905
FXUS66 KPDT 061123
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
423 AM PDT Sun Sep 6 2026

.KEY MESSAGES...
- Increasing rain shower chances as an upper low swings across
  the area through Monday

- Low chances (15-25%) of isolated thunderstorms mainly across
  mountainous terrain through Monday

- Mostly dry conditions with light winds and seasonable
  temperatures the remainder of next week

&&

.DISCUSSION...Satellite imagery tonight continues to show a
well defined closed low over the PacNW, with a shortwave impulse
pivoting around the low and into south central OR at this time.
Ahead of the trough, regional radar shows light to moderate
showers extending northward towards the OR/WA border, with light
showers from the OR Cascade crest to the OR Columbia Basin and
eastern mountains.

Today, showers will continue to spread north and east as the
shortwave impulse followed by the upper low push north and east
across the PacNW. Both CAMs and deterministic models do show
that shower activity will mostly be focused across northeast OR
and far eastern WA, with any shower activity along the WA
Cascades to end by the evening hours. As for thunderstorm
activity, weak instability (150-350 J/kg) building along
elevated terrain from the OR Cascade crest to far northeast OR
will result in isolate thunderstorm development in these areas
this afternoon. As for precipitation amounts, there is a 70-95%
chance of 0.25 inches of precip across areas of central OR, The
OR Cascade crest, and the Blue mountains today. In the WA
Cascades, Wallowa County, and the Blue mountain foothills there
is a 40-70% chance of 0.1 inches of precip with less than a 40%
chance elsewhere.

Late tonight through tomorrow, the upper low will swing
northeast out of the forecast area. Shower activity tonight will
remain focused along the Blues and spread across the elevated
terrain in far northeast OR as the low continues northeast.
There remains a 70-90% chance of 0.25 inches of precip across
the Blues and Wallowa county through Monday`s shower activity,
with a 30-50% chance of 0.1 inches across the remainder of the
eastern mountains and the Blue mountain foothills. The upper
level low passing over the Cascades will also result in a
tightening of the cross-Cascade surface pressure gradient,
resulting in breezy to locally gusty winds (15-30mph with gusts
30-45mph) through the Cascade gaps and portions of the Columbia
Basin. By Monday evening, what is left of the upper level low
will have pushed east of the region, with showers diminishing
area wide.

Tuesday through Wednesday, ensemble members are in good
agreement of weak upper troughing developing from the PacNW into
the eastern Pacific, placing the region under a split flow
aloft. By Wednesday, flow becomes more zonal as the trough
becomes a cut-off low well offshore CA. During this period, dry
conditions and light winds will persist across the forecast
area with near to a few degrees above normal temperatures.

Thursday into early Friday, ensemble members start off in good
agreement that an upper trough will swing south across BC into
the PacNW, with light mountain showers returning Friday morning
(confidence 55-60%). However the remainder of Friday into
Saturday, ensemble members are split in the evolution of the
synoptic pattern. Half the ensemble members from the ECMWF,
Canadian, and GFS show the cut-off low offshore (now merged with
remnants of TS Marie) merge into the upper trough dropping out
of BC and stall offshore the PacNW, while the other half of the
members have the upper trough swing through the PacNW sometime
during this period without merging with the cutoff low
(disagreement in timing and southward extent). The latter
solution would result in light mountain showers and cooler
temperatures across the area, while the former solutions would
keep light to moderate showers with isolated thunderstorms
across the Cascades and continued mild to warmer temperatures.
Forecast confidence during this period is very low (15%) owing
to continued disagreement amongst ensemble members in the
evolution of the synoptic pattern and run to run variability.
Lawhorn/82

&&

.AVIATION /12Z TAFS THROUGH 12Z MONDAY/...Mostly VFR conditions
to prevail through the period. Vsby/CIGs may become MVFR or
lower under moderate to heavy precip this morning at sites
RDM/BDN (confidence 30-40%). Otherwise, upper low impacting the
region will bring shower impacts to most sites throughout today,
with stratiform rain this morning becoming showery in the
afternoon. Did include a prob30 group for thunder at site BDN
for this afternoon, with chances being low (<15%) at all other
sites. Winds will remain light, 12kts or less, through the
period. Lawhorn/82

&&

.Preliminary Point Temps/PoPs...
PDT  68  48  74  45 /  30  50  10   0
ALW  70  53  74  51 /  20  60  30   0
PSC  73  50  78  47 /  10  20  10   0
YKM  71  52  77  50 /  40  10   0   0
HRI  71  49  77  46 /  40  20  10   0
ELN  71  47  71  43 /  20  20   0   0
RDM  60  36  67  35 /  80  40   0   0
LGD  72  46  70  42 /  50  80  40  10
GCD  69  46  70  43 /  70  80  10   0
DLS  69  53  74  49 /  80  10   0   0

&&

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

&&

$$

DISCUSSION...82
AVIATION...82