227
FXUS66 KPDT 101809
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
1109 AM PDT Thu Sep 10 2026

.KEY MESSAGES...

- Breezy winds and dry conditions today - Elevated Fire Weather

- Isolated thunderstorms Saturday

- Widespread showers Saturday through Sunday morning

&&

.DISCUSSION...
SHORT TERM: Current radar and infrared satellite imagery shows
dry conditions as high level clouds stream across the Kittitas
Valley and into the Columbia Basin from the north. This is in
response to an approaching upper level trough and associated
weak cold front that will move into the area this afternoon. The
system will provide an increase in cloud cover, slightly cooler
afternoon temperatures, light evening showers over the northern
Blue Mountains/foothills, and breezy conditions across the
Columbia Gorge, Simcoe Highlands, and the Kittitas Valley. The
overall trend in both the light rainfall and breezy winds have
decreased over the past 24 to 48 hours as guidance has trended
toward a weaker cold front, upper level shortwave, and pressure
gradient developing along the Cascades. Sustained west-northwest
winds of 15-25 mph with gusts of up to 40 mph will be possible
across the aforementioned areas, with gusts of 20-25 mph
expected over lower elevations of the Columbia Basin, Blue
Mountain foothills, and Central Oregon. Winds peak between 1PM
and 5 PM. Advisory-level winds are not anticipated as the NBM
suggests only a 6% chance across Ellensburg, which is down from
9% yesterday and 22% on Tuesday. However, fire weather concerns
arise as afternoon humidities are projected to drop back into
the mid-teens across Central Oregon and the John Day Basin.
Winds are expected to be just shy of critical values at lower
elevations where humidities are lowest, but may still lead to
fire spread concerns through the afternoon and early evening.
Due to the present dry airmass, rain amounts are expected to be
minimal and only 0.01-0.05" at higher elevations through the
Blue Mountains after 5PM. The HREF and NBM both highlight a
10-30% chance of measurable rainfall (0.01") over these areas,
with highest amounts in and around the Tollgate area.

The trough broadens tonight into Friday morning as a strong
upper level shortwave pushes offshore just north of Vancouver
Island and strengthens to a low pressure system as it continues
its descent. It reaches the Washington and Oregon coasts early
Saturday before moving onshore and into the region Saturday
evening into Sunday morning. The developing southwest flow and
efficient moisture supply from another closed low pressure
system located in the Eastern Pacific, will initiate showers
along the Cascade and eastern mountains Saturday afternoon
before becoming widespread after 5PM. Guidance is in good
agreement with this feature, and has trended slower and wetter
in the previous 24-48 hours - which is directly related to the
strength of the closed low over the Eastern Pacific. This slower
progression of the incoming system will provide a slight gap
between this system and the exiting system bringing today`s
breezy conditions to the area. This gap will allow a transient
ridge of high pressure to develop between these two systems,
which will keep humidities at lower elevations rather dry and in
the 30-40% range early Saturday afternoon. As a result, rain
amounts across lower elevations of the Columbia Basin/Gorge,
Central Oregon, and Blue Mountain foothills may struggle to
reach a wetting rain (0.10" or greater). Higher elevations are
expected to receive a wetting rain (60-70% chance via the NBM),
with up to 0.50" over the Cascade crest and Wallowa County. The
developing ridge will also lead to the potential for isolated
thunderstorms to develop through the afternoon and evening
across the east slopes of the Cascades, John Day Basin, and the
Blue Mountains (15-25% chance). The main thunderstorm concerns
include gusty outflow winds, frequent lightning, and brief
downpours as both the AI ECWMF and GFS ensembles suggest
precipitable water amounts of 0.88-0.98" - which is 125-155% of
normal. 75

LONG TERM: During the day on Sunday, models are in good
agreement that rain chances will start to decrease throughout
the day as the closed upper low moves into the region. Latest
LREF clusters show that this solution is supported by around 75%
of the members. Due to the slowing trend mentioned earlier, the
precip will likely linger through the morning hours and into
the afternoon. Current NBM probabilities for measurable precip
(>0.01 in) during the morning are highest in the Blue and the
Wallowa Mountains with probabilities between 70-80%. Elsewhere
in the CWA, probabilities are generally between 40-50%. The
exception is in the lee of the Cascades south of I-84, where the
NBM has probabilities between 5-20%. Most of the region will
have a good chance of seeing a wetting rain (>0.1in) from this
system with probabilities from the NBM between 50-60% for lower
elevations and 80-90% in higher elevations. Latest model
guidance does develop very small amounts of instability during
the day on Sunday over the higher terrain so an isolated
thunderstorm could develop (10-20%).

It is worth noting that there has been an increase in support
since the last run of the LREF for an outlier scenario where
the trough doesn`t come ashore until midday Sunday. Currently
only around 12% of the members support this solution (The vast
majority of them come from the GEPS ensemble). In this solution,
the rain chances don`t come in until Sunday (instead of
Saturday) and the overall QPF amounts are lower.

After Sunday, models indicate weak large scale troughing to be
present with the possibility of a weak cutoff low forming off
the coast of California. However, given the quiet nature of
the upper-level flow, impacts seem limited at this time with
temperatures running around or slightly below normal (1-6F). 26

&&

.AVIATION /18Z TAFS THROUGH 18Z FRIDAY/...
VFR conditions are forecast at all sites through the period,
though there is a very low (<30%) chance of sub-VFR CIGs
overnight into Friday morning at BDN/RDM due to low stratus.
Winds will increase in magnitude through the day, generally
15-20 kts with gusts of 20-30 kts. Aside from a chance (30%) of
light rain showers at PDT/ALW late afternoon and evening, no
precipitation is forecast.

&&

.Preliminary Point Temps/PoPs...
PDT  84  48  73  46 /   0  20   0   0
ALW  84  53  74  52 /   0  30   0   0
PSC  87  53  78  51 /   0  10   0   0
YKM  82  51  74  54 /   0   0   0   0
HRI  87  53  77  49 /   0  20   0   0
ELN  74  47  70  48 /   0   0   0   0
RDM  85  46  76  41 /   0   0   0   0
LGD  86  48  75  46 /   0  30   0   0
GCD  86  49  77  46 /   0   0   0   0
DLS  83  52  76  54 /   0   0   0   0

&&

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

&&

$$

SHORT TERM...75
LONG TERM...26
AVIATION...86