825
FXUS66 KPDT 260350
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
850 PM PDT Sat May 25 2024

.SHORT TERM UPDATE...A strong northwest flow aloft and a tight
pressure gradient will gradually weaken and relax through the
night. Surface winds are gusting to 25-35 mph this evening...a
little longer than currently in the forecast. The breezy winds in
the forecast were extended up to 06Z before decreasing. No
precipitation is occurring in the forecast area, although WSR-88Ds
are picking up on a few weak returns over the central WA Cascades.

The wind advisory for the Kittitas Valley will be allowed to
expire at 11 AM. Winds are just under advisory criteria in this
area now.

The flow will become more anticyclonic with the building upper
level ridge on Sunday for continued dry and warmer conditions.
There will be a 8-12 degree increase in afternoon temperatures on
Sunday compared to today. The warming trend continues into
Memorial Day with afternoon temperatures climbing to above
seasonal average. One minor change to the forecast was to expand the
areal coverage of thunderstorms on Memorial Day. The update was
made to give heads up for the potential (20-25%) chance in the
eastern mountains at the end of the holiday weekend. The upper
level ridge will shift eastward, and the upper flow will increase
from the southwest. Meanwhile, the inverted surface thermal trough
will also shift eastward. This will steepen lapse rates and the
increasing mid-level moisture will bring at least a slight chance
of thunderstorms. Wister/85



&&

.PREV DISCUSSION... /issued 445 PM PDT Sat May 25 2024/

SHORT TERM...This afternoon through Monday night...Isolated
light showers will taper off over the Blue Mountains this
afternoon and evening, followed by mostly dry conditions as a
weak shortwave ridge moves overhead tonight through Monday
morning. Any light precipitation that develops overnight through
Sunday should remain pinned to the Washington Cascade crest as
orographic lift wrings some moisture out of the entrenched marine
layer.

Breezy to windy westerly winds have indeed materialized this
afternoon across climatologically windy regions such as the
Kittitas Valley, lower Columbia Basin of Oregon, eastern Columbia
River Gorge, and foothills of the Blue Mountains. While winds
reaching sustained advisory criteria are not anticipated (outside
of the Kittitas Valley), confidence is high (70-90% chance) that
limited portions of the eastern Columbia River Gorge, lower
Columbia Basin, and foothills of the Blue Mountains will see winds
of 25-35 mph with gusts of 40-45 mph this afternoon and evening.
In the Kittitas Valley, winds have been oscillating around
advisory criteria all morning and early afternoon, primarily with
respect to sustained winds (25-35 mph) as gusts above 45 mph have
been brief. Will maintain the Wind Advisory since rationale from
the morning update still appears valid -- in fact, cross-Cascade
pressure gradients have strengthened to 5-5.5 mb.

Sunday through Monday, mostly quiet weather, albeit with a
warming trend, is forecast. Winds will slacken compared to today
with just localized breeziness.

Monday evening into Monday night, as flow aloft turns
southwesterly ahead of an approaching offshore trough, guidance
is advertising a surge of moist, slightly unstable, mid-level air
into eastern Oregon and southeast Washington. While synoptic lift
appears weak, indicators for elevated instability and shower
development look good -- negative mid-level theta-e lapse rates
overlap an RH field in excess of 70% in the vicinity of the right
entrance region of a 300-200 mb jet. At minimum, expecting the
formation of an altocumulus field; at most, some isolated
thunderstorms may develop along with scattered rain showers
(10-20% chance). Plunkett/86

LONG TERM...Tuesday through Saturday...Models in good agreement
at the onset of the forecast period, with a system moving into
the PacNW Tuesday afternoon/evening. Moisture over the area will
produce light showers over the Blue Mountains ahead of this
arriving trough. As the trough axis advances over the west coast
on Tuesday afternoon, the Cascade Mountains will see an increasing
opportunity (40-70% chance) for high elevation showers. While most
will be rainfall, some mixed precipitation will be possible in
the cooler pockets.

The advancement of the trough across the forecast area on
Wednesday will continue the threat for high elevation
precipitation, with moisture also developing over the areas of
central Oregon. Confidence is not as significant with these
showers, with around a 10-20% chance of development on Wednesday
afternoon, decreasing through the evening. The departure of the
trough will bring a drier west to northwest flow across the area
late Wednesday. Through this point, models remain in generally
decent agreement, with cluster analysis demonstrating only minor
variations. Cooling temperatures noted with this set-up, with
values dropping 5-10 degrees between Tuesday and Wednesday. The
level to which these resultant temperatures drop below normal
overall mirrors their value drop (5-10 degrees below normal).

Models begin to diverge Thursday, with disagreements becoming
apparent in cluster analysis quickly. From the numerical guidance
perspective, there are two scenarios possible for the end of the
week. The first is a drier northwest flow persisting to keep
showers  generally at bay. The second is a trough and generally
northerly flow into Friday with some lingering moisture. The NBM
base of the forecast puts a lean on the slightly more active
scenario, with showers lingering over the high terrain. Regardless
of solution, a warming trend begins on Thursday.

By Friday, however, models come into better alignment with an
amplifying ridge. The degree of amplification does vary between
solutions, along with the pace of progression of the ridge through
the forecast area. The next system will advance in/towards the
PacNW on Saturday. The arrival time of the system again varies
with the pace of the departing ridge, with limited solutions
bringing precipitation into the forecast area. Current forecast
has a slight chance (10-20%) of showers over the higher peaks on
Saturday, with otherwise dry conditions and continued warming
temperatures. Branham/76

AVIATION...00Z TAFs...Winds remain the primary aviation concern
this evening, but winds will decrease overnight.  Westerly winds
sustained at 15-25 kt gusting to 30-40 kt will decrease to 8-15kt
before midnight. Skies are SCT-BKN between 5-10 kft with higher
clouds above with some clearing overnight. Wister/85

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  40  71  45  78 /   0   0   0  10
ALW  44  73  49  81 /   0   0   0   0
PSC  46  76  51  83 /   0   0   0   0
YKM  40  73  43  79 /   0   0   0   0
HRI  44  76  48  82 /   0   0   0   0
ELN  43  68  44  76 /   0   0   0   0
RDM  33  72  40  79 /   0   0   0   0
LGD  36  69  43  78 /   0   0   0  10
GCD  35  71  43  81 /   0   0   0  10
DLS  46  74  49  79 /   0   0   0   0

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...Wind Advisory until 11 PM PDT this evening for WAZ026.

&&

$$

SHORT TERM...86/85
LONG TERM....76
AVIATION...85