878
FXUS66 KPDT 192135
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
235 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 Plateua 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


&&

.AVIATION...18Z TAFs (previous discussion)...VFR conditions are
expected through the next 24 hours. Expect mainly FEW-SCT clouds
at 5K-10K feet AGL though some clouds at around 4K feet AGL at
KALW will linger for a few more hours. KDLS will have west winds
of 10-20 kts with gusts to 30 kts through 04Z then start
decreasing and drop below 12 kts after 11Z. KPDT and KALW will
continue at 10-15 kts with gusts to 25 kts through 22Z then drop
off to below 12 kts. KRDM, KBDN and KYKM will have northwest winds
at 10-15 kts with gusts to 25 kts through 05Z-08Z before dropping
off to below 10 kts. KPSC will remain below 12 kts for the next
24 hours. 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 /  10   0   0  20
HRI  42  75  48  69 /  10   0   0  10
ELN  41  69  44  63 /  10   0   0  20
RDM  31  65  36  62 /  20   0   0  10
LGD  36  62  38  65 /  10  10   0  10
GCD  34  63  37  65 /  20  10   0  10
DLS  44  71  48  64 /  10   0   0  30

&&

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

$$

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