028
FXUS66 KPDT 211123 AAA
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
423 AM PDT Tue May 21 2024

Updated Aviation Discussion

.SHORT TERM...Today through Thursday...Little to note across the
forecast area this early morning as a dry northwest flow aloft
keeps conditions cool and quiet. While area radar imagery shows
very little, nighttime satellite imagery is showing scattered
bands of high clouds beginning to filter into the PacNW ahead of
the next weather system.

The quiet and dry conditions across the forecast area will
continue through the morning hours, but by the afternoon a warm
front arriving ahead of an upper low will begin to spread light
to moderate rain across the forecast area, with the heaviest
rainfall occurring along the Cascade crest, Blues, and the
Wallowas. Steady rainfall will continue through the overnight
hours across the aforementioned mountain areas, with probabilities
of at least 0.5 inches of rainfall between 50-60%, and at least
0.75 inches between 20-30%. Breezy to locally breezy winds will
also develop with the frontal passage this afternoon, with
sustained winds between 15-20 mph and gusts 25-30 mph in the lower
elevations. As the upper closed low drops into the PacNW with the
left entrance region of a stacked jet between 250mb-500mb over
northeast OR, precipitation will switch from steady stratiform to
persistent light to moderate showers during the morning hours
Wednesday and continue into the evening hours. While the cold core
of the upper low will allow for graupel to develop within the
showers, increasing instability and steepening mid- level lapse
rates across the eastern mountains will lead to a slight chance
(15-20%) of isolated thunderstorms developing. Snow levels will
drop from 6kft-7kft Tuesday night to 4kft-4.5kft Wednesday morning
across the mountains, introducing chances of snow showers across
mainly the higher mountain areas with accumulations up to 1 inch
near Tollgate. The arrival of the upper low overhead will also
lead to breezy to windy westerly winds developing through the
Cascade gaps and into central OR, Simcoe Highlands, and the OR
Columbia Basin. Along the Simcoe Highlands and the OR Columbia
Basin, NBM probabilities for sustained winds 30mph or greater
reach into the 60-75% range, with 70-80% probabilities for gusts
to exceed 45mph in the afternoon hours.

As the upper low moves into central ID Wednesday night, a dry
northwest flow aloft will begin to develop over the PacNW, pushing
any remaining shower chances to far southeastern WA and
northeastern OR. By Thursday afternoon, only a few isolated
showers will be possible over Wallowa county, with dry conditions
elsewhere. Winds will have also weakened area wide for Thursday,
with only locally breezy west to southwest winds through the
Columbia Basin and along the Blue mountain foothills. Lawhorn/82

.LONG TERM...Friday through Tuesday...Deterministic models and
ensembles are in very good agreement on the 500 mb pattern for
Friday through the weekend then subtle differences develop
early next week on how close an upper level trough will
approach the PACNW coast. On Friday an upper low will move SEWD
across British Columbia and then across WA/ORE on Saturday. This
will result in increasing chances for showers with 30-50% POPS over
the mountains and foothills and 10-20% POPS elsewhere beginning
Friday night and continuing through Saturday afternoon/early
evening. QPF amounts are forecast to be modest with NBM 50th
percentile amounts of .10-.25 inches for the 24 hour period ending
00Z Sunday for the northeast mountains and Cascade crest. One caveat
to this is that both the deterministic GFS and ECMWF are showing
heavier amounts of QPF with this system. Thus QPF forecasts will
likely need to be fine tuned as the event gets closer.

Best potential for thunderstorms (15-20%) will be over the eastern
mountains on Saturday afternoon and early evening as the upper low
passes over the region. Forecast soundings are not very impressive.
Though there are steep low to mid level lapse rates, CAPE values are
only around 200 J/kg. Vertical shear is also quite weak so expect
disorganized and isolated storms with only sporadic LTG strikes. It
will also be breezy to locally windy on Saturday especially in those
lower elevation locations that are windy under westerly flow
regimes. Almost all of the 50 ensemble members of the ECMWF show
peak wind gusts of 35-45 mph at KELN on Saturday afternoon with a
mean of 40 mph. Likewise the NBM 4.2 shows a peak wind gust of 40
mph at KELN at 23Z on Saturday. Thus there is some potential (40-
45%) that a wind highlight may be needed for the Kittitas Valley on
Saturday.

After the upper low exits the area Saturday evening the ensembles
show a rather pronounced upper level ridge building over the Rockies
with an upper trough gradually approaching the coast. This will put
the region in a drier, warming SW flow aloft for Sunday into early
next week. High temperatures will warm from a few degrees below
normal on Sunday to as much as 10 degrees above normal by Tuesday.
On Tuesday high temperatures will be well into the 80s in the
Columbia Basin. The NBM Probabilities of high temperatures exceeding
90 degrees on Tuesday for the Columbia Basin and Yakima Valley are
currently 20-30%. The ensemble clusters mostly support this dry/warm
scenario for early next week but one of the clusters that is heavily
weighted by the Canadian model moves the offshore trough closer to
the coast sooner which could result in increasing chances for
showers as early as Monday afternoon or Monday night. The
probability of this occurring is around 20%. Thus for now went with
the NBM which holds off on any precipitation east of the crest until
Tuesday evening. 78


&&

.AVIATION...12Z TAFs...VFR conditions highly likely (95-100% chance)
at all sites through Tuesday early afternoon, though an incoming
weather system will usher in BKN mid and high-level clouds. This
afternoon and evening, OVC CIGs and -RA will spread across eastern
Oregon and eastern Washington. Chances of MVFR CIGs increase to 60-
70% tonight. Forecast soundings and NBM suggest the best chances of
BKN-OVC MVFR CIGs would be at PDT/ALW/KBDN/KRDM. Winds will pick up
this morning and generally be 10-15 kt sustained with gusts 15 to 25
kt through the remainder of the period, though the gustiness in the
winds will tend to subside after sunset. 78

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  68  46  57  42 /  20  90  70  50
ALW  70  47  60  45 /  20  90  80  50
PSC  71  53  64  48 /  20  80  50  30
YKM  67  42  61  41 /  50  50  20  10
HRI  71  50  63  46 /  20  90  50  30
ELN  61  39  56  43 /  60  60  30  10
RDM  65  43  53  33 /  20  70  20  10
LGD  66  43  53  39 /  10  90  80  70
GCD  66  43  53  37 /  10  90  80  70
DLS  66  48  62  47 /  50  70  30  10

&&

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

$$

SHORT TERM...82
LONG TERM....78
AVIATION...86