Area Forecast Discussion
Issued by NWS Pendleton, OR

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400
FXUS66 KPDT 291638
AFDPDT

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

.MORNING UPDATE...Water vapor imagery shows the upper-level trough
axis over eastern Oregon rapidly shifting east this morning. Day
Cloud Phase RGB imagery reveals some wave-drive cirrus to the lee
of the Cascade crest (mainly Washington) and FEW-BKN low-level,
orographically driven clouds over the Blue Mountains and their
adjoining foothills. As the day progresses, we are expecting
mostly shallow convection to redevelop across the region as low-
level lapse rates steepen with daytime heating. Convection will
be further aided by an approaching mid/upper-level jet and some
upper-level PVA over south-central/southeast WA and far northern
OR this afternoon as a vorticity maximum slides overhead. 12Z
HREF soundings show marginal profiles for any thunder today due to
limited vertical ascent (up to around 500 mb) of lifted parcels
in an environment characterized by maximum 2-6 km lapse rates of
6-7 C/km (per current mesoanalysis and forecast soundings) and up
to 250 J/kg surface-based CAPE (per the 12Z HREF 50th
percentile).

As far as forecast updates, we have expanded PoPs to "slight
chance" (15-24% PoPs) of showers across a broader region and added
a slight chance of thunder for areas of the foothills of the
Blues of WA and into the northern reaches of our Washington
Columbia Basin zone.

Winds are still anticipated to be breezy to windy across the
region through the day, but no highlights have been issued.
Plunkett/86

&&

.AVIATION...18Z TAFS...VFR conditions are expected to continue
over the next 24 hours. There will be some SCT-BKN CIGs through
the day and evening 050-090 mainly impacting PDT and ALW then
clouds dissipate overnight. As a result of the passing upper level
trough, breezy winds will occur at all sites with sustained winds
10-20kts and gusts around 30kts out of the west-northwest. Winds
will decrease overnight.

&&

.PREV DISCUSSION... /issued 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

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  64  40  68  42 /  20  20   0   0
ALW  66  43  70  45 /  20  20   0   0
PSC  71  45  74  48 /  20  20   0   0
YKM  68  37  73  41 /  10   0   0   0
HRI  69  43  74  45 /  20  20   0   0
ELN  62  40  68  41 /  20   0   0   0
RDM  61  33  69  39 /   0   0   0   0
LGD  60  37  65  39 /  20  10   0   0
GCD  63  36  67  40 /  10   0   0   0
DLS  66  43  73  47 /  10   0   0   0

&&

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

$$

MORNING UPDATE...86
SHORT TERM...90
LONG TERM....75
AVIATION...91