777
FXUS66 KPDT 250946
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
246 AM PDT Tue Jun 25 2024

.SHORT TERM...Today through Thursday night...Weather concerns for
the short term period revolve around a developing low pressure
trough off the eastern Pacific that will be pushing eastward and
increasing the instability and moisture over the area. Winds will
also increase ahead and behind the front, although winds will not
be particularly strong (confidence 80%). The trough will track
across WA/OR on Thurs bringing cool temperatures and mountain
showers. Overall confidence in the short term forecast is
moderate, as uncertainty lies in the convective activity on Wed.
Latest models are trending stronger with the approaching trough
and embedded shortwaves associated with it.

Today will be a hot and dry day with the HeatRisk in the minor to
moderate category. A large upper level ridge over the four corner
states has amplified over eastern WA/OR. The inverted surface
thermal trough has strengthened as well with 850mb temps rising to
around 20-25C by the afternoon. It is highly likely for many of
our lower elevations to observe mid-90s today and a few spots in
the John Day Basin--Monument and Spray--are forecast to observe
100. The ridge will shift eastward tonight as a deep upper low
approaches the coast. There is increasing instability aloft over
southeast Oregon, and latest models are showing signs of a weak
embedded disturbance that will bring a chance (30%) of high based
thunderstorms south and east of John Day after midnight.

As stated previously, the low is coming in stronger and is even
showing signs of a negative tilt as it moves inland tomorrow.
Confidence is high that showers and thunderstorms will develop as
early as sunrise with the threat continuing through the day, but
the questions remain where will convection likely occur, will
storms be strong or severe, and how much precipitation to expect.
Fortunately, the CAMs now run as far out as Wed afternoon and have
proven helpful on the details. The areas with the best chance of
showers and thunderstorms will be along the east slopes of the WA
Cascades, the Blue Mtns, and the Wallowas where PoPs are 30-60%.
There is a slight chance (10-20%) of showers and thunderstorms
elsewhere. PWATs increase from around 0.7-0.8 in the morning to a
little over an inch during the day, therefore wetting rain can be
expected. NBM and HREF means probability of 0.1" over the Blues
and Wallowas as well as the east slopes of the WA Cascades north
of Mt Adams ranges 30-60%, and the probability of 0.25" or more in
these same areas is around 20%. SBCAPES have increased to 300-450
J/kg which isn`t significant but has doubled from the CAPEs in
previous model runs. 0-6km shear is strong--ranging from 45-60
kts. This will mean that some storms will be strong with
sustained updrafts and will be capable of heavy downpours, hail
and strong outflows. Wallowa and Union Counties are shown to have
the highest CAPES and impressive bulk shear. Meanwhile, a tight
pressure gradient will set up with a marine push increasing winds
across the Columbia Basin and surrounding areas. The eastern CR
Gorge will be windy on Wed. RHs will be in the 20s and 30s for
most of the forecast which is not critically low to warrant fire
weather highlights.

The marine push will help to stabilize the atmosphere so that
Thurs will be more showery rather than convective due to cool air
at the surface. It will be breezy to windy, however. The
Cascades, mainly over WA, will have numerous showers (60-70%
chance) while showers over the northeast mountains will be widely
scattered (20-30% chance). The upper trough will arrive into ID
Thurs night followed by a drier northwest flow aloft. Wister/85


.LONG TERM...Thursday through Tuesday...The long term starts off
quiet, but by the weekend we should see a bit more active weather
including both the return of breezy winds as well as some lower
elevation rain and mountain thunderstorms while temperatures
generally hover around normal, varying by a few degrees each day.

The overall pattern should be fairly simple, as we see transient
ridging quickly moving across Friday into Saturday. Then by
Saturday evening, a deep trough will move inland, keeping the
weather active during the remainder of the weekend into early next
week. Thereafter, we should remain in between separate weather
features, with a deep ridge offshore of California and troughing
to our east, causing zonal to northwesterly flow through the end
of the long term.

In regards to sensible weather, any concerns with temperatures
should be fairly nil as highs remain mostly near normal through
the period. This should mean a smattering of upper 70`s to low
80`s in Central Oregon, with more widespread low to upper 80`s
for the remainder of our population centers. The warmest day is
expected to be Saturday during the last day of the transient
ridge, where the NBM shows a a 20-40% chance of seeing highs of or
above 90 degrees for the Columbia Basin of Washington and down to
the Foothills of the Blues including Walla Walla and Pendleton.
Otherwise the passing of the trough over the weekend and then the
westerly to northwesterly flow thereafter should help to keep us
temperate and mostly near normal.

With the passing trough, two concerns should occur with regards
to breezy winds and showers/thunderstorms. For winds, breezy
conditions can be expected both Sunday and again Monday. The NBM
indicates around a 20-70% probability at gusts of 45+ mph for our
usual breezy locations (Lower Columbia Basin/Foothills of the
Blues, Simcoe Highlands, and Kittitas Valley) for both days but
highest chances on Monday in the post-frontal environment. This
system will also be conducive of precipitation, with PWAT values
on Sunday around 100-150% of normal according to ECMWF ensembles.
This combined with modest instability of around 300-600 J/kg in
the eastern mountains of Oregon should promote thunderstorms for
this region, while much of the rest of the CWA has a chance at a
few showers. QPF amounts should still remain on the low side
though, with the aforementioned eastern mountains of OR the only
area likely to see more noteworthy amounts as the NBM expects a
20-40% probability of 12-hour QPF amounts 0.1 inch or more. Then
through early next week, concerns for precipitation are minimal to
nil, with the expected flow pattern causing rain shadowing from
the Cascades, possibly allowing for some isolated activity into
the eastern mountains. Goatley/87

&&

.AVIATION...12Z TAFs...VFR conditions expected over the next 24
hours. Upper level cloud deck moves in across the region tomorrow
morning at FEW to BKN. Winds 10 knots or less all sites except
BDN/RDM, where a few breezes 15-20 knots may be possible this
afternoon. Goatley/87


&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  91  61  86  53 /   0  10  20  10
ALW  95  63  91  57 /   0  10  20  10
PSC  95  64  93  59 /   0  10  20   0
YKM  91  60  88  52 /   0   0  20  10
HRI  95  63  90  58 /   0  10  20   0
ELN  90  60  82  53 /   0   0  20  20
RDM  92  56  83  47 /   0  10  10   0
LGD  90  60  86  53 /   0  10  30  20
GCD  95  60  90  52 /   0  10  20   0
DLS  94  63  81  58 /   0  10  20   0

&&

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

$$

SHORT TERM...85
LONG TERM....87
AVIATION...87