448
FXUS66 KPDT 252353
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
453 PM PDT Tue Jun 25 2024

.AVIATION...00Z TAFs...VFR conditions are forecast for all sites
through the period (>95% probability of occurrence). Increasing
mid-level moisture and elevated instability will lead to
increasing cloud cover and may facilitate high-based showers and
isolated thunderstorms during the late-evening and overnight hours
(10-40% chance, highest across south-central WA). For TAF sites,
the best chances of measurable precipitation will be at YKM
(30-40%) during the late night and early morning hours. Confidence
is lower at all other sites (<30% chance). Afternoon convection
may affect KPDT/KPSC/KALW/KYKM Wednesday, but confidence on
timing/location precludes mention in the 00Z TAFs.

Diurnally driven winds will diminish in magnitude and gustiness
this evening. Westerly winds will increase to 12-22 kts with
gusts of 20-35 kts for all sites Wednesday morning and afternoon.
Plunkett/86

&&

.PREV DISCUSSION... /issued 208 PM PDT Tue Jun 25 2024/

SHORT TERM...Tonight through Thursday...Currently there is an
upper level ridge of high pressure over the region while an
upper level trough approaches the Pacific Northwest coast. This
will provide slightly warmer than normal temperatures overnight.
There is also some convective moisture over northern California
and northwest Nevada lifting north into southern Oregon. So far
this is mainly showers but expect convection to continue to
intensify with some thunderstorms and showers moving into
southeast Oregon overnight. Some of this could clip Grant county
late.

Will see a shortwave rotating around the bottom of the upper level
trough late tonight approaching the Cascades and then swing across
the forecast area through Wednesday morning and midday. This
shortwave begin to trigger some convection beginning around 12Z
along the east slopes of the Cascades roughly from Redmond
northward then proceed northeastward through the rest of the
morning and early afternoon. This will at least provide a slight
chance (10-15%) of thunderstorms across the forecast area with the
eastern slopes of the Washington Cascades being the most likely
(15-25% chance). This convection will shift northeast and out of
the Columbia Basin in the early afternoon while the east slopes of
the Cascades will see some lingering convection through the
afternoon.

Convection will begin to increase over northeast Oregon and
southeast Washington (mainly the mountains 20-30% chance) in the
afternoon and then continue overnight. This includes a chance of
thunderstorms (15-25%) through the evening. The shortwave will
also usher in some breezy 15-25 mph westerly winds (70-90%
probability) through the day bringing slightly cooler conditions.
See more discussion regarding winds in the Fire Weather section
below.

The upper level trough will move inland and across the region on
Thursday. This will further cool temperatures and maintain breezy
conditions. Showers are expected along the Cascade crest as well
as over the mountains of northeast Oregon and southeast
Washington until the evening and then come to an end.

LONG TERM...Friday through Tuesday...Weather in the long term
period will be generally benign though a trough crossing the area
Sunday and Monday will raise a few concerns. Temperatures will be
within a couple of degrees of normal though a transitory ridge
crossing the area Saturday will lead to temperatures 3 to 5
degrees above normal.

Friday will be a quiet day as a departing trough moves through
eastern Montana into North Dakota and Saskatchewan while another
trough begin to move out of the Gulf of Alaska. Deterministic
models are in good agreement in having a modest ridge develop
off the coast and this will warm temperatures around 5 degrees
from Thursday to the upper 70s to mid 80s with 70s in the
mountains. It should be sunny without any chance of precipitation
and winds will be no more than 10 mph.

Models agree in having the ridge strengthen Friday night into
Saturday with the ridge axis over Idaho and Montana while the
trough moves to just offshore by Saturday afternoon. The ridge
and the resulting southwest flow will warm temperatures another 2
to 5 degrees with highs in the 80s with 90 possible in the Tri-
Cities (47% chance) and Hermiston (36% chance). The mountains will
be in the mid 70s to lower 80s. This will be the warmest day of
the long term period. Cloudiness will be increasing in the
afternoon and there will be a chance of light rain showers along
the Cascade crest.

On Saturday night and Sunday, models are in reasonably good
agreement in having the trough move over the area with a shortwave
crossing the area. This will lead to a chance of showers over the
mountains and the Blue Mountain Foothills and a slight chance of
showers elsewhere. A slight chance of thunderstorms will be
confined to Wallowa county. Rain amounts will be just a few
hundredths of an inch though the highest terrain of the eastern
mountains could get up to a quarter inch of rain. It will also be
breezy to windy in the afternoon with 20 to 30 mph winds
anticipated in the Eastern Columbia Gorge, the Kittitas Valley and
Oregon Blue Mountain Foothills. In those areas, the NBM has a 55
to 85 percent chance of gusts to 40 mph. Temperatures will cool
3-4 degrees back to the upper 70s to mid 80s with 70s in the
mountains.

The shortwave will move to the east Monday and models have some
differences about the location and strength of an upper low off
the British Columbia. This leads to some differences about
whether we will have a northwest or westerly flow overhead. Either
way, it will be a sunny and dry day with temperatures a degree or
so cooler than Sunday. Wind will again be the main concern with
20 to 30 mph winds in the Kittitas Valley and 15 to 25 mph winds
in the Columbia Gorge, Simcoe Highlands and Blue Mountain
Foothills. NBM probabilities show a 97 percent chance of 40 mph
wind gusts and a 30 percent chance of 49 mph gusts in the Kittitas
Valley while the other locations mentioned above have a 50 to 70
percent chance of 40 mph wind gusts.

By Tuesday, deterministic models are widely differing. The GFS has
a closed low and trough over the area, the ECMWF has a low and
trough over central British Columbia with a zonal westerly flow
over our area while the Canadian has the low and trough in the
Gulf of Alaska with a northwesterly flow over our area. Have kept
the forecast sunny and dry with highs a couple of degrees warmer
than Monday. Perry/83

FIRE WEATHER...As mentioned in the Short Term discussion,
westerly winds will increase on Wednesday becoming 15-25 mph
(70-90% probability) along the east slopes of the Cascades and
across the Columbia Basin. The strongest of these winds are
expected across the Lower Columbia Basin from The Dalles to
Hermiston. Meanwhile, minimum humidities are expected to range
from the lower 20s to mid 30s with those lower range humidities
mainly to be north of the Horse Heaven Hills. The minimum
humidities between The Dalles and Hermiston are expected to be in
the mid 20s to lower 30s. The combination of winds and low
relative humidities does not quite reach Red Flag Warning
criteria. But none-the-less, any fine fuel fires experiencing
these winds will likely see rapid spread and precautions should be
taken to avoid sparking or starting a fire.

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  58  83  52  74 /  10  20  10  10
ALW  59  88  55  78 /  10  20  10  20
PSC  61  89  59  80 /  10  20   0  10
YKM  60  85  53  76 /  10  40  10   0
HRI  61  87  56  80 /  10  20   0  10
ELN  57  79  55  71 /  10  50  10  10
RDM  57  80  46  72 /  10  20   0   0
LGD  58  84  50  70 /  10  20  20  20
GCD  59  90  49  73 /  10  20   0  10
DLS  61  81  57  75 /  10  20   0   0

&&

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

$$

SHORT TERM...91
LONG TERM....83
AVIATION...86
FIRE WEATHER...91