926
FXUS66 KPDT 291131
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
431 AM PDT Wed May 29 2024

.Updated Aviation Discussion.



.SHORT TERM...Today through Friday...

Bottom Line Up Front: (BLUF)

1. Breezy to windy conditions through the Cascade Gaps today.

2.Mountain rain with thunderstorms in far WA Cascades and far eastern
mountains of Wallowa County.

3. Dry and warm conditions to return tonight.

Models are in firm agreement with the upper level trough that
remains overhead. The front has moved past the region which has
allowed the winds to tapper off ever so slightly. However, much like
ahead of the front, winds will still remain elevated through this
evening becoming a bit more regular. Raw ensembles show 60% to 80%
chances of >25 mph through the Simcoe Highlands, lower Columbia
Basin of OR, foothills of the Blue Mountains as well as Kittitas
Valley. Ensembles also show 60-80% probabilities of >39 mph gusts in
the same areas. After the trough traverses the remainder of the
region, models are in firm agreement with the leading edge of an
upper level trough moving overhead. Clusters agree with the
positioning and timing of the ridge and favor the EURO ensembles
with the only variance being with the amplitude.

With the upper level trough continuing overhead, mountain rain and
isolated thunderstorms will continue to linger. Especially across
the WA Cascades around Stampede Pass and Snoqualmie and then the
far eastern mountains of Wallowa along the Snake River. Looking at
model derived soundings, instability will remain across the areas
with CAPE values lingering between 150-250 J/kg, lapse rates of 7.5
C/km and precipitable water values of near 0.50 inches. While these
ingredients are not stellar for thunderstorm development, the
orographic lift provided along the mountains will help enough that
some isolated thunderstorms cannot be ruled out. However, ensembles
show

By tonight the models show the leading edge of an upper level ridge
making its way into the region. The upper level trough will be to
the east across Idaho and western Montana. Northwesterly flow aloft
will dominate the region bringing the daily diurnal winds through
the Cascades Gaps. Dry conditions will be ahead of the upper level
ridge with warm temperatures making their way back to the region.
Temperatures today will be cooler today due to the frontal passage.
However, a warm up will begin Thursday and continue through Friday.
60% of the raw ensembles show the foothills, central and north
central OR to be in the upper 50s/low 60s and the Basin, Gorge,
Kittitas and John Day Basin in the low 70s. Temperatures will
increase each day with raw ensembles showing over 50% the majority
of the region in the mid to upper 70s. Bennese/90

.LONG TERM...Saturday through Wednesday...

Key Messages:

1. Breezy afternoon winds, peaking Saturday and Monday.

2. Afternoon thunderstorms possible Saturday and Monday.

3. Near normal temperatures through Monday before warming.

The extended period is characterized by a strong upper level low
pressure located over the Gulf of Alaska, spinning multiple
shortwave troughs into the Pacific Northwest that will lead to
breezy afternoon winds, widespread showers Sunday and Monday, and
the potential for thunderstorms along the Blue Mountains and east.
The primary concern through the period resides with breezy winds
associated with the passage of a cold front both Saturday and
Monday, as areas experiencing the highest gusts include the Simcoe
Highlands, Kittitas Valley, and portions of the lower Columbia Basin
of Oregon. Gusts of 25-35 mph are forecast on Saturday and 30-40 mph
on Monday. Confidence in these values is moderate to high (60-80%)
as the GFS suggests a pressure gradient between Portland and Spokane
of 8-9 mb on Saturday and 9-10.5 mb on Monday, which is shy of the
normal advisory threshold of 12 mb. There is slightly higher
confidence in Monday`s winds as the ECMWF EFI showcases
unclimatologically elevated winds and gusts through the Basin and
along the east slopes, with 60-80% of ensembles in agreement. In
comparison, the ECMWF EFI does not indicate any unclimatologically
high wind speeds or gusts on Saturday. The NBM is also in agreement
with higher winds Monday afternoon and early evening as the
probability of 40 mph wind gusts is a 75-95% chance over the Simcoe
Highlands, a 60-80% chance across Kittitas Valley, and a 55-75%
chance along the Lower Basin of Oregon and the northern Blue
Mountains foothills; compared to a 70-85% chance for the Simcoe
Highlands, a 55-75% chance for the Kittitas Valley, and a 50-70%
chance along the Lower Basin of Oregon and the northern Blue
Mountains Saturday afternoon and early evening. At this time, there
is low to moderate confidence (40%) in advisory level winds Monday
afternoon/evening as the NBM indicates a 30-50% chance of 50 mph
winds or greater over the Simcoe Highlands and across the Basin, and
a 10-20% across the Kittitas Valley. Breezy afternoon winds are also
anticipated Sunday and Tuesday afternoon, but gusts are only
expected to stay between 15 and 25 mph, with pockets of up to 30 mph
in elevated and exposed areas in the Simcoe Highlands, Kittitas
Valley, and Basin/foothills.

The secondary concern in the long-term period resides with
developing thunderstorms Saturday and Monday afternoon as the cold
front and associated upper level trough pass through the area. The
primary areas of concern include Wallowa County on Saturday and the
Blue Mountains, John Day-Ochoco Basin/Highlands, Grande Ronde
Valley, and Wallowa County Monday afternoon/evening. Much like with
winds, the upper level trough and cold front on Monday look
stronger. However, guidance is not in agreement with trough strength
on Monday as the GFS hints at a weaker trough and less instability
over the eastern mountains. The ECMWF indicates very little surface
based CAPE (<50 J/kg) Saturday and more substantial CAPE values (150-
350 J/kg) on Monday compared to the GFS CAPE of 100-200 J/kg
Saturday and 50-100 J/kg on Monday. Currently, more confidence is in
a weaker system on Saturday as 64% of ensembles are advertising this
outcome, which are highly favored by the ECMWF and CAN. The main
uncertainty with Monday`s system is timing, as indicated by 57% of
ensembles with the trend in a later arrival (32% versus 25%). This
is also visible when viewing the 12Z and 00Z runs of the GFS and
ECMWF, which have both slowed down about 6 hours and have
strengthened slightly. In regards to trough strength, ensembles are
still in equal disagreement with 22% hinting at a weaker shortwave
and 21% aligning with a stronger shortwave. Thus, it is expected
that the strength of Monday`s system is less likely to change in
relation to the timing of the system.

Behind the initial cold front and shortwave Saturday, flow aloft
will turn more from the southwest as the next system approaches and
drops from the parent system located in the Gulf of Alaska. This
will enhance moisture transport to lead to widespread showers
starting Sunday afternoon as the next system approaches the coast.
Rain chances will peak overnight into Monday morning before slowly
tapering off through the afternoon and evening. At this time, rain
chances peak between 5 PM Sunday and 5 PM Monday. During this
timeframe, the NBM highlights a 25-35% chance over the Lower
Columbia Basin (including Tri-Cities), Eastern Gorge, Central Oregon
(Bend/Redmond), and the Yakima/Kittitas Valleys; a 55-70% chance
across the northern Blue Mountain foothills; and a 65-85% chance at
elevation over the Cascade and Blue Mountains of 0.25 of an inch of
rain or more. As indicated in the previous section, guidance is in
disagreement with timing of this system. This leads to the
expectation that the peak timing of rain chances may arrive earlier
Sunday afternoon as observed via ensembles. This will have to be
further monitored as the event nears.

Due to the overall agreement in guidance in Saturday`s system and
more concern residing with timing associated with the second system
on Monday, confidence is moderate to high (50-70%) regarding
temperatures in the extended period. Overall, high temperatures will
stay near to slightly above normal until late in the period, with
temperatures briefly dipping below normal on Monday. The warmest
temperatures are anticipated to occur on Wednesday as an upper level
ridge builds in the wake of the departing upper level trough. There
is high confidence (80-90%) in this building ridge, as 85% of
ensembles are indicated this pattern midweek. This would lead to
highs breaking into the low to mid-80s across the Basin, northern
Blue Mountain foothills, Central Oregon, and the Yakima Valley. 75


&&

.AVIATION...12Z TAFS...VFR conditions currently at all sites, which
will stay the course through the period. The only exception is KBDN,
which is currently MVFR due to reduced ceilings of OVC030. This
should be brief as SCT-BKN cloud cover is anticipated shortly.
Current BKN-OVC ceilings of 050-080 will eventually lift Monday
evening in the wake of the exiting system. As a result of the
passing upper level trough, breezy winds will occur at all sites as
gusts of between 20-30kts are anticipated out of the west-northwest.
Winds will slacken along with breaking clouds as an upper level
ridge approaches the coast into early Thursday. 75


&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  64  38  68  43 /  10  10   0   0
ALW  66  42  70  45 /  20  20   0   0
PSC  71  44  74  48 /  20  10   0   0
YKM  68  35  73  41 /  10   0   0   0
HRI  69  41  74  45 /  10   0   0   0
ELN  62  37  68  43 /  20   0   0   0
RDM  61  32  70  38 /  10   0   0   0
LGD  60  36  65  39 /  10  10   0   0
GCD  63  35  67  40 /  10   0   0   0
DLS  66  43  73  48 /  10   0   0   0

&&

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

$$

SHORT TERM...90
LONG TERM....75
AVIATION...75