475
FXUS66 KPDT 301110
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
410 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 /12Z TAFS THROUGH 12Z MONDAY/...
VFR conditions are forecast through the period. Diurnally
driven winds of predominantly 10 kts or less, with periodic
higher gusts, anticipated at all sites.

&&

.Preliminary Point Temps/PoPs...
PDT  76  45  86  54 /   0   0   0   0
ALW  76  51  89  59 /   0   0   0   0
PSC  79  48  88  53 /   0   0   0   0
YKM  79  53  85  57 /   0   0   0   0
HRI  78  48  88  54 /   0   0   0   0
ELN  75  47  80  53 /   0   0   0   0
RDM  78  40  86  45 /   0   0   0   0
LGD  78  43  89  51 /   0   0   0   0
GCD  79  46  91  53 /   0   0   0   0
DLS  80  55  87  61 /   0   0   0   0

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
WA...None.
OR...Air Quality Alert until noon PDT Monday for ORZ041-505-506-
     509>511.
     Freeze Warning until 9 AM PDT this morning for ORZ050.

&&

$$

DISCUSSION...86
AVIATION...86