090
FXUS66 KPDT 171005
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
305 AM PDT Sat Aug 17 2024

.SHORT TERM...Overnight satellite imagery reveals the calm before
the storm: mostly clear skies over the forecast area, while a
deep low pressure system churns just off the coast of the PacNW.
This system will bring a myriad of impacts to us today, including
strong to severe storms across the Cascade range, possibly
extending into the east slopes, as well as a burst of hot and dry
weather across the lower Columbia Basin eastward. The weather
looks quieter after today, but Saturday does look to be awfully
active.

Looking at the timing of showers and storms today, CAMs more or less
agree in cells materializing around 1 to 3 PM this afternoon, which
is a bit later than previous guidance had suggested. Storms will
then lift northward along the spine of the Cascades, reaching
Washington by around 4 to 6 PM, lasting well into the evening and
with some CAMs suggesting isolated activity across the central WA
Cascades through the overnight hours heading into Sunday. Confidence
is high in storms being strong to severe over the Cascades, as the
upper-level dynamics are good with strong orographic lifting and
plentiful moisture to work with (PWATs are as high as an inch to 1.5
inches, which is impressive across a high mountain range). Bulk
shear is formidable as well at over 55 kts in some spots, which
makes sense given the strong SW to S flow ahead of the low. SPC has
painted the Cascades under a slight risk of severe storms as a
result, with damaging winds and hail being the primary risks. QPF
ranges from two tenths to even three quarters across the Cascades,
so wetting rains should be widespread enough to preclude any fire
weather headlines.

Biggest challenge comes in predicting just how far east storms will
extend beyond the Cascades. The 00z HREF reveals that CAM members
are in a bit better agreement in storms spreading into the
Washington Basin and not much south of that, however it is a bit
concerning that models like the NAMNEST reveal scattered storms
across much of the Columbia Basin in the evening, while the HRRR
remains fairly dry east of the Cascades. Decent bust potential is in
the forecast as a result, but more HREF members do appear to be
siding with storms spreading eastward, so did opt to upgrade the
Basin zones under a Fire Weather Watch into a Red Flag Warning. The
synoptic setup is strong enough that current thinking is that it`s
better to err on the side of caution.

That being said, the stark drop-off in mid-level moisture according
to models like the NAM12 reveal the more southerly orientation of
the winds aloft as you trek eastward into the Interior Northwest.
Southerly flow aloft will advect hot and dry Great Basin air into
this area, with the NBM suggesting that even the Tri-Cities may see
highs approaching the century mark. Down south, toward OR642, winds
will be gusty, and instability elevated, so a Red Flag Warning
remains in effect there. Thinking that the forecast area east of the
Tri-Cities remains warm and dry with locally gusty winds, but
critical fire weather conditions due to winds and low RH appear to
be isolated to around southeastern Oregon for today.

A blocking pattern keeps this low in place for the next several
days, leaving us under amplified SW flow, however moisture aloft
drops off, thus a dry forecast with seasonable temps prevails for
Sunday and Monday. While we`re under this pattern, however,
intermittent chances for high mountain showers will prevail so long
as favorable moisture advection takes place under this pattern.
Evans/74

.LONG TERM...Tuesday through Saturday...Models remain in good
agreement about maintaining the current persistent weather pattern
of an upper low and trough along the Pacific Northwest coast and a
ridge over the Rockies and the Midwestern states through the long
term period. Differences between models are fairly minor Tuesday
through Thursday then begin to increase next Friday and Saturday.
This will give us cooler than normal temperatures and showery
conditions mainly in and near the Cascades most days. Thunderstorm
activity will be fairly limited away from the Washington Cascade
crest. The Kittitas Valley and eastern Columbia Gorge will have
breezy 15-20 mph winds each afternoon though winds will be nothing
unusual in the rest of the area. The Extreme Forecast Index is
indicating little in the way of unusual weather aside from the below
normal high temperatures in central Oregon Tuesday through Thursday.

Models show the upper low in the offshore trough opening up into a
wave Tuesday and coming ashore in the evening while another upper
low moves south along the British Columbia coast. This will bring a
chance of showers to the Washington Cascades and a slight chance of
showers to most of the rest of the area in the late afternoon and
Tuesday night. The Washington Cascades and Simcoe Highlands could
get up to a tenth of an inch of rain but the rest of the area will
have minor amounts of rain. NBM probabilities show less than a 15
percent chance of a tenth of an inch of rain over the entire area.
Models aren`t showing any CAPE and LIs are +1.5 and higher, so do
not expect any thunderstorms. Temperatures will be in the mid 70s to
mid 80s. The Tri-Cities has a 41 percent chance of 90 degree
temperatures and Hermiston and Hanford 21 percent but the rest of
the area is under 10 percent.

Wednesday will have the trough move overhead while the next upper
low continues south along the British Columbia coast. This will keep
a a chance of showers along the Washington Cascade crest with a
slight chance extending into the Kittitas and Yakima Valleys. Models
hint at enough instability along the immediate Cascade crest in the
late afternoon, so have a slight chance of thunderstorms there. Due
to the colder air with the trough, temperatures drop about 4-5
degrees to the upper 60s and 70s with a few places reaching the
lower 80s in the Columbia Basin. Chances for 90 degree temperatures
are virtually zero and chances for 80 degrees are 80-100 percent in
the Columbia Basin and 45-70 in the rest of the lower elevations.

Thursday will see the deepening low arrive in the base of the trough
off the Pacific Northwest coast. A slight chance of showers with
very light rain amounts will continue mainly along the Cascades
though a few showers will develop in central Oregon in the late
afternoon. Models indicate some instability in the late afternoon in
central Oregon but the window appears to be short and the NBM
doesn`t add any thunderstorms so have kept them out of the forecast
for central Oregon. The chances of a tenth of an inch of rain are no
more than 15 percent along the Cascade crest. Temperatures will warm
1-2 degrees to the 70s and lower 80s. The Tri-Cities and Hermiston
will have a 15-25 percent chance of 90 degrees but chances will be
under 10 percent elsewhere.

By Friday, models have a deep trough but differ significantly as to
the location. About 60 percent of the model ensemble members have
the trough a few hundred miles offshore, 20 percent right at the
coast and the remaining 20 percent have a weaker trough centered
near the Cascades. The NBM favors the majority and has the southwest
flow developing a slight chance of showers along the Cascades and
central Oregon in the afternoon then into the eastern mountains in
the evening. Central Oregon has a slight chance of afternoon
thunderstorms as well. Rainfall amounts look to be very light with
chances of a tenth of an inch no more than 15 percent. Due to
cloudiness and showers, temperatures remain stationary in the mid to
upper 70s in central Oregon but the rest of the area warms another 3
degrees or so to the lower to mid 80s with mainly 70s in the
mountains. The Columbia Basin has a 30-55 percent chance of reaching
90 degrees while the rest of the area is under 25 percent.

On Saturday, model clusters show four significantly varying
solutions, each with support from 23-28 percent of the members, so
no one solution appears more likely than the others. The choices are
a trough with a strong closed low off the Oregon/California border,
a trough with a weaker low a few hundred miles further south off the
northern California coast, a strong trough with the low opening into
a wave at the northern California coast and a shallow trough with
its axis near the Idaho border. Most of these favor continued
showery conditions and the NBM has a chance of showers over the
mountains and a slight chance of showers elsewhere. It also have a
slight chance of afternoon thunderstorms over the eastern mountains.
It drops temperatures 3-5 degrees due to the clouds and showers to
the 70s and lower 80s.

Through the long term, the NBM brings breezy winds of 15 to 20 mph
through the eastern Columbia Gorge and Kittitas Valley, but the
chances of 25 mph winds in those locations are only 30-50 percent
Tuesday, Wednesday and Saturday and under 30 percent Thursday and
Friday. The rest of the area will have normal afternoon wind speeds
under 15 mph. Perry/83

&&

.AVIATION...06Z TAFs (Previous Discussion)...Quiet weather and
VFR conditions, consisting of winds of 10 kts or less coupled with
sky cover ranging from SKC to FEW-SCT mid-high cloud will persist
overnight through Saturday morning.

Saturday afternoon, confidence is very high (>90% chance) that
thunderstorms will initiate over the Cascades and propagate
northward through evening. Along the east slopes, confidence is
lower in shower and thunderstorm coverage...generally 30% or less
for central Oregon. By evening, showers and thunderstorms will
likely expand northward to north-central Oregon and south-central
Washington, impacting DLS and YKM (>50% chance of precipitation).
Afternoon thunderstorms along the Cascades are expected (>75%
chance) to produce outflow winds of 40-50 mph, with lower chances
(<20%) of 60 mph. These outflow winds will manifest as a stark
wind shift at DLS/PDT, and outflow may also reach the remainder of
TAF sites, though confidence in magnitude/timing is lower.
Showers and thunderstorms will be capable of producing temporary
MVFR or lower conditions. Plunkett/86

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  93  56  84  52 /  10  20   0   0
ALW  99  62  88  57 /   0  20   0   0
PSC  99  65  89  60 /   0  30   0   0
YKM  93  58  84  54 /  10  80  10   0
HRI  98  62  88  56 /   0  30   0   0
ELN  93  59  84  56 /   0  80  10   0
RDM  87  47  82  46 /  40  60   0   0
LGD  94  53  89  51 /   0  10   0   0
GCD  93  52  88  50 /   0  10   0   0
DLS  95  61  83  60 /  20  80  10   0

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Red Flag Warning from 2 PM this afternoon to 11 PM PDT this
     evening for ORZ641.

     Red Flag Warning from 1 PM this afternoon to 8 PM PDT this
     evening for ORZ642.

WA...Red Flag Warning from 5 PM this afternoon to 2 AM PDT Sunday for
     WAZ691.

     Red Flag Warning from 8 PM this evening to 5 AM PDT Sunday for
     WAZ690.

     Flash Flood Watch from 2 PM PDT this afternoon through late
     tonight for WAZ522-523.

&&

$$

SHORT TERM...74
LONG TERM....83
AVIATION...86