014
FXUS66 KPDT 170520 CCA
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
1019 PM PDT Thu May 16 2024

Updated Aviation Discussion

.EVENING UPDATE...Despite decently strong cross-Cascade surface
pressure gradients (5 mb SEA-ELN and 10-11 mb PDX-GEG), winds
have dropped below advisory criteria for all sites this evening,
and 00Z HREF probabilities indicate a very good chance (80%) that
winds outside of the Kittitas Valley will remain below advisory
criteria through the night. Delving further into observations,
VAD wind profiles indicate a low-level jet of 40-45 kts between
3-4 kft at KPDT, though winds reaching the surface are
significantly weaker at 15-25 kts with gusts of 20-35 kts. Surface
observations along area ridgetops corroborate the low-level jet;
North Pole Ridge RAWS and Rattlesnake Mountain both show sustained
winds of 30-35 kts with gusts of 40-45 kts. Given the lack of
support in observations and high-resolution guidance, have opted
to cancel all existing Wind Advisories except for the one in
effect for the Kittitas Valley. Breezy to windy westerly winds are
still forecast overnight through Friday morning, just not strong
enough to warrant wind highlights. Plunkett/86

&&

.AVIATION...06Z TAFS...VFR conditions will prevail through the
period. Clouds are developing and expanding across the area and SCT-
BKN clouds at 040-070 are expected until around 12Z followed by
clearing skies. It will be mostly clear on Friday with increasing
mid clouds after 03Z Saturday mainly at KYKM and KDLS. Winds have
slowly diminished this evening but expect sustained winds of 10-20
kt and gusts 20-30kt to persist for much of the period. There could
be a little lull in the winds toward daybreak as the boundary layer
decouples followed by an increase again after sunrise. 78

&&

.PREV DISCUSSION... /issued 449 PM PDT Thu May 16 2024/

SHORT TERM...Today through Saturday...

Key Messages:

1. Windy conditions through Friday afternoon.

2. Elevated river levels on the Naches River.

3. Near to below normal high temperatures beginning Friday.

An upper level shortwave and an accompanying cold front will be
passing through our area this evening through Friday morning.
These approaching synoptic features have already developed a
pressure gradient along the Cascades, which have elevated winds
across the Kittitas Valley, Simcoe Highlands, Eastern Gorge, and
the northern Blue Mountain foothills as gusts of between 40 and 46
mph have already occurred at the Dalles, Pendleton, and
Ellensburg airports. The current pressure gradient between
Portland and Spokane is 10.8 mb, and is expected to continue to
tighten through the afternoon to between 11.9 and 13.6 mb via the
GFS, NAM, and SREF. Guidance suggests that the pressure gradient
will tighten even further Friday morning to between 12.0 and 14.8
mb early Friday morning before slowly slackening into the
afternoon. These gradient values are associated with winds of up
to 50 mph, which has warranted the extension of the Wind Advisory
over the aforementioned areas to 5 PM Friday. Confidence is high
(85-95%) regarding advisory level winds as the NBM showcases an
80% to 95% chance of wind gusts of 47 mph or greater and a 50% to
70% chance of 55 mph wind gusts or greater across the earlier
mentioned areas. There is also the potential for blowing dust to
occur across areas where the advisory is active, which may
decrease visibility and cause for potentially hazardous travel -
especially for large profile vehicles.

Snow melt due to the event earlier in May coupled with persistent
above normal temperatures have allowed some rivers to rise. At
this time, all rivers are forecast to stay below action stage
with the exception of the Naches River at Cliffdell. The current
level at this reach is 29.08 feet, and is forecast to peak this
evening around 11 PM at 29.17 feet before slowly subsiding to
below action stage (29 feet) Saturday evening by 5 PM. The Naches
River at Naches is forecast to stay about 6 inches below action
stage (16 feet), peaking this evening before slowly dropping over
the weekend.

This passing shortwave today will also bring with it mountain rain
along the Cascades this evening before extending into the Blue
Mountain overnight into early Friday morning. A transient upper
level ridge will keep conditions dry on Friday ahead of another
dropping upper level shortwave that digs over the area and ejects
to our east as a trough, bringing more breezy conditions and
mountain rain across the Cascades and northern Blue Mountains
Saturday afternoon and evening. Lower elevations will stay dry
through the period, but these features will usher in a cooler
airmass associated with persistent northwest flow that will
enhance on Saturday. High temperatures will drop 10 to 15 degrees
Thursday to Friday as below normal high temperatures continue over
the weekend and into the extended period. 75

LONG TERM...Sunday through Friday...The low over western Canada
persists this work week as the NW flow aloft moves over PacNW with
another potential low over Gulf of Alaska moving southerly.
Chances of precip remain on the low end (<15%) but increases to
10-20% in the low elevations and 20-40% at mountain areas with a
cold front associated with the low over Gulf of Alaska moving
southerly Tuesday onward. This will bring orographic lifting into
the moisture-starved trough hence more showery activity.
Thunderstorms are possible but, chances are slim as this pattern
doesn`t seem to provide enough instability (<20%). Very light QPF
amounts in the low elevations including Cascades but slightly more
at the Blue Mountains to around 0.03-0.12 inch.

Gusty winds will continue this week at around 20-30kts mainly along
Simcoe Highlands, Kittitas and Yakima Valley with strong surface
pressure gradient as the front passes. The NBM probabilities of
wind gusts >=22 mph shows high chances greater than 70%, but
40-60% over Ochoco-John Day Highlands. The agreement with the
models and ensembles are improving with this pattern including
potential showery precip impacting mountain areas. Temperatures
will be slightly below or near normal, mainly 50s-60s within the
mountains. Snow levels for this week will be 3000-5000ft but
increase Thursday afternoon as precip dries out. Feaster/97

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  43  64  39  67 /  10   0   0   0
ALW  48  68  45  70 /  20   0   0   0
PSC  50  72  46  73 /  10   0   0   0
YKM  41  69  40  69 /   0   0   0   0
HRI  47  70  43  71 /  10   0   0   0
ELN  41  62  41  63 /  10   0   0   0
RDM  42  65  34  66 /   0   0   0   0
LGD  45  60  36  66 /  20  10   0   0
GCD  46  63  36  69 /   0   0   0   0
DLS  48  66  46  67 /  10   0   0   0

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...Wind Advisory until 2 PM PDT Friday for WAZ026.

&&

$$

EVENING UPDATE...86
SHORT TERM...75
LONG TERM....97
AVIATION...78