000
FXUS66 KPDT 111018
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
318 AM PDT Thu Apr 11 2024

.SHORT TERM...Today through Saturday...The short term is getting
active as we see a deep upper low follow the coastline just
offshore of the region, causing our flow to turn southwesterly to
southerly bringing moisture and instability that will fuel shower
and thunderstorm chances into the weekend.

Thursday is expected to be the quietest day of the forecast period
as we see cloud cover and moisture begin moving into the region
ahead of the approaching offshore low. This should be beneficial
in regards to limiting significant convective activity as we move
into the weekend, but for today this should produce some rain
showers across the mountains along with very limited and light
snow for the highest peaks in the Cascades. This aforementioned
cloud cover has moved in a bit slower than models expected though,
and the clear overnight skies have cooled our temperatures
significantly faster this morning than models expected. Have had
to temper down this morning`s lows drastically as a result, but
still expecting our highs for the day to peak in the mid 60`s to
low 70`s despite the incoming clouds as the thermal trough should
help to raise daytime warmth.

Friday is where the forecast becomes the most interesting as the
progress of the low southwards continues to drive moisture into
our region. Sounding profiles indicating favorable conditions for
high-based thunderstorm structures are present in models, with a
wide swath of instability profiles across these solutions. Some
CAMs are indicating the potential for CAPE amounts between
1000-1500 J/kg, but a more reasonable amount seems to be around
500-800 J/kg, with a bullseye of the strongest instability across
Wallowa County and into the Blue and Strawberry Mountains. The
HREF is indicating probabilities of around 30-60% at CAPE values
in excess of 500 J/kg, giving moderate confidence to the strength
of the instability present. These profiles would be most conducive
of strong and damaging wind gusts, though marginally severe hail
would also be possible just before sunset. The region of interest
for this severe activity is expected to be limited from just east
of John Day up through La Grande and then northeastward through
most of Wallowa County.

As the offshore low continues its descent southeastwards and into
California, moisture begins to depart with it and cloud cover
will wane. This is when the ridging, still holding on to most of
our area, will make itself the most known, as temperatures
outside of Central Oregon soar, promoting very warm highs in the
mid to upper 70`s, around 10 to 13 degrees above normal for this
time of year. Tri-Cities may be able to break 80 for the day,
while Central Oregon should have enough cloud cover to keep the
area a bit cooler, generally in the mid to upper 60`s. A few more
rounds of showers and storms from Central Oregon up through the
Wallowas are once again possible, but thankfully our convective
threat isn`t expected to be as robust as Friday, with only general
thunderstorms currently anticipated. Goatley/87

.LONG TERM...Sunday through Thursday...It will be an active start to
next week with several weather concerns. Sunday begins with a large
upper low over the Great Basin. Although the ensembles shows this as
a broad upper low, the deterministic are not as washed out and
advertise two upper lows pinwheeling around each other with the low
that brings unsettled weather to eastern OR and far southeast WA
centered along the ID/NV border. Confidence is around 50% for at
least scattered showers. The NBM also has 20-30% ProbThunder over
Wallowa, Union, and Grant Counties with SBCAPES as high as 300 J/kg.
The strongest shear is south of the forecast area, so there is low
confidence at this time for strong or severe storms. Little to no
showers are forecast for our northern zones, however there will be
an increasing westerly flow aloft and at the surface.  Winds will be
increasing ahead of the next system dropping south from British
Columbia.  A strong marine push will set up with a tight cross
Cascade gradient with very windy conditions through the Cascade
gaps. Based on the 25th percentile of the NBM, there is a good
chance of 40-50 mph gusts in the Kittitas Valley and the eastern
Columbia River Gorge.

Monday will be a day of stronger winds covering a larger area due to
a cold front accompanied by a strong WNW flow aloft progged to sag
southward across the PacNW.  Models have been consistent showing
this pattern, so confidence is high (80%) that it will be very windy
in many areas. The NBM 25th percentile shows 50-60 mph gusts.  Not
only will there be the typical impacts with winds of these
magnitudes, blowing dust near newly plowed fields in April is often
a result of strong winds and will also be included in the forecast.
Snow levels will fall significantly with the front--down to 3000
feet Monday and 2000 feet Monday night.  Preliminary snowfall
amounts are 1-4" in the northern Blues and Wallowas as well as along
the Cascade crest.  A westerly flow is not favorable for
precipitation in the lower elevations, so snow amounts in the valley
floors are light--one inch or less--for Monday and Monday night.

Tuesday will be met with the cold low over ID and MT. That is based
on the majority of the ensembles, although there is a small
percentage that pushes the low eastward and leaves WA/OR under a
northwest flow aloft.  There are also variations in the models on
the amplitude of the low to our east. If models come into better
agreement with the low and continue to show wrap around
precipitation, the one area of concern will be Wallowa County as
snow accumulations could warrant winter highlights.  It`s too early
at this point, and there is uncertainty as well.  I did bump up
PoPs, QPF and snow amounts for the Wallowa Valley for Tuesday and
may need to adjust more.  There is a 60% chance of 1-3" snow over
the northern Blues on Tuesday.  Max temperatures across the area on
Tuesday will be 5-10 degrees below seasonal average.

Details in the forecast get more hazy for Wednesday and Thursday due
to model differences. Cool and unsettled weather is expected,
although there remains uncertainty on what to expect with the upper
trough to the east. The majority of the ensembles place the forecast
area on the backside of the large trough and under a NNE flow aloft
for Wednesday. The chance of precipitation will only be 20-30% over
the mountains and adjacent valleys. There are large variations on
whether an upper ridge will follow on Thursday or the forecast area
remains on the backside of the trough.  Wister/85

&&


.AVIATION...06Z TAFS (previous discussion)...VFR conditions for all
sites during the rest of this period, with prevailing high clouds
(FEW to OVC, 100-250 kft) and light winds. KRDM/KBDN will remain
breezy Thursday afternoon up to 18 kts until becoming light
overnight into Friday morning. Feaster/97


&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  64  43  62  43 /   0  10  30  20
ALW  67  47  66  46 /   0  30  40  20
PSC  70  49  68  46 /   0  10  10  10
YKM  66  43  65  41 /  10  10   0   0
HRI  69  47  66  45 /   0  10  10  10
ELN  63  42  64  41 /  10  10   0   0
RDM  65  39  58  38 /   0  20  30  10
LGD  67  42  66  44 /  10  30  50  40
GCD  70  42  70  43 /  10  20  60  40
DLS  65  48  66  48 /  10  10  10  10

&&

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

$$

SHORT TERM...87
LONG TERM....85
AVIATION...97