Area Forecast Discussion
Issued by NWS Pendleton, OR

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000
FXUS66 KPDT 190533
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
1033 PM PDT Wed May 18 2022

Updated Aviation Discussion

EVENING UPDATE...The first in a series of low pressure systems
has exit the region this evening with shower activity having
diminished outside of the central and northern WA Cascades.
Westerly winds have also begun to weaken across the region as we
approach sunset and the effects from the cold front passage
earlier today are starting to diminish. Tonight and tomorrow,
surface pressure gradients will continue a gradual weakening
trend, while a closed-low currently off the coast of central BC
will dive southeast across the PacNW as it becomes an open-wave
shortwave trough. The Cascades will see an uptick in shower
activity overnight, then spread to the northeast mountains
tomorrow with the passage of the shortwave trough. With colder air
aloft, instability will increase just enough to allow a chance of
an isolated thunderstorm to develop over the northeast mountains.
Snow levels will be dropping to near or just above pass level in
the mountains, so some showers may consist of a rain/snow mix or
light snow. Breezy but sub- advisory winds will persist through
tonight and tomorrow as surface pressure gradients will be slow to
weaken across the region. The strongest winds will be expected
through the Cascade gaps as the cross-Cascade pressure gradients
will still be fairly tight, though confidence at this time is low
for any wind advisories needing to be reissued, especially for the
Kittitas valley. Afternoon temperatures tomorrow will be in the
50s to mid 60s, with 40s in the mountains. Lawhorn/82

AVIATION...06Z TAFs...VFR conditions to prevail through the
period. CIGS will continue to be sct-bkn AOA 5kft to 15kft AGL
through the period. Another weather system will impact the area
tomorrow, with VCSH and possibly a stray shower impacting sites
PDT/RDM/BDN/ALW in the late morning through early afternoon. Winds
have diminished across most sites, with mainly 12-20kts and gusts
up to 25kts. This trend will continue into the early morning.
Sites DLS/PDT will see winds increase to 20-25kts with gusts
30-35kts after 17Z, with all other sites mainly 15-20kts with
gusts 25-30kts possible. Winds will gradually decrease into late
afternoon and evening tomorrow. Lawhorn/82

PREV DISCUSSION... /issued 452 PM PDT Wed May 18 2022/

SHORT TERM...Tonight through Friday night...Multiple waves will
impact our CWA during the short term period with widespread breezy
to windy conditions tonight and Thursday, below-normal temperatures
through Friday night, and chances of rain for the lower elevations
with some light snow for the mountain passes and higher peaks.

The closed upper-level low currently located off the British
Columbia coast will transition into an open shortwave trough as it
drops southeast into the Pacific Northwest tonight through Thursday.
The cold frontal passage today and tonight will drop snow levels to
near pass levels overnight, and some lingering shower activity in
the mountains will lead to light accumulations of a trace to a
couple inches, primarily for the Cascade Passes. While there are no
plans to extend the current Wind Advisories that are in effect until
10 PM tonight, sub-advisory winds will likely continue overnight in
the wake of the cold front and in advance of the next shortwave.

Thursday will see a slight chance of shower activity associated with
lift from the incoming shortwave and general instability from
daytime heating. The best chances for activity will be for the
mountains where snow levels of generally 4000 feet or under will
allow some additional light snow accumulations. Given the showery
nature of the precipitation, some areas may see little to no
accumulation while others may see brief heavier showers. Afternoon
highs from the mid 30s to mid 40s in the mountains may limit snow
accumulation. Outside of the mountains, some rain showers are
forecast for portions of north-central OR, valleys of northeast OR,
and the foothills of the Blue Mountains. Could also see an isolated
thunderstorm across the northeast mountains, and with cooler air
aloft, small hail may also accompany lightning, gusty winds, and
locally heavy downpours. In addition, widespread breezy to windy
conditions will continue Thursday with the strongest winds expected
through the Kittitas Valley. While current HREF guidance indicates
sub-advisory winds, forecast cross-Cascade pressure gradients are
stronger than today. The Kittitas Valley tends to experience strong
northwest winds in these situations so will continue to monitor in
case an advisory is needed.

As the upper-level shortwave exits to the south Thursday evening and
night, shower activity should diminish and pressure gradients will
slacken as higher pressure builds over the region. This will lead to
mostly dry conditions for Thursday night, and with the calmer winds
and clearing skies allowing more efficient radiational cooling,
morning lows may approach or reach freezing in colder spots.
Moreover, high mountain valleys and central Oregon will likely see
morning lows in the 20s. Frost may also occur Friday morning.

As Friday afternoon rolls around, daytime heating may allow a few
showers to develop over the Washington Cascades and northeast
mountains. Yet another upper-level low is forecast to drop south
from British Columbia Friday night. Plunkett/86

LONG TERM...Saturday through Wednesday...The forecast area will
be on the west side of a large scale upper trough, with the
trough`s axis over the Rocky Mountains. This will result in mostly
a northwest flow over the forecast area. However, the latest
deterministic runs have a portion of the cold pool with the trough
aloft over the forecast area. This will result in instability due
to steep temperature lapse rates. This combined with available
moisture will result in afternoon showers and thunderstorms over
the eastern/northeast mountains as well as the WA Cascades east
slopes on Saturday. The deterministic models are in fair to good
agreement with each other on both timing/phase and amplitude of
the of the trough axis, and are in good agreement with the
ensembles. The cluster analysis shows low variance for both EOF 1
and EOF 2. Therefore confidence is high enough to mention a slight
chance of thunderstorms in the afternoon for these areas.

On Sunday, the trough axis is progged to shift eastward with a
more pronounced north to northwest flow over the PacNW and CWA,
especially over western areas of the CWA. This will result drier
and more stable conditions over western areas. However, there
could still be a few showers over the northern Blues and Wallowa
Mountains. Also cannot rule out some thunder and will include a
slight chance of thunder with these showers on Sunday as well due
to lingering moisture and instability. However, confidence is not
as high for thunderstorms on Sunday.

After Sunday, an upper ridge will build over the region on Monday
with drier, more stable conditions, and warmer temperatures. Then
there is some larger disagreements going into Tuesday between the
deterministic models. The GFS has a back door upper trough
swinging down in a northwest flow aloft, while the ECMWF has more
of a ridge pattern going into Tuesday. Day 7 Cluster Analysis
shows a fair amount of variance on Tuesday, which causes less
confidence in the day 7`s forecast (Tuesday). The NBM has a
slight chance of showers over the Cascades east slopes and the
northeast mountains. Will lean toward the NBM solution for now for
Tuesday and leave these showers in the forecast due to the
uncertainty. On Wednesday, there is better agreement between the
deterministic models with a dry, more stable, and warmer upper
ridge pattern over the forecast area. Winds will be light
through most of the extended period, though there will be locally
breezy winds on Monday afternoon and early evening, and then more
so on Tuesday and Wednesday afternoons. 88


&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  40  58  36  64 /  10  30   0   0
ALW  41  59  38  66 /  10  30   0  10
PSC  44  64  41  70 /   0  10   0   0
YKM  39  61  35  68 /  10   0   0  10
HRI  43  63  41  69 /  10  10   0   0
ELN  38  55  38  65 /  20  10   0  10
RDM  34  54  28  62 /   0  30   0   0
LGD  35  50  33  59 /  10  50  10  10
GCD  36  49  33  61 /  10  40   0  10
DLS  44  60  42  69 /  20  30   0   0

&&

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

$$

SHORT TERM...82
LONG TERM....99
AVIATION...82


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