704
FXUS66 KPDT 170951
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
251 AM PDT Sat May 17 2025

.SHORT TERM...Today through Monday...Nighttime satellite and
radar imagery tonight shows an upper level trough offshore the
PacNW pushing showers across western OR/WA, while showers have
begun to develop across eastern OR. Some isolated thunderstorms
have also begun developing south of the forecast area this early
morning, with isolated thunderstorm activity expected to develop
within the forecast area in the next several hours.

Shower chances will continue to increase from southwest to
northeast across the forecast area this morning as the upper
trough offshore continues to push inland. Weak elevated
instability with modest mid-level lapse rates will develop out
ahead of the trough axis this morning, allowing slight chances
(around 15%) of isolated thunderstorms to develop mainly over the
southern third of the forecast area.

Today, the upper trough will continue to dive southeast into the
Great Basin later this morning, allowing a secondary upper trough
to dive across the PacNW later today through Sunday. Guidance
does indicate that the upper trough will develop into a closed low
as it enters the PacNW during it`s southeastward descent. Shower
activity will continue area-wide through later this afternoon,
with shower activity diminishing across the lower elevations as
the upper low moves overhead tonight and Sunday morning. Increasing
surface based instability(200-500 J/kg) and modest low to mid
level lapse rates will result in isolated to widely scattered
thunderstorm chances (15-30%) mixing in with showers across the
forecast area through late this evening (confidence 60-85%).
Through tonight, there is a 75-90% chance for rain accumulations
of 0.25 inches across the mountain zones, including the Grande
Ronde valley, with a 50-60% chance of 0.5 inches along the cascade
crest and northern Blues...55-85% chance for 0.1 inches in the
lower elevations. As showers continue through tonight, snow levels
will drop to 4kft-5kft, resulting in snow showers developing
across the Cascade crest and the northern Blues through Sunday
afternoon. With the showery nature of the incoming precipitation,
snow accumulations will be hit or miss across the mountain areas,
but areas below 5.5kft will see snow accumulations around 2 to 4
inches (confidence 50-70%).

Winds will increase through the Cascade Gaps and into portions of
the Columbia Basin and Central OR later this afternoon as cold
air advection under the upper trough/low passages will result in a
strengthening of the cross-Cascade surface pressure gradient.
Winds today will increase to 20-30mph with gusts up to 40mph
through the Cascade gaps, and 15-25mph with gusts up to 35mph
elsewhere in the lower elevations (confidence 80-90%).

Sunday, rain/snow showers will mainly be confined to the Cascades,
Blues, and Wallowa county as the upper low continues to push
southeast. By Sunday late afternoon and evening, shower activity
will diminish across the aforementioned areas as the upper low
crosses into the Great Basin to the southeast. Surface winds will
continue to strengthen Sunday, with a 80-90% chance that wind
gusts will reach or exceed 45 mph through the Kittitas valley and
a 60-75% chance through the eastern Columbia River Gorge. A wind
advisory was issued for the Kittitas valley for Sunday morning
through the evening as wind gusts are expected to exceed the 45
mph mark. Elsewhere in the lower elevations, winds will continue
to be 15-25mph with gusts up to 35mph (confidence 70-80%).

Sunday night into Monday morning, transient upper level ridging
will quickly move across the PacNW, keeping the region dry and
bringing an end to breezy winds impacting the the Cascade gaps
and lower elevations. By Monday late morning, the upper ridge will
flatten as an upper shortwave trough approaches the PacNW. The
shortwave trough will move across the PacNW Monday afternoon
through Monday night, providing another round of mountain
rain and high elevation snow showers. This system will be much
weaker than it`s predecessors from this weekend, but will still
result in breezy winds developing through the Cascade gaps into
the lower elevations Monday. Lawhorn/82

.LONG TERM...Tuesday through Saturday...Models in solid agreement on
the synoptic pattern through at least Friday, before solutions start
to diverge. For the first stretch of the period, a relatively benign
zonal flow pattern will be in place, with the only real sensible
weather threats being breezy winds through the Cascades and light
PoPs for our high mountain zones. By around Friday, however,
deterministic guidance suggests an oncoming trough that could
potentially bring showers and thunderstorms to the forecast area,
while ensembles generally lean toward a continuation of zonal flow,
perhaps even the introduction of a high pressure ridge. Given these
discrepancies, forecast confidence after next Thursday is low (20-
30%).

No real disagreement across guidance Tuesday into Wednesday, as
zonal flow will be driven by high pressure centered over BC. Winds
will be breezy through the Cascade Gaps and east slopes as a result,
as is typical under these patterns. Models introduce low-end PoPs
(20-40% across the WA Cascade crests and eastern mountains of OR
during the day Tuesday and Wednesday, but the expectation is that
any precip that occurs under this pattern will be very light.
Deterministic guidance hints at a shortwave embedded within the
zonal flow regime impacting the forecast area late Wednesday into
Thursday, which also shows up across two-thirds of GEFS members, but
all other ensemble guidance doesn`t really reflect this, so the
forecast for Wednesday into Thursday is largely dry.

By Friday morning, deterministic guidance introduces a trough into
the PacNW which, at least according to the GFS, would bring storm
chances to the forecast area. However, the ECMWF depicts a weaker
trough, and the majority of ensemble guidance depicts no trough at
all, rather a continuation of zonal flow. Will lean toward what the
ensembles are suggesting for now, which would mean more dry weather
(outside of perhaps the Cascades) through the start of next weekend,
but so long as these model discrepancies exist, can`t speak with too
much confidence in the forecast beyond the midweek. Evans/74


&&

.AVIATION...12z TAFs...A weather system passing through the region
today will make for shower and thunderstorm chances for most sites
through the period. Showers will start early this morning, generally
on the lighter end, before scattered shower and storm cells pop up
during the late morning and into the afternoon hours, starting
around 18z in BDN/RDM and spreading eastward into PDT/ALW/PSC by
00z. Made liberal use of PROB30 for -tsra as coverage of
shower/storm activity is expected to be sporadic, therefore
confidence is low on which sites will see what impacts and when.
Conditions may briefly drop to IFR under storm cells. Clearing is
then expected by the late evening and overnight hours heading into
Sunday, however winds will start to pick up once shower/storm
activity ends, gusting as much as 30 kts for sites such as DLS and
PDT, mainly out of the W/NW. Evans/74


&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  64  43  60  38 /  70  70  20  10
ALW  65  44  61  42 /  70  80  30  10
PSC  70  46  67  42 /  70  60  10   0
YKM  68  43  65  41 /  50  20   0   0
HRI  69  45  65  42 /  60  50  10   0
ELN  63  41  59  39 /  70  20   0   0
RDM  60  36  56  33 /  70  20   0   0
LGD  57  40  52  34 /  90  80  40  20
GCD  55  37  52  32 /  90  70  30  10
DLS  68  48  63  44 /  60  10   0   0

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...Wind Advisory from 8 AM to 8 PM PDT Sunday for WAZ026.

&&

$$

SHORT TERM...82
LONG TERM....74
AVIATION...74