830
FXUS66 KPDT 232136
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
236 PM PDT Tue Jul 23 2024

.SHORT TERM...Tonight through Thursday...Smoky to mostly clear
conditions across the forecast area this afternoon as the PacNW
sits between an upper low in the northeast Pacific and an upper
ridge extending from the Desert Southwest into the northern
Rockies. Thanks to the upper low and an associated marine push
yesterday, temperatures have cooled significantly across the
region, with this trend expected to continue through the remainder
of the week.

Through Thursday, the upper low will be the main driver of
activity across the region as it slowly pushes east into western
Canada. Through the rest of today, however, relatively cooler
and quiet conditions will persist through tonight, with locally
breezy winds through the Cascade Gaps.

Wednesday, the upper low will push an upper shortwave trough into
the PacNW with a southwest flow aloft being maintained throughout
the day. As a result of the trough/flow regime, a modest plume of
mid-level moisture will be pulled across portions of eastern OR
and far southeastern WA. As far as moisture goes, HREF members
show mean PWATS ranging from 0.85 to 1.1 inches across the eastern
mountains and the Blue Mountain foothills in the afternoon, with
probabilities of 1 inch of PWATS ranging from 70-90% in this area.
With the southwest flow aloft, instability will also be increasing
from south central OR to far southeast WA. Both NBM and HREF means
show surface CAPE values between 500-750 J/kg across this area with
the addition of a developing mid to upper level jet producing
0-6km shear values between 35-50kts. These conditions combined
with the increased moisture lead to high confidence(70-90%) in
isolated to widely scattered thunderstorms developing in the
afternoon through the late evening. These thunderstorms will be
capable of producing abundant lightning, small hail, strong gusty
outflow winds, with brief downpours. There is a low threat (10-15%
chance) for flash flooding from individual storms in canyon and
burn scar areas, though multiple storms training over the same
area may lead to localized flash flooding. The approaching upper
shortwave trough will also tighten the cross Cascade pressure
gradient Wednesday, resulting in breezy winds developing through
the Cascade gaps and into portions of the Columbia Basin.

Thursday, the upper low will be directly north of the PacNW
throughout much of the day, placing the region under a west
southwest flow aloft. This will effectively cut-off any mid to
upper level moisture support into the PacNW, resulting into very
little to no thunderstorm chances. However, pressure gradients
across the Cascades will remain tight, resulting in another day of
breezy winds through the Cascade gaps into portions of the
Columbia Basin. Lawhorn/82

.LONG TERM...Friday through Tuesday...

Key Messages:

1. Temperatures within a couple of degrees of normal Friday through
Tuesday.

2. Breezy afternoon winds through the Cascade gaps each day.

3. Little chance of showers or thunderstorms except for a 10-15
percent of very light showers over the Washington Cascade crest
Monday night and early Tuesday morning.


Models are in good agreement in having a trough over the Pacific
Northwest through next weekend before a few differences develop
Monday, Differences increase on Tuesday as models split between
having a ridge building into the area from the desert southwest or
continuing to have our area under the trough. The Extreme Forecast
Index indicates little in the way of unusual weather in the long
term period. Temperatures will be a couple of degrees below normal
Friday and Saturday then warm to a couple of degrees above normal
early next week. Breezy winds are expected through the Cascade gaps
each afternoon though chances of 25 mph winds will be negligible
except in the Kittitas Valley (20-40 percent) and the Columbia Gorge
(15-25 percent). Both of those locations have 20-50 percent chances
of 35 mph wind gusts but no more than 10-15 percent of 40 mph wind
gusts.

Friday will see a trough over western Canada and the Pacific
Northwest with a strong low in northern Alberta and Saskatchewan.
This gives us a generally westerly flow over our area. Models also
have a weak closed low or open wave off the central California coast
which should not affect our area during the day. Friday night the
upper low moves eastward but the trough remains overhead, The weak
low/wave moves into northern California Friday night and gets
absorbed into the main flow which turns more southwesterly. Highs on
Friday will be in the mid to upper 80s with mid 70s to lower 80s in
the mountains. Columbia Basin and central Oregon locations have a 30-
55 percent chance of reaching 90 degrees. Friday night lows will be
in the mid 40s to mid 50s.

On Saturday, the upper low in Canada moves east and ceases
influencing our weather. The Pacific Northwest remains under a broad
and shallow trough with a west to southwest flow aloft. Temperatures
warm a couple of degrees to the upper 80s to lower 90s with upper
70s to mid 80s in the mountains. Chances of 90 degree temperatures
rise to 60-90 percent in the Columbia Basin and 35-65 percent in
central Oregon. Overnight lows Sunday morning will be similar to
Saturday morning except in the Columbia Basin which will be a couple
of degrees warmer. Winds will continue to be generally light though
10 to 20 mph winds will develop in the afternoon through the Cascade
gaps and in central Oregon.

Sunday is very similar to Saturday as models maintain the trough
over our area. Model ensemble member clusters show very few
differences as to the strength and location of the trough.
Temperatures are little changed from Saturday though afternoon winds
look to be a couple mph faster. The chances for 90 degree
temperatures are similar to Saturday. As with Friday and Saturday,
there is very little chance of showers and thunderstorms.

On Monday, changes begin to manifest themselves in the models. A
ridge approaches the Gulf of Alaska while a weak closed low develops
about 1000 miles west of northern California (GFS and ECMWF) or
about 500 miles west of the Oregon/California border. 55 percent of
the model ensemble members favor the more distant closed low which
keeps a trough overhead with the trough axis offshore. The other 45
percent have a much shallower trough over our area. Models warm
temperatures a couple of degrees to the upper 80s to mid 90s with
upper 70s to mid 80s in the mountains. The chances of 90 degree
temperatures rise to 65 to 95 percent in the lower elevations. There
will continue to be 10 to 20 mph winds in the afternoon. Models show
a wave moving through British Columbia Monday night with a few light
rain showers possible along the Washington Cascade crest.

By Tuesday, model clusters trend towards having a ridge over the
desert southwest building towards the Pacific Northwest (69 percent
of model ensemble members) while the others keep a trough at the
coast or just offshore. The possibility of ridging has warmed
temperatures a degree or two. The uncertainty in the models lowers
the chances of 90 degree temperatures to 55-85 percent despite the
forecast temperatures being slightly warmer. There will be a very
slight chance of showers along the Washington Cascade crest in the
morning. Once again, west to northwest winds of 10 to 20 mph are
expected in the afternoon. Perry/83


&&

.AVIATION...Previous Discussion...18Z TAFs...Clear skies and VFR
conditions will be prevalent though SCT high clouds above 20K feet
AGL will begin moving into KRDM and KBDN after 09Z tonight. Hazy
conditions from area wildfires will continue with possible thick
smoke layers aloft. Winds will be mainly 5-15 kts though KDLS will
have northwest winds of 10-20 kts with gusts to 30 kts from
22Z-10Z. Perry/83


&&

.FIRE WEATHER...Red Flag Warnings in effect from Wednesday
afternoon through evening for fire weather zones OR642, 645,
WA692, and 693 for abundant lightning. Fire Weather Watches are in
effect from Wednesday afternoon through evening for fire weather
zones OR643 and 644 for abundant lightning. An upper level low
pressure system approaching the region will lead to the
development of isolated to widely scattered thunderstorms across
mainly the Blue mountains. Thunderstorms will be capable of gusty
outflow winds, especially across the southern Blues, Strawberrys,
and portions of the Wallowa district. Abundant lightning, small
hail, and brief downpours will also be possible with any
developing thunderstorms.


&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  57  92  56  80 /   0  10   0   0
ALW  62  95  60  84 /   0  10  10   0
PSC  61  96  62  85 /   0   0   0   0
YKM  59  93  56  82 /   0   0   0   0
HRI  60  95  59  85 /   0   0   0   0
ELN  58  86  54  77 /   0   0   0   0
RDM  53  93  50  82 /   0   0   0   0
LGD  58  97  55  82 /   0  30  30   0
GCD  61 101  57  87 /   0  40  20   0
DLS  59  88  59  81 /   0   0   0   0

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Fire Weather Watch from Wednesday afternoon through Wednesday
     evening for ORZ643-644.

     Red Flag Warning from noon to 11 PM PDT Wednesday for ORZ642-645.

WA...Red Flag Warning from 2 PM to 11 PM PDT Wednesday for WAZ692-693.

&&

$$

SHORT TERM...82
LONG TERM....83
AVIATION...83