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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS66 KPDT 171121
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
421 AM PDT Wed Apr 17 2024

Updated Aviation Discussion

.AVIATION...
-- Changed Discussion --
12Z TAFS...VFR for all sites for this period with
mostly clear skies and some clouds (FEW to BKN at 250 kft). Clouds
should be cleared out during the day with winds remaining light.
Feaster/97
-- End Changed Discussion --

&&

.SHORT TERM...Today through Friday night...Near- to sub-freezing
morning low temperatures remains the biggest highlight of the
short-term period as multiple public forecast zones became
eligible for freeze warnings on April 15th. Otherwise, offshore
winds, locally breezy in north-central OR and south-central WA,
are forecast to develop this afternoon through Friday as a
surface thermal trough sets up west of the Cascades in response to
an offshore upper-level ridge. Our forecast area will remain
under a northwest flow aloft on the back side of an upper-level
trough over the northern Rockies through Friday before the ridge
axis pushes inland Friday night. The result will be mostly dry
weather aside from a slight chance (15% chance) of isolated
afternoon showers this afternoon and Thursday afternoon for the
northern Blues, Wallowas, and Washington Cascades.

Have opted to issue a freeze warning for the foothills of the
northern Blues of OR this morning as numerous surface
observations had approached or dipped below freezing as of 2-3am.
Confidence is very high (>90% chance) that cold-prone locations of
the zone will see sub-freezing temperatures this morning.
Elsewhere, in Pendleton temperatures have yet to drop below 34 as
a weak cross-Cascade pressure gradient has kept winds just strong
enough to mix the boundary layer. One other potential mode of
failure for the warning is a mid-level cloud deck that remains
draped over the northern Blues. The westward extent of the cloud
deck has intermittently blanketed the foothills, thus keeping a
lid on what would otherwise be a good setup for radiational
cooling with low PWATs (0.2-0.3") and surface dew points that are
substantially below freezing area-wide. Thus, areas that do
manage to see clear skies and decouple will likely drop to or
below freezing.

Thursday morning appears to be another potential freezing morning
for the Kittitas and Yakima valleys, lower Columbia Basin of
OR/WA, and the foothills of the northern Blues of OR/WA. NBM
probabilities paint the best chances of sub-freezing temperatures
along the northern Blue Mountains of OR - broadly 30-60%, and low
chances (<30%) for the other forecast zones. Moreover, the
synoptic setting is supportive with a dry air mass (PWATs of
0.15-0.3") and weaker surface pressure gradients near morning
leading to higher confidence in sustained decoupling and
subsequent near-surface inversion development. That said, much
like this morning, some low cloud cover may remain over the region
and is the most likely reason temperatures would remain warmer
than forecast.

Offshore-oriented pressure gradients are anticipated to strengthen
Friday, so while the NBM is advertising similar probabilities of
freezing morning lows compared to Thursday, forecaster confidence
is lower (<30% chance). Plunkett/86

&&

.LONG TERM...Saturday through Monday...The long term will be
characterized as a typical spring set of pattern shifts. First, an
upper level ridge will be over the region Saturday keeping the
region dry followed by a relatively weak upper level trough
Saturday night into Sunday morning. Snow levels will be above 5000
ft Saturday lowering to near 3500 ft as the upper level trough
moves into the PacNW Saturday night. The trough will bring
mountain precipitation Sunday before giving way to the next
incoming upper level ridge. Southwest to westerly flow will
dominate the region through Monday.

Models are in relatively good agreement through the period. The
upper level ridge will be in place over the region Saturday with
dry conditions in place. Models show that by Saturday night weak
upper level trough will move southeastward towards the region and
onshore. The ensembles show a 30-50% chance of precipitation
amounts to 0.10 inches of rain with a dusting of snow along the
crests into Sunday morning. This will quickly drop off as the
leading edge of another upper level ridge traverses into the
region Sunday afternoon and continuing through Monday. Models
remain in firm agreement and looking at the deterministic
guidance, the main variances are in the timing of the systems.
Upper level winds will be primarily southwest to west and
diurnally breezy with the highest winds occurring Sunday as the
upper level trough moves across the Cascades. Winds will be breezy
with only 10-20% of the raw ensembles showing peak sustained
winds being above 20 mph during the trough passage.

Lastly, temperatures will average in the low to mid 70s through
the Basin and adjacent valleys with 50% of the ensembles in
agreement. 50-60% of the ensembles show mid to upper 60s through
the Gorge, Central/North Central OR, foothills of the Blues and
finally, 62% of the ensembles show low to mid 50s for the
mountains. 40-50% of the ensembles show it will be chilly with the
average nightly temperatures to be in the upper 30s to low 40s
across much of the region and low to mid 30s along the crests.
Bennese/90

From Tuesday morning, a upper-level ridge continues passing over
PacNW eastward, becoming a zonal pattern by Tuesday night as a
trough moves near the OR coastline Wednesday into Thursday
morning. Dry conditions will remain throughout Tuesday until
Wednesday night when the trough brings rain/snow in the low
elevations and mountain areas, with snow accumulation and QPF
reaching less than 1 inch (30- 40% confidence). Snow levels will
be around 5000-8000ft Tuesday into Thursday morning the ridge
passes over the PacNW before the trough arrives. Temperatures will
warm several degrees these few days with 50s along the crests of
the Cascades and 60s to mid 70s at the mid-low elevations. Breezy
conditions will occur with wind gusts up to about 24 mph across
the Simcoe Highlands Tuesday until Wednesday late morning as winds
become light again. Feaster/97

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  55  31  60  32 /   0   0   0   0
ALW  59  34  63  35 /   0   0   0   0
PSC  63  36  67  39 /   0   0   0   0
YKM  61  32  64  34 /  10   0   0   0
HRI  61  33  65  35 /   0   0   0   0
ELN  59  33  61  33 /  10  10   0   0
RDM  53  26  60  29 /   0   0   0   0
LGD  51  28  57  29 /  10   0   0   0
GCD  52  27  59  31 /   0   0   0   0
DLS  62  38  66  40 /   0   0   0   0

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Freeze Warning until 8 AM PDT this morning for ORZ507.

WA...None.
&&

$$

SHORT TERM...86
LONG TERM....90/97
AVIATION...97