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000
FXUS66 KPDT 121025
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
325 AM PDT Fri Apr 12 2024

.SYNOPSIS...A deep low offshore of the PacNW will fuel a multi-day
thunderstorm event this weekend for the inland PacNW with a low-
end potential for severe weather possible. Thereafter, the low
will move inland across California and help to sweep high pressure
ridging eastward allowing for a cooler northwesterly to northerly
flow as another system moves inland across Canada and then down
into the Northern Rockies during the first half of next week. This
will help to transition our concerns from weekend thunderstorms to
gusty winds and cooler than normal temperatures.

.SHORT TERM...Today through Sunday...A busy weekend is ahead for
our CWA as multiple days of shower and thunderstorm activity are
on tap including a low-end potential for severe weather.

The main weather feature driving this weekend`s thunderstorm
potential is a deep upper low that will skirt offshore southward
through Saturday before making its way inland across Central
California to end the weekend.

For Today - Ongoing cloud cover plus short wave ridging are
expected to help temper overall instability for the day. That
being said, CAPE values are still expected to range between
500-1000 J/kg, with some of the CAM models even indicating we
could rise as high as 1500-1700 J/kg for eastern portions of the
CWA. This combined with a strong shear environment of widespread
bulk shear values of 30-60 knots and PWAT values in excess of 0.5
inch will help support thunderstorms that do form. Sounding
profiles indicate storms across Grant County up through
southeastern Umatilla and into Union and Wallowa counties will be
able to produce isolated activity capable of strong wind gusts and
possibly some marginally severe hail. Once the sun sets and
heating ends, expect the severe threat to decay into the evening
hours.

For Saturday - A more limited severe threat is anticipated despite
a more widespread appreciable area for thunderstorm potential.
With the low moving southward, the exit region of the mid- level
jet will also settle further southwards with potions of Central
and North-Central Oregon seeing the potential for new rounds of
stronger thunderstorm activity for the day. CAPE values will be a
bit more limited than compared to Friday, but with stronger shear
still intact and capable of supporting updrafts, an instance or
two of large hail may be possible. Daytime high temperatures are
also of note as clearing across northern Oregon and southern
Washington will allow for significant warming, with highs ranging
across the 70`s, and portions of the Columbia Basin nearing 80
degrees for the day.

For Sunday - The severe threat will be declining on Sunday with
the low moving inland across California, but wrap around moisture
across the top of the low plus some modest instability should
continue to contribute to further thunderstorms from Central
Oregon up through the northeastern mountains of Oregon and
southeastern Washington. Otherwise, our focus shifts once again to
the temperatures as another warm day is on tap for northern
Oregon and southern Washington, including some highs in the low
80`s across the Columbia Basin. Tightening pressure gradients with
significant differences of 10-15 mb from Portland to Spokane give
confidence that we may see wind advisory level conditions (gusts
45+ mph) overnight into Monday, possibly even as strong as high
wind warnings (gusts 58+ mph). An eventful weekend leads into
continued concerns to start next week and the long term.
Goatley/87

.LONG TERM...Monday through Friday...A strong cold front is progged
to move across the forecast area on Monday, and confidence is high
that its impact over the forecast area will be strong winds and
significant lowering of the snow levels. Models have been consistent
advertising the front, but latest runs differ on the timing and
strength. Some are now about 6 hours slower and not as strong with
the cold air advection (approx. 3% GEFS, 16% EC ENS, 55% GEPS from
the cluster analysis). NBM is trending stronger with the winds and
now shows 75% probability of gusts meeting advisory criteria for the
Kittitas Valley, Simcoe Highlands, and portions of the Lower
Columbia Basin and Blue Mtn Foothills (mainly the Oregon side) and a
50% chance of high wind gusts for these same areas. Forecast
currently shows patchy blowing dust (BLDU) and will keep this with a
good chance of BLDU near newly plowed fields. Snow levels will lower
to around 2500 feet behind the front.  The westerly flow is not
favorable for precipitation in the lower elevations, therefore
little to no accumulations are forecast in the valleys.  Drier air
behind the front will also keep snow accumulations around 1-4 inches
in the mountains with the exception of the WA Cascades north of I-90
where models bullseye moderate to heavy snow that is likely
associated with a Puget Sound Convergence Zone setting up in the NW
flow aloft.

Models are in general agreement deepening the trough along the
Rockies which leaves eastern WA/OR on the backside of the trough and
under a NNW flow aloft.  Once again, models vary on the depth of the
trough. Typically, one would expect large spreads in the max and min
temps for Tues and Tues night due to these different solutions, but
the NBM is not showing large spreads in the box whisker plots.  It
will be colder Tues and Tues night with snow levels around 2500-3000
feet. There is a 40-60% chance of snow showers over the mountains
and adjacent valleys with up to 3 inches of accumulation. Confidence
in any significant snow in the Blues and Wallowa County is low
(30%), as a majority of ensembles are no longer showing this as a
wrap-around event.

The weather is less active Wednesday-Friday, but there remains model
discrepancies.  High pressure will build along the eastern Pacific
on Wednesday, resulting in a northwest flow aloft in the forecast
area.  There will be orographic showers over the Cascades and
eastern mountains (30-40% chance), continued cool and locally breezy
winds.  The ridge is progged to move eastward across WA/OR Thursday,
although there are model differences on the timing and amplitude of
the ridge.  Models also differ on whether the ridge will retrograde
and allow a shortwave to dive south along the front side of the
ridge or the ridge will strengthen over the PacNW on Friday.
Wister/85

&&


.AVIATION...06Z TAFS (previous discussion)...VFR conditions are
expected through the TAF period. Mainly dry conditions are expected
through the night and into Friday morning everywhere. However, there
are a few showers on radar around PDT and ALW. These -SHRA could
affect these locations until around 12/08-09Z. Will see where the
showers are around TAF issuance time to determine if a mention in
the TAF is warranted. Additional -RA is is expected to develop at
PDT and ALW again Friday afternoon. There are also rain chances at
BDN and RDM. Winds are expected to be 10 kts or less through the
period.


&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  63  43  72  48 /  40  40  10  10
ALW  68  46  77  51 /  40  50  10  10
PSC  69  47  78  51 /  10  20   0   0
YKM  66  41  75  46 /  10   0   0   0
HRI  67  45  76  50 /  30  30   0  10
ELN  65  41  74  47 /  10   0   0   0
RDM  55  39  63  43 /  40  10  40  50
LGD  68  44  73  47 /  60  40  10  20
GCD  70  44  74  46 /  70  30  30  40
DLS  66  46  74  52 /  20  20  10  10

&&

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

$$

SHORT TERM...87
LONG TERM....85
AVIATION...77