Area Forecast Discussion
Issued by NWS Pendleton, OR

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FXUS66 KPDT 230336

Area Forecast Discussion
National Weather Service Pendleton OR
836 PM PDT Fri Sep 22 2023

.EVENING UPDATE...Quiet night in store for the forecast area as we
find ourselves in a dry slot wedged between two upper-level lows
to our east and west. As a result, mostly clear skies prevail
according to latest satellite imagery. The system to our west will
lift and skirt us just to our north overnight, causing some mid-
level clouds to build, but otherwise expecting a dry forecast
until Sunday, when the weather pattern shifts to a robust
SW/atmospheric river pattern that will keep us cool and damp for
much of the next week.

Made only minor adjustments to overnight lows and sky cover.

.AVIATION...06z TAFs...VFR conditions expected for the period.
Expect light, terrain-driven winds across all sites, save for some
locally breezy conditions at the DLS. Clear skies overnight will
give way to bkn-ovc cigs around 15 kft by Saturday afternoon.


PREV DISCUSSION... /issued 205 PM PDT Fri Sep 22 2023/

SHORT TERM...Tonight through Sunday...The closed upper level low
pressure system over SE Oregon this morning has rapidly moved east
into eastern Idaho this afternoon. Still seeing some wrap around
moisture in the form of clouds impacting eastern Washington and NE
Oregon. Otherwise, forecast area is mostly clear and cool as a
transitory ridge of high pressure builds over the region. This ridge
will move east over the northern Rockies overnight allowing a
deepening upper level trough to set up over the eastern Pacific. One
small shortwave coming through the trough will transition through
western Washington late tonight and Saturday. This will produce
precipitation mainly from the Cascades westward but very little to
none for the rest of the forecast area but it will increase cloud

The upper level trough off the coast will continue deepening as a
closed low forms in the Gulf of Alaska and takes up position a few
hundred miles off the Pac NW coast by late Sunday. This will allow
for some tropical moisture to be tapped into the southwest flow in
the form of a weak atmospheric river. This moisture will begin to
impact the forecast area in the form of a warm front lifting up the
coast late Sunday and Sunday night clipping the western portion of
the forecast area. This will increase the chance of precipitation
mainly to the western portion of the forecast area. Will also see an
increase in southerly winds associated with this frontal passage
mainly across eastern Oregon Sunday night into Monday. Temperatures
will be back to near normal over the weekend.

LONG TERM...Monday through Friday...An active early fall weather
pattern is expected next week. A large trough will be controlling
our weather with cool temperatures and a chance of light rain each

Models are in very good agreement about the trough with few
differences Monday through Tuesday night and from Wednesday onward
some differences develop as is usually the case, though changes to
the forecast don`t occur until Thursday night and Friday. The
Extreme Forecast Index mainly highlights windy conditions Monday in
central Oregon and the eastern mountains and breezy winds in general
on Tuesday. Otherwise, conditions aren`t too unusual for late
september. Cluster phase analysis shows good agreement between model
ensemble members and deterministic runs are well centered within
their respective ensemble members. Overall forecast confidence is
good then becomes normal Thursday and Friday.

On Monday, models are in agreement in having a deep trough centered
off the west coast with a deep low off of the northern tip of
Vancouver Island. The trough will be sending a front ashore and into
our area with a south to southwesterly flow aloft. Models show the
front being fed by an atmospheric river, which will bring generous
amounts of rain west of the Cascades but significant rain in our
area will be confined to the Cascade crest. Rain will be likely in
the Cascades and from central Oregon north to the Kittitas Valley
but further east will just have a slight chance to chance of rain.
Rain amounts will be up to a third of an inch in the Cascades and
generally less than a tenth of an inch elsewhere. Temperatures will
be in the mid 60s to mid 70s. In the afternoon, the front will bring
southwesterly winds increasing to 20 to 25 mph with gusts to 35 mph
central Oregon, the Grande Ronde Valley and the eastern mountains
with 10 to 20 mph winds in the rest of the area.

Tuesday will see the upper low weaken and move a few hundred miles
up the British Columbia coast while the atmospheric river weakens
and no longer affects our area. The flow over the area will turn a
little more westerly but still from the southwest. The area will
have another chance of rain though in the lower elevations it will
be barely measurable with up to a tenth of an inch in the mountains.
Temperatures will cool to the mid 60s to lower 70s with mid 50s to
mid 60s in the mountains. Breezy southwest winds at 10 to 20 mph are
expected in the afternoon.

On Tuesday night and Wednesday, models differ about the evolution of
the upper low but by the afternoon have it back it where it was
Tuesday afternoon with a similar strength. The flow over the area is
now westerly and a shortwave will move into the area. This gives us
a 40-60 percent chance of rain across the area with a few hundredths
of an inch in the lower elevations, up to a quarter inch in the
Cascades and up to a tenth of an inch in the rest of the mountains.
Temperatures will drop further to the lower to mid 60s with 50s in
the mountains.

On Thursday and Friday, the models agree in having reinforcing
energy strengthening the trough though with differences. The GFS
brings a closed low in the trough Thursday and centers the trough
over our area through Friday while the ECMWF brings the low along
the British Columbia coast and deepens the trough offshore by
Friday. The Canadian brings in a shallower trough Thursday and moves
it to the Rockies Friday. Have kept a chance of rain over the area
each day with very light rain amounts. Temperatures Thursday and
Friday will be similar to Wednesday. Perry/83


PDT  43  72  48  70 /   0   0   0  10
ALW  47  75  52  73 /   0   0   0  10
PSC  48  76  55  72 /   0   0   0  10
YKM  45  73  46  68 /   0   0  10  20
HRI  46  75  52  73 /   0   0   0  10
ELN  45  71  47  65 /   0  10  10  20
RDM  40  71  42  67 /   0   0  10  20
LGD  40  74  44  71 /   0   0   0   0
GCD  40  76  43  73 /   0   0   0   0
DLS  50  73  52  70 /   0   0  10  20




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