 20 percent).

Temperatures will continue to be about 5 to 10 degrees above normal,
especially on Tuesday. Highs are expected to be in the mid 40s to
around 50 today and Monday, and in the upper 40s to mid 50s on
Tuesday. In the mountains, highs will be in the upper 20s to mid 30s
today and Monday, and then in the mid 30s to lower 40s on Tuesday. 88

.LONG TERM...Wednesday through Sunday...The long term is
characterized by a departing upper level trough and cold front,
with precipitation also exiting the region while temperatures cool
several degrees. High pressure then takes over, bringing mostly
quiet conditions to the bulk of the long term forecast period.

Wednesday starts with an upper level trough just east of our
region, with precipitation expected to be lingering in the
mountains. This should subside by the afternoon, with only light
accumulations expected as the bulk of the primary QPF occurs just
before this time period. Cooler northwesterly flow behind a cold
frontal passage should help lower temperatures back down to near
average. Beyond this, high pressure is expected to overtake the
PacNW, and should be the primary weather influence through most of
the remaining forecast period, which will shield the region from
precipitation and significant weather highlights. The exceptions
will be some light precipitation possible across the Cascades on
Friday into Saturday as a deepening low approaches the coastline
of the PacNW, with this low then becoming somewhat of a wild card
as models diverge on the expected track and strength of this
system during the end of the period.

Analyzing the ensembles shows good agreement on the exiting
initial trough and subsequent ridging pattern through Friday,
with some mild variation on the strength and exact positioning of
the ridge. Thereafter, the approaching and deepening upper low
introduces a bit more variation as some models expect it to be
diverted southward towards California, while others expect it to
move inland a bit quicker and hold more of an influence over our
region. Analysis of the ensemble clusters versus their
deterministic counterparts does not present a standout choice, as
variation remains daily across all members. For this forecast run,
have elected to follow the NBM guidance, as this does keep us in
the drier pattern through next weekend with the bulk of the
precipitation and low's influence kept to the south over
California. Goatley/87

&&

.AVIATION...12Z TAFs...Conditions remain mixed between VFR to MVFR
from passing rainfall, which should continue through the afternoon
hours. Rain should pass DLS/YKM/BDN/RDM through the beginning of
the afternoon, with rain ending for PSC/ALW/PDT during the
afternoon into the evening. Strong winds overspread through the
day as well, gusting to 25-40kts, strongest at PDT. Site YKM
should remain under 20kts. Goatley/87

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  49  33  49  35 /  80  40   0   0
ALW  49  33  49  36 /  80  40  10   0
PSC  50  34  51  39 /  70  10   0   0
YKM  46  29  48  33 /  50  10   0   0
HRI  50  35  51  37 /  70  20   0   0
ELN  44  33  46  32 /  40  10  10  10
RDM  45  26  46  30 /  40  10   0   0
LGD  43  31  42  30 /  80  60  10   0
GCD  43  26  41  28 /  70  40  10   0
DLS  50  38  50  36 /  70  10  10   0

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Wind Advisory from 4 PM this afternoon to 7 AM PST Monday for
     ORZ044-507-508.

     Winter Storm Warning until 4 AM PST Monday for ORZ502-503.

     Winter Weather Advisory until 4 AM PST Monday for ORZ506-509.

WA...Winter Storm Warning until 4 AM PST Monday for WAZ030.

&&

SHORT TERM...88
LONG TERM....87
AVIATION...87