273
FXUS66 KPDT 121127
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
327 AM PST Wed Feb 12 2025

.SHORT TERM...Today through Friday night...

Key Messages:

1. Very cold, record-breaking temperatures persist.
   *Cold Weather Advisories Active*

2. Widespread snow chances Thursday and Friday.

3. Breezy to windy conditions Thursday.

Current infrared satellite imagery showing clear skies as a very
dry airmass is currently draped across the region. This is in
response to the upper level trough that dropped through the
Pacific Northwest on Tuesday, allowing flow aloft to be from the
north. This initial trough is currently in the Four Corners region
as a second upper level shortwave slides down the synoptic flow to
enhance cold air advection across our area. The enhancement will
lead to the coldest morning and afternoon temperatures in the
forecast, which has also brought a slight uptick in winds along
the foothills of the northern Blue Mountains. These winds will
allow wind chill temperatures to drop to -10 or below, which has
warranted to inclusion of the northern Blue Mountain foothills in
the already active Cold Weather Advisory. However, the advisory
over the Blue Mountain foothills is only valid through 8 AM this
morning, whereas the existing advisory across the Blue Mountains,
Central Oregon, John Day-Ochoco Highlands, Wallowa County, and the
east slopes of the Washington/Oregon Cascades extends to Thursday
morning. Low temperatures both this morning and Thursday morning
over these areas could reach to -20. Confidence in these low
temperatures is high (90%) as 2 AM observed wind chill values are
between -6 and -9 across the northern Blue Mountain foothills and
Central Oregon, with -15 degree values across Union and Wallowa
Counties. Upper level flow will become more from the west later
today in the wake of the departing shortwave and as an incoming
transient ridge quickly slides through the area late tonight.

The upper level ridge pushes to our east overnight as an upper
level low begins to open up as it proceeds onshore. Light snowfall
will begin across Deschutes county during the early morning hours
on Thursday, reaching the Gorge and Southern Blue Mountains by 10
AM before encompassing the entire area through the afternoon. 1-3
inches of snowfall is expected on Thursday for most areas, with
3-6 inches expected for elevations above 4000 feet. The only
exception looks to be the Yakima/Kittitas Valleys which should
experience less than 1 inch of snow. The upper level trough`s axis
will be overhead into early Friday morning before exiting to our
east Friday evening. This will bring another day of light snowfall
on Friday, with the majority of snow falling at elevation over the
Cascades and Blue Mountains. However, the foothills of the Blue
Mountains may experience an additional 1-2 inches due to
upslope/orographic lifting. These snow amounts do reach advisory
thresholds for portions of Central Oregon and the northern Blue
Mountain foothills, but current confidence is low (30%) as the NBM
suggests a 10% chance of 2 inches or more (advisory-level) for
the Bend/Redmond area and a 20-30% chance along the northern Blue
Mountain foothills.

The incoming system will also lead to breezy winds as a pressure
gradient sets up along the Cascades and Blue Mountains. This
gradient is due to the high pressure exiting to our east as strong
surface low pressure moves onshore to our west. Breezy east winds
will first be experienced through the Gorge and Basin as the
pressure gradient between Portland (KPDX) and Hermiston (KHRI)
reaches 10 to 14 mb via the GFS and NAM. Winds are expected to
peak in these areas between 1 AM and 7 AM Thursday morning as wind
gusts of up to 25 mph will be possible. Winds will also be breezy
through the Grande Ronde Valley as a 6 to 9 mb pressure gradient
is advertised by the GFS, NAM, and SREF between Meacham (KMEH) and
Baker City (KBKE), peaking between 4 AM and 10 AM Thursday
morning with gusts of 40 mph. These gusts do approach advisory
criteria (45 mph or greater), but confidence is still moderate
(50-60%) as the NBM suggests a 55-80% chance of gusts of 40 mph or
greater, but only a 25-60% chance when increasing to 45 mph or
greater. Thus, additional analysis will be needed as the event
approaches. 75


.LONG TERM...Saturday through Wednesday...

Bottom Line Up Front

1. Unsettled weather through the period

2. Light lowland snow gives way to rain with continued mountain snow

3. Temperatures begin to shift more towards seasonal normal

The long term will continue the unsettled weather beginning with an
upper level shortwave bringing snow across the vast majority of the
region. By the end of the weekend, models remain in decent agreement
with a transient ridge making its way over the PacNW. Saturday night
the models are in agreement with an upper level trough makes its way
towards the region with a warm front out ahead. Southwest flow aloft
will bring in more precipitation as the trough collides with a weak
atmospheric river. With the warm air pushing in causing the snow
levels to rise and much of the precipitation will fall as rain
across the lower elevations. As we move into the beginning of the
week, models show the trough to continue to traverse east while
another upper level ridge makes its way onshore. This will bring
with it a bit drier weather. Clusters show the variances in the
models to be primarily to be with the amplitude of the systems with
timing issues becoming a bit more apparent later in the period.
Overall, confidence in the forecast is relatively high at 80%
confidence.

The lowland snow that is expected to accompany the system Saturday
shows that the lower elevations of the Basin will see between 0.10-
0.20 inches of snowfall on Saturday with the majority of the
snowfall amounts being above 2000 feet with the exception of central
OR. Models show central OR will see little to no snow accumulation
during this event. However, 60-80% show areas around Snoqualmie,
Tollgate and Santiam Pass seeing snow amounts well above 4-8 inches
over the 48 hour event. The I-84 corridor has 60-70% probabilities
of seeing near 4 inches as well. Higher amounts are expected at the
crests. By Monday ensembles show the mountains will see continued
snowfall while the lower elevations will see a transition to rain.
Monday 60-80% of the raw ensembles show an additional 2 inches
across the I-90 corridor, additional 4-5 inches across Santiam Pass
and an additional 4-6 inches across the Norther Blues above 3000
feet. Rain amounts will vary through the day transitioning from
light snow showers in the early morning before transitioning to rain
Sunday afternoon. 30-40% of the raw ensembles show the Basin will
receive 0.10-0.20 inches of rain while 60-70% show the foothills
will receive 0.20-0.30 inches. Moving into Monday and Tuesday,
precipitation will tapper off along the lower elevations bringing
only snow showers to the crests of the mountains.

EFI shows the temperatures to steadily increase towards near normal
through the period. Temperatures to start the period will remain 10-
15 degrees below seasonal normal before increasing to 2-5 degrees
below. NBM shows the majority of the region will see temperatures in
the 40s beginning Monday and remaining as such through the remainder
of the period. Temperatures along the higher terrains of the
mountains will see highs in the low to mid 30s. Bennese/90


&&

.AVIATION...12Z TAFs...All TAF sites are currently VFR and guidance
shows the sites to remain VFR through the period. Confidence in the
conditions remaining VFR is high as guidance is in agreement the
northerly flow will keep the region free and clear of low and mid
level clouds and precipitation over the next 24 hours. Guidance
shows winds will vary through the period and remain below 10 kts.
Bennese/90


&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  23   4  25  18 /   0   0  60  50
ALW  21   6  26  21 /   0   0  60  50
PSC  27   6  29  20 /   0   0  60  50
YKM  26   6  26  18 /   0   0  50  40
HRI  26   4  27  17 /   0   0  70  50
ELN  25   5  26  18 /   0   0  40  40
RDM  27   4  29  15 /   0  10  70  40
LGD  19  -1  27  22 /   0   0  60  60
GCD  26   3  30  23 /   0   0  80  70
DLS  31  11  29  20 /   0   0  70  50

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Cold Weather Advisory until 7 AM PST Thursday for ORZ049-050-502-
     503-506-509-511.

     Cold Weather Advisory until 8 AM PST this morning for ORZ507.

WA...Cold Weather Advisory until 7 AM PST Thursday for WAZ030-522.

     Cold Weather Advisory until 8 AM PST this morning for WAZ029.

&&

$$

SHORT TERM...75
LONG TERM....90
AVIATION...90