572
FXUS66 KPDT 252108
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
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


&&

.AVIATION...18Z TAFs...VFR conditions will continue for the next 24
hours.  Skies will remain clear through around 04Z-07Z when BKN mid
to high level clouds will move in as an upper trough approaches the
coast with increasing moisture and instability. Light showers and a
few thunderstorms will cross the area with the main impacts over the
mountains though there will be a possibility at all TAF sites 11Z-
18Z. Due to low confidence, limited mention to VCSH and -SHRA at
Columbia Basin sites after 14Z. Winds will remain below 12 kts
through 15Z tomorrow morning except at KRDM and KBDN which will have
gusts to 20 kts from 22Z-04Z this afternoon. After 15Z, winds will
be increasing to 10-15 kts with gusts to 25 kts at KDLS. 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  59  83  52  74 /   0  20  10  10
ALW  59  88  55  78 /  10  20  10  20
PSC  61  89  59  80 /   0  20   0  10
YKM  60  85  53  76 /  10  30  10   0
HRI  61  87  56  80 /  10  20   0  10
ELN  57  79  55  71 /  10  30  10  10
RDM  57  80  46  72 /  10  20   0   0
LGD  58  84  50  70 /   0  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....90
AVIATION...90