628
FXUS66 KPDT 191145
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
345 AM PST Sun Jan 19 2025

.SHORT TERM...Today through Tuesday night...

Key Messages:

1. Frigid temperatures today through Monday.

2. Breezy winds across the Grande Ronde Valley Tuesday morning.

Current radar and infrared satellite imagery shows light returns
over portions of the northern Columbia Basin and northern Blue
Mountains under partly to mostly cloudy skies. The radar returns
are not expected to lead to any precipitation as forecast
soundings indicate substantially dry air below 10,000 feet under
a shallow moist layer in the mid-levels, which should only result
in an increase in cloud cover. These conditions are a result of a
weak upper level shortwave traveling down the front side of the
upper level ridge, which is located just offshore. The shortwave
will pass over the area later today, keeping partly cloudy skies
across the Blue Mountains and foothills and also providing a
slight chance (10-15%) of flurries during the late afternoon and
early evening for the aforementioned areas.

Our location to the ridge has kept upper level flow coming from
the northwest, but today`s weak shortwave will enhance flow and
shift it from the northwest to the north. This shift in flow aloft
will more efficiently bring colder, arctic air into the region
tonight into Monday, which is when temperatures are expected to
bottom out and be their coldest. As a result, high temperatures
today will break into the low to mid-30s across the Columbia
Basin, Blue Mountain foothills, and Central Oregon, dropping 2-5
degrees on Monday into the upper 20s to low 30s. Chilly morning
temperatures are again expected today as values will drop into the
upper teens to low 20s across the Basin and foothills, and in the
low to mid-teens over Central Oregon. The coldest temperatures
will occur overnight Sunday into Monday as morning temperatures
will plummet into the low to mid-teens across the Basin and
foothills, and dropping into mid-single digits over Central
Oregon. Confidence in these temperatures is high (70-80%) as the
NBM suggests a wide spread of a 30-80% chance low temperatures of
17 degrees or below across the Basin and foothills, and a 40-70%
chance of morning temperatures of 10 degrees or lower across
Central Oregon. This is a result of guidance trending a little hot
in terms of temperatures as a result of an unclimatologically
cold airmass dropping south and into our region. This is
advertised by the ECMWF EFI, which highlights areas of the Basin,
east slopes of the Cascades, and the Blue Mountain foothills with
unclimatologically cold low temperatures on Monday. An example of
guidance trending hot occurred Saturday morning, as temperatures
at Pasco (KPSC) dropped to 17 degrees, which was well below the
advertised 26 degrees via the NBM and ECMWF ensemble and 25
degrees via the ECMWF deterministic. Another upper level shortwave
is poised to briefly flatten the offshore ridge on Tuesday,
leading to flow aloft shifting from the north to the west to bring
warmer temperatures on Tuesday as highs break back into the mid-
to upper 30s across the Columbia Basin and Blue Mountain
foothills. Tuesday morning temperatures will be slightly warmer
from Monday morning, with values dropping into the upper teens to
low 20s across the Basin, foothills, and Central Oregon.

Tuesday`s upper level shortwave will also bring breezy conditions
over the Grande Ronde Valley as a pressure gradient is expected
to set up across the Blue Mountains. Winds will be peaking between
9 PM Monday through 11 AM Tuesday as the GFS, NAM, and SREF
advertise pressure gradients of 6-7 mb between Baker City (KBKE)
and Meacham (KMEH) and 7-7.5 mb between Ontario (KONO) and Meacham
(KMEH). The pressure gradient to Baker City is just above the
normal advisory threshold of 6 mb, but the gradient to Ontario is
below the normal advisory threshold of 10 mb. Thus, sustained
south winds of 20-30 mph with gusts of up to 40 mph will be
possible early Tuesday morning. Confidence in these winds is high
(80%) as the NBM suggests a 60-65% chance of gusts reaching 35 mph
or greater, a 35-40% chance of gusts of 40 mph or higher, and a
5-10% chance of advisory level winds (45 mph or greater). 75


.LONG TERM...Wednesday through Sunday...Wednesday, ensemble mean 500-
mb heights indicate high likelihood (>95% chance) in a broad trough
downstream over the Great Plains, with a ridge of high pressure
located just offshore of OR/WA. While ensemble means and all
scenarios retain above-normal heights over the PacNW through
Thursday, a subset of members (21%) advertise a shortwave trough by
Thursday afternoon. Of note, all but 16% suggest at least some
flattening/suppression of the ridge. The trough scenario would
facilitate light precipitation across the Cascade crest and Blue
Mountains, while the other solutions would be dry.

Friday, a breakdown of the ridge is likely as all ensemble clusters
suggest some flavor of a trough over the PacNW and/or Nrn Rockies.
By Saturday, solutions begin to diverge with 57% of ensemble members
indicating the aforementioned trough digging southward such that
below-normal 500-mb heights are present over the forecast area. The
remaining members take the trough farther east with a drier
northerly flow building overhead. By Sunday, dry conditions are
likely (>75% chance) as ensembles favor either a dry northerly to
northeasterly flow aloft.

The return to a troughing pattern, however brief, supports
precipitation chances Friday into Saturday. That said, precipitation
will likely be light  NBM probabilities of exceeding 0.1" for the
lowlands range from 5-40%, and 30-70% for the mountains. More
significant precipitation is unlikely  less than 5% chance of 0.5"
for the lowlands, and 5-30% for the mountains.

So far this winter, the lowlands have received little snow relative
to normal. Will that finally change? Well, during the 48-hr period
between 4AM Friday and 4AM Sunday, there is a 20-50% chance of
measurable snow (0.1") for the lowlands. Probabilities of 1" and 2"
of snow drop to 5-20% and 10% or less, respectively. With regard to
mountain snowfall, NBM probabilities are currently not supportive of
high confidence in reaching advisory amounts; for example, there is
a 10-40% chance of 6" for the WA/OR Cascade crest and the northern
Blue Mountains. The probability of warning-level totals is less than
15% for all mountain zones.

NBM probabilities of advisory-level wind gusts (45 mph or greater)
are low (25% or less) late Thursday through Friday. The track of the
trough will modulate how much wind the forecast area receives, and
from which direction (westerly vs northerly). Plunkett/86


&&

.AVIATION...12Z TAFs...VFR conditions are likely through the day for
all sites, with FEW-BKN mid-level clouds. Late afternoon and
evening, chances of sub-VFR CIGs increase as a shallow layer of low-
level moisture combined with upslope flow may facilitate temporary
BKN-OVC conditions at PDT/ALW/RDM/BDN (20-40% chance). Elsewhere,
chances of sub-VFR CIGs are very low at DLS (10%). While no
measurable precipitation is forecast, could see some light flurries
at PDT/ALW (10-15% chance).

Sustained winds of 10 kts or less are forecast for all sites through
the period. Intermittent afternoon and evening gusts of 10-15 kts
are likely (60-80%) at BDN/RDM/PSC and possible at YKM (30%), but
have not included mention in the TAFs. Plunkett/86


&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  29  15  27  20 /   0  10   0   0
ALW  30  19  29  22 /   0  10   0   0
PSC  34  20  31  23 /   0   0   0   0
YKM  32  12  28  18 /   0   0   0   0
HRI  32  19  30  22 /   0   0   0   0
ELN  31  10  27  17 /   0   0   0   0
RDM  30   7  33  19 /   0   0   0   0
LGD  31   9  29  17 /   0  10   0   0
GCD  34   8  33  15 /   0   0   0   0
DLS  35  22  33  25 /   0   0   0   0

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
&&

$$

SHORT TERM...75
LONG TERM....86
AVIATION...86