903
FXUS66 KPDT 232231
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
231 PM PST Sat Nov 23 2024

.SHORT TERM...THIS AFTERNOON THROUGH MONDAY NIGHT...A shortwave
trough embedded within southwest flow on the east side of a
broad, offshore closed low will combine with some weak instability
(HREF-advertised MUCAPE of 100-250 J/kg among CAMs that are
handling boundary layer moisture well) to generate showers across
the forecast area this afternoon and evening. Forecast soundings
suggest at least some potential for isolated lightning with more
robust updrafts, but have opted to omit mention from the gridded
forecast due to medium confidence in location/timing. Highest
chances of an isolated strike are 5-14% for the Blue Mountains
region where some early afternoon clearing has occurred and weak
showers are ongoing. Elsewhere, chances are very low in the
Columbia Basin and along the Blue Mountain foothills (5-10%).

Snow showers over the Blues are not expected to generate enough
accumulation for any winter highlights, but sudden drops in
visibility may occur in the more intense showers. Additionally,
1-3" of snow is forecast for the region including much of Wallowa
County.

A break in precipitation is likely for areas east of the Cascade
crest overnight into Sunday as a transient shortwave ridge tracks
overhead. Have included mention of patchy fog in the forecast for
some basin and valley locations where low-level inversions are
expected, though some mid-level cloud is anticipated overnight,
which may disrupt a more organized stratus/fog layer from
developing.

Sunday evening, ensemble and deterministic NWP guidance are in
good agreement that the remnants of the low currently offshore
near 39N, 134.6W will track onshore and facilitate some breezy
winds along the Blue Mountain foothills as surface pressure
gradients tighten. Probabilities of advisory-level wind gusts are
low (<30%) for wind-prone slopes. The same system will induce
low-level upslope flow for the east slopes of the Washington
Cascades where there is a 20-50% chance of 6" of snow below 5000
ft. Current forecast amounts are lower at 2-5", below advisory
criteria.

Precipitation chances will persist through Monday night as the
offshore closed low finally moves inland, tracking across
southwest Oregon and northwest California. Plunkett/86

&&

.LONG TERM...TUESDAY THROUGH SATURDAY...No major weather concerns
are anticipated for the long term period, including Thanksgiving.
A shortwave trough is progged by the models to track from the
northwest to the southeast on Tuesday, bringing scattered to
numerous mountain showers with snow levels around 3500 feet. There
is moderate confidence (60% confidence) that this weak and fast
moving system will do little in snow accumulations as QPF will be
less than 0.1 inch. This will be followed by a drier northwest
flow aloft Tuesday night through Thanksgiving Day. This will bring
cool days with chilly nights  nothing extreme but temperatures
falling about 2-5 degrees below seasonal average. There is only a
15-30% chance of orographic light snow along the WA Cascades and
the northern Blue Mountains during this time, so it`s looking
quite tranquil for weather on Thanksgiving Day. The one fly in the
ointment will be fog, freezing fog and low clouds that may
develop in the Columbia Basin and some of the wind protected
valleys, but confidence is relatively low (20%) as the ensembles
agree that 2m RHs will be less than 90% across the board. Since
the forecast area is on the east side of an offshore ridge and
under a northwest flow aloft, this pattern is not favorable for
widespread fog.

While most of the ensembles are in agreement with the offshore
ridge amplifying on Friday and pushing tracking eastward across
the PacNW on Saturday, there is a small percentage that retrograde
the ridge on Friday then flatten the ridge. This results in a
westerly flow aloft over the region. Differences aren`t major but
does impact the sensible weather. For now, a persistent forecast
will be shown for Saturday (Day 7). Wister/85

&&

.AVIATION...00Z TAFs...Aviation forecasting has been a bit of a
challenge due to the fog and low clouds that developed over the
Washington TAF sites as well as PDT earlier today but dissipated
with the afternoon sun.  The one exception has been YKM where low
clouds remain in the Yakima Valley. Increasing mid level clouds
will limit radiational fog tonight, and the chance for fog to
return at PDT, ALW and PSC is relatively low. However, it cannot
be ruled out as the humidity is high after last night`s rainfall.
An easterly gradient setting up along the eastern CR Gorge will
provide low level upslope and stratus clouds at DLS tonight. RDM
and BDN will remain VFR with bases varying between 5-15 kft.
Southwest winds are breezy at RDM and BDN and will decrease
overnight. Other sites will observe winds 10 kts or less.
Wister/85

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  29  49  34  48 /  20  10  20  30
ALW  34  49  35  50 /  40  10  20  40
PSC  35  46  35  46 /  10  10  20  20
YKM  28  41  29  42 /  10  40  40  30
HRI  32  49  35  47 /  10  10  20  20
ELN  26  41  30  42 /  10  50  50  40
RDM  26  46  32  46 /  20  20  20  40
LGD  26  42  32  42 /  70   0  20  50
GCD  26  44  31  46 /  40  10  40  60
DLS  33  47  37  46 /  20  40  70  70

&&

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

$$

SHORT TERM...86
LONG TERM....85
AVIATION...85