393
FXUS66 KPDT 162320
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
420 PM PDT Fri May 16 2025

.UPDATED AVIATION...


&&

.PREV DISCUSSION... /issued 229 PM PDT Fri May 16 2025/

SHORT TERM...Tonight through Sunday night...

Key Messages:

1. Showers and a 10-30% chance of thunderstorms tonight through
Sunday.

2. Breezy to windy conditions through the Cascade gaps and across
portions of the Columbia Plateau Saturday and Sunday.

3. Light snowfall for mountain passes Saturday night and Sunday
morning.

Tonight, the first of back-to-back waves is expected to arrive
from the northeast Pacific (>90% confidence). This wave is
currently visible on water vapor imagery west of Vancouver Island,
diving southeast towards the PacNW. The second vorticity maximum
located in the Gulf of Alaska, is not advertised by ensemble NWP
to arrive until Saturday night through Sunday, and is the main
driver of uncertainty in the short-term forecast; its track will
have implications on precipitation and wind.

Shower chances will increase tonight, with some weak elevated
instability facilitating low-end chances (10-15%) of embedded
thunderstorms for portions of central and east-central Oregon,
mainly in Grant County.

Area-wide shower chances will increase Saturday through Saturday
night as the first wave provides ample forcing, and as the second
wave digs southeast into the PacNW. Have maintained an area-wide
slight chance (15-24% probability) to chance (25-35% probability)
mention of thunder in the forecast Saturday. Best chances of
thunderstorms will be over the Blue Mountains and their foothills
as well as portions of the lower Columbia Basin during the
afternoon (25-35% probability) .

Delving into clusters of ensemble solutions does reveal some
uncertainty in the evolution of the system as the two waves
interact Saturday night through Sunday; ensemble members are still
uncertain whether the second wave will become a slower-moving
closed low overhead late Saturday through Saturday night or if it
will remain a weaker open wave trough with a quicker exit through
Sunday.

Westerly winds will increase in magnitude Saturday afternoon through
the Cascade gaps as cross-Cascade pressure differences increase
and a robust low-level jet (35-45 kts) sets up. NBM probabilities
of exceeding advisory-level gusts (45 mph) through the eastern
Columbia River Gorge, Kittitas Valley, and wind-prone portions of
the Columbia Basin/Blue Mountain foothills are roughly 40-70%.

Saturday night through Sunday, the likelihood of reaching
advisory criteria is high (60-95% chance) through the Kittitas
Valley and eastern Columbia River Gorge. Moreover, a low-medium
chance (30-60%) of warning-level wind gusts (58 mph or greater)
is advertised by the NBM for the Kittitas Valley. In other wind-
prone portions of the Columbia Basin/Blue Mountain foothills, NBM
probabilities of reaching advisory criteria are medium-high
(40-70%). Due to the aforementioned uncertainty in the precise
track of the second wave, have held off issuing any wind
highlights with the afternoon forecast package.

Lastly, will mention measurable snowfall (0.1" or greater) is
more likely than not (50-85% chance) across mountain passes
Saturday night through Sunday morning as colder air associated
with the trough/closed low spreads over the region. In fact, NBM
probabilities place medium (40-60%) chances of 1" of snowfall or
more for the higher passes of the Cascades and Blue Mountains.
However, snow accumulation on roadways may be limited due to warm
antecedent conditions. Plunkett/86

LONG TERM...Monday through Friday...Westerly flow over the area
will allow for a weak system to move across the region on
Monday. It will bring mainly light rain to the mountains.
Lingering moisture in the westerly flow will keep precip chances
in the mountains on Tuesday.

The rest of the extended period looks to be dry, with some
transitory ridging Tuesday into Wednesday and more zonal flow
Wednesday into Thursday then more ridging Thursday into Thursday
night.

Monday and Tuesday will see diurnally breezy winds, with Tuesday
possibly breezier. Winds will mainly be in the 25 to 30 mph range
in the normally gusty areas such as the Kittitas Valley, Simcoe
Highlands, Columbia Basin and Foothills of the Blue Mountains.

On Monday, NBM probabilities of wind gusts >= 25 mph is quite high
generally 90 percent or greater in these areas. However, the
probabilities of wind gusts >= 39 mph is 20 to 50 percent. On
Tuesday. NBM probabilities of wind gusts >= 25 mph is about the
same as Monday. However, the probabilities of wind gusts >= 39 mph
is a bit higher30 to 60 percent and much higher in the Kittitas
Valley (80 percent).

&&

.AVIATION...00Z TAFs...VFR conditions expected through the
period with the exception of a brief period of MVFR for BDN/RDM
near 13Z. DLS will remain windy through 7Z before a brief settle
down between 07-17Z before increasing 15-19G20-25kts. Otherwise,
winds will remain below 10kts through 18Z. Bennese/90

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  48  64  43  62 /  20  90  70  30
ALW  49  64  44  61 /  10  80  90  40
PSC  48  69  45  69 /  10  70  60  10
YKM  49  69  41  67 /  30  70  10   0
HRI  49  69  44  67 /  20  80  50  20
ELN  47  65  41  61 /  40  80  20   0
RDM  42  62  34  57 /  30  80  20  10
LGD  42  58  39  54 /  30  90  90  50
GCD  43  56  36  52 /  70 100  80  30
DLS  51  68  46  64 /  30  60  10  10

&&

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

$$

SHORT TERM...86
LONG TERM....77
AVIATION...90