764
FXUS66 KPDT 012146
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
246 PM PDT Sat Jun 1 2024

.SHORT TERM...Tonight through Monday...Satellite and radar imagery
this afternoon show scattered cloud cover across the forecast
area, with the central WA Cascades reporting any precip in the
last 3 hours. While there are light radar returns moving across
the area, the surface has been too dry for any precipitation to
reach the ground.

Through this evening, a weak shortwave trough and attendant
surface cold front will continue to push east across the PacNW,
producing light mountain showers and breezy conditions through the
lower elevations. A brief dry spell with light winds will follow
the exit of this coupled system tonight, though cloud cover will
increase in the pre-dawn hours tomorrow.

Active weather once again returns by mid tomorrow morning as an
upper trough with leading frontal boundaries pushes into the PacNW
from the northeast Pacific. Rain bands will spread across eastern
WA/OR throughout Sunday and continue well through Monday morning
as the warm and cold fronts sweep across the region. These showers
will also be supported by an accompanying Atmospheric River that
will bring PWATS between 0.75-1.25 inches (around 200% of normal)
across the region, with GFS and ECMWF ensemble mean IVT values
between 500-750 kg/m*s. This will translate to rainfall
accumulations between 1.25-1.75 inches across the Northern Blues,
Cascade crest and upper slopes, and the Wallowas from 5AM Sunday
to 11AM Monday; 1 to 1.25 along the Cascade lower slopes and
immediate foothill locations along the Northern Blues; 0.5 to 1
inches across the northern Blue mountain foothills, Wallowa/Grande
Ronde valleys, and portions of the southern Blues; 0.1 to 0.25
inches across the remainder of the areas with locally higher
amounts approaching the Cascade east slopes and the foothills of
the Blues (confidence 60-80%). While rainfall may be heavy at
times across the mountain locations, area rivers and streams have
been low enough to abate the concern for flooding, however,
smaller streams and areas of poor drainage could see localized
flooding across portions of the eastern mountains and Wallowa
county. There will also be potential for isolated thunderstorms to
develop across the eastern mountains and Wallowa county Monday,
with GFS and NAM guidance showing most unstable CAPE between
250-750 J/kg and 0-3km shear between 30-50kts over these areas.
The main threats with any developing cells will be heavy rainfall,
small hail, gusty winds, and frequent lightning.

Besides rainfall, the cold front boundary and subsequent
trough passage Monday will also tighten surface pressure gradients
along the Cascades. This will result in windy to gusty conditions
developing through the Cascade gaps and into the lower elevations
late Monday morning through the evening. Sustained winds of
20-30 mph and gusts up to 45mph will be possible across the lower
elevations, with higher values across the Simcoe Highlands, OR
Columbia Basin, and the Kittitas valley. NBM currently shows a
80-95% 24-hr probability of max sustained winds reaching 30 mph
and gusts reaching 45 mph across the OR Columbia Basin, Simcoe
Highlands, and portions of the Kittitas valley. While
probabilities are high, will wait for high-res guidance before
potentially issuing wind advisories in the aforementioned areas.
Lawhorn/82


.LONG TERM...Tuesday through Saturday...A period of weather more
akin to summer looks to be in store for the region later this work
week, however model discrepancies continue to hinder forecast
confidence. Guidance consistently depicts a broad ridge establishing
itself over the PacNW by the midweek, but where this ridge axis
ultimately centers itself remains up in the air, as just a slight
eastward shift in its position would invite SW flow aloft into the
forecast area, and thus a cooler temperature forecast as well as a
chance for thunderstorms.

The long-advertised Atmospheric River that will make for a wet
forecast in the short term will start to relent early Tuesday,
before high pressure slides in by Wednesday. Even this close in the
long term, however, the deterministic GFS and ECMWF diverge on how
the high pressure ridge establishes itself, with the GFS favoring a
more negative tilt and thus more SW flow over the forecast area,
while the ECMWF has a bit of a flatter ridge, which would lead to a
warmer forecast. Ensembles seem to lean more towards the ECMWF
solution, but confidence levels on the temp forecast are around
moderate (50-60%) based on deterministic model differences. NBM
probabilistic remains bullish on the potential for warm temps by the
end of the work week, broadly depicting a 60-80% chance for highs
above 90 on Friday for our lower elevation zones, but that could
very well chance should the GFS solution start to win over in the
forecast over the coming days.

Forecast confidence is low (<30%) after Friday, as guidance exhibits
wide variability. Ensemble clusters waver between enhanced SW flow
and continued ridging. The former solution is what the NBM is
currently leaning towards, which would result in shower and
thunderstorm chances across the forecast area, while the latter
would mean more heat for the forecast area. A bit of a tricky
forecast thus far, but fortunately precip chances look to be limited
until at least next weekend. Evans/74


&&

.AVIATION...18z TAFs (Previous Discussion)...VFR conditions expected
today, with bkn-ovc cloud cover prevailing at most sites at around
10-15 kft. Winds will be moderately breezy at some sites, namely DLS
and PDT, with gusts approaching the 25-30 kt range at times this
afternoon, with all other sites at around 10-15 kts. Direction will
generally be W/NW. A weather system will then start to move in
towards the very end of the TAF period, threatening sites with rain
by late tomorrow morning. Evans/74


&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  52  69  54  66 /  10  60 100  80
ALW  55  71  57  68 /  10  60 100  90
PSC  58  71  59  73 /  10  60  80  50
YKM  51  68  50  69 /  10  70  80  40
HRI  56  72  56  71 /  10  50  70  60
ELN  49  65  50  65 /  10  70  80  50
RDM  46  68  53  63 /  10  40  70  50
LGD  47  69  54  63 /  10  40 100 100
GCD  46  71  55  66 /  10  30 100  90
DLS  56  67  58  66 /  10  70  90  60

&&

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

$$

SHORT TERM...82
LONG TERM....74
AVIATION...74