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000
FXUS66 KPDT 181719
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
1019 AM PDT Thu Apr 18 2024

Updated aviation discussion

.AVIATION...18Z TAFS...VFR conditions with clear skies today but
then some increasing high clouds for RDM and BDN late tonight into
Friday. Winds will become N-NE 5-15kts by afternoon and then become
less than 5 kts overnight.

.UPDATE...Generally quiet weather for today and tonight.
Temperatures have risen fairly quickly this morning and are now
mainly from the mid to upper 30s into the 40s in the freeze warning
area, so the freeze warning has been cancelled. Will assess the need
for additional freeze warnings tonight as the morning model data
arrives. Otherwise, few changes were made to the current forecast.

&&

.PREV DISCUSSION... /issued 415 AM PDT Thu Apr 18 2024/

SHORT TERM...Today through Saturday night...Nighttime microphysics
RGB imagery reveals mostly clear skies across the forecast area
early this morning. The region is located under a dry northwesterly
flow aloft on the back side of a broad trough downstream over the
Northern Great Plains. Offshore, an upper- level ridge axis is
apparent and is tracking slowly eastward.
Coupled with low PWATs, low surface dew points, and a relatively
weak offshore-oriented surface pressure gradient translating to
light winds (<10 kts for most non-ridgetop areas), temperatures
have dropped to near freezing for most lower-elevation zones. The
freeze warnings for the Yakima and Kittitas valleys, lower
Columbia Basin of OR/WA, and foothills of the northern Blues of
OR/WA remain in place through 9am this morning.

This afternoon, isolated showers are forecast to develop,
supported by weak instability and some modest upper-level forcing,
but chances of measurable precipitation are very low (5-14%)
forecast for the WA Cascades and Blues of NE OR and SE WA.

As the inverted surface pressure trough strengthens this
afternoon and Friday, offshore east to northeast winds are
forecast to become locally breezy for north-central OR and south-
central WA especially along ridgetops.

Friday morning, chances of freezing temperatures are lower than
this morning, but the same zones in this morning`s freeze warnings
have a 20-60% chance of freezing temperatures per NBM
probabilities so additional highlights may be needed.

Saturday morning, the upper-level ridge axis is expected to move
over the PacNW. Saturday afternoon through Saturday evening,
ensemble guidance is showing a pattern change with an upper-level
trough moving into the PacNW. While ensemble clusters are not
available for analysis this morning, ensemble mean H500 and
vorticity are exhibiting some differences in timing and location.
The best chances of precipitation will be across the mountains
(25-50% for the Blues, 55-90% for the Cascade crest and eastern
slopes) with lower chances (10-40%) for the lower elevations.
Breezy westerly winds are forecast with the trough passage, but
confidence is currently low (<40% chance) in reaching advisory
criteria. Plunkett/86

LONG TERM...Sunday through Tuesday...The long term will begin
with the remnants of the weak upper level trough that will move
over the region. The precipitation with this upper level shortwave
will be light an mainly across the Cascades bringing a light
dusting of snow. By Sunday night the shortwave will dissipate
making way for the leading edge of an upper level ridge to make
its way towards the region bringing in the dry and warm
temperatures through Wednesday.

Sunday models are in relatively firm agreement through the period
with the only variances being in the positioning of the upper
level trough with a very slight timing variance. With this upper
level system, a brief down tick in the snow levels will allow
Snoqualmie Pass to see snow. 70-90% of the raw ensembles show up
to 0.3 inches of snow along Snoqualmie Pass with higher amounts
along the crests. Much of the energy associated with this system
is poised across the WA Cascades with raw ensembles showing less
than 30% chances of up to 0.10 inches along the OR Cascades and
the Blues. Pressure gradients will continue to tighten as the
trough slips over the mountains and raw ensembles show 30-50%
probabilities winds will be sustained winds of 20 to 25 mph along
the lower Columbia Basin and the foothills of the Blues.
Temperatures will remain in the upper 50s to low 60s across the
region and mid 60s through the Basin, with the majority (90%) of
the ensembles in agreement.

Monday through Tuesday the models remain in firm agreement with
the upper level ridge making its way onshore. Models show the
leading edge of the ridge to be encroaching the region Monday with
the variances being in the timing and amplitude of the ridge. Dry
and warming conditions will accompany the ridge with global
ensembles all showing agreement to an uptick in temperatures. Raw
ensembles show over 60% probabilities Central/N Central OR and the
foothills of the Blues will see temperatures in the mid 60s, over
50% show the Basin in the high 60s to low 70s. Over 50% of the
global ensembles show that Tuesday will see temperatures rise
another 2-5 degrees. Winds will be mostly from the southwest and
becoming diurnally breezy. Bennese/90

Wednesday morning to Thursday night, a closed low moves over from
the OR coast with precipitation that will impact the PacNW as it
continues to move southerly towards the CA coast. Mostly rain will
be impactful in the low elevations with some very light snow in
the Cascades crests and Blue Mountains, with snow accumulations
less than 0.01 inches and QPF less than 1 inch (30-40%
confidence). Snow levels will be around 5000-7000ft Wednesday into
Thursday night as the trough passes. Temperatures continue being
in the 50s along the crest of the Cascades and 60s to low 70s at
the low elevations (>60% confidence). Breezy conditions will occur
with wind gusts up to about 22 mph across the Simcoe Highlands
Wednesday afternoon but might come and go as throughout Thursday.
Feaster/97

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  59  31  63  35 /   0   0   0   0
ALW  62  34  66  38 /   0   0   0   0
PSC  65  38  68  40 /   0   0   0   0
YKM  64  34  65  34 /   0   0   0   0
HRI  64  34  68  37 /   0   0   0   0
ELN  61  33  61  33 /   0   0   0   0
RDM  60  28  62  33 /   0   0   0   0
LGD  57  29  60  34 /   0   0   0   0
GCD  58  30  63  36 /   0   0   0   0
DLS  66  39  67  42 /   0   0   0   0

&&

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

$$

SHORT TERM...77
LONG TERM....90
AVIATION...90