942
FXUS66 KPDT 211744
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
944 AM PST Fri Feb 21 2025

.UPDATED AVIATION...18Z TAFs...LIFR conditions at ALW/PSC with BR.
Conditions are expected to lift by this afternoon between 21-23Z.
Confidence in timing is moderate (30-50%). Conditions will
deteriorate again at ALW/PSC near 04Z. The remaining TAF sites
are VFR and will remain as such through the period. There is a
chance (30%) that YKM will see a rain shower around 03Z, however
this will not affect VIS. CIGs will primarily be SCT- BKN above
15kft with winds below 10 kts. Bennese/90


&&

.PREV DISCUSSION... /issued 317 AM PST Fri Feb 21 2025/

SHORT TERM...Today through Sunday...Satellite imagery tonight
shows a narrow band of high level cloud cover pushing across
eastern OR/WA, while low stratus and freezing fog has developed in
the lower Yakima valley and portions of the WA Columbia Basin. The
fog and stratus is expected to expand in areal extent slightly
across these areas through the morning hours and lifting before
noon.

A transient ridge over the PacNW tonight is expected to push east
later this evening, allowing a weak shortwave and surface
warm front passage to bring light rain to the Cascade and Blue
Mountain foothills, with high elevation mountain snow. The warm
front passage overnight will begin a period of warm air advection
into the intermountain PacNW that will persist through Sunday. By
Saturday morning, snow levels are expected to rise to between
4kft-5kft along the Blues and 5kft to 6kft across the Cascades.
Precipitation will briefly taper off late Saturday morning through
the afternoon behind the exit of the shortwave.

By Saturday evening, a west-southwest flow aloft with a modest
jet streak will be setup over the PacNW, which will allow a series
of shortwave troughs to lift across the region through early next
week.

The first, and arguably wettest, in a series of shortwave trough
systems will arrive to the PacNW by Saturday evening, and will be
accompanied by a weak to moderate atmospheric river (AR) and
surface warm front. Snow levels will start off between 5kft to
6.5kft Saturday evening and will rise to 6.5kft to 8kft as the
associated warm, moist airmass filters into the region behind the
warm front. This will facilitate mostly rain below the highest
mountain peaks along the Cascade crest and eastern mountains
through Sunday. There has been little change since 24 hours ago
amongst guidance, with the expectation that there will be plenty
of moisture to work with as the AR with this system will be
pointed right into the PacNW. Both the GFS and ECMWF IVT ensemble
suites continue to show an 80-100% chance of IVT values greater
than 250 kg/m/s during a 24 hour window making it east of the
Cascade crest, providing enough moisture for moderate to heavy
precipitation accumulations along the Cascade crest and the
interior northern Blues, with light to locally moderate
accumulations in the lower elevations. Late Sunday, the next
shortwave trough is expected to quickly pass through the PacNW and
exit by Monday morning, with the AR effectively ending early
Sunday night. The NBM 48-hr probabilities ending Monday morning
for precipitation accumulations shows a 95-100% chance along the
Cascade crest/east slopes and a 60-80% chance along the interior
northern Blues for 1 inch of rain accumulations. Looking at 2
inches or greater, chances are between 70-90% along the Cascade
crest, with less than 25% chance along the interior Blues. In the
lower elevation, there continues to be a 60-90% chance of 0.25
inches of rain across central/north central OR, the
Yakima/Kittitas/Grande Ronde valleys, and along the Blue Mountain
Foothills; there is a 40-60% chance across the remainder of the
lower elevation zones.

Breezy to locally windy conditions will also develop Saturday
afternoon ahead of the parade of shortwave troughs expected
through the weekend. Surface pressure gradients between the Blue
Mountain foothills to the Lower Treasure valley will increase to
around 10-12mb by Saturday evening, facilitating the strongest
winds (gusts between 45-55mph) through the Grande Ronde valley
and along the Blue Mountain foothills through Sunday night. From
central OR to the Columbia Basin, breezy to locally breezy
southwest to west winds will also develop overnight Saturday. Peak
winds in the lower elevations along mountain ridges will coincide
with the development of a low to mid level jet Sunday morning
through late afternoon. Winds will begin weakening Sunday night
into Monday morning.

The persistent west to southwest flow through the weekend will
allow temperatures to warm region-wide. By Sunday afternoon, high
temperatures will peak into the upper 50s to mid 60s, with upper
30s to 40s in the mountains. Lower elevation morning low
temperatures will also be trending upwards, with upper 30s to 40s
expected by Sunday morning.

With the expected rainfall and subsequent snowmelt/runoff in the
mountains, area rivers and streams will begin to rise Sunday, and
will continue to rise into early next week. At the moment, most
rivers are expected to remain within bankfull through Sunday,
though the Naches, Yakima, Klickitat, Walla Walla, and Umatilla
rivers will begin to reach bankfull by early next week. Smaller
streams will also see rises, and those streams that have not been
cleared of debris may see minor flooding into adjacent low lying
areas. Lawhorn/82

LONG TERM...Monday through Friday...Sensible weather concerns
through the extended forecast will revolve around a shortwave
trough passage late Monday through Tuesday, with upper level
ridging building back over the region through most of next week.
Hydrological concerns will also arise into the extended forecast.

Ensemble guidance begins in good agreement early Monday in a quick
moving transient ridge moving across the PacNW with a brief lull
in precipitation. During the second half of Monday into Tuesday,
ensemble guidance is in good agreement that the last in the train
of shortwave troughs will bring another round of precipitation to
the region, with snow levels dropping back down to between 4kft to
5.5kft (gradient west to east) Monday night, then dropping
further to between 3kft to 4kft by Tuesday afternoon. This will
result in a return of moderate snow accumulations (5-10 inches)
mainly above 5.5kft in the Cascades, with lighter snow
accumulations (2-5 inches) below 5.5kft in the Cascades and
northern Blues. But all that said, ensemble cluster guidance
continues to be disagree on the timing of the system. About 60%
of members favor a Monday evening arrival and early Tuesday
morning exit, with the other 40% of members favoring a Monday
night arrival and Tuesday exit.

After Tuesday afternoon, ensemble guidance comes back into great
agreement in upper level ridging building over the PacNW with
mostly dry conditions through the end of the work week. That said,
about 75% of ensemble cluster members shows some form of a weak
shortwave flattening the upper ridge early Thursday, with light
mountain precipitation developing Thursday morning into the
afternoon. By late Thursday, all ensemble cluster solutions show
the ridge rebounding and persisting over the region through
Friday.

Lastly, by Monday morning, many of the rivers and streams across
the forecast area are forecast to see peak flows from the
continued runoff of snowmelt/rain from area mountains. With
colder conditions in the mountains and area-wide drier conditions
developing by Tuesday, river and stream flows are expected to
gradually decrease through the remainder of the week. Lawhorn/82

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  48  36  58  46 /   0  30  30  80
ALW  45  36  54  44 /  10  40  40  90
PSC  44  35  58  46 /  10  40  20  70
YKM  43  32  51  41 /  20  30  20  70
HRI  46  35  59  46 /  10  30  20  70
ELN  41  32  46  37 /  30  50  30  70
RDM  53  34  59  46 /   0  20  10  60
LGD  41  33  44  38 /   0  30  30  90
GCD  47  33  52  41 /   0  20  20  80
DLS  48  39  53  46 /  10  30  30  80

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...Freezing Fog Advisory until 11 AM PST this morning for WAZ028-
     029.

&&

$$

SHORT TERM...82
LONG TERM....82
AVIATION...90