868
FXUS66 KPDT 290953
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
253 AM PDT Sat Jun 29 2024

.SHORT TERM...Today through Monday...Nighttime satellite imagery
tonight shows mid to upper level cloud cover spreading across WA
and western OR. Meanwhile, water vapor imagery shows a transient
ridge exiting into ID/MT while an upper trough develops in the
northeast Pacific.

Today, the transient ridge will continue to push east of the
region, while a developing trough offshore places the PacNW under
an increasing southwest flow aloft. In this flow, a shortwave
trough will lift across the PacNW this afternoon and evening.
While previous runs showed this feature being relatively dry, most
current runs of short-term and global deterministic guidance have
switched course, with scattered showers developing across the WA
Cascades this afternoon, then spreading to the eastern mountains
and Blue Mountain foothills later in the evening and overnight
hours. With the support of the left exit region of an upper jet
and modest low level lapse rates, there will be slight chances
(15-20%) of isolated thunderstorms across the eastern mountains
through early Sunday morning. Winds through the Cascade gaps will
also be on the increase later this afternoon the cross Cascade
pressure gradient tightens with the shortwave passage.

Sunday, the trough offshore will push across the PacNW, resulting
in lingering scattered shower chances across the central WA
Cascade crest and Wallowa county. Lingering surface based
instability will also result in the development of an isolated
thunderstorm or two across eastern Wallowa county Sunday
afternoon. An attendant surface cold front with the trough passage
will help to strengthen a marine push through the Cascade gaps,
resulting in winds increasing further through the Kittitas valley,
eastern Gorge, Simcoe Highlands, and the OR Columbia Basin. Winds
in these areas will generally be 20-30mph with gusts up to around
40mph throughout Sunday, though probabilities of 24-hr max wind
gusts exceeding 45mph in the aforementioned areas is 60-80%. While
the breezy winds will bring elevated fire weather concerns, the
marine push will help to raise minimum relative humidities above
critical levels into the 20s to mid 30s, keeping conditions from
reaching red flag criteria.

The upper trough will slowly make it`s way into ID/MT through
Monday, with northwest flow aloft developing over the PacNW by
Monday evening. Surface pressure gradients will remain tightened
across the Cascades as the trough continues to transit the region,
resulting in another day of breezy winds between 15-25mph with
gusts up to 40mph across the lower elevations. Locally windy
conditions will also be possible through the Gorge, Kittitas
valley, Simcoe Highlands, and the OR columbia Basin, where
probabilities of 24-hr max wind gusts exceeding 45mph are 75-90%.
Afternoon relative humidities will remain above critical levels
throughout Monday, though fire weather concerns will remain
elevated.

Temperatures will moderate through the beginning of next week,
with afternoon highs generally in the upper 70s to 80s in the
lower elevations, and 60s to mid 70s in the mountain areas.
Lawhorn/82

.LONG TERM...Tuesday through Saturday...Ensemble clusters show
little spread in solutions on Tuesday. Confidence is high (>95%
chance) that the synoptic pattern will consist of downstream
troughing over the Northern Plains and Northern Rockies, with an
anomalously strong upper-level ridge in place upstream in the
Pacific Ocean. This will place the forecast area under a dry
northwest flow aloft. Precipitation chances are extremely low (<5%
chance for the Wallowas and Washington Cascade crest). Westerly
winds are forecast through the Cascade gaps, strongest for the
Kittitas Valley and eastern Columbia River Gorge. NBM probabilities
show a 50-90% chance of reaching advisory-level gusts through the
Kittitas Valley on Tuesday, while probabilities are low (<20%
chance) elsewhere.

Wednesday, little overall change is anticipated in the longwave
pattern. Deterministic guidance advertises a shortwave sliding
southeast across eastern WA and northeast OR during the day. The dry
air mass should preclude any significant chances of precipitation
(<5% area-wide) as the shortwave traverses the region, but another
day of breezy to locally windy gap winds is likely (>90% chance)
through the Kittitas Valley and eastern Columbia River Gorge.
Currently, no Red Flag conditions are forecast, and NBM
probabilities of advisory-level wind gusts drop to 30-70%
through the Kittitas Valley.

Ensemble clusters begin to show diverging solutions by Thursday.
While the favored solution is an eastward-propagating ridge (56%
chance), a minority of members advertise a return to troughing for
the PacNW (11%), and the remaining 33% keep the ridge axis well
offshore. The result is reasonably high confidence in warming
temperatures accompanied by Moderate HeatRisk for the lower
elevations on Thursday. Moreover, with the ridge axis favored to
shift eastward over the forecast area, onshore gap winds would
diminish in magnitude.

By Friday and Saturday, ensemble clusters reveal continued
divergence in member solutions. 85% of ensemble members are
advertising an amplified upper-level ridge in the vicinity of the
PacNW. The unlikely solution (13-15% chance) is troughing over the
PacNW. Should troughing materialize, much cooler temperatures
relative to what is forecast would result. Moreover, it would
support light precipitation for the Cascade crest and northern Blue
Mountains. The strong ridging solutions would support continued
warming across the region. This anticipated warming trend would
facilitate widespread Moderate HeatRisk for the lower elevations
Friday and Saturday. To provide some tangible values and illustrate
uncertainty based on the aforementioned scenarios, the NBM is
advertising a 60-85% chance of exceeding 90 degrees, and a 10-30%
chance of exceeding 100 degrees for our main population centers
Friday and Saturday. While very unlikely at this point, the lower
elevations have a 1-7% chance of reaching or exceeding 110 degrees
on Saturday. Plunkett/86


&&

.AVIATION...12Z TAFs...VFR conditions are likely (90% chance or
greater) for all sites through the period. A weak system passing
over the region this afternoon into tonight will facilitate
increasing cloud cover (BKN-OVC) and low chances of showers for all
sites (5-15% chance, except 35-45% chance at PDT/ALW). Sustained
winds are anticipated to remain 12 kts or less, with gusts of 20 kts
or less, except at KDLS where afternoon and evening winds of 12-17
kts with gusts of 20-25 kts are forecast. Plunkett/86


&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  82  58  83  54 /  10  30   0   0
ALW  86  60  86  57 /  10  30  10   0
PSC  87  63  89  59 /  10  10   0   0
YKM  84  57  85  54 /  20  20   0   0
HRI  86  61  88  58 /  10  20   0   0
ELN  81  56  78  54 /  20  20   0   0
RDM  80  50  79  47 /  10  10   0   0
LGD  83  56  79  50 /  10  30  10   0
GCD  87  53  81  49 /  10  40  10   0
DLS  85  60  83  59 /  10  10   0   0

&&

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

$$

SHORT TERM...82
LONG TERM....86
AVIATION...86