361
FXUS66 KPDT 162325
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
425 PM PDT Wed Sep 16 2026

.KEY MESSAGES...
- Slight thunderstorm chances today, higher chances Thursday

- Light showers Thursday, drying out through the end of the week

- Warming and drying trend into next week

&&

.DISCUSSION...
Current radar and visible satellite imagery shows very light
returns across Central Oregon and the John Day Basin under
mostly clear skies. This is in response to the upper level low
pressure system, located to our southwest, digging south into
Northern California before ejecting east through Central and
Eastern Oregon Thursday into Friday. As the system stalls over
the next 24 hours, a transient ridge of high pressure will build
through Idaho and western Montana. This ridge will keep skies
mostly clear today, but clouds will increase overnight into
Thursday as the ridge begins to break down in accordance with
the shifting upper level low pressure. The eroding of the ridge
will lead to the potential for weak instability today, with much
more potential on Thursday. Today`s threat of thunderstorms
is expected to primarily stay confined across South-Central
Oregon, but a 10% chance of an isolated thunderstorm is possible
over Deschutes and southern Crook counties between 3-8 PM. The
low is expected to begin its easterly shift overnight tonight,
allowing the thunderstorm threat to begin across the southeast
tip of Grant county around 2 AM. This threat will progress
slowly north and extend to include Deschutes, Grant, Crook,
Union, Wallowa, Southern Gilliam, Southern Morrow, and Southern
Umatilla counties by the early afternoon. Storm chances will
continue to increase through the afternoon before peaking
between 5-8 PM as the extent pushes into Southern Walla Walla
and Southern Columbia counties. Primary concerns will include
frequent lightning, brief downpours, and gusty/erratic winds.
There is low agreement with ensemble members related to any
storms beginning before 5 AM on Thursday, with only 25% of
members suggesting that early of a start to convection. However,
all members expect isolated development before 11 AM, with 60%
of members advertising enhanced afternoon development with
surface CAPE values at or above 500 J/kg. This leads to a high
likelihood of developing storms with breezy winds, frequent
lightning, and small hail possible. The thunderstorm threat
wanes overnight into Friday morning, but briefly returns Friday
afternoon across Wallowa, Union, and Eastern Grant counties.

Light showers are also expected overnight tonight through
Thursday across Central and Eastern Oregon associated with the
passing low pressure system. Both the ECWMF and GFS ensembles
highlight precipitable water amounts of 0.7-0.9" which is
150-180% of normal. This amount of incoming moisture relates to
a 50-85% chance of receiving a wetting rain (0.10" or greater)
across Central and Eastern Oregon on Thursday, with best chances
(70-85%) across Grant, Crook, and Wheeler counties. Rain chances
will peak along with thunderstorm chances, between 5-8 PM
Thursday. Expected rain amounts over elevated terrain are
0.10-0.20", with lower elevations and areas along the foothills
staying below 0.10" as only a 0.01-0.05" of rain is likely along
lower elevations of the Blue Mountain foothills.

Guidance is in good agreement with the upper level low pressure
system exiting into Idaho Friday morning and continuing to track
east across Idaho into Saturday morning. This will open the door
to an upper level ridge moving onshore and over the region, with
rain tapering off by Friday afternoon/evening over Eastern
Oregon. A warming and drying trend will begin on Friday and
persist through Monday as the ridge axis shifts over the Pacific
Northwest and upper level flow becomes more from the southwest
due to a upper level trough slowly passing through the Gulf of
Alaska and along the northern British Columbia coast. As a
result, afternoon humidities will dip into the teens Saturday
and mid-to upper teens on Sunday across portions of Central
Oregon and the John Day Basin, with high temperatures breaking
into the mid-80s across lower elevations of the Columbia Basin.
Temperatures are anticipated to peak on Monday as the upper flow
shifts from southwest to more from the west, but afternoon
humidities are expected to stay in the upper teens through
midweek. 75

&&

.AVIATION /00Z TAFS THROUGH 00Z FRIDAY/...
VFR conditions will largely prevail at the terminals through
the period. The exception will be where showers develop later
tonight through Thursday that could bring reduced VIS and
perhaps some increased winds in the heavier showers/storms.
Currently expecting low to medium chances (20-60%) of showers
through the day on Thursday along a BDN to ALW line, with higher
probabilities to the southeast. There is also a low chance
(10-15%) of embedded thunderstorms in the afternoon for these
locations as well. Areas to the northwest will likely remain
dry. Outside of showers/storms, northerly winds will generally
become light this evening and persist through much of the day
tomorrow.

&&

.Preliminary Point Temps/PoPs...
PDT  47  72  51  77 /   0  20  40   0
ALW  52  73  55  77 /   0  10  50   0
PSC  50  79  52  81 /   0   0  20   0
YKM  50  79  52  79 /   0   0   0   0
HRI  49  76  52  80 /   0  10  30   0
ELN  47  77  50  76 /   0   0   0   0
RDM  40  66  41  75 /  10  50  30   0
LGD  47  73  48  74 /   0  50  70  20
GCD  46  71  49  73 /  20  70  70  20
DLS  56  84  58  81 /   0   0   0   0

&&

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

&&

$$

DISCUSSION...75
AVIATION...99/CGAA