Sisters, Oregon Weather Forecast Discussion

983
FXUS66 KPDT 281143
AFDPDT
Area Forecast Discussion
National Weather Service Pendleton OR
443 AM PDT Sun Sep 28 2025
.Updated Aviation Discussion.
.AVIATION...12Z TAFS...VFR conditions currently across all sites,
which will stay the course through the period. Light winds below
10 kts will persist, with KRDM/KBDN being the exception as winds
will hover around 10 kts this afternoon. Cloud cover will increase
in the evening as a system approaches from the west, with
ceilings of 25kft. 75
&&
PREV DISCUSSION... /issued 341 AM PDT Sun Sep 28 2025/
DISCUSSION...
Key Messages:
1. Warm and dry day ahead.
2. Showers and isolated thunderstorm threat through week.
Current radar and infrared satellite imagery shows dry conditions
as a thin veil of high level clouds slowly cross through the area.
This is in response to the incumbent ridge slowly beginning to
depart to our east as an upper level trough approaches the coast
later today. Southwest flow aloft will enhance today to provide a
slightly warmer day with highs peaking in the low to mid-80s
across the Columbia Basin and lower elevations of Central Oregon.
These values are about 10 degrees above normal for this time of
year, which will also be coupled with afternoon humidities dipping
down between 15-25%. Lower humidity values are expected to occur
across the John Day Basin, Northern Blue Mountains/Foothills,
Grande Ronde Valley, and lower elevations (0.10") is
anticipated to occur through much of the area (40-60% chance via
NBM) with the exception of the Tri-Cities area (24%) and Yakima
(31%). The highest rain amounts of around 0.20" will occur over
the John Day Highlands and the northern Blue Mountains with 50-60%
confidence. Rain amounts Tuesday and Wednesday will struggle to
reach wetting rain amounts outside of the Cascade crest, east
slopes of the Cascades, Elkhorns, and higher elevations of the
Blue Mountains.
The incoming system will also bring with it the chance for
isolated thunderstorms as it erodes the backside of the upper
level ridge. The best chances will occur along the east slopes of
the Oregon Cascades into the US-97 corridor and across Grant,
Union, and Wallowa counties as the NAM advertises 100-200 J/kg of
surface CAPE. The NAM also hints at this same amount of CAPE over
Wallowa County on Tuesday afternoon, and the GFS shows 50-150 J/kg
of CAPE across the Cascades, Columbia Basin, and the northern Blue
Mountains/Foothills on Wednesday. At this time, Wednesday does
look to have more of a widespread thunderstorm threat, however,
models and ensembles are in disagreement on the timing and
progression of the upper level low pressure as it moves onshore.
Currently, ensemble members are split between the low opening up
and moving inland along the British Columbia coast Wednesday or
pushing onshore along the Oregon/California coasts late Thursday.
Currently, the former scenario is the front-runner, which would
lead to a better potential for isolated thunderstorms as
additional CAPE would be available. Even with the potential for
afternoon thunderstorms, they are expected to stay discrete and
sub-severe (

NWS PDT Office Area Forecast Discussion