515
FXUS66 KPDT 250539
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
939 PM PST Sun Nov 24 2024

.Updated Aviation Discussion.


.AVIATION...06Z TAFS...VFR conditions currently at all sites, but
will degrade to MVFR due to reduced visibilities of 4SM for
KRDM/KBDN and lowered ceilings of OVC025 for the remaining sites.
The only exception will be KYKM which will experience IFR conditions
due to reduced visibilities of 3SM and ceilings of OVC015 toward the
end of the period. Light rain will be impacting KDLS/KPDT/KALW
overnight and KRDM/KBDN Monday afternoon as a weakening surface low
and upper level shortwave move into the Pacific Northwest. Winds
will stay variable and below 10 kts for most sites, with KALW being
the exception with gusts of up to 25 kts during the overnight
period. There is also a slight chance (30-40%) for fog developing in
and around KPSC during the early morning hours on Monday, but did
not include in the taf due to the low probability. 75



&&

.PREV DISCUSSION... /issued 655 PM PST Sun Nov 24 2024/

UPDATE...Latest satellite imagery shows an upper low offshore of
the PACNW with a shortwave trough moving NE along the Oregon
coast. A cloud shield and associated precipitation is over
western and southern Oregon. Took a look at the latest NBM QPF and
amounts look light and spotty through the day on Monday (mostly
less than .10 inch liquid). A bit more robust and organized
precipitation/QPF is expected Monday night and Tuesday as the
parent upper low finally swings inland across Oregon. Its a bit
breezy this evening along the foothills of northern Oregon near
the base of the Blue Mountains. The KMEH to KPDT pressure gradient
peaks at around 3.7 mb at 06Z so winds could approach advisory
levels for a short time. The current thinking is that a wind
advisory is not needed. However will monitor wind trends during
the remainder of the evening. The current forecast is on track for
tonight with only a few minor changes made to POPS and low
temperatures. 78

PREV DISCUSSION... /issued 253 PM PST Sun Nov 24 2024/

SHORT TERM...Tonight through Tuesday night...A large low pressure
trough off the eastern Pacific has circulated a series of
embedded shortwaves across the PacNW with periods of snow and
rain. This pattern will continue through Monday night, then the
trough and its parent low will move inland Monday night then
across WA/OR Tuesday. By Tuesday evening, the trough will exit the
region into ID followed by a drier northwest flow aloft. One of
the forecast challenges in the short term is the timing and
location of individual shortwaves. Models have been in agreement
with the overall synoptic pattern, but the devil has been in the
details. The good news is that rain and snow amounts will be
relatively light. 850-700 mb winds are from the south-southwest
and not favorable for significant upslope over the mountains, and
upper level support is relatively weak with no indications of an
atmospheric river. The likelihood of any winter highlights through
Tuesday night is low (20%) but one area that will need to be
monitored for snow accumulations will be along the John Day-Ochoco
Highlands Monday night and Tuesday.

Currently, a shortwave is off the central OR coast, as evident on
water vapor loops. There has been steady light precipitation along
the southern Washington Cascades, and occasional rain/snow mix
has been observed in the Kittitas and Yakima Valleys. Snoqualmie
Pass at 3000 feet has received 3 inches since last night. The
shortwave will lift north across the northern half of WA tonight
with precipitation tapering off. Another wave will follow right at
the heels and over western Oregon and the Cascades on Monday. The
heaviest snow will be on the west facing slopes due to the
southwest flow aloft. The chance of precipitation will increase
over eastern Oregon Monday night and Tuesday, especially from the
Blue Mtns eastward and south of Condon.

As previously mentioned, the area that will need to be monitored
for snow accumulations meeting advisory criteria will be in the
John Day-Ochoco Highlands. Precipitation will be steady Monday
night and Tuesday, as the main upper low tracks across the
southern half of Oregon. Snow levels will lower from near 5000
feet to 4000 feet,and there is around a 40% chance that this area
will observe 3-6 inches above 4500 feet. If confidence increases
and/or snow levels are forecast to be lower, a winter weather
advisory may be needed. Otherwise, snow accumulations in the
mountains/valleys will be light Monday night and
Tuesday...generally 3 inches or less (confidence 50-70%).
Wister/85

LONG TERM...WEDNESDAY THROUGH SUNDAY...Ensemble guidance is in very
good agreement that the remnants of the closed low currently
offshore will morph into an open wave trough as it tracks onshore,
exiting the region by early Wednesday. Operation guidance hints at a
weak shortwave trough embedded in the northwesterly flow aloft. This
may facilitate some orographic showers across the Cascade crest and
northern Blue Mountains through the day.

Uncertainty in the longitudinal placement of an upper-level ridge
axis appears to be the main driver of variance among ensemble NWP
Thursday through Saturday. Ensemble clusters suggest the most likely
scenario (65-75% of members) is dry conditions with the ridge axis
centered just offshore or over the PacNW. This solution would also
favor basin/valley fog coupled with cool to cold temperatures for
the entire forecast area. The alternate scenario (25-35% of members)
places the ridge axis a bit farther offshore with a shortwave trough
digging southeastward amidst northwesterly flow aloft. This solution
would keep persistent PoP chances across area mountains, with
temperatures cool enough for snow. Both scenarios favor mostly dry
conditions for the lower elevations.

Sunday, the most likely scenario provided by ensemble clusters is
persistent upper-level ridging over the PacNW, albeit with some
variance in strength (57% chance of a strong ridge, 20% chance of a
weaker offshore ridge axis). The remaining members (13%) advertise a
return of precipitation with an atmospheric river directed into the
PacNW.

Due to the aforementioned uncertainty, the NBM places slight chance
to chance PoPs across the Cascades and northern Blues through much
of the forecast period. Plunkett/86


&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  35  49  33  42 /  20  30  60  40
ALW  38  50  35  44 /  20  40  60  60
PSC  34  45  37  46 /  20  20  40  10
YKM  31  43  27  45 /  50  30  20  10
HRI  33  47  35  45 /  20  30  50  20
ELN  32  43  28  44 /  70  40  30  10
RDM  32  47  27  42 /  30  40  50  20
LGD  34  42  31  41 /  10  40  60  70
GCD  34  46  30  42 /  20  50  80  70
DLS  36  47  36  49 /  60  50  40  30

&&

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

$$

UPDATE...78
SHORT TERM...85
LONG TERM....86
AVIATION...75