854
FXUS66 KPDT 120524
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
1024 PM PDT Sun Aug 11 2024

Updated Aviation Discussion

.AVIATION...06Z TAFs...VFR conditions expected over the next 24
hours outside of YKM where brief periods of MVFR could be possible
due to localized haze and smoke. Otherwise look for breezy winds
to be possible again tomorrow all sites except YKM/PSC, with
expected gusts between 20-30 knots, strongest DLS. Goatley/87


&&

.PREV DISCUSSION... /issued 425 PM PDT Sun Aug 11 2024/

SHORT TERM...Tonight through Tuesday...A troughing pattern is
expected (>95% chance) to persist through Tuesday. Current
satellite imagery shows a mid/upper-level low over Vancouver
Island with a modest mid/upper-level ridge axis placed over
eastern Oregon into western Idaho and northeast Washington. This
translates to weak southwesterly flow aloft. Some mid-level
moisture is apparent on satellite imagery with clouds increasing
in coverage. Additionally, cumulus fields have developed over the
Blue Mountains and Washington Cascades. Confidence in any showers
and thunderstorms is low (5-15%) this afternoon and evening due to
weak synoptic forcing.

This evening through Monday morning, there is a non-zero chance
(5-15%) that elevated showers and an isolated thunderstorm will
develop across the Blue Mountains and adjoining foothills. While
some weak synoptic forcing and elevated instability will be
present, mid-level moisture appears marginal (50-70%) in forecast
soundings. Have maintained a "slight chance" mention of showers
in the forecast, but actual chances of measurable precipitation
are likely on the low end of the aforementioned range.

Attention shifts to the evolution of a compact closed low centered
off northern California for Monday afternoon through Tuesday.
Guidance is nearly unanimous in advertising the closed low
transforming into an open-wave shortwave trough as it swings
northeast across northern California, northwest Nevada, and
eastern OR late Monday evening through Monday night. The broad
forcing for ascent (evident in PVA and q-vector fields) associated
with this shortwave, coupled with ample mid-level moisture
(700-500 mb RH >70%) and negative mid-level (700-500 mb) theta-e
lapse rates should facilitate nocturnal showers accompanied by
embedded thunderstorms. The best chances for convection will be
across the Blue Mountains, perhaps extending as far west as the
foothills of the Blue Mountains. Notably, chances of wetting rain
(0.10" or greater) for eastern Grant, Union, and Wallowa counties
are 20-40% from Monday evening through Tuesday morning. Looking
west, probabilities drop to 5-20% for the western Blue Mountains
and adjoining foothills.

Breezy to locally windy onshore winds are forecast Monday
afternoon/evening and Tuesday afternoon/evening through the
Cascade gaps. Confidence is currently too low to advertise any
wind highlights, but the pattern is conducive to sustained winds
of 15-25 mph and gusts of 30-45 mph through climatologically windy
locations. Plunkett/86

LONG TERM...Tuesday Night through Sunday...Models are in good
agreement in having a trough influencing our weather through the
long term period though there are differences as to the exact
location and strength of the trough for any particular time.
Initially, the trough will be fairly shallow and weak, giving us
quiet and benign weather but by Thursday, an upper low over
British Columbia will start deepening and strengthening the trough
and this will give us a slight chance of showers and mountain
thunderstorms through next weekend. With the trough over the area,
temperatures will be below normal, especially next weekend, where
high temperatures may be 10 degrees below normal. The trough will
also bring stronger pressure gradients and it is expected that
there will be breezy Cascade gap winds each afternoon. The NBM
gives 70-95 percent chance of 20 mph winds in the Columbia Gorge
and Kittitas Valley each afternoon through the period.

The Extreme Forecast Index is indicating little in the way of
unusual weather except for below normal temperatures Saturday
onward with values of -0.70 to -0.75 centered over the Oregon
Cascades and central Oregon. Clusters show a shallow trough
centered offshore Tuesday night and Wednesday in 75 percent of the
model ensemble members though a minority of the Canadian and
ECMWF members favor slight ridging and a westerly flow. Weather
will be generally quiet though a slight chance of showers and
thunderstorms will linger in the early evening over Wallowa
county. The trough will also provide tight pressure gradients and
15 to 25 mph winds are expected in the Columbia Gorge and Kittitas
Valley. Lows will be in the 50s in the Columbia Basin and in the
60s elsewhere though higher mountain valleys will dip into the
30s. Wednesday`s highs will be a little warmer than Tuesday with
highs in the mid 70s to mid 80s. Not expecting much in the way of
showers though a slight chance of showers is possible south of
Bend.

Thursday will see energy diving southwestward out of northern
British Columbia and beginning to deepen the trough. Models differ
as to the location of the trough axis though about two thirds have
it along the coast and the rest further inland. A generally
southwest flow will warm most of the area a couple more degrees to
the upper 70s and 80s. A southwest flow will bring moisture and
increased instability. The NBM shows 100-300 j/kg of CAPE and have
a slight chance of thunderstorms over the eastern mountains and
central Oregon with a slight chance of showers over the Washington
Cascade crest, eastern Columbia Basin and the Blue Mountain
Foothills. The Cascade gaps will see breezy winds of 15-20 mph in
the afternoon.

Friday will see the arrival of a closed low off the Pacific
Northwest coast. 85 percent of the model ensemble members have the
trough axis along the coast or just offshore with the remainder
having it further offshore. Colder air with the low will cool
temperatures about 5 degrees to the lower 80s in the Columbia
basin and in the 70s elsewhere. There will again be enough
instability for a chance of showers and a slight chance of
thunderstorms in central Oregon and the eastern mountains with
rain amounts of up to a tenth of an inch. The rest of the area
will have a slight chance of showers with generally a few
hundredths of an inch.

Saturday, model clusters continue to show a strong closed low and
trough over the area though they differ as to whether the trough
will be centered offshore or onshore. This will continue the
showery pattern with a slight chance of afternoon thunderstorms in
the eastern mountains. By Sunday, the low moves north to near
Vancouver Island and this reduces showers over most of the area.
Temperatures will be mainly in the 70s Saturday with a few 80s in
the Columbia basin. The NBM is showing a 60-85 percent chance of
reaching 80 degrees in the Columbia Basin and only about 30-50
percent elsewhere. With low moving north on Sunday, temperatures
warm a few degrees to the mid 70s to mid 80s though NBM chances of
reaching 80 degrees are only slightly higher. Perry/83

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  58  85  54  79 /  10  20  20  10
ALW  64  89  58  83 /  10  20  20  10
PSC  66  90  61  84 /   0   0  10   0
YKM  58  88  53  82 /  10   0   0   0
HRI  63  88  58  83 /   0   0  10   0
ELN  59  85  55  80 /  10   0   0   0
RDM  50  83  45  78 /   0   0  10   0
LGD  55  88  53  81 /  20  20  40  20
GCD  55  90  53  83 /  10  20  20  20
DLS  61  81  59  77 /   0   0   0   0

&&

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

$$

SHORT TERM...86
LONG TERM....83
AVIATION...87