388
FXUS66 KPDT 210509
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
1009 PM PDT Thu Aug 20 2026

.KEY MESSAGES...
- Hot temperatures through Friday -- Heat Advisories in effect.

- Very low to slight chance (5-20 percent) of high-based
  showers and thunderstorms Friday and Saturday.

- Widespread breezy to windy westerly winds coupled with low
  relative humidity Saturday -- Fire Weather Watches in effect.

&&

.DISCUSSION...
Water vapor imagery shows a large, closed low offshore in the
Pacific. Meanwhile, an upper-level ridge of high pressure
extends from the Four Corners into the Pacific Northwest. The
interaction of these two synoptic-scale features will drive the
bulk of our weather over the next several days.

Under the ridge, widespread hot conditions are in place with
afternoon high temperatures already in the low/mid-90s early
this afternoon. Temperatures are expected to continue climbing
into the mid/upper 90s this afternoon, with localized areas
exceeding 100 degrees such as in Hells Canyon and Warm Springs
area. Heat Advisories have been issued for the majority of
our forecast area, in effect through 10 PM Friday. Friday is
forecast to be the hotter day across the northeast half of our
forecast area from south-central Washington to northeast Oregon,
with widespread upper 90s to lower 100s forecast. While
uncertainty in precise temperatures does exist due to forecast
high clouds and wildfire smoke, confidence remains medium-high
(40-80%) in Major HeatRisk (level 3 of 4) for the majority of
our population centers within the aforementioned area. This
level of heat affects anyone without cooling/hydration. In areas
that area not likely (20% chance or less) to see Major
HeatRisk, Moderate HeatRisk (level 2 of 4) is forecast (70-90
percent confidence); areas include central Oregon and portions
of the Blue Mountain foothills in the southwest half of our
forecast area.

Ensemble NWP guidance is in concensus that the upper low will
slowly track north-northeast overnight through Friday and remain
offshore. This places the forecast area under southerly to
southwesterly flow aloft. Synoptic-scale forcing coupled with
increasing mid-level moisture may facilitate some high-based
showers and thunderstorms overnight through Friday and into
Saturday morning, but probabilities are quite low (5-15
percent), highest for the Cascades.

The low is then expected (95 percent confidence) to track inland
across northern Oregon and Washington during the day Saturday,
morphing into an open-wave trough as it moves inland. Its
attendant cold frontal passage during the day will help drive
breezy to windy westerly winds through the Cascade gaps as well
as across the Columbia Basin and portions of the Blue Mountains.
While confidence was only medium (40-60%) in reaching RH
thresholds, due to the magnitude of the forecast winds (10-25
mph with gusts of 25-40 mph, locally higher through especially
wind-prone areas), have issued Fire Weather Watches due to wind
and low relative humidity for the majority of our non-mountain
fire weather zones, in effect Saturday afternoon through
Saturday evening. Additional headlines may be needed if
confidence increases in overlap of critical wind/RH thresholds.

Some breezy conditions are forecast to linger Sunday as the
trough exits to the east.

&&

.AVIATION /06Z TAFS THROUGH 06Z SATURDAY/...
VFR conditions persist through the forecast period with
lingering elevated smoke in the region. RDM/ALW is 4SM and MVFR
due to the smoke settling and is expected to lift in the next
few hours. OTherwise there is a small chance (5-10%) that
elevated smoke will makes it way down to the surface at the
remaining sites. Winds will be mostly less than 12 knots
sustained.

&&

.Preliminary Point Temps/PoPs...
PDT  60  96  63  90 /   0   0  10  10
ALW  65 101  70  93 /   0   0  10  10
PSC  69 100  66  95 /   0   0  10  10
YKM  68  98  70  95 /   0   0  10  20
HRI  67  97  68  92 /   0   0  10  10
ELN  62  96  67  88 /   0   0  10  30
RDM  60  93  54  88 /   0   0  10   0
LGD  58 101  65  95 /   0   0  10  20
GCD  63 100  63  95 /   0   0  10  10
DLS  72  96  66  85 /   0  10  10  10

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
WA...Air Quality Alert until 1 PM PDT Friday for WAZ028>030.
     Heat Advisory until 10 PM PDT Friday for WAZ024-026>030-521-
     523.
     Fire Weather Watch from Saturday afternoon through Saturday
     evening for WAZ690-691.
     Fire Weather Watch from Friday evening through Saturday
     afternoon for WAZ695.
OR...Air Quality Alert until 1 PM PDT Friday for ORZ041-050-502-505-
     506-509>511.
     Heat Advisory until 10 PM PDT Friday for ORZ041-044-049-050-
     502-503-505>508-510-511.
     Fire Weather Watch from Saturday afternoon through Saturday
     evening for ORZ691-696-698-700-703.

&&

$$

DISCUSSION...86
AVIATION...90