048
FXUS66 KPDT 152248
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
348 PM PDT Tue Sep 15 2026

.KEY MESSAGES...
- Low thunderstorm chances Thursday into early Thursday night
  across the eastern mountains (30%) into possibly the northern
  foothills (15%).

- Warming temperatures into and through the weekend with highs
  reaching slightly above normal in tandem with dry conditions.

&&

.DISCUSSION...
Latest water vapor imagery shows a deepening
upper-level trough dropping into the PacNW coast. Its main
shortwave trough in the imagery is currently offshore OR with
its dry slot and moist ascent apparent downstream in western OR.
Radar mosaic shows some areas of precip, however, dry air at
the lower levels is limiting it reaching the ground that could
be attributable to the modest moisture being transported in and
the dry air aloft. Ensemble guidance is in fair agreement with
this becoming a closed low moving to be near the northern CA
coast by late Wednesday. Broad scale ascent will spread and
result in spill over precipitation across the Cascades and into
portions of central OR late this afternoon/tonight as the
shortwave dives towards northern CA. The best chances for light
rain will be across central OR and the surrounding mountains on
the east slopes of the Cascades and Ochocos with chances as high
as 20-40% for a tenth of an inch or more at Madras, Mitchell,
and Bend. Otherwise, chances will diminish overnight with dry
conditions tomorrow and highs near to slightly below average.

Looking ahead, much of the spread revolves around this upper
troughs amplitude late Wednesday-Thursday as it evolves. The
upper low is anticipated to begin weakening Thursday as it
starts to lift northeastward. Thereafter guidance s in fair
agreement with it ejecting as an open wave into the Northern
Intermountain West Friday with the pattern becoming more
progressive. The overall quantity of moisture and available
instability are the main forecast challenges on Thursday. Of
which, clustering scenarios show two leading outcomes where
sufficient instability and residual moisture will support
isolated, embedded thunderstorm potential across the eastern
mountains and foothills of the Blues (~50%) versus a lower
precipitation/less supportive convective environment scenario
(50%). The former involves a slower, deeper evolution of the
upper trough compared to the weaker, faster evolution seen in
the latter. Regardless, wetting rain and light to locally
moderate rainfall is more likely than not across the eastern
mountains. The main sensible weather spread revolves around how
far west precip will extend into the eastern Lower Basin and
chances for isolated thunderstorms in the foothills of the
Blues. Confidence is presently low (20-25%) for lightning risk
at this time.

Afterwards, a more progressive synoptic pattern is seen in the
eastern North Pacific as a shortwave ridge fills in behind with
increasing 500 mb height rises spreading into the region.
Guidance hints at another shortwave trough moving to the West
Coast late this weekend. Outside lingering eastern mountain
rain chances Friday as it departs, a warming trend is
anticipated this weekend with highs few to several degrees above
normal forecast by Sunday with dry conditions in place
currently. Ensemble guidance is in fair agreement this weekend
with the second shortwave trough tracking and impacting northern
CA than the PacNW. Low confidence in precipitation occurring at
this time over the forecast area this weekend.

&&

.AVIATION /00Z TAFS THROUGH 00Z THURSDAY/...
Flight categories should remain fairly VFR through the TAF
period. Broken-overcast skies are forecasted through the early
evening hours at all airports, as well as some few-scattered
cloud cover as we move into the overnight hours. There is a
high chance (confidence 80-85%) for light showers near RDM/BDN
beginning at 00Z, and a low probability of light rain showers
anytime between 06Z-10Z




&&

.Preliminary Point Temps/PoPs...
PDT  45  74  46  73 /  10   0   0  40
ALW  48  77  51  74 /  10   0   0  30
PSC  47  81  49  80 /  10   0   0  10
YKM  47  75  49  80 /  10   0   0   0
HRI  46  78  48  77 /  10   0   0  20
ELN  41  71  45  77 /  10   0   0   0
RDM  40  68  39  69 /  40  10  10  40
LGD  46  77  46  76 /  10   0   0  60
GCD  45  78  46  73 /  10   0  20  70
DLS  54  80  55  84 /  20   0   0   0

&&

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

&&

$$

DISCUSSION...80
AVIATION...01