866
FXUS66 KPDT 291113
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
413 AM PDT Tue Sep 29 2026

.KEY MESSAGES...
- Light to moderate mountain showers across the WA Cascades and
  Northern Blues today

- Breezy winds through the Columbia Basin and Eastern Columbia
  River Gorge this afternoon

- Near to above normal temperatures, mostly dry conditions, and
  light winds develop into early next week

&&

.DISCUSSION...Nighttime satellite imagery tonight shows
increasing mid to upper level cloud cover from northwest to
southeast. Meanwhile radar is showing returns mostly over
northwest WA at this time, with returns slowly expanding south
and east.

Today and Wednesday: Zonal flow aloft will remain over the
forecast area throughout today and Wednesday, however an upper
shortwave trough passing through BC will clip the WA
Cascades and northern Blue mountains. This will result in
light to moderate rain showers developing across these areas.
Since 24 hours ago, chances for 0.1 inches of precip in the
Northern Blues has fallen quite a bit, with only a 10-25% chance
through tonight. Meanwhile, there is a 70-95% chance of 0.25
inches along portions of the WA Cascades and east slopes
(highest chances in central WA Cascades, lowest south central WA
Cascades). In addition to the shortwave trough, a weak cold
front boundary will move across WA during the day. This will
result in breezy west to southwest winds (15-25mph with gusts
25-40 mph) in the Columbia Basin and Eastern Gorge developing
throughout the day, with light to locally breezy winds
elsewhere.

Flat upper level ridging in the eastern Pacific will build into
the western CONUS by Wednesday morning, resulting in a return to
dry conditions and light winds.

Thursday and Friday: Ensemble guidance are in good agreement
that near zonal flow will persist over the PacNW as a flat ridge
remains just south of the region. This will allow a subtle
shortwave to slide across the PacNW Thursday, with very light
showers developing across the WA Cascades during the day
(confidence 55-65%).

Friday, ensemble and deterministic guidance are in favor of
upper level ridging beginning to build into the PacNW from the
south, with a drying and warming trend beginning. However, about
22% of members made up from the ECMWF, GFS, and Canadian
ensemble suites favor another shortwave trough quickly sliding
across the area, resulting in another round of showers across
the Cascade crest/east slopes and the northern Blues Friday. At
this time, confidence is low (20%) in this solution developing.

Saturday through Monday: Upper level ridging will continue to
build over the PacNW over the weekend and into early next week
(confidence 50-70%). Temperatures will be on a gradual warming
trend, with a return to 80 degree temps next Monday (50-75%
chance). Otherwise, dry conditions with light winds will prevail
during this period. Lawhorn/82

&&

.AVIATION /12Z TAFS THROUGH 12Z WEDNESDAY/...VFR conditions to
prevail through the period. CIGs are expected to remain 5kft
AGL or higher as a system moves through the area. Winds will
increase to 12-17kts with gusts 20-30kts at sites
DLS/PDT/ALW/PSC between 19Z and 1-3Z. Otherwise, winds will be
light through the period. Lawhorn/82

&&

.Preliminary Point Temps/PoPs...
PDT  76  49  72  46 /   0  10   0   0
ALW  75  54  72  50 /   0  10  10   0
PSC  78  49  75  45 /   0  10   0   0
YKM  74  50  71  48 /  10  10   0   0
HRI  77  51  75  45 /   0  10   0   0
ELN  66  44  66  42 /  20  20   0   0
RDM  75  38  74  39 /   0   0   0   0
LGD  75  47  70  44 /   0   0  10   0
GCD  75  43  74  45 /   0   0   0   0
DLS  74  53  76  49 /   0  10   0   0

&&

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

&&

$$

DISCUSSION...82
AVIATION...82