065
FXUS66 KPDT 042141
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
241 PM PDT Sat May 4 2024

.SHORT TERM...Tonight through Monday night...A deep low currently
centered about 130 miles west of Coos Bay has brought widespread
overcast skies and a band of light precipitation over south
central WA and north central OR since early this morning. This
weak deformation band is only a precursor to a stronger band
from the Cascades westward where numerous rainfall amounts of 1-2
inches have been observed over the past 24 hours. This is all
associated with an AR in the range of 500-600 kg m-1 s-1 aimed
directly over the OR Cascades. Areas east of the Cascades will not
observe as much precipitation as this band tracks to the east,
but this is definitely one to bring above average rainfall to the
area. For the next 24 hours, many areas will have 0.25-0.75" and
locally around 1". The east slopes of the OR Cascades have already
received 0.25-0.5" already, and the Mt. Wilson RAWS site in Wasco
County measured 0.78".

A Winter Weather Advisory is in effect for the Cascade east
slopes of OR above 4000 feet (public zone OR509) through 11 AM
Sunday. Based on current temperatures and dewpoints, the snow
levels in OR509 are around 5000 feet--a little higher than
previously expected. However, temperatures at Santiam Pass are in
the mid 30s and precipitation will soon change over the snow as
snow levels fall to around 3500 feet tonight. As the low travels
to the ESE, colder air behind the system will cause snow levels to
lower to around 3500-4500 feet over the southern Blues (public
zone OR503) and the John Day- Ochoco Highlands (public zone
OR506). A Winter Weather Advisory is also in effect for OR506
above 4000 feet tonight through 11 PM Sunday. OR503 will have snow
accumulations around 3-6 inches, but mainly along the higher
peaks. The only location along HWY395 with several inches of snow
in the forecast is Long Creek Mountain. The northern Blue Mtns
will also have around 3-6 inches of snow, but mainly above 5000
feet which includes Tollgate. There are no plans to issue any
Winter Weather Advisories for the Blue Mtns. It`s important to
note that the probability progs from the NBM and HREF use a 10:1
ratio which will indicate higher than the current forecast, as
10:1 is a little too high with our warm temperatures we have
observed today.

The upper low will weaken as it travels east across southern OR
and NV and into southern ID tomorrow. This will be followed by a
northwest flow aloft Sunday night. Precipitation will decrease
during this time, but numerous snow and rain showers will continue
over the mountains and valleys. Snow levels will be around
3500-4500 feet. A second system will follow as a shortwave trough
on Monday, keeping snow levels down around 4000 feet along with
light orographic showers. Even though QPF and snow amounts will be
light, snow covered roads in early May will catch some unprepared
travelers by surprise and this will be addressed the next couple
of days. The air mass will be marginally unstable, and an isolated
thunderstorm cannot be ruled out, mainly over southeast OR.

Not only will forecasters focus on precipitation amounts and snow,
but wind is another concern. A Wind Advisory is in effect Monday
for many areas across south central WA and north central OR.
Surface gradients have been tight today, and the increasing winds
aloft and tighter gradients on Sunday will result in very windy
conditions gusting to 45-50 mph. Confidence in the wind advisory
is around 80% but warning criteria (i.e. 58 mph or stronger gusts)
is low (20%). Wister/85

.LONG TERM...Tuesday through Saturday...Cool, breezy, and showery
weather is forecast on Tuesday with a significant warming and drying
trend through the remainder of the period, especially Thursday
through Saturday.

Synoptic Overview: The period will start out on Tuesday with a broad
upper-level closed low downstream over the Great Plains and an upper-
level ridge building in upstream offshore in the Pacific. This will
place the forecast area under a cool northwest flow with light rain
and snow showers forecast for the mountains. Wednesday, flow aloft
turns north to northeast, and any lingering shower activity will
likely be confined to the northeast mountains, mainly in Wallowa
County. Thursday, ensemble guidance is in excellent agreement that
the low downstream over the Great Plains will eject eastward with a
piece of shortwave energy likely (>80% chance) facilitating the
formation of another closed low over the Great Basin into
California. Ensemble clusters show some spread in solutions
regarding the development and evolution of the closed low over
California, primarily with regard to the magnitude of the 500-mb
heights over northern California and southern Oregon. Pattern
details aside, all clusters are advertising some flavor of a Rex
block pattern setting up Thursday and persisting through Friday
(>80% chance) or Saturday (75-80% chance). This will favor a warming
trend as the upper-level ridge/high sets up over the PacNW to the
north of the closed low. There is a low chance (22% of members) that
the closed low will migrate far enough north over NW CA/SE OR that
showers will develop on Saturday over the mountains (mainly the
Blues.

Regarding the chances of any watches, warnings, or advisories, the
NBM is suggesting a 60-95% chance of advisory-level winds Tuesday
for the Kittitas Valley, Simcoe Highlands, Columbia Gorge, foothills
of the Blue Mountains of Oregon, and portions of the lower Columbia
Basin of Oregon.

The probability of maximum temperatures exceeding 80 degrees
increases Thursday, Friday, and Saturday (peaking Saturday at 60-90%
chance for the Columbia Basin and adjacent valleys and foothills).
Moreover, the Columbia Basin and Yakima Valley have a 45-65% chance
of highs exceeding 85 degrees on Saturday, and there is a very low
chance of exceeding 90 degrees (<15% chance). While NWS HeatRisk
values increase Thursday, Friday, and Saturday, they are currently
forecast to stay at a category 1 (minor). This level of heat
primarily affects individuals extremely sensitive to heat,
especially when outdoors without effective cooling and/or adequate
hydration. The chance of any heat-related highlights is extremely
low (<5% chance). Plunkett/86


&&

.AVIATION...18Z TAFs (previous discussion)...A deep upper-level
closed low, currently centered offshore, will continue to
influence the weather at all sites through the TAF period as it
tracks southeast across SW Oregon and NW California. Confidence is
high (80-90% chance) in at least temporary MVFR conditions at
BDN/RDM due to reduced VSBYs and/or CIGs through the period.
Elsewhere, chances of MVFR conditions ramp up to 50-60% this
evening into Sunday morning as rain becomes more widespread. Due
to the location of a surface low in eastern Oregon, wind direction
and magnitude will vary across TAF sites through the morning
before switching to southwesterly to westerly at 10-20 kts
sustained with gusts of 15-35 kts by late afternoon and evening
for all sites. Plunkett/86


&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  39  50  40  56 / 100  90  70  60
ALW  44  52  44  58 / 100 100  80  60
PSC  47  59  47  65 / 100  90  40  20
YKM  44  61  39  63 /  90  70  10  20
HRI  42  56  43  62 / 100  90  40  40
ELN  45  58  40  60 /  60  60  20  20
RDM  33  49  34  52 /  70  60  10  60
LGD  39  48  38  52 / 100  80  90  80
GCD  34  44  35  50 / 100  90  70  90
DLS  45  57  46  57 /  80  60  30  50

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Wind Advisory from 2 AM to 11 PM PDT Sunday for ORZ041-044-507-
     508-510.

     Winter Weather Advisory from 5 PM this afternoon to 11 PM PDT
     Sunday for ORZ506.

      Winter Weather Advisory until 11 AM PDT Sunday for ORZ509.

WA...Wind Advisory from 2 AM to 11 PM PDT Sunday for WAZ024-521.

&&

$$

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