607
FXUS66 KPDT 210509
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
1009 PM PDT Sun Sep 20 2026

.KEY MESSAGES...
- Warming and drying trend through Monday.

- Mountain showers Tuesday, widespread rain late in the week.

- Breezy winds Tuesday and Friday.

&&

.DISCUSSION...
Current radar and visible satellite imagery shows dry conditions
and clear skies as an upper level ridge of high pressure slowly
moves to our east. This has allowed high temperatures to
increase 4 to 6 degrees from yesterday as highs peak in the
mid-80s across the Columbia Basin, Gorge, and Central Oregon.
High temperatures will continue to climb a couple more degrees
on Monday as flow aloft becomes more from the southwest and
brings warmer air across the region ahead of the next system on
Tuesday. Low afternoon humidities are also present through
Monday with values dropping into the mid-to upper teens across
Central Oregon and the John Day Basin. Humidities will be slowly
improving through the week as multiple systems eye the Pacific
Northwest.

The first of these systems, an upper level shortwave, will move
onshore over Central British Columbia on Tuesday. The best
chance (15%) for light showers and isolated thunderstorms will
occur over the John Day Highlands. 60% of ensemble members
showcase CAPE values of 250-350 J/kg, which extends across
southern Union and Wallowa counties on Tuesday afternoon.
However, the limiting factor may be a moisture trigger as only
51% of ensembles suggest measurable precipitation (0.01" or
greater) over these areas, with best chances across the John Day
Highlands. The lack of shear may also attribute to minimal
thunderstorm development as only 50% of ensemble members show
shear of 20 knots or more, with only 21% of members showcasing
both elevated CAPE values of around 300 J/kg and shear of 20
knots or greater. As a result of the incoming system and
departing ridge, breezy winds are expected through the Columbia
Gorge and Kittitas Valley with west-northwest winds of 20-25
mph with gusts of 30-35 mph possible.

A ridge of high pressure will build across northern California
and into Idaho Wednesday and early Thursday ahead of the next,
more robust system Thursday afternoon. Guidance is coming into
slightly better agreement with the timing and strength of this
system, as 64% of ensemble members advertise showers arriving
along the Cascades Thursday afternoon, with 92% of members
suggesting widespread rainfall by Friday afternoon. The AI
ECMWF and GFS ensembles highlight precipitable water of 0.7-0.9"
across the Columbia Basin, which is 120-140% of normal and
aligns with more of a wetter scenario than what guidance was
suggesting just 24 hours ago. In response to this recent shift
in guidance, confidence is still quite low related to specific
rain amounts. The highest rain amounts are expected to occur
across the Blue Mountains and Wallowa/Union counties Thursday
through Friday. Due to the building ridge of high pressure to
our south, breezy winds through the Columbia Gorge and Kittitas
Valley are likely. Current guidance hints at slightly lower
winds than on Tuesday, but this will have to be analyzed further
as the event nears. 75

&&

.AVIATION /06Z TAFS THROUGH 06Z TUESDAY/...
VFR conditions will persist through the forecast period. Winds
will be light and variable with no CIGs expected.

&&

.Preliminary Point Temps/PoPs...
PDT  52  84  52  76 /   0   0   0   0
ALW  57  85  57  77 /   0   0   0   0
PSC  53  86  54  80 /   0   0   0   0
YKM  54  85  54  79 /   0   0   0   0
HRI  54  85  55  78 /   0   0   0   0
ELN  52  80  50  70 /   0   0   0   0
RDM  46  83  44  75 /   0   0   0   0
LGD  51  84  52  79 /   0   0   0   0
GCD  49  85  51  80 /   0   0   0   0
DLS  60  86  56  77 /   0   0   0   0

&&

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

&&

$$

DISCUSSION...75
AVIATION...90