885
FXUS66 KPDT 081752 AAA
AFDPDT

Area Forecast Discussion...Updated
National Weather Service Pendleton OR
952 AM PST Wed Jan 8 2025

Updated aviation discussion

.UPDATE...
Made a few adjustments to wind based on current observations and
latest guidance, otherwise current forecast looks on track.

&&

.AVIATION...18Z TAFs...Low stratus continues over parts of the
Columbia Basin and have kept MVFR cigs at KYKM and KPSC for a few
more hours. Otherwise have sct-bkn cigs at 040-080 through this
evening. Low stratus expected return 03Z-09Z with MVFR/IFR cigs at
all TAF sites except KRDM and KBDN. Kept vsby at MVFR at KYKM, KALW
and KPDT but wouldn`t be surprised to see lower vsby at those sites.
Did not have the confidence to add IFR or lower vsbys. Have other
sites staying at VFR vsby. Winds will remain below 8 kts for the
next 24 hours. Perry/83

&&

.PREV DISCUSSION... /issued 430 AM PST Wed Jan 8 2025/

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

Key Messages:

1. Near normal high temperatures today before cooling Thursday.

2. Mountain snowfall and lower elevation rain Friday.

3. Breezy winds Friday afternoon and evening.

Current radar and infrared satellite imagery showing light returns
along the Washington Cascades and extending east into the lower
Columbia Basin and the Yakima/Kittitas Valleys under mostly cloudy
skies. These conditions are in response to the upper level
shortwave that is passing just to our north, which is suppressing
the present upper level ridge. This quick moving shortwave will
pass to our east by mid-morning to allow the upper level ridge to
rapidly rebuild over the area, bringing clearing skies and dry
conditions to the area today. High temperatures will break into
the low to mid-40s across the Basin, Gorge, and foothills, which
is near normal for this time of year. The ridge will rebuild this
afternoon and evening, in the wake of the departing shortwave, as
its axis slowly moves into the region on Thursday. This will allow
for the potential of fog to form across portions of the Basin and
foothills, as both surface and upper level high pressure will be
present. The best chances for fog development tonight into
Thursday morning will be across the Blue Mountain foothills,
Central and north-central Oregon, and the Yakima/Kittitas Valleys.
Overnight lows will be a couple degrees cooler than Wednesday
morning as values will drop into the upper 20s to low 30s. These
temperatures add to the fog concern Thursday morning as the
majority of fog will occur in freezing temperatures, which may
lead to slick roadways as ice deposits on surfaces. Confidence in
fog developing is moderate to high (60-80%), however, confidence
in where exactly the fog will develop is low to moderate (30-50%).
Thus, ensure extra travel time Thursday morning, and keep up to
date on any fog advisories that may be active before you depart.
Flow aloft will turn from the west this morning to being from the
northwest by mid-morning and then from the north by Thursday
morning. This will lead to much cooler high temperatures on
Thursday as flow aloft from the north brings in a cooler air mass
as the ridge rebuilds from the coast. As a result, high
temperatures on Thursday will be about 5 degrees cooler than on
Wednesday, with values peaking in the mid-to upper 30s across the
Basin, Gorge, and foothills and in the low to mid-40s over
Central Oregon.

The upper level ridge will begin to suppress again Thursday
evening as an upper level trough begins to approach the British
Columbia coast. This system will not only shift flow aloft to be
more out of the west-southwest on Friday, bumping up high
temperatures about 10 degrees from Thursday, but will also bring
with it a return to mountain snowfall and lower elevation rain.
Precipitation chances (70-80%) will materialize along the
Cascades and east slopes Friday morning, extending across the
Basin and foothills through the afternoon as chances peak
(30-90%). Chances will then confine to the foothills (20-50%),
Cascades and east slopes (40-70%) and the eastern mountains
(50-70%) through the evening before only encompassing the
mountainous terrain of the Cascades and northern Blue Mountains
by Friday night. Snow levels will be between 4000 and 5000 feet
through the day, dropping to between 3000 and 4000 feet by Friday
night. Rain amounts of 0.03-0.15" will be possible along the Blue
Mountain foothills and over the east slopes of the Cascades, with
0.01-0.06" anticipated across the John Day-Ochoco Basin.
Confidence in these rain amounts is moderate (40-60%) as the NBM
suggests a 25-65% chance of a wetting rain (0.10" or greater)
along the northern Blue Mountain foothills and a 40-70% chance of
0.03" or more over the John Day-Ochoco Basin. Dry conditions are
expected over much of the Columbia Basin, Central Oregon, and the
Yakima/Kittitas Valleys with this event. Snow accumulations will
primarily occur at elevations above 4000 feet, but only amount to
totals of 1 to 5 inches. Confidence in these snow amounts is
moderate (50-60%) as the NBM highlights a 60-80% chance of 2
inches of snow or more across the Cascades and northern Blue
Mountains above 4500 feet, which drops to a 30-45% chance of 4
inches of snow or more. These snow amounts are well below any
winter highlight criteria.

The upper level trough and attendant frontal system on Friday will
allow a pressure gradient to develop along the Blue Mountains,
leading to breezy winds across the Grande Ronde Valley and the
Blue Mountain foothills. The GFS, NAM, and SREF highlight a
pressure gradient of 6-8.5 mb developing between Baker City (KBKE)
and Meacham (KMEH), which is well above the normal 5 mb advisory
threshold. The GFS, NAM, and SREF also advertise an 8-9.5 mb
pressure gradient from Ontario (KONO) to Meacham (KMEH), which is
just below the normal 10 mb advisory threshold. Both of these
cross-sections suggest winds reaching near advisory-level,
peaking between 7 AM and 1 PM Friday. Confidence in these wind
gusts reaching advisory level is moderate (50-60%) as the NBM
suggests a 50-65% chance of 45 mph wind gusts or greater Friday.
When analyzing winds along the northern Blue Mountain foothills,
it is the GFS and SREF that suggests near advisory-level winds
with a gradient of 9.5-11 mb between Portland (KPDX) and Spokane
(KGEG). The GFS is alone in advertising that the pressure
gradient will reach above the normal advisory threshold of 10 mb,
which is suggested to occur between 4 PM and 11 PM on Friday.
Confidence in advisory-level winds along the northern Blue
Mountain foothills is moderate (40-60%) as the NBM highlights a
30-60% chance of wind gusts reaching 45 mph or higher. Thus,
continued analysis will be necessary as the event nears.

LONG TERM...Saturday through Wednesday...Moisture associated with
the system from Friday will linger on Saturday. This will keep a low
chance (15-30%) or continued precipitation across the Cascade and
Blue Mountains. Though snow levels will be between 2-3 kft, guidance
shows a 40-60% chance of 2 inches of snowfall, dropping to less than
25% over the highest peaks for 4 inches of snowfall. In addition,
areas of gusty wind will be found, especially across the Columbia
Basin with a high chance (60-70%) of gusts to 25 mph through the
afternoon hours. These are anticipated to be the strongest winds
through the forecast period, with generally light winds returning
Sunday morning.

Another wave is anticipated to move south into the forecast area on
early Sunday, spreading additional moisture across the area. This
quick moving system is favored to have the greatest impact through
the mid-morning hours of Sunday when a 20-40% chance of
precipitation is found reaching across much of northeastern Oregon,
and across portions of Walla Walla and Columbia Counties in
Washington. Snow levels remain around 2 kft on Sunday, and are
favored to produce 1-2 inches of snowfall (30-50% chance), mainly
over the Blue Mountains and the Washington Cascades across the
northwest corner of Kittitas County. The brunt of this system should
move outside of the forecast area by late Sunday afternoon, with
widespread dry conditions anticipated by Sunday night.

The ridge will then regain influence over the region late Sunday and
into Monday. Drier conditions will then persist through Wednesday.
Cluster analysis is in fairly good agreement through the entirety of
the forecast period with the progression of systems.

Temperatures will generally cool through the period, though areas
will see some warm-ups during the ridging periods. Temperatures are
generally anticipated to remain within a few degrees of normal
through the forecast period. Branham/76

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  28  36  27  44 /   0   0   0  60
ALW  29  37  29  43 /   0   0   0  70
PSC  33  39  32  45 /   0   0   0  30
YKM  29  38  28  42 /   0   0   0  30
HRI  30  38  30  46 /   0   0   0  40
ELN  27  36  27  41 /   0   0   0  40
RDM  27  43  29  46 /   0   0   0  20
LGD  25  39  28  41 /   0   0   0  70
GCD  25  43  27  44 /   0   0   0  50
DLS  34  41  33  48 /   0   0   0  70

&&

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

$$

SHORT TERM...77
LONG TERM....75
AVIATION...83