831
FXUS66 KPDT 050505
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
1005 PM PDT Sat May 4 2024

Updated Aviation Discussion

.AVIATION...06z TAFs...Light to moderate rain showers will impact
all sites through the period, leading to MVFR and even IFR
conditions at times as showers lead to reduced cigs and even
lowered vsby`s at times. The bulk of the rain associated with this
ongoing weather system is expected to occur during the overnight
hours, with locally heavy showers possible for PDT and ALW around
sunrise Sunday. Elsewhere, showers will be of a lighter intensity,
however cigs for most of the period are expected to be bkn-ovc
between 2 and 7 kft. Rain chances will end earliest across sites
near the Cascades, spreading eastward throughout the period,
before rain is largely expected to end for all sites by nightfall
Sunday night. Winds will remain gusty at times as well, peaking
during the day Sunday as many sites see westerly gusts up to 35
kts, especially DLS and PDT. Evans/74

&&

.PREV DISCUSSION... /issued 854 PM PDT Sat May 4 2024/

EVENING UPDATE...A wet and windy 24 hours is in store for the
forecast area as a deep low pressure system continues to move
onshore right around the Oregon/California coastline. The main
band of wraparound moisture has been late to materialize based on
what models originally foresaw, but latest radar imagery this
evening shows the band finally starting to develop, with much of
the forecast area expected to be enveloped with rain by midnight
tonight, gradually spreading northward as we head into tomorrow
morning.

Latest HREF has come in a bit wetter than previous runs, with the
latest CAMs putting quite the bullseye over the Oregon Basin early
tomorrow morning. NBM QPF is not quite as bullish, but this wrap
around banding does have the potential to produce pretty
respectful amounts of moisture potentially exceeding half an inch
by tomorrow afternoon in the Oregon Basin. Whether that actually
materializes is another question, as CAMs have been a bit off with
regards to the start of rainfall, but did update QPF based on
latest HREF as well as recent trends.

Overall messaging with the winds and mountain snow remains the
same. Webcams show accumulating snow starting to occur over the
Oregon Cascade passes, with snow expected to spread into the John
Day/Ochoco Highlands as this oncoming low pulls in colder air into
the forecast area. Wind gusts also look fairly consistent compared
to the latest HREF. Overall changes to the forecast package
focused primarily on updating PoPs to reflect the drier-than-
anticipated conditions this evening, but that is expected to
change quickly as this band of wraparound moisture is expected to
materialize overnight. Evans/74

PREV DISCUSSION... /issued 443 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

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  39  50  40  56 / 100  90  70  60
ALW  44  52  44  58 /  90  80  80  60
PSC  47  59  47  65 /  80  90  40  20
YKM  44  61  39  63 /  40  60  10  20
HRI  42  56  43  62 / 100  90  40  40
ELN  45  58  40  60 /  40  40  20  20
RDM  33  49  34  52 /  70  70  10  60
LGD  39  48  38  52 / 100  80  90  80
GCD  34  44  35  50 /  90 100  70  90
DLS  45  57  46  57 /  90  70  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 until 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...74
LONG TERM....86
AVIATION...74