889
FXUS66 KPDT 152158
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
258 PM PDT Wed May 15 2024

.SHORT TERM...Tonight through Friday Night...

Key Points for the next few days:

1. An upper trough will bring windy conditions to the Kittitas
Valley, Blue Mountain Foothills of Oregon, the Simcoe Highlands and
the Eastern Columbia Gorge Thursday and Thursday Night with breezy
conditions elsewhere.

2. The trough will bring light rain and snow showers to the Cascades
and northern Blue Mountains Thursday night and cool temperatures
about 10 degrees on Friday.

3. Water levels will remain high along the Naches River for the next
several days.

A large upper ridge remains over the region this afternoon giving us
generally clear skies aside from some high cirrus. Pressure
gradients remain tight along the Cascades and that is giving the
Kittitas Valley northwest 15 to 25 mph winds with gusts to 35 mph
and the Yakima Valley is similar but a few mph slower. Winds will
diminish tonight but still remain at 10 to 20 mph in the Kittitas
Valley. Temperatures will run a few degrees warmer than last night
with lows in the lower to mid 50s in the Columbia Basin and in the
mid to upper 40s elsewhere.

However, overnight an upper trough will be sinking south through
British Columbia and by tomorrow morning, the ridge will be
flattening and pressure gradients will be tightening up further.
Temperatures will cool a few degrees from today with highs in the
upper 70s to mid 80s and in the upper 60s to mid 70s in the
mountains. The big concern will be winds, which look to be just shy
of wind advisory levels in the Kittitas Valley, the Blue Mountain
Foothills of Oregon, the  Simcoe Highlands and the Eastern Columbia
River Gorge, mainly near Maryhill. NBM wind probabilities show a 70-
90 percent of 25 mph sustained winds but only a 10-20 percent chance
of 35 mph sustained winds. Wind gusts are a similar story with a 70-
90 percent chance of 38 mph wind gusts but only a 25-50 percent
chance of 45 mph wind gusts. With these probabilities, do not have
the confidence to put out a wind advisory, though it looks close.
Will hold off and later shifts can monitor for any increases in wind
guidance.

The upper trough will cross the area Thursday night and Friday
morning. West to northwest winds will be strong (15 to 25 mph)
Thursday night through Friday morning then drop off in the afternoon
as the trough departs. It will also bring a chance of light rain and
snow showers in the Cascades late Thursday afternoon and night and
too the eastern mountains after midnight. Snow levels will drop from
7000-8000 feet to 3500-4500 feet. Rain amounts in the Cascades will
be up to a tenth of an inch with less than an inch of snow possible.
The eastern mountains will get just a few hundredths of an inch of
rain. Thursday night lows will drop to the 40s with mid 30s to lower
40s in the mountains.

By Friday afternoon and night, the upper trough will be centered
over the Rockies with the upper ridge having shifted away from the
west coast but still promoting a northwest flow over our area. Winds
will drop off and precipitation will have ended. Lows will cool
further to the upper 30s to mid 40s with 30s in the mountains.
Perry/83


.LONG TERM...Saturday 12Z(Saturday) to Wednesday
06Z(Wednesday night)...

The NW flow aloft passes over PacNW Saturday with a cutoff low
Southwest CA and a deep trough over western Canada into Sunday
morning before entering a shortwave onward. Chances of precip will
be low (<15%) for the weekend due to less moisture within the cold
air mass but increases next week with the trough moving over PacNW.

Thunderstorms might be possible for Tuesday and Wednesday night at
the Wallowa county, but chances are slim (<20%) due to weak
instability. QPF amounts will be a few hundredths in the low
elevations starting Tuesday, but increases around the mountain areas
up to 0.25 inch along the crest of the Cascades and between around
0.02-0.45 inch in the Blues and Wallowas Wednesday.

Gusty winds will occur Saturday with the EFI values of 0.6-0.7
showing an unusual event for wind gusts. The NBM probabilities of
wind gusts >=22 mph shows greater than 80% across most areas, but 50-
70% over Ochoco-John Day Highlands and Wallowa County with wind
gusts >=34 mph across Simcoe Highlands ranging between 60-90% and 40-
93% across the Kittitas and Yakima Valley. This is influenced by
strong surface pressure gradient. Wednesday and Thursday might
experience gusts but confidence is low (<20%).

Great agreement for this weekend pattern until late Sunday onward
when some uncertainties began to show with about 49% total cluster
members favoring a shortwave whereas 35% total members showing more
of a ridge occurring over the PacNW. The models also diverges when
low splitting appear in GFS as it moves over PacNW with ECMWF
showing a trough occurring sooner. Confidence is high for the
weekend pattern (>70%), but decreases for next week as the timing
differs and the low over Canada making the forecast challenging (40-
50%).

High temperatures will be slightly below normal roughly by 4-10
degrees Monday into Wednesday as the cooling period occurs with
trough over PacNW. Snow levels from Saturday into Monday night will
be around 3000-4500ft as the trough passes with limited precip but
increases to 5000ft or higher Tuesday onward. Feaster/97

&&

.AVIATION...Previous discussion...18Z TAFS...VFR conditions are
expected through the TAF period. Cloud coverage should clear up
this morning into tonight/Wednesday morning. Winds will be light
for now, but expect winds to increase for most sites starting this
evening at around 10- 15kts with gusts up to 25 kts. After a
brief lull in winds overnight, gusty winds will return after 12Z
Wednesday. Feaster/97

&&

.HYDROLOGY...With the warmer temperatures, snowmelt is creating
some rises on area rivers and streams though generally not high
enough to be a concern. The exception is the Naches River at
Cliffdell, which is expected to rise above bankfull late this
evening and remain just above bankfull through Sunday morning. The
probabilistic forecast gives a greater than 95 percent of this
happening. Further downstream at Naches, the river is expected to
remain well below bankfull and the probabilistic forecast gives
the Naches River at Naches a less than 5 percent chance of
reaching bankfull. Perry/83

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  52  78  44  66 /   0   0   0   0
ALW  57  81  48  69 /   0   0   0   0
PSC  57  85  50  73 /   0   0   0   0
YKM  50  81  40  71 /   0   0   0   0
HRI  56  82  47  72 /   0   0   0   0
ELN  53  73  41  64 /   0  10  10   0
RDM  46  78  42  66 /   0   0   0   0
LGD  49  76  44  61 /   0   0  10   0
GCD  47  79  45  64 /   0   0   0   0
DLS  55  75  49  68 /   0   0   0   0

&&

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

$$

SHORT TERM...83
LONG TERM....97
AVIATION...97
HYDROLOGY...83