542
FXUS66 KPDT 200534
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
1034 PM PDT Sun May 19 2024

.AVIATION...06Z TAFs...VFR conditions are likely through the
period (90-100% chance) with very low chances (10% chance or less)
of MVFR CIGs due to a passing shower overnight (PSC/ALW) or
temporary BKN morning stratocumulus (PDT/ALW/BDN/RDM/DLS).

Isolated, light rain showers north of PSC and ALW are anticipated
to diminish in coverage over the next several hours with no rain
expected at TAF sites.

Aside from some northwest gusts of 15-25 kts at YKM until roughly
08Z, winds of 10 kts or less are forecast for all sites
overnight. Later Monday morning through afternoon, expect some
mixing of higher momentum air down to the surface as the
convective boundary layer grows. This will result in gusts of
12-25 kts, strongest at YKM/DLS. Plunkett/86

&&

.PREV DISCUSSION... /issued 829 PM PDT Sun May 19 2024/

EVENING UPDATE...Isolated showers are diminishing in coverage
across south-central WA and north-central to northeast OR as
instability wanes. Of note, the 00Z OTX sounding recorded over 600
J/kg of surface-based CAPE, though values were likely less over
our forecast area based on observed vertical extent of the
stronger cells and RAP-based mesoanalysis. Still, where convective
activity was more robust over the east slopes of the Washington
Cascades, some lightning registered on lightning products.
The evening update reflects minor changes to PoPs, removed
thunder for Wallowa County and added it for the Wenatchee
Mountains north of Ellensburg from 5PM-8PM. The threat of
lightning is rapidly tapering off as sunset approaches so do not
have it in the 8PM-11PM weather. Plunkett/86

PREV DISCUSSION... /issued 420 PM PDT Sun May 19 2024/

SHORT TERM...Tonight through Tuesday night...A positive tilt
longwave trough extends across most of Canada and across the
northwest interior of the US, and eastern WA/OR is on the backside
of the trough and under a northerly flow aloft. The air mass is
slightly unstable over WA and northeast OR, as evident by the
diurnal showers increasing in coverage north and west of our
forecast area. The trough is sagging southward, therefore showers
will travel south with widely scattered showers by early this
evening over the forecast area. The best chance of showers
(40-60%) is over Wallowa County. The NBM places Wallowa County in
a 20% chance of t-storms this evening as well, although the CAMS
have limited instability and shear and there is a lot of cloud
cover at this time to prevent surface heating. Will keep a slight
chance there but confidence is low. Showers will end after sunset,
and the rest of the night will be clear to partly cloudy and
chilly. Subfreezing temperatures will once again be observed in
the mountains and some of the adjacent valleys.

The instability associated with the upper trough is preventing
surface winds from decoupling from winds aloft, and this has
resulted in the relentless breezy winds. Since little change in
the weather pattern is expected on Monday, the widespread gusts to
20-30 mph (locally higher) will continue. While a drier northerly
flow will keep most of the forecast area precipitation free,
instability and moisture remain east of the Blue Mtns where PoPs
of 20-40% are forecast for Monday afternoon along with a slight
chance of thunderstorms in Wallowa County.

A return to cool and wet conditions over the PacNW is set for
Tuesday and Tuesday night. A deep closed low over B.C. will
make its appearance and provide numerous showers. Precipitation
chances increase in our area late Tuesday afternoon and evening
to around 60-90%. The only exception will be over south central WA
and along the Columbia Deschutes Plateau where rain shadowing will
keep most locations dry or limit rainfall, and PoPs will show
20-40%. Although rainfall amounts will not be substantial, wetting
rain will be observed over most forecast zones. NBM shows around
a 40-60% chance of 0.5" or more rainfall in the Blues, Wallowas,
and the eastern Cascade valleys (20% along the Blue Mtn foothills
and Grande Ronde Valley) with this approaching system. Wister/85

LONG TERM...Wednesday through Sunday...Unsettled and cool weather
is expected through the long term period as a couple of troughs
move across the area Wednesday and Wednesday night and again
during the Friday night to Sunday period. This will bring showery
weather and a slight chance of mainly mountain thunderstorms. The
Extreme Forecast Index highlights rain Wednesday along the
Cascades, eastern mountains and Blue Mountain Foothills (which has
a shift of tails raising a low possibility of an abnormally wet
day there). It also highlights winds on Wednesday over most of the
Oregon portion of the area and the Simcoe Highlands. On Thursday,
while the EFI doesn`t have significant snow, it does have a shift
of tails as high as 1 over Wallowa county and with a slight
chance of late afternoon thunderstorms there, that appears to be
suggesting a low chance of heavy convective snow showers Thursday
evening. In addition, the EFI has generally cool temperatures
Wednesday in Washington and in the eastern mountains on Thursday.

Cluster analysis shows model ensemble members starting out in good
agreement on Wednesday in having an upper low and trough over the
area though the GFS deterministic run ejects it into Idaho faster
than the ECMWF and Canadian. Agreement remains good on Thursday in
having the trough over Idaho though there are differences as to
whether the flow over our area will be north, northeast of
northwest. On Friday about 80 percent of the models have a shallow
weaker trough over the Rockies while the other 20 percent have a
stronger trough still centered over Idaho and down into
California. On Saturday, models have 55 percent of the ensemble
members with a closed low and trough over our area while the rest
have a much shallower and weaker trough. On Sunday, 66 percent of
the models have a trough over our area or just to the east in
Idaho, 22 percent have a very shallow trough over Idaho and a weak
ridge along the PacNW coast and the rest have an approaching
trough offshore with a southwesterly flow over our area.

Wednesday will have a chance of rain over most of the area with
the heaviest rain over the mountains with up to a half inch
while the Blue Mountain Foothills will get one to two tenths of an
inch and the rest of the area just a few hundredths of an inch.
Instability from the low overhead will give the eastern mountains
a slight chance of thunderstorms in the afternoon and expanding
into the Blue Mountain Foothills and north central Oregon in the
late afternoon. Snow levels will be around 4500-5000 feet, so up
to an inch or two of snow will be possible on the higher mountain
peaks. Tight pressure gradients will create west to northwest 20
to 30 mph winds in most of the lower elevations with the Kittitas
Valley, Simcoe Highlands and the southern Blue Mountain Foothills
having a 45 percent chance of gusts to 40 mph. Temperatures will
be in the mid 50s to mid 60s with 40s and lower 50s in the
mountains.

Thursday, with the trough and low moving east, will have a chance
of mountain rain with snow above 6000 feet. Showers will taper
off in the evening and the lower elevations will be mainly dry.
Being closer to the low, Wallowa county will have enough
instability in the afternoon for a slight chance of thunderstorms.
Rain amounts in the mountains will be up to a tenth of an inch.
Temperatures will warm 6-8 degrees from Wednesday.

On Friday, the NBM compromises between the model solutions and has
a slight chance of rain in the mountains becoming a chance in
the mountains and Blue Mountain Foothills Friday night. The
eastern mountains will have a slight chance of thunderstorms in
the late afternoon. Rain amounts are less than a tenth of an inch.
Temperatures will be near normal in the upper 60s to mid 70s with
60s in the mountains.

Next weekend, with wide differences between the models, have a
chance of rain in the mountains and the Blue Mountain foothills
with the rest of the area mainly dry. Rain amounts are up to a
tenth of an inch Saturday and just a few hundredths of an inch on
Sunday. Thunderstorm chances will be limited to a slight chance
each day over the higher terrain of the eastern mountains.
Temperatures are 3-5 degrees cooler Saturday and then warm back
up 3-5 degrees on Sunday. Perry/83

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  39  68  42  65 /  10   0   0  10
ALW  43  71  47  68 /  10  10   0  10
PSC  46  76  50  70 /  10   0   0  10
YKM  39  75  45  67 /  20   0   0  20
HRI  42  75  48  69 /  20   0   0  10
ELN  41  69  44  63 /  10   0   0  20
RDM  31  65  36  62 /  10   0   0  10
LGD  36  62  38  65 /  10  10   0  10
GCD  34  63  37  65 /  10  10   0  10
DLS  44  71  48  64 /  10   0   0  30

&&

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

$$

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