788
FXUS66 KPDT 301715
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
1015 AM PDT Sun Aug 30 2026

.KEY MESSAGES...
- Warming afternoon high temperatures, albeit with chilly
  morning lows for valley locations, and mostly dry through
  Tuesday under a building upper-level ridge.

- Active weather associated with a couple low-pressure systems
  is likely (80% confidence) to return late Tuesday and persist
  through the week.

&&

.DISCUSSION...
Water vapor imagery shows a closed upper-level low departing to
the east over the northern Rockies, leaving a cool northwesterly
flow over the region in its wake. Meanwhile, a shortwave ridge
is located just upstream, extending from offshore in the Pacific
to inland over western British Columbia. This ridge will
continue to slide eastward into the Pacific Northwest through
Monday, resulting in dry conditions and warming daytime highs.
That said, valley locations will continue to observe chilly
morning temperatures owing to a dry air mass and predominantly
light winds overnight under the ridge. Confidence is low
(10-30%) in observing near-freezing temperatures Monday morning
for cold-prone valleys of central and northeast Oregon.

Confidence is high (80%) in a return to more active weather as
a couple upper-level lows influence the Pacific Northwest`s
weather Tuesday afternoon through the end of the week. Ensemble
guidance takes the shortwave trough in the Gulf of Alaska
southeast along the coast through Tuesday, resulting in
amplifying southwesterly flow aloft over the forecast area.
Coupled with an increase in PWATs and some weak instability,
there is a slight chance (10-20%) of thunderstorms currently
forecast over the Blue Mountains region Tuesday afternoon.
Ensemble guidance is in loose agreement that the wave will then
eject northeast across the Pacific Northwest Tuesday night
through Wednesday, with continued chances of precipitation.

The main attraction, an anomalously deep, closed upper-level
low continues to be advertised by all clusters of ensemble
solutions late Wednesday through the remainder of the week.
Ensemble systems have yet to come to a concensus on exact time
of arrival, or if/when the low comes onshore; differences in
location/timing among ensemble solutions accounts for
approximately 40-60% of variance through the weekend. Regarding
solutions, roughly half of the 00Z ensemble members take the
low inland, or partially inland, by late Thursday and Friday,
while the remainder keep it centered just offshore. The former
solutions would be cooler with well-below-normal temperatures
and widespread showers while the latter would be warmer and
drier, increasingly so the farther offshore the low is centered.

It`s too early to talk about exact precipitation amounts, but
will note the NBM is advertising a widespread 50-80% chance of a
wetting rain (0.10" or greater) for the lower elevations and
60-95% chance for the mountains between Tuesday night and
Thursday night.

&&

.AVIATION /18Z TAFS THROUGH 18Z MONDAY/...
VFR conditions with mostly light winds at less than 12 knots.
Not expecting any major impacts from smoke or haze, but the
small chance (5 to 15 percent) still persists for all TAF sites
seeing smoke makes it way down to the surface from the overnight
to early morning hours.

&&

.Preliminary Point Temps/PoPs...
PDT  76  44  86  51 /   0   0   0   0
ALW  76  50  89  57 /   0   0   0   0
PSC  79  49  89  54 /   0   0   0   0
YKM  79  54  86  59 /   0   0   0   0
HRI  78  49  88  55 /   0   0   0   0
ELN  75  47  81  54 /   0   0   0   0
RDM  78  41  87  46 /   0   0   0   0
LGD  78  44  89  52 /   0   0   0   0
GCD  79  47  92  54 /   0   0   0   0
DLS  80  55  88  62 /   0   0   0   0

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
WA...None.
OR...Air Quality Alert until noon PDT Monday for ORZ041-505-506-
     509>511.

&&

$$

DISCUSSION...86
AVIATION...95