849
FXUS66 KPDT 070524
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
1024 PM PDT Mon May 6 2024

Updated Aviation Discussion

.EVENING UPDATE...Radar and satellite imagery this evening show
convective type showers have diminished area wide, with some light
returns noted along the Cascade crest, Blues, and Strawberry
mountains. Through tonight, a shortwave trough is expected to
continue sliding east across the PacNW, with increasing shower
activity along the OR Cascade crest and the higher terrain across
the Blues. The most recent 00Z run of the HREF shows short-term
high res members are also in agreement with the uptick in snow
shower potential through early tomorrow morning. Expect an
additional 2 to 5 inches in portions of the northern Blues and the
OR Cascade crest above 4.5kft through 5AM tomorrow. As a result,
have extended the winter weather advisory for the OR Cascade east
slopes to 5AM as well. Otherwise, a strong surface pressure
gradient will develop along the Cascade crest, resulting in
another day of breezy winds in through the Cascade gaps and lower
elevation zones. The strongest winds are expected through the
Kittitas valley tomorrow, where sustained northwest winds of 25
to 35mph and gusts up to 50mph are forecast (confidence 70-80%).
Updated winds through tomorrow morning, as well as snow amounts in
the mountains, with rest of the afternoon forecast package still
on track. Lawhorn/82

.AVIATION...06z TAFs...VFR conditions to prevail through the
period. Light showers will still be possible across the
mountainous terrain tonight, especially east of sites PDT/ALW and
west of site DLS. Otherwise, precipitation not expected to impact
sites through the remainder of the period. Breezy west winds of
around 20kts with gusts 30-35kts possible will persist at site PDT
through the period. Site ALW will see south/southwest winds of
12-17kts with gusts up to around 25kts through tomorrow evening.
Winds will be 12kts or less at sites DLS/RDM/BDN/YKM/PSC through
tonight, then increasing after 15Z-16Z to 12-20kts with gusts up
to 30kts possible mostly from the west/northwest, except at site
PSC where winds will be west/southwest. Lawhorn/82

PREV DISCUSSION... /issued 216 PM PDT Mon May 6 2024/

.SHORT TERM...
Tonight through Wednesday night...
The short term period will start off with several items of
interest then end on the quiet side.

First, another upper low will affect the Pacific Northwest through
Tuesday morning, with the majority of the impacts later this
afternoon into tonight, and mainly over the mountains. This upper
low will move down from British Columbia and across Washington,
before moving east into Idaho on Tuesday.

Snow levels which are initially 4000-5000 feet, except for below
4000 feet in portions of the Washington Cascades will decrease to
below 3000 feet by Tuesday morning. Snow levels will rise again to
above 4000 feet by Tuesday afternoon, except for portions of the
Washington Cascades.

Winter Weather Advisories remain in effect for the Oregon Cascades
and Northern Blue Mountains and these still look good. QPF values
have come up a bit with the morning model runs, but the advisory
still looks good and again with the time of year any
precipitation that falls during the day will have a much harder
time accumulating.

There are thunderstorm chances (20-40%) across much of the region
today, with the best chances across the Blue Mountains, Wallowas
and John Day Highlands. CAPE values are forecast to be in the
300-400 J/kg range with LI values -3 to -4 degrees C. Guidance
also suggests activity will decrease after 07/00Z.  Current
satellite imagery shows abundant cloud cover across much of the
area, with more breaks in Washington and portions of the Columbia
Basin. Will the clouds prevent thunderstorm development?

Next topic is breezy wind gusts which should mainly be in the 25
to 35 mph range with gusts as high as 40 mph through tonight and
into Tuesday before decreasing Tuesday evening. The Columbia
Basin, Foothills of the Blue Mountains, Columbia Gorge, Simcoe
Highlands, Kittitas Valley and Yakima Valley are the most likely
areas to see the strongest winds. The NBM probabilities of winds
>=39 mph across much of the aforementioned areas are 80% to 100%.
The NBM probabilities of winds >=47 mph drops to between between
30% and 60% on average. The ECMWF EFI is keying in on wind gusts
across the Basin, but more so across the Blue Mountains and
Wallowa COunty, with values of 0.8 to 0.9. Also, another area
across the Yakima and Kittitas Valley.

By later Tuesday, any precipitation will be confined to the
eastern oregon mountains and by Wednesday morning, dry north to
northwesterly flow will build in followed by a ridge. This will
bring increasingly warm and dry weather for the rest of the
period.

Finally, high temperatures will warm each day, from the 50s and
60s on Tuesday to the 60s and 70s on Wednesday. The ECMWF EFI is
focusing on high temperatures on Tuesday, with much of Oregon
below normal, in the -0.6 to -0.7 range and a large swath of
eastern Oregon in the -0.7 to -0.8 range with some areas of the
southern Blue mountains in -0.8 to -0.9.

.LONG TERM...
Thursday through Monday...Models in good agreement through the
period. However, they do diverge slightly on how fast the upper
level ridge weakens and moves east. EOFs show very little model
differences through Saturday, but diverge on how quickly the ridge
weakens and moves east thereafter. Looking at WPC cluster
analysis, there is a good chance (50-70%) the ridge will hold on
(but weakens) through the period and a low chance (15-20%) of a
weak trough clipping Washington by Monday. The ensemble members
paint a similar picture with the ECMWF weakening the ridge quicker
allowing a weak trough to move through the area late in the
period while the GFS is slower and develops general zonal upper
level flow Sunday and Monday. The deterministic models on pretty
good alignment through the period, but differ mainly in timing of
a weak shortwave moving through Washington Monday. NBM is a bit
drier for Monday which is likely a good bet given the cluster
analysis, but it may start trending a little "wetter" Monday over
Washington if the disturbance pans out. Daytime highs will be
above normal through the period, but expected to peak Saturday
15-20 degrees above normal.

There will be some hydrological concerns over the weekend as the
strong upper level ridge develops off the coast. This will provide
ideal conditions for melting some of our winter snowpack. All rivers
will see rises, but portions of the Naches River may approach (or
exceed) bankfull by Saturday or Sunday.

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  36  56  34  62 /  20  10   0   0
ALW  40  59  39  64 /  30  20   0   0
PSC  43  65  41  71 /  20   0   0   0
YKM  35  63  34  72 /  10   0   0   0
HRI  40  63  38  68 /  10  10   0   0
ELN  36  57  37  69 /  10   0   0   0
RDM  29  50  26  59 /  30   0   0   0
LGD  34  50  32  57 /  60  40   0   0
GCD  31  47  31  57 /  70  40  10   0
DLS  42  60  40  70 /  20   0   0   0

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory until 5 AM PDT Tuesday for ORZ502-509.

WA...Winter Weather Advisory until 5 AM PDT Tuesday for WAZ030.

     Wind Advisory from 8 AM to 11 PM PDT Tuesday for WAZ026.

&&

$$

EVENING UPDATE...82
SHORT TERM...77
LONG TERM....81
AVIATION...82