816
FXUS66 KPDT 050533
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
1033 PM PDT Fri Sep 4 2026

.KEY MESSAGES...
- Much cooler than average temperatures persist into the weekend
  as much as 8-12 degrees with a warming trend beginning Monday
  into mid-week.

- Unsettled weather with low chances of thunderstorms (15-25%)
  persisting into early next week.

- Little to low fire weather concerns owing to elevated
  afternoon humidity and showery weather.

&&

.DISCUSSION...
Latest water vapor imagery shows a complex mid-
level closed low over northwest OR with a shortwave trough
moving into southeast BC. Additional embedded disturbances are
rotating around its southern base with shortwave troughs
presently moving into eastern OR and into northern CA. Latest
radar display shows some newly developed convection near/off
Rattlesnake Ridge and Indian Rock with some infrequent lightning
in the last hour. Initial forecast concerns turn to thunderstorm
chances across much of the area, albeit isolated coverage
(15-25%), with the activity happening a few hours earlier than
expected. Latest SPC mesoanalysis shows ~500 J/kg MUCAPE/SBCAPE
being realized there west into the Cascades with the convection
aided by orographic ascent in those aforementioned areas. Latest
Day Cloud Phase RGB imagery shows the additional cu development
is less impressive after the initial CI over Rattlesnake Ridge
and Indian Rock so additional lightning may be hard to
materialize.

Latest ensemble clustering and deterministic guidance show
decent agreement increasing overall confidence in the large
scale pattern evolution through Monday. Latest EPS means show
scant movement east the next 48-hrs as it fills in with embedded
shortwave troughs ejecting and elongate it to extend to offshore
northern CA by Saturday. Lingering residual moisture with
limited additional moisture being transported into the area will
keep the weather unsettled and cloudy as this upper trough
evolves. In general, the cooler air mass will remain in place
with highs generally 8-12 F below average this weekend across
much of the lower elevations. Of which, record cool highs will
be approached for KHRI and KRDM at least Saturday and Sunday,
respectively. While areas saw as much as a half inch to
approaching 1 over the last 24-hrs this morning in Wallowa and
parts of the extreme eastern mountains, lesser rainfall totals
are expected this weekend in comparison over any 24-hr period.
With the showery precip expected, chances for 0.5 or more
largely peak only 30-50% in the Cascades and higher peaks of the
eastern mountains.

As the upper trough starts to make progress east, 500 mb grand
ensemble members are still agreeing on a weak neutral trough
over Oregon and Washington though spread is increasing. Moving
forward in time, the main low in the trough begins to split
before it lifts and translates east. The southernmost low gets
ejected and opens into a wave and, during this period of
movement, the embedded short wave trough moves through the
greater flow. Timing and the intensity of the low in the
deterministic and ensembles then become more in disagreement.

Comparing ECMWF and GFS ensembles, both location and strength
of CAA and WAA are in great disagreement in the beginning and
this disagreement continues though the forecast. Deterministic
cold air advection remains over the area for the entire forecast
period.

Monday morning temperatures for the lower elevations in the
foothills and Columbia Basin should be mid 50s, progressing to
higher elevations around La Grande sub-50s is likely. Bend and
the southern border of the CWA will see low to mid 40s. Monday
afternoon we continue our unseasonable cool temps with highs in
the mid to upper 70ds in the columbia basin and foothills, the
Blue and Strawberry mountains will remain much cooler then its
lower elevation neighbors with areas like Bend and La Grande
being closer to the mid and upper 60s.

The WPC ERO Day 3 has the CWA in a marginal for excessive
rainfall. WPC is discussing upgrading the area to a slight as
model guidance becomes clearer. Higher amounts of precipitation
on the east side of the Blues with a modest rain shadow to the
west. NBM has chances peaking up to 67% for a 0.25" of precip or
more mainly on or past the Blues on the east side. Holistically,
the forecast area has a 24% chance or higher for at least a
.01" that increases as you travel south on 84. We are in
agreement with the WPC the amount of moisture that gets
transported from Hurricane Marie will heavily influence what
precip amounts will be.

Monday night into Tuesday shows the main trough traveling
eastward into Montana and tracking into Alberta/Saskatchewan,
while a secondary trough forms off the Oregon/California coast
being influenced by Tropical Storm Eduardo. While the Ensembles
agree on the timing of the movement of both 500mb troughs,
there is uncertainty of the position and strength of the
offshore secondary trough going into Tuesday afternoon. Ensemble
members show showers from Monday afternoon into early Tuesday
(73% of members) with the mid to late Tuesday showing no
precipitation. Temperatures present should be near normal
values, with drier conditions present as the trough moves NE.

&&

.AVIATION /06Z TAFS THROUGH 06Z SUNDAY/...
VFR conditions with on and off shower activity through
tomorrow. Not expecting much heavy precip with this system
through tomorrow, but there is a small chance (5 to 15 percent)
that a moderate shower could pass through a TAF site and bring
temporary MVFR/IFR conditions. Winds in general will be light in
nature with most sustained winds less than 12 knots.

&&

.Preliminary Point Temps/PoPs...
PDT  42  70  44  68 /  30  40  20  40
ALW  48  72  49  71 /  20  30  30  20
PSC  45  75  46  74 /   0  10  20  20
YKM  49  72  49  74 /   0  30  20  30
HRI  45  73  46  72 /  10  10  20  30
ELN  42  69  43  71 /  10  40  20  30
RDM  34  67  37  63 /  60  30  40  80
LGD  40  72  42  72 /  30  40  20  60
GCD  41  70  43  69 /  30  40  10  80
DLS  52  74  52  74 /  30  30  20  30

&&

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

&&

$$

DISCUSSION...80
AVIATION...95