130
FXUS66 KPDT 312245
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
345 PM PDT Mon Aug 31 2026

.KEY MESSAGES...
- Drying and warming trend will continue through early Tuesday
  as upper level high pressure moves over the region

- A series of upper level low pressure systems will bring
  periods of rain, breezy westerly winds, and isolated
  thunderstorm chances Tuesday into Friday

&&

.DISCUSSION...
Much like yesterday, little to talk about this
afternoon as visible satellite imagery shows mostly clear skies
across the region. While not evident on satellite, however,
layers of near surface smoke have filtered into portions of
central OR and along the Blue mountain foothills last night
resulting in a light haze in these areas.

Today through early Tuesday: An upper level ridge sliding across
the PacNW today has resulted in modest warm-up across the
forecast area, with most locations looking at afternoon
temperatures 7 to 10 degree warmer than yesterday. Otherwise,
winds will remain light through early tomorrow.

To the west of the upper level ridge, an upper shortwave trough
has already made it`s way south out of the Gulf of Alaska, and
per water vapor imagery is around 550 miles west of the PacNW at
this time. The upper trough will continue to amplify southward,
with southerly flow developing over the region by tonight.

Tuesday through Friday: Confidence remains high (80-90%) that
an active period of weather will be in store for the forecast
area as two low pressure systems impact the area.

Tuesday afternoon, the aforementioned shortwave trough offshore
the PacNW will lift northeast with scattered rain showers
developing along the Cascades, central OR, and the Blue
mountains through Tuesday evening. The deep southerly flow ahead
of the trough will result in weak instability developing over
the area, though isolated thunderstorm chances (10-20%) will be
limited to far northeast OR Tuesday afternoon. The approaching
upper low will also result in pressure gradients tightening
across the region, with breezy westerly winds developing in the
lower elevations by Tuesday afternoon. Breezy winds will overlap
with low relative humidity in the Harney Basin, promoting
elevated fire weather conditions along the southern periphery of
the Southern Blue mountains and central OR fire weather zones.

Wednesday, there is good agreement amongst ensemble guidance
that the upper trough will lift northeast across the PacNW,
providing the best chance of widespread rain showers across the
mountains and lower elevations through the remainder of the
period. During transit, the upper trough will take on a negative
tilt that will promote isolated to widely scattered
thunderstorms (10-25% chance) developing across the Blue
mountains in the afternoon and early evening. Breezy west to
southwest winds will develop across the lower elevations
throughout Wednesday, but the fire weather threat will remain
low owing to cooler temperatures and higher relative humidity
across the area. The probability of wetting rains (>0.1 inches)
will be 50-80% in the lower elevations, and 85-100% in the
mountains for Wednesday. Meanwhile, ensemble cluster solutions
remain in good agreement that a potent upper low will dive south
along the BC coastline and arrive offshore Vancouver Island by
Wednesday evening.

Thursday through Friday, ensemble cluster solutions are in
agreement that the upper low will park along the PacNW coast
through this period, resulting in light to moderate
rain showers across the mountains and lower elevations. The
extent of the cold core of the upper low over the region and
weak surface instability developing over the mountains will also
result in isolated to scattered thunderstorm development
Thursday afternoon. By Friday, overall ensemble members favor
the low weakening, limiting activity to just rain showers over
the mountains, however ensemble cluster solutions show a
disagreement on whether the closed low remains or if it becomes
an open wave, with the open wave solution resulting in lower
precipitation amounts.

Saturday and Sunday: Disagreement grows amongst ensemble members
in regards to the evolution of the upper low/trough over the
PacNW. About 60% of members favor the low remaining centered
along the PacNW coastline, resulting in continued light showers
over the mountains. Whereas the remaining 40% of the members
favor the upper trough pushing inland, with precipitation
diminishing west of the trough axis. Confidence in forecast
details over the weekend is low (15-20%). Lawhorn/82

&&

.AVIATION /00Z TAFS THROUGH 00Z WEDNESDAY/...
VFR conditions prevail through the period. Low chances (5 to
15%) that surface smoke will push conditions to MVFR temporally.
Winds will start to strengthen in the Central Oregon sites,
also including the DLS with a weather system approaching. Gusts
could reach 20 to 30 knots at times, especially during the later
part of the TAF period.

&&

.Preliminary Point Temps/PoPs...
PDT  52  84  49  76 /   0   0  20  30
ALW  58  85  55  77 /   0   0  30  40
PSC  54  88  53  78 /   0   0  20  30
YKM  59  88  56  74 /   0   0  20  30
HRI  56  88  53  79 /   0   0  20  30
ELN  54  84  50  71 /   0   0  20  30
RDM  46  80  41  67 /   0  20  30  70
LGD  54  87  50  75 /   0   0  60  50
GCD  55  88  51  73 /   0   0  20  30
DLS  61  81  57  73 /   0   0  10  80

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
WA...Air Quality Alert until 10 AM PDT Wednesday for WAZ026-522-523.
OR...Air Quality Alert until noon PDT Wednesday for ORZ041-505-506-
     509>511.

&&

$$

DISCUSSION...82
AVIATION...95