420
FXUS66 KPDT 222328
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
428 PM PDT Thu May 22 2025

.UPDATED AVIATION DISCUSSION...


.AVIATION...00Z TAFs...VFR conditions for all sites with winds less
than 12 kts and mid to high clouds. Feaster97


&&

.PREV DISCUSSION... /issued 241 PM PDT Thu May 22 2025/

.SHORT TERM...today through Saturday...Radar shows the line of
showers that passed through the CWA earlier today has mostly
dissipated and is heading off east towards Idaho. A few showers
remain possible across the Blues and parts of the WA Cascades as
we head through the rest of the night. A stray lightning or two
from a shower is possible across parts of the Wallowas later this
afternoon. Yakima/Ellensburg have slight chances (15-25% chance)
to see an isolated thunderstorm as clouds cleared out from
earlier in the day, allowing for some additional instability to
re-enter the atmosphere. As the shortwave exits the region, precip
chances and sky cover decrease headed into Friday. High
temperatures across the board Friday rise to the low 70s to mid
70s. Higher elevations will see low 60s as their high. A secondary
shortwave develops later into Friday just south of the CWA, but
some moisture advection will make it`s way into the southern parts
of the region. Coupled with daytime heating with highs in the
high 60s and low 70s and moisture entering southern parts of the
CWA, there`s a slight chance (15-25% chance) for an isolated
thunderstorm to pop-up in the Southern Blues and eastern mountains
Friday afternoon.

Ridging will return Saturday and into Sunday morning. The ridge
axis will be placed over the region late Saturday morning/early
after- noon. By nature, this pattern will set-up for a clear,
hotter profile heading into the holiday weekend. Highs will depart
5-15 degrees above the normal for this time of year on Saturday,
starting the weekend in the mid to low 80s across parts of the
Basin and high 70s across higher elevation areas. Winds will
remain light with diurnal patterns, with sustained up to 15
knots.

.LONG TERM...Sunday through Wednesday...

Key Messages:

1. Isolated thunderstorms Sunday and Monday.

2. Breezy winds Monday afternoon and evening.

3. Thunderstorm chances return Wednesday afternoon and evening.

The extended period is characterized by an upper level trough
passing through the Pacific Northwest Sunday and Monday before upper
level ridging builds through the midweek. The late weekend trough
will bring with it periods of light rain across the area Sunday
evening through Monday evening as rain amounts of up to 0.10" will
be possible for elevations below 4000 feet, and 0.15-0.25" of rain
for elevations above 4000 feet. The primary concern associated with
this incoming trough will be the potential for isolated
thunderstorms as the trough erodes the backside of the retreating
ridge of high pressure. CAPE values of 150-250 J/kg are present
along the east slopes of the Cascades and across Central Oregon
Saturday and from Central Oregon through northeast Oregon on Sunday.
Ensembles depict variability on both the incoming trough and present
ridge strength, relating to 76% of ensemble members suggesting
similar or slightly more CAPE values with 55% of members suggesting
an additional 100-300 J/kg Sunday. This suggests that there is
slightly more confidence in increasing thunderstorm potential on
Sunday afternoon and evening, predominately after 5 PM. Members also
suggest that the best probability of increased CAPE would be over
the east slopes of the Oregon Cascades extending east through
Deschutes and Crook Counties on Sunday. 55% of ensemble members for
Monday`s potential for thunderstorms suggest an additional 50-200
J/kg of CAPE than what is currently forecast. However, only 19% of
members suggest more than the 50 J/kg increase, which is based on a
much stronger upper level trough developing and eroding the ridge.
The main hazards associated with these storms will be gusty/erratic
winds and the potential for small, pea-sized hail.

The incoming trough and the associated cold front will allow for a
pressure gradient to develop along the Cascades to allow for breezy
winds along the east slopes of the Cascades, Kittitas Valley, Simcoe
Highlands, Eastern Gorge, and the Southern Blue Mountains/foothills.
Sustained west-northwest winds of 15 to 25 mph with gusts of up to
35 mph will be possible, peaking between 4-9 PM Monday. Confidence
in these wind values is moderate (50-60%) as the GFS suggests a
pressure gradient of 9-10 mb between Portland and Spokane (PDX-GEG),
which is below the normal wind advisory threshold of 12 mb. Ensemble
clusters are also split as 53% of members hint at slightly higher
winds than what is currently forecast in relation to a stronger
incoming trough. At this time, there is low confidence (20-30%) in
wind gusts reaching advisory-level (45 mph or greater).

Long-range guidance struggles with the weather pattern after Monday,
either exiting the trough to our east or dropping it to our south as
a closed low pressure system. Either way, it does allow for upper
level ridging to occur across the Pacific Northwest, but how much
ridging is able to develop will be a result from which solution ends
up panning out. The dropping closed low scenario poses the best
chance for isolated thunderstorms to occur across the John-Day
Basin/Highlands, Southern/Northern Blue Mountains, and Wallowa/Union
Counties Wednesday afternoon and evening as around 500 J/kg (ECMWF)
of CAPE would be present versus 200 J/kg (GFS) with the eastward
departure scenario. Currently, 58% of ensemble members are
suggesting near or slightly more CAPE available across the
aforementioned areas, which would align slightly more with the
closed low scenario. Hazards will be similar to the thunderstorms
Sunday and Monday, with gusty/erratic winds and small (pea-sized)
hail being the primarily concern. 75

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  45  74  46  81 /   0   0   0   0
ALW  47  73  49  80 /   0   0   0   0
PSC  45  78  45  84 /   0   0   0   0
YKM  45  77  47  82 /   0   0   0   0
HRI  47  78  46  84 /   0   0   0   0
ELN  45  74  45  81 /   0   0   0   0
RDM  37  73  39  83 /   0   0   0   0
LGD  40  70  41  77 /  10  10   0   0
GCD  39  71  41  80 /  10  20  10   0
DLS  47  75  46  84 /   0   0   0   0

&&

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

$$

SHORT TERM...95
LONG TERM....75
AVIATION...97