257
FXUS66 KPDT 271851
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
1151 AM PDT Mon May 27 2024

.MORNING UPDATE...Aside from increasing mid- and high-level clouds
through the day, quiet weather is anticipated through mid-
afternoon as the axis of an upper-level ridge continues to amplify
as it tracks eastward over the northern Rockies.

Later this afternoon, surface-based convection may develop (10-20%
chance) over the mountains of central and east-central OR, though
12Z HREF members are not in good agreement on coverage due to a
wide spread in forecast boundary layer moisture and CAPE. Best
chances for showers and isolated thunderstorms will be over the
high terrain, namely the Ochocos, Elkhorns, and other sub-ranges
of the central and east-central OR Blue Mountains. East of the
Cascade crest, isolated activity is forecast over the Newberry
Crater area.

As we transition to the evening and overnight hours, CAMs are in
excellent agreement that surface-based instability from daytime
heating will wane, though a spread of solutions is evident when
examining mid-level moisture. Any subsequent instability as
southwesterly flow aloft advects moisture and increasingly steep
lapse rates and northward across central/north-central OR and
limited portions of south-central WA and northeast OR. Should the
moister solutions with near-saturated mid-levels materialize,
showers and a few isolated thunderstorms may develop this evening
and track northward overnight (10-20% chance PoPs). Global
guidance is highlighting areas of near zero to negative Theta-E
lapse rates coincident with 70-95% 700-500 mb RH, increasing
confidence in at least some altocumulus and showers, though
synoptic-scale lift is not robust until later Tuesday morning.

The forecast was updated to reflect more aggressive CAM solutions
and a slight chance of thunder was also expanded this evening and
overnight. Plunkett/86

&&

.PREV DISCUSSION... /issued 905 AM PDT Mon May 27 2024/

SHORT TERM...Today through Wednesday night...An upper ridge will
be located over the Pacific Northwest early today. This ridge will
move eastward later in the day, allowing for southwesterly flow to
establish itself ahead of an approaching trough and cold front.


The trough and associated cold front will move onshore Tuesday
afternoon and cross the region through Tuesday night.

There could be lingering precipitation over the mountains into
Wednesday afternoon, then dry conditions Wednesday night. However
it will be cooler on Wednesday.

In the developing southwesterly flow this afternoon/evening, CAPE
will increase to over potentially over 1000 J/KG in spots,
especially across the Blue Mountains and LI values will be in the
range of -1 to -4 degrees Celsius. However, moisture is quite
lacking. During the evening hours, there is at least some better
moisture. So it looks as if there could be some isolated
thunderstorm potential during the afternoon and evening hours, but
overall potential is low (<20%). The Storm Prediction Center has
placed a large portion of central and northeast Oregon and
southeast Washington in general thunderstorms today, though any
activity looks to be later in the afternoon into the evening. The
ECMWF EFI is not all that excited about thunderstorm potential
today.

The better chance of showers in the mountains and thunderstorms is
on Tuesday in increasing southwesterly flow ahead of the advancing
trough. The Storm Prediction Center has placed much of Wallowa
County and a very small portion of Union County in Marginal Risk
of severe thunderstorms, with general thunderstorms expected west
of this area from about Walla Walla to Pendleton, including Grant
County and points eastward.

CAPE values on Tuesday will range from 200 to over 1000 J/kg with
LI values -1 to around -6 degrees Celsius. The highest values will
be further east. As the cold front works its way eastward,
conditions will stabilize, and based on current timing it appears
that any thunderstorm activity should be winding down by early
evening, with CAPE decreasing and LI values becoming positive
behind the passage of the cold front. The ECMWF EFI does key in on
thunderstorm potential on Tuesday mainly across the Blue
Mountains and Wallowas on Tuesday, with a large area of 0.6 to 0.7
for CAPE and some embedded areas of 0.7 to 0.8.

The other concern on Tuesday will be wind with the front/trough.
It does look to be breezy, especially in the Oregon Basin,
Columbia Gorge, Simcoe Highlands, Kittitas Valley. NBM
probabilities of wind gusts >=39 mph are 80 to 90%. However, NBM
wind gust probabilities of wind gusts >= 47 mph drop to 40% or
less everywhere except in the Kittitas Valley, where they are
about 50%. If a wind advisory is needed anywhere, the most likely
location would be the Kittitas Valley. The ECMWF EFI ranges from
0.5 to 0.7 across the Columbia Basin on Tuesday. On Wednesday
(which would mainly be Wednesday morning), it has values of 0.6 to
0.7 across the Simcoe Highlands and 0.7 to 0.8 across the Kittitas
Valley and 0.6 to 0.8 across the Blue Mountains and portions of
Wallowa County.

High temperatures will be about 5 to 10 degrees above normal today
and will generally be in the upper 70s to lower 80s. Highs will be
cooler on Tuesday and close to normal, with values ranging from
the low 70s to lower 80s. Wednesday, will be even cooler, with
high temperatures ranging from the mid 60s to the lower 70s. These
temperatures are 5 to 10 degrees below normal.

LONG TERM...Thursday through Monday...Models start out in good
agreement in having a trough moving off to the east with a ridge
building over area from the eastern Pacific. On Friday, model
clusters show general agreement in having the ridge building
further with the ridge axis just off the coast. The trough moves
further east in 80 percent of the model ensemble members while the
remaining 19 percent keep the trough over Idaho and still having
an influence over Wallowa County. By Saturday, models develop
greater differences though each case keeps dry weather over our
area. 48 percent have a flat ridge centered offshore or just at
the coast with a westerly flow over head and the other 52 percent
have the ridge (with varying amplitude) centered over the Rockies
and a shallow trough just off the coast. On Sunday, 63 percent
have a ridge centered over the Rockies with a southwest flow over
our area, 23 percent have a ridge centered along the coast and 14
percent have the ridge centered several hundred miles offshore
with a weak trough centered along the Idaho border. On Monday,
models favor a strong ridge with significant differences as to the
location of the ridge axis. 38 percent have the ridge axis at the
coast while the other 62 percent have the ridge centered over the
Rockies with a trough approaching the coast. However, the ECMWF
and Canadian deterministic runs have a trough near the coast with
a front moving through the area Sunday night into Monday. The
extreme Forecast Index indicates little in the way of unusual
weather aside from cool temperatures Thursday into Friday with
values of around -0.75.

The result of all this favors mainly dry weather Thursday through
Sunday aside from a slight chance of rain showers along the
Cascades and in the higher terrain of Wallowa County at times.
There will also be a warming trend. Thursday will have a slight
chance of light rain showers from the departing front over the
Wallowa mountains and the higher terrain near Hells Canyon. Winds
will be generally light, though the Kittitas Valley will have
northwest winds around 20 mph in the afternoon. The Columbia Basin
will have highs in the upper 60s to mid 70s with mid 50s to mid
60s elsewhere. Lows Thursday night will be in the upper 30s to mid
40s in the Columbia Basin and in the lower to mid 30s elsewhere.

Friday and Saturday will remain dry with mainly light winds.
Temperatures Friday will warm to the 70s in the lower elevations
with upper 50s and 60s in the mountains. Saturday will rise to the
upper 70s to mid 80s with upper 60s and 70s in the mountains.

On Sunday, the NBM brings a chance of rain showers to the Cascades
and a slight chance to the eastern mountains in the late afternoon
and Sunday night reflecting the ECMWF and Canadian deterministic
runs. The ECMWF brings in the cold front with the bulk of the
precipitation on Monday, so have a chance of rain in the mountains
and a slight chance in the lower elevations. Temperatures Sunday
rise a couple of degrees from Saturday. Monday begins to show some
cooling with temperatures dropping back to near Saturday`s
values. Overnight low temperatures will show a similar warm up
Friday night through Sunday night, ending up in the mid 50s to
lower 60s in the Columbia Basin and in the mid 40s to lower 50s
elsewhere. Perry/83

AVIATION...18Z TAFs...VFR conditions will prevail through the
period. A weak disturbance will be moving through the area
overnight. Clouds will increase, but only expecting mid to high
level clouds at this time. A few showers may develop near KPDT and
KALW after midnight, but should remain light. Winds will be 10 kts
or less with some higher afternoon gusts. Earle/81

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  78  53  73  45 /   0  20  20  10
ALW  81  56  78  49 /   0  20  20  20
PSC  82  58  81  50 /   0  20  20   0
YKM  80  50  76  40 /   0   0  10   0
HRI  82  56  78  48 /   0  20  20   0
ELN  76  52  70  42 /   0  10  10   0
RDM  78  46  69  40 /   0  10  10   0
LGD  78  50  73  45 /   0  10  50  50
GCD  81  49  77  43 /  10  10  60  40
DLS  79  55  70  48 /   0   0  10   0

&&

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

$$

MORNING UPDATE...86
SHORT TERM...77
LONG TERM....83
AVIATION...81