480
FXUS66 KPDT 190521
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
1021 PM PDT Sat May 18 2024

Updated aviation discussion

.EVENING UPDATE...Mid-level water vapor imagery clearly reveals a
trough digging ESE across the PacNW this evening. Mid/upper-level
PVA over the Columbia Basin and the foothills of the northern
Blue Mountains is likely aiding the isolated to scattered
showers that are apparent on radar, perhaps further enhanced by
the left jet exit and modestly steep mid-level lapse rates (00Z
HREF-advertised 7 C/km). Along the Cascade crest, any showers
will be more orographically driven.

While no significant lightning threat is anticipated with shower
activity, GLM did briefly highlight some activity with the
strongest cell of the evening northwest of Boardman. Have opted to
keep a mention of thunder out of the forecast given low
confidence in any further activity as any lingering weak
instability wanes overnight. Plunkett/86

&&


.AVIATION...06Z TAFs...VFR conditions are likely with just a few
showers impacting PDT ALW and PSC over the next 2-4 hours with CIGs
of 5-10 kft expected. Westerly to northwesterly winds of 10-20
kts with gusts of 15-30 kts will continue decreasing overnight
then being 5-15kts on Sunday.

PREV DISCUSSION... /issued 247 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 capable of thunderstorms 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

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  40  63  39  69 /  20   0  10   0
ALW  43  66  43  72 /  40  10  10  10
PSC  47  70  46  77 /  20   0   0   0
YKM  39  68  41  75 /  10   0   0   0
HRI  43  69  43  75 /  20   0   0   0
ELN  39  64  39  71 /  10   0   0   0
RDM  33  57  31  66 /   0   0   0   0
LGD  35  58  36  64 /  20  10  10  20
GCD  36  58  34  64 /  10   0  10  10
DLS  46  65  44  72 /  10   0   0   0

&&

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

$$

EVENING UPDATE...86
SHORT TERM...85
LONG TERM....83
AVIATION...86