280
FXUS66 KPDT 151626
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
926 AM PDT Wed May 15 2024

.MORNING UPDATE...A fairly quiet day is anticipated as high
pressure remains over the west coast. Satellite imagery shows high
cirrus streaming across the region in a northerly flow aloft,
particularly on the eastern half of the area. Looking upstream, it
should continue into this afternoon, so have made some sky cover
adjustments today. Due to the ridging, temperatures will be about
5 degrees warmer today with highs in the lower to mid 80s and in
the 70s in the mountains. This is about 7-9 degrees above normal.
Winds will be the main forecasting challenge for the next few
days, though more on Thursday and Friday than today. Today, breezy
to windy conditions will occur mainly in the Kittitas Valley with
current northwest 15 to 25 mph winds increasing to 20 to 30 mph
this afternoon. Did make a few minor wind adjustments. Tomorrow
an upper low and trough will approach out of British Columbia and
tighten pressure gradients. This will give the Columbia Basin 20
to 30 mph winds and possibly requiring a wind advisory if models
show winds any stronger than that. These winds will continue on
Friday as well, though confidence is less and models indicate
winds to be slightly weaker. Forecast update already out.

On the hydrological front, the Naches River at Cliffdell remains
just below bankfull though it is expected to rise above bankfull
this afternoon. It is forecast to remain above bankfull through
Sunday though it never gets more than a few inches above
bankfull, so impacts should be minor. The Naches River at Naches
continues to run high but is expected to remain below bankfull.
Probabilistic forecasts give it just a 9 percent chance of
reaching bankfull in the next 10 days. Perry/83

&&

.PREV DISCUSSION... /issued 150 AM PDT Wed May 15 2024/

SHORT TERM...
Today through Friday night...A strong ridge of high pressure off
the coast will keep dry conditions over the region through early
Thursday.

Then, the high will be suppressed south as a decent shortwave and
upper trough approach the coasts of Oregon and Washington. These
features will move onshore Thursday night and across the area
Friday morning before exiting to the east. Dry northwesterly flow
will return behind the departing trough for most of Friday into
Friday night.

As the trough approaches the region on Thursday, two things will
occur.

First, there will be some precipitation chances across the
Cascades. The Washington Cascades will see precipitation chances
first, on Thursday followed by the Oregon Cascades later THursday
into early Friday.

QPF amounts are expected to be light, generally a few hundredths
of an inch to possibly around 0.10 inches in the Washington
Cascades. Snow levels will drop by Friday morning from over 7000
feet to around 3500 feet in the Washington Cascades and between
4500 and 5500 feet in the Oregon Cascades. However, with the light
QPF, any snow accumulations will be very light.

The second impact to occur Thursday into Friday as the trough and
shortwave move across the region will be a tightening pressure
gradient and a subsequent increase in winds. This will occur
especially Thursday afternoon and evening, with the strongest
winds looking to be Thursday night into Friday morning. Friday
looks to remain breezy, but winds will decrease, especially Friday
evening into Saturday morning. The Columbia Basin, Columbia Gorge,
Foothills of the Blue Mountains, Simcoe Highlands, Yakima Valley
and Kittitas Valley will likely have the strongest winds but most
places should at least be breezy. Winds will gust 25 to 35 mph,
with gusts as high 45 to 50 mph possible in the strongest areas
Thursday night.

NBM probabilities of wind gusts >= 39 mph on Thursday are
basically 80 to 90% if not higher in most of the aforementioned
locations. NBM probabilities of wind gusts >= 47 mph are 50 to
80% across the same region. NBM probabilities >= 55 mph decrease
to 50% or less, and in most places 30% or less. Wind advisories
may end up being needed for a Thursday evening/night into Friday
morning for at least a portion of the area, and we will continue
to monitor.

On Friday, NBM probabilities of wind gusts >= 39 mph are again
fairly high 80 to 90% across most of the area mentioned above.
However, the NBM probabilities of wind gusts >=47 mph are
generally 50% ore less and over a smaller area than on Thursday.

LONG TERM...
Saturday through Wednesday...Ensemble guidance coming into better
agreement on the synoptic pattern this weekend onward, which
generally looks to be breezy with below-normal temps and some
chances for showers, primarily in the mountains. Deterministic
guidance falls a bit out of sync by the start of next work week,
but the overall message remains the same.

A band of amplified NW flow aloft will be over the PacNW Saturday as
the region finds itself between a cutoff low southwest of SoCal and
a deep low over western Canada. The airmass overhead still looks to
be moisture-starved, so outside of some light orographic shower
activity across mainly the WA Cascades and the eastern mountains of
OR, the main effect of this NW flow aloft will be to mix down gusty
winds to the surface. NBM probabilistic guidance suggests a 40-50%
chance for wind gusts to exceed advisory criteria across the usual
suspect areas, namely gap wind regions such as the Gorge, Oregon
Basin, Simcoe Highlands, and the Kittitas Valley. Friday looks to be
the more windy day as a result of this pattern, however. Breezy
conditions are then expected to linger into early next week as the
relatively progressive synoptic pattern with NW flow aloft continues
to support gap flows.

Deterministic guidance diverges late Sunday onward, stemming from
how to handle the broad Canadian low. GFS seems to suggest that the
low will largely remain in place and circulate another trough down
into the PacNW, while the ECMWF shows the low splitting, with the
westward remnant diving down into the PacNW. Ensemble clustering
shows most members depicting the forecast area enveloped by some
kind of trough pattern next week, with some slight variations on
timing. The ECMWF would bring troughing over the region a bit sooner
than the GFS. Overall effect would be that of cooler temps, with
overnight lows in the 30s and 40s around Monday night onward,
potentially below freezing across our elevated valleys and central
Oregon. Should the trough center itself over the forecast area,
light convective showers could also develop during the day around
the tail end of the period.

Overall forecast confidence is on the higher end (60-70%) thanks to
models trending toward a more common solution. Biggest sources of
uncertainty lie mainly in the forecast Monday onward, with details
surrounding how cold temps will get and how much of a widespread
precip threat there will be with troughing overhead eluding us for
now. Evans/74

AVIATION...
12z TAFs...VFR conditions expected. Sct high clouds will prevail
over all sites at times today, with most sites seeing terrain-
driven winds less than 12 kts. Exception will be YKM and DLS,
which will see breezy gap winds out of the NW, gusting around 20
kts at times during the period. All sites expected to see lighter
winds by nightfall Wednesday night. Evans/74

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  79  52  78  45 /   0   0   0   0
ALW  81  57  81  49 /   0   0   0   0
PSC  87  57  85  50 /   0   0   0   0
YKM  86  50  81  41 /   0   0   0   0
HRI  85  55  83  47 /   0   0   0   0
ELN  80  53  73  42 /   0   0   0   0
RDM  80  45  78  41 /   0   0   0   0
LGD  76  49  77  46 /   0   0   0  10
GCD  78  48  79  44 /   0   0   0  10
DLS  84  55  75  50 /   0   0   0   0

&&

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

$$

MORNING UPDATE...83
SHORT TERM...77
LONG TERM....74
AVIATION...74