301
FXUS66 KPDT 032130
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
230 PM PDT Mon Jun 3 2024

.SHORT TERM...Tonight through Wednesday...Radar imagery this
afternoon shows showers mainly across the WA Cascade crest, while
water vapor satellite imagery shows an upper level trough moving
across the PacNW. At the surface, much of the forecast area are
currently under winds of 15-30 mph, with gusts 30-50mph.

By this evening, the upper trough will have exit into ID and
western MT, placing the PacNw under a west northwest flow aloft.
While much of the forecast area will remain dry, the flow aloft
will result in upslope showers across the Cascades continuing
through tonight. Surface winds will also decrease this evening
behind the upper trough exit, with light to locally breezy winds
overnight.

Tomorrow morning, an upper low in the northeast Pacific will
swing a shortwave coupled with a weak surface cold front across
the PacNW. This will result in an increase in shower activity
across the Cascades early tomorrow morning, while light to
moderate rain showers associated with the frontal passage will
develop across portions of the eastern mountains and eastern
portions of the Columbia Basin. Rain accumulations will be
heaviest across the Cascade crest/peaks as well as the northern
Blues, where 0.25-0.75 inches is anticipated. Along the foothills
and eastern portions of the Basin, expect up to 0.1 inches with
any showers that pass overhead. The cold front and shortwave
passage will also result in pressure gradients tightening again
across the intermountain PacNW, however, deterministic guidance
shows the pressure gradient between Portland and Spokane only
peaking up to 9mb. This would translate to winds mainly 15-25mph
with gusts up to 40mph across central and north central OR, the eastern
Columbia River Gorge, and the Columbia Basin and adjacent
foothills/valleys. The breezy winds and mountain showers will
persist into the evening, but will quickly diminish overnight.

By Wednesday morning, dry conditions and light winds will return
to the PacNW as a broad upper level ridge builds in from the
Desert Southwest. Afternoon temperatures will be on the increase
as well for Wednesday, with a return to upper 70s and low 80s
across the lower elevations. Lawhorn/82


.LONG TERM...Thursday through Monday...Models are in overall good
agreement in the weather pattern in the long term period though
small differences initially become larger next weekend into early
next week and reduce confidence in temperatures and the possibility
of thunderstorms Sunday and Monday. Model ensemble cluster analysis
shows excellent agreement Thursday in having a ridge over the
Rockies and a southwest flow over our area. Friday is similar though
models begin to have differences in the strength of the ridge which
would affect the warming temperatures slightly. On Saturday, the
model strength differences increase due to uncertainty about troughs
in the Gulf of Alaska and from Hudson Bay down into the eastern
CONUS, though overall 500 MB heights are higher and temperatures
should peak that day. On Sunday, models bring a trough towards the
west coast with widely varying strengths for the trough. This does
reduce the influence of the Rocky Mountain ridge over our area and
should cool temperatures. Models then differ greatly on Monday with
the trough becoming a barely noticeable feature as most of the
energy goes far to our south and forms a cut off low off of Southern
California or Baja California. 55 percent of the models have the
weak feature over or area and the other 45 percent have a weak ridge
offshore with flow mainly westerly in most of the model clusters.
The main concerns in the long term period are the building heat
Thursday into Saturday and then how temperatures begin to cool
Sunday and Monday and also the extent of showers and thunderstorms
Friday afternoon onward. Overall forecast confidence starts out good
and becomes normal by Sunday and then below normal on Monday.

Thursday will be a sunny day with temperatures warming 7-9 degrees
in general, though only around 5 degrees in central Oregon. This
will give us highs in the mid to upper 80s in the lower elevations
and in the 70s and lower 80s in the mountains. This pattern will
continue on Friday with highs in the Columbia Basin warming another
5 or so degrees and about 3 degrees in the rest of the area. This
will warm the Columbia Basin to the lower to mid 90s and the rest of
the area will be in the upper 70s and 80s. The Columbia Basin and
Columbia Gorge have a 60-85 percent chance of reaching 90 degrees
while the Blue Mountain Foothills and central Oregon have a 35-45
percent chance of reaching 90 degrees. The southwest flow aloft will
bring some moisture into the area and heat will provide the
instability for a slight chance of late afternoon thunderstorms over
the Ochoco-John Day Highlands and the John Day Basin.

On Saturday, with the ridge at its strongest, the Columbia Basin
will warm a couple more degrees while elsewhere, increasing cloud
cover will cool temperatures elsewhere a couple of degrees. While
chance of reaching 90 degrees in the Columbia Basin remains at 85-
100 percent and at 40-60 percent in the Blue Mountain Foothills,
chances of reaching 100 degrees are 20-25 percent in the Tri-Cities
and Hermiston and 10 percent or less elsewhere. Increasing moisture
and instability will bring a chance of rain showers and a slight
chance of thunderstorms to the eastern mountains and the central
Oregon Cascades with a slight chance of rain showers along the rest
of the Cascade crest and the Blue Mountain Foothills.

On Sunday, the ridge weakens and with a trough offshore, models
bring a marine push through the Cascade gaps with the Columbia Gorge
and Kittitas Valleys having westerly winds of 15 to 25 mph in the
afternoon. This cools temperatures 5 to 8 degrees to the 80s with
mainly 70s in the mountains. This will also lead to another slight
chance of showers and thunderstorms mainly in the eastern mountains.

Conditions become less certain on Monday, so have gone with
temperatures a degree or so cooler than Sunday and once again have
afternoon 15 to 25 mph winds in the Columbia Gorge and Kittitas
Valley. Have also continued a slight chance of afternoon showers in
the Northern Blue Mountains and Wallowa County. Perry/83


&&

.AVIATION...Previous Discussion...18Z TAFs...Rain showers have
moved into the eastern mountains this morning, so do not
anticipate showers at any TAF site this afternoon. SCT-BKN skies
at 050-090 will become FEW-SCT this afternoon. After 00Z, a weak
disturbance will move over the area with skies becoming BKN-OVC at
035-060. Showers are possible after 06Z, but have limited mention
of -SHRA to KDLS, KPDT and KALW after 08Z-10Z. Have used periods
of VCSH at other TAF sites due to low confidence. Strong west to
southwest winds are expected to continue at 15-30 kts with gusts
to 40 kts through this afternoon before decreasing after 03Z.
Winds will begin increasing again at KRDM and KBDN after 15Z
tomorrow morning. Perry/83


&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  48  71  49  75 /  30  20   0   0
ALW  51  71  51  77 /  30  40   0   0
PSC  55  75  52  79 /  20  20   0   0
YKM  48  73  44  77 /  20  30   0   0
HRI  52  75  51  80 /  30  20   0   0
ELN  44  68  43  74 /  20  30   0   0
RDM  46  73  45  79 /  10  10   0   0
LGD  46  71  48  76 /  30  40  10   0
GCD  47  76  47  81 /  20  10   0   0
DLS  52  70  52  78 /  40  40  10   0

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Wind Advisory until 11 PM PDT this evening for ORZ041-044-507-
     508-510.

WA...Wind Advisory until 11 PM PDT this evening for WAZ024-026>029-
     521.

&&

$$

SHORT TERM...82
LONG TERM....83
AVIATION...83