Area Forecast Discussion
Issued by NWS Pendleton, OR

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FXUS66 KPDT 281616
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
916 AM PDT Thu Mar 28 2024

.SHORT TERM UPDATE...It will be another day of rain and mountain
showers as the forecast area lies under a broad cyclonic flow that
extends from the eastern Pacific across the western U.S. Mid-lapse
rates are currently 7.5C/km, based on SPC Mesoscale Analysis. Due
to more instability today, there is a slightly better chance for
thunderstorms compared to yesterday. SBCAPES from CAMs are around
150-200 J/kg along with 30kts of shear between 0-6km. However, the
composite reflectivity progs from the HREF members are not
impressive and the probability of lightning is only around 20%.
Some of these showers could bring brief periods of moderate
precipitation, and this includes the snow in the mountains. This
was observed yesterday with showers that developed over the
northern Blues and the foothills. Special weather statements may
be required if heavy showers develop. Wister/85

&&

.PREV DISCUSSION... /issued 449 AM PDT Thu Mar 28 2024/

Updated Aviation Discussion.

SHORT TERM...Today through Saturday night...A deep, closed
upper-level low continues to churn in the Pacific, currently
centered at roughly 47N, 131W, about 350 miles offshore. Isolated
light shower activity is evident on radar, but most areas will
remain dry into morning. Ensemble and deterministic NWP guidance
are in excellent agreement that the upper low will remain stalled
offshore through the day with the forecast area under a west to
southwest flow aloft. The main talking point for today`s forecast
is another round of showers for the forecast area, focused mostly
on the mountains with 75-100% PoPs for the Cascades, 55-85% PoPs
for the Blues, with lower chances for the foothills and lower
elevations (mostly 25-50% PoPs). With steepening low- and mid-
level lapse rates this afternoon, and 00Z HREF-advertised MUCAPE
of roughly 200-400 J/kg, have opted to include a slight chance
mention of thunder with afternoon and evening shower activity for
the Blue Mountains and adjacent foothills. Soundings suggest
shallow convection with echo tops of 20-25 kft, and NBM
probabilities are low (10-20%) for thunder at any given
location.

Friday, confidence is high (>95% chance) that the upper-level low
offshore will fill and weaken while a secondary vorticity max
sliding down from the northwest prompts the development of a new
circulation off the California coast. Another round of isolated
afternoon showers is forecast for the mountains and foothills with
drier conditions (aside from a 5-15% chance of isolated showers in
the mountains on Saturday afternoon) returning for Friday night
through Saturday night. Plunkett/86

LONG TERM...Sunday through Thursday...The extended period is
characterized by an upper level ridge building through Monday
before a pair of shortwaves midweek bring a return to rain and
high elevation mountain snow chances to the area. The ridge will
bring a quick warming trend through Tuesday as high temperatures
may break into the low to mid-70s across lower elevations of the
Basin. The primary weather concern during this period resides with
elevated winds across Central Oregon and the Grande Ronde Valley
Sunday, with breezy conditions returning Wednesday along the
Columbia Basin, Eastern Gorge, and the Kittitas Valley.

Deterministic models and 51% of ensemble members highlight an
upper level shortwave diving south early Sunday, ahead of the
building upper level ridge that encroaches into the Pacific
Northwest from the west late Sunday into Monday. This synoptic
feature will bring breezy north winds across the area, but will
also produce a pressure gradient across Blue Mountains. The GFS
showcases a 4.15 mb difference between Meacham and Baker City
Sunday afternoon, which correlates to gusts of 25-30 mph.
Confidence in these wind gusts are moderate to high (60-80%) as
the NBM highlights an 82% and 85% chance of 25 mph gusts at John
Day and La Grande, respectively. Elevated winds may also extend
further west, as the NBM suggests a 55-60% chance of 25 mph wind
gusts reaching the Bend/Redmond area Sunday afternoon.

Ensembles are in good agreement regarding the upper level ridge
building in from the west Sunday afternoon through Monday. This
will relate to high temperatures reaching into the 70s for lower
elevations of the Basin Monday and Tuesday before the ridge
shifts east and weakens. Models and ensemble members are also in
agreement that Tuesday will be the warmest day, as the NBM
suggests 76 degrees in the Tri-Cities, 74 in Hermiston, and 73 for
Yakima and Pendleton - which is 10-13 degrees above normal.
Confidence in these high temperatures are moderate to high
(75-90%) as the ECMWF EFI showcases above climatological high
temperatures across the area, and the NBM highlights a 85-95%
chance of highs reaching 70 degrees or above across the Basin.

The upper level ridge weakens and slides east as a weak shortwave
passes to our north late Tuesday ahead of a stronger shortwave
poised to move onshore Wednesday afternoon. The first shortwave
will allow for a chance (15-40%) of precipitation over our
mountain zones Tuesday afternoon before chances sneak down to
lower elevations of the foothills and east slopes on Wednesday
with the arrival of the stronger shortwave. Limited moisture will
be present with either feature, with the main impact being
elevated winds through the Basin, Gorge, and Kittitas Valley.
Gusts of 30-40 mph out of the west will be possible as the NBM
suggests a 50-70% chance of wind gusts of 40 mph or greater across
the Simcoe Highlands, Eastern Gorge, Blue Mountain foothills, and
the Yakima/Kittitas Valleys. Confidence in these wind gusts is
moderate (50-60%) as the GFS highlights a 12 mb pressure gradient
setting up between Portland and Spokane and ECMWF ensembles
suggest a 6-hour max gust of 38 mph at the Dalles Wednesday
afternoon/early evening.

The uncertainty is timing and strength of the ridge and subsequent
shortwaves, as the ECMWF brings a much more substantial shortwave
through midweek along with a weaker ridge early in the period.
This would relate to cooler and windier conditions with the ECMWF
than the GFS. Current analysis between deterministic and ensemble
solutions do align more with the ECMWF result via the cluster
phase space. Thus, the NBM was utilized through the extended
period to provide an applicably weighted scenario. 75

AVIATION...12Z TAFS...VFR Conditions currently across all sites,
which will stay the course through the period. Light showers will
linger through the morning for KALW and extend into the early
evening. Light showers will also be possible for KPDT and KDLS
through the afternoon before dissipating into the early evening.
These showers will drop ceilings to between 5kft-7kft, but will
improve to 10kft toward the end of the period. Winds will increase
in the afternoon for KRDM/KBDN/KYKM to 20-25kts out of the
southwest, and will linger through much of the evening for KYKM. 75

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  54  33  54  32 /  40  30  20   0
ALW  56  37  57  34 /  50  40  40  10
PSC  61  39  61  36 /  20  10   0   0
YKM  56  33  59  30 /  20  10  10   0
HRI  60  35  59  34 /  20  20  10   0
ELN  53  31  56  30 /  30  20  10   0
RDM  50  30  52  27 /  30  20  10  10
LGD  50  31  51  31 /  60  60  40  10
GCD  48  29  50  29 /  60  40  40  10
DLS  56  38  59  37 /  60  30  10   0

&&

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

$$

SHORT TERM...85
LONG TERM....75
AVIATION...75


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