529
FXUS66 KPDT 182145
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
245 PM PDT Sat May 18 2024

.SHORT TERM...Tonight through Monday night...Any concerns for the
short term period are minor and not worthy of any highlights at
this time. Currently, the PacNW is under a broad cyclonic flow
aloft. Washington and northern Oregon are on the north side of an
upper level jet that is rotating along the bottom of a shortwave
trough. The shortwave trough will track across the region this
evening. Doppler Radars are detecting increasing showers over the
northern half of Washington at this time, and the showers will
develop over the Washington and northern Oregon Cascades as well
as the Blue Mountains and Wallowas this evening. HREF shows these
as light showers in our CWA with QPF amounts less than a tenth
and little to no snow accumulation with the exception of the east
slopes of the WA Cascades. White Pass has about a 40% of 1" of
snow but only 10% of 2" of snow on HREF. Since snow levels will be
around 4000 feet, Snoqualmie Pass will have rain or a rain/snow
mix. Any instability with this approaching shortwave will be north
of the forecast area. There have been mixing of winds aloft to
the surface that will continue through the evening hours with
numerous stations reporting gusts 25-35 mph...locally 40 mph. The
probability of gusts greater than 40 mph lowers dramatically, so
no wind advisories this evening.

Any showers on Sunday will be widely scattered and primarily in
Wallowa County. The chance of showers is only 20-30%. On Sunday,
the forecast area will be on the backside of the trough and under
a northerly flow aloft. There are no embedded waves or upper level
support to speak of. It will be a chilly morning and the
afternoon will be about 5-10 degrees below seasonal average.

The weather pattern does not change much on Monday, but there will
be a slightly better chance (30-40%) of rain and mountain snow in
Wallowa County and a slight chance of rain over the northern
Blues. I cannot rule out a thunderstorm or two in Wallowa County
where SBCAPES are as high as 200 J/kg (Canadian GEPS mean is
around 350 J/kg), but the mean shear is very weak. The upper low
to the east is progged to strengthen with some wrap around
moisture. Not all models agree, as the NAM is particularly dry
over northeast Oregon. The deterministic GFS is the most bullish
on precipitation, but nothing significant. Monday will be warmer
than Sunday but still slightly below seasonal average. Wister/85


.LONG TERM...Tuesday through Saturday...Unsettled weather is
expected through the long term period as a couple of troughs move
across the area with only a brief break in between. Model agreement
has improved in the last couple of runs though the normal deviations
grow by late in the week mainly about the strength and timing of the
trough and upper low arriving Friday into Saturday. The Extreme
Forecast Index indicates below normal temperatures in the long term
with values of -0.70 to -0.75 Tuesday through Thursday though it
only highlights temperatures in the Washington Cascades on
Wednesday. Rainfall is highlighted with a value of 0.67 on Wednesday
in the Washington Cascades, the eastern mountains of Oregon and the
Blue Mountain Foothills. However, it does have a shift of tails of 0
over the Washington Cascades and the Blue Mountain Foothills, so
there is some possibility of a more extreme rain event. Finally, it
also highlights windy conditions mostly in Oregon on Wednesday with
a value of 0.70.

Tuesday starts out with models in good agreement in having an upper
low and trough over the northern British Columbia coast then having
it move south to near the US/Canadian border by Tuesday evening and
into western Washington overnight. It will be pushing a front ahead
of it into our area in the late afternoon. During the day, rain will
be confined to the Cascade crest but Tuesday night will see a chance
of rain over the entire area. The mountains will get a quarter to a
half inch of rain since snow levels are 6000-7000 feet. The Blue
Mountain Foothills will get 1 to 2 tenths of an inch while the rest
of the lower elevations will just have light rain mountains. The
Columbia Basin will have westerly winds of 10 to 20 mph.
Temperatures will be a few degrees below normal with highs in the
upper 60s to lower 70s in the Columbia Basin and in the upper 50s to
mid 60s elsewhere.

On Wednesday, 75% of model ensemble members will have the trough
overhead before moving into Idaho Wednesday night. This will give us
another chance of rain across the area and the NBM puts a slight
chance of afternoon and early evening thunderstorms over the eastern
Oregon mountains. The remaining 25% have a very weak crossing the
area a few hours faster than the other ensemble members. Rainfall
amounts will be up to a quarter inch in the mountains and a tenth of
an inch or less elsewhere. Winds will again be breezy with 15 to 25
mph in the afternoon. Temperatures will cool around 5 degrees to a
well below normal 60s in the Columbia Basin and in the 50s in the
mountains.

Thursday will be a bit of a break though 83% of the model ensemble
members keep the trough over Idaho and close enough to cause some
issues. Have kept a 30-40% chance of rain showers (snow levels
around 6000 feet) in the mountains and a 10-20% chance in the Blue
Mountain Foothills with the rest of the area dry. Rain amounts will
be very light. Temperatures will warm 2-4 degrees from Wednesday.

Models diverge on Friday with the ECMWF and Canadian deterministic
runs having a trough with a deep closed low arriving either Friday
afternoon or night while the GFS arrives a few hours earlier with a
much weaker low. The NBM appears to favor the former though rain
amounts are light. A southwesterly flow ahead of the trough will
warm temperatures 3-5 degrees from Thursday to the 60s and lower 70s
with mid to upper 50s in the mountains. By Saturday, 71% of model
clusters and the ECMWF and Canadian deterministic runs keep the
trough over the area while the GFS deterministic and the rest of the
ensemble members have the low and trough having moved off to the
east. Have kept a chance of light rain showers over the mountains
next Saturday with temperatures a degree or two cooler than Friday.
With a trough crossing the area, unstable conditions will allow for
a slight chance of thunderstorms over the eastern mountains each
day. Perry/83


&&

.AVIATION...18Z TAFs (previous discussion)...VFR conditions are
expected through the next 24 hours with FEW-SCT mid and high level
clouds though Columbia Basin sites KALW, KPDT and KPSC will have
BKN-OVC clouds at around 5K-8K feet AGL late this afternoon and
evening as a disturbance tries to generate showers. Expect that
little will reach the ground but have -SHRA at KALW and KPDT from
04-09Z and VCSH at KPSC from 02Z-07Z. Breezy to windy conditions
will occur again today as winds will increase in the next couple
of hours to west to northwest at 15-25 kts with gusts of 30-35 kts
through 06Z-08Z before decreasing overnight and tomorrow morning
to around 10 kts with higher gusts. Perry/83


&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  39  63  39  69 /  20   0  10   0
ALW  43  66  43  72 /  40  10  10  10
PSC  45  71  46  77 /  20   0   0   0
YKM  38  68  41  75 /  10   0   0   0
HRI  42  69  43  75 /  10   0   0   0
ELN  40  63  39  71 /  10   0   0   0
RDM  33  58  31  66 /   0   0   0   0
LGD  37  58  36  64 /  10  10  10  20
GCD  36  58  34  64 /  10   0  10  10
DLS  45  66  44  72 /   0   0   0   0

&&

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

$$

SHORT TERM...85
LONG TERM....83
AVIATION...83