782
FXUS66 KPDT 180514
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
1014 PM PDT Tue Sep 17 2024

Updated Aviation Discussion

.AVIATION...06Z TAFs...VFR conditions expected over the next 24
hours. With winds lessening, all sites expected to become 10 knots
or less overnight. DLS should see gusts around 20-25 knots
tomorrow after 21Z. Cloud decks expected to lift to become high
decks FEW to BKN, or possibly SKC. Goatley/87

&&

PREV DISCUSSION... /issued 258 PM PDT Tue Sep 17 2024/

SHORT TERM...Today through Thursday night...

Key Messages:

1. Breezy to windy afternoon conditions each day, peaking today.

2. Light showers and isolated storms possible this evening.

3. Steady warm-up through period, eventually reaching near normal.

Current radar and visible satellite imagery showing light returns
along the northern Blue Mountains and foothills under partly to
mostly cloudy skies east of the Hermiston/Boardman area. These
conditions are in response to an upper level low pressure system
to our southeast beginning to lift into eastern Idaho and western
Wyoming. Another upper level system is approaching the coast, and
a weak transient ridge has developed between these two upper
level systems. This ridge has allowed clear to mostly clear skies
to occur along the east slopes of the Cascades and the Gorge. The
incoming system has also allowed a pressure gradient to develop
along the Cascades, attributing to breezy to windy conditions
through the Gorge, Blue Mountain foothills, and Kittitas Valley.
Gusts of 44 mph and 38 mph out of the northwest have been
reported this afternoon at The Dalles Airport and Ellensburg
Airport respectively, which is expected to persist until 7 PM.
Winds will slowly slacken this evening as the exiting system
pushes into easter Montana and the coastal system digs south along
the northern California coast. Sustained winds of 15-25 mph and
gusts of 35-45 will be possible through 7 PM over these areas.
Confidence in these wind values is moderate to high (60-80%) as
the HREF and NBM showcase the probability of winds staying below
advisory criteria (Sustained winds of 30 mph or greater and/or
gusts of 45 mph or greater) of 20-40% over these areas. Elsewhere,
winds will stay elevated with sustained winds of 10-20 mph and
gusts of 20-30 mph. An upper level ridge will infiltrate into the
area from the west on Wednesday ahead of a weak shortwave riding
over the top of the ridge on Thursday, with both features
providing windy afternoon conditions as a pressure gradient
redevelops along the Cascades. Elevated winds are expected to be
less than today and more confined to the Gorge and Kittitas Valley
with sustained winds of 20-30 mph and gusts of 30-40 mph likely
both days. The NBM is in agreement with this assessment, as the
probability of sustained winds of 30 mph or greater is less than
10%, and gusts of 45 mph or greater is 10-20% for isolated areas
of the Gorge and Kittitas Valley on only Thursday.

These two upper level low pressure systems have kept periods of
light showers occurring across the northern Blue Mountains and
foothills through the morning and early afternoon. However, drier
conditions will ensue through the remainder of the afternoon and
into the evening as the system departs to our east. Only rain
amounts of 0.01-0.03" are expected across the Blue Mountains and
east slopes of the Oregon Cascades, with wetting rain only
possible at higher elevations of the Elkhorns and eastern Wallowa
County. There is still a potential for an isolated thunderstorm
over these areas as the NBM shows surface CAPE of around 200 J/kg,
but that chance is quickly dissolving as the departing upper
level system loses its influence over the area. There has yet to
be any lightning associated with today`s showers, but a strike or
two will be possible (~15%) over the next couple hours for
southern Wallowa county.

Northwest flow aloft will turn more westerly on Wednesday before
incurring a more southwest component on Thursday. This will allow
temperatures to slightly increase through the period, with the
greatest increase expected on Thursday. Even with this slight
warm-up, high temperatures will only reach near normal or slightly
above normal by Thursday with values breaking into the low 80s
across lower elevations of the Basin and into the Yakima Valley.
Confidence in highs breaking into the 80s for these areas is
moderate to high (60-70%), as the NBM advertises a 50-70% chance.
75

LONG TERM...Friday through Tuesday...Models are showing better
agreement through the long term period and are beginning to show
some consensus for early next week. High pressure offshore and
troughs to our east will promote a northwest flow over the area
through much of the period though model clusters show varying
degrees of ridging over the area with a persistent 10-15% of model
solutions showing a more trough like solution rather than a ridge.
The Extreme Forecast Index shows little in the way of significant
weather aside from some marginally breezy winds through the Cascade
gaps Friday afternoon. Temperatures will be near normal Friday and
Saturday then will warm a few degrees to the mid to upper 70s Sunday
through Tuesday.

Friday will start out with high pressure in the eastern Pacific with
a trough stretching from Alberta south to the desert southwest. The
low pressure will be too far away to affect us other than to provide
the pressure gradient for breezy winds around 20 mph in the Columbia
Gorge and Kittitas Valley in the afternoon. The NBM shows both of
those locations having a 65-70 percent chance of wind gusts greater
than 25 mph. Temperatures will be in the lower to mid 70s with only
the warmest locations in the Columbia Basin having as much as a 25
percent chance of reaching 80 degrees.

Saturday will see the trough moving further east and this will allow
the ridge offshore to strengthen and expand over our area though
there are differences about the strength of the ridge. Temperatures
will warm a degree or so. The northwest flow will create some light
upslope showers along the Washington Cascade crest in the late
afternoon and evening.

Sunday will see the ridge continuing offshore though again there are
differences. 65 percent of the model ensemble members have a strong
ridge with a northwest flow overhead, 25 percent have a weaker ridge
further offshore with a zonal westerly flow into our area and 10
percent have a trough riding over the ridge and through the area.
The NBM favors a chance of rain along the Washington Cascade crest
with just a few hundredths of an inch expected. The rest of the
mountains and most of the Columbia Basin will have a slight chance
of showers with minimal rain amounts. Temperatures warm a few
degrees to the mid to upper 70s with mid 60s to lower 70s in the
mountains.

Monday will see the ridge axis move closer to the coast and this
keeps showers out of the forecast with temperatures similar to
Sunday. Tuesday the trough will be overhead according to 70 percent
of the model ensemble members with the other 30 percent keeping it
just offshore. It looks like another dry day aside from a slight
chance of showers along the Washington Cascade crest. Temperatures
will warm a few degrees to the mid to upper 70s with many lower
elevation locations having a 50-75 percent chance of reaching 80
degrees. Perry/83

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  47  74  45  76 /  10   0   0   0
ALW  52  76  50  79 /  10   0   0   0
PSC  55  79  53  80 /  10   0   0   0
YKM  47  79  46  80 /   0   0   0   0
HRI  53  78  50  80 /  10   0   0   0
ELN  52  74  48  79 /   0   0   0   0
RDM  42  71  40  77 /  20   0   0   0
LGD  44  72  44  77 /  10   0   0   0
GCD  42  71  42  77 /  10  10   0   0
DLS  55  78  51  79 /  10   0   0   0

&&

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

$$

SHORT TERM...75
LONG TERM....83
AVIATION...87