000
FXUS66 KPDT 161644
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
944 AM PDT Tue Apr 16 2024

.UPDATE...Current radar and visible satellite imagery showing
spotty light returns along the west slopes of the Cascades as
high level clouds attempt to exit to our east. These conditions
are a response from an upper level trough that is dropping through
Washington today before slowly shifting to our east later in the
week. Due to this synoptic feature, tightening pressure gradients
coupled with westerly flow aloft will lead to another breezy day
across the east slopes and portions of the Lower Columbia Basin.
Winds are expected to be lower than yesterday, but will still
reach advisory criteria across the Kittitas Valley as the winds
slowly increase this morning and peak through the afternoon and
evening. Thus, a Wind Advisory has been issued for the Kittitas
Valley between 1 PM and 8 PM later today as wind gusts of up to 50
mph out of the northwest will be possible. Confidence is high
(80-90%) in these gusts as the NBM showcases a 61% chance of wind
gusts of 47 mph or greater for Ellensburg this afternoon, with the
GFS, NAM, and SREF all suggesting a pressure gradient of 5-7 mb
between Seattle and Pasco. A gradient of 4.1 mb was observed
earlier this morning, which coincided with observations at the
Ellensburg airport reaching gusts of between 30-40 mph.

A secondary weather concern resides with frigid temperatures
tonight into Wednesday. High temperature today will be 4-8 degrees
cooler than on Monday, with values peaking in the upper 50s to low
60s across the Lower Columbia Basin and in the mid-50s through
Central Oregon. Flow aloft will shift from the west to the north-
northwest this evening as clouds attempt to clear and winds
slacken in the Basin. These conditions will allow for efficient
radiational cooling to occur along the foothills of the northern
Blue Mountain of Oregon, but the limiting factor will be how
quickly cloud cover clears. This looks to be delayed and may not
fully clear in some areas, which would keep temperatures slightly
warmer and above freezing. Confidence is further gleaned from the
NBM, which highlights only a 15-30% chance of Wednesday morning
temperatures dropping to 32 or below across the northern Blue
Mountains. The NBM also suggests a 25th and 75th percentile of 33
and 36 degrees at Pendleton, with the deterministic solution
forecasting a low temperature of 33 degrees. Winds will keep the
lower atmosphere mixed over the Kittitas and Yakima Valleys, so
Wednesday morning temperatures are expected to stay above
freezing or briefly touch 32 degrees in isolated, protected areas.
At this time freeze highlights are not anticipated for the area,
but will continue to analyze through the morning to confirm
guidance initializes well with expected cloud cover. 75


&&

.PREV DISCUSSION... /issued 421 AM PDT Tue Apr 16 2024/

SHORT TERM...Today through Thursday night...Although winds have
decreased overnight, breezy to windy conditions remain for areas
in and around the Lower Columbia Basin. A strong westerly flow
aloft and a tight westerly pressure gradient will be present
throughout the day. An upper level trough along the WA/BC border
will sag southward today which will continue the cold air
advection. The Kittitas Valley observed high winds yesterday.
Winds will not be as strong as yesterday, but there is high enough
confidence to issue another Wind Advisory for this afternoon
through early evening covering I-90 from Ellensburg to Vantage.
Moisture ahead of the trough mostly high level and shallow, but
there is a Puget Sound Convergence Zone that has set up in the WNW
flow, and KATX Doppler Radar shows light precipitation north of
Snoqualmie Pass. Current temperatures east of Snoqualmie Summit
are in the mid 30s and road temps are in the mid 40s, so it is
highly unlikely for snow accumulations to be observed east of the
summit this morning.

Cold and drier air has spread across the forecast area with
dewpoints currently in the 20s to lower 30s. One forecast
challenge tonight will be whether the Kittitas Valley, Yakima
Valley, and the Oregon portion of the northern Blue Mtn Foothills
will have freezing temps--locations where the growing season
usually begins in mid April. Forecast has been consistently
showing overnight lows between 31-33F for these zones. I`ve
decided to not issue Freeze Warnings for two reasons--winds and
cloud cover. The Kittitas Valley will have winds to provide mixing
until late in the evening, and the Yakima Valley will have
overnight lows around 32F with low probabilities for subfreezing
temps (30% for min temps less than 32F and 5% for 28F or less
per the NBM). Cloud cover will be the inhibiting factor for the
Pendleton, Athena, and Milton-Freewater areas to observe
subfreezing temperatures, but there is some uncertainty. Clouds
will be broken-overcast through midnight with clearing early
Wednesday morning. For now, will cover the cold overnight lows
with weather stories and social media posts.

There will be slight warming Wed-Thur with chilly overnight lows.
Surface winds will shift to the north and will be locally breezy.
The PacNW will be on the bottom of the broad trough and under a
cyclonic flow aloft. Any showers will be widely scattered over
the northeast mountains. Once again, moisture is very limited,
thus any snow amounts will be very light. Wister/85

LONG TERM...Friday through Tuesday...Overall benign weather is
currently forecast for the period with no good ensemble agreement
in any highlight-worthy or climatologically unusual events.

Ensemble guidance is in good agreement that the forecast area will
be on the back side of a departing upper-level trough Friday, and
a shortwave ridge axis will be placed just offshore. Differences
among ensemble members are small and dry weather is anticipated
area-wide (<5% PoPs). A surface thermal trough on the west side of
the Cascades will support easterly to northeasterly winds for the
Columbia Plateau. Forecast soundings show a low-level jet of
25-35 kts from north-central OR to central WA. While wind
highlights appear unlikely at this time, NBM probabilities of
exceeding 24-hr maximum gusts of 45 mph are 30-50% for ridges and
highlands prone to northeast winds.

Over the weekend, uncertainty in the pattern grows, though the
upper-level ridge axis is favored (>50% chance) to slide
overhead. Uncertainty in the pattern peaks late Saturday as
guidance is advertising a range of solutions. Broadly speaking, a
trough is forecast (87% of members) to approach BC or the PacNW
while the remaining members (13%) retain an amplified ridge. While
59% of members suggest the trough will impact BC, 28% track it
across the PacNW.

Sunday afternoon through Monday, ensemble clusters show some
differences in the depth of the trough as it tracks inland across
the Northern Rockies and Northern Plains - 56% of members favor an
upper-level ridge building into the PacNW while 22% suggest a
deeper trough downstream with northwest flow lingering across
the PacNW.

Tuesday, ensemble solutions range from a deep trough approaching
the PacNW (9% of members) to an amplified ridge overhead (29% of
members). The remaining members are indicating a solution
somewhere between the two. Plunkett/86

&&

AVIATION...12Z TAFs...VFR conditions are forecast and very likely
(95-100% chance) through the period. W/NW winds of 10-15 kts with
gusts of 20-25 kts are forecast for all sites this afternoon, and
expected to slacken later this evening to 10 kts or less. Clouds,
predominantly SCT-BKN 150-250 across the region this morning are
anticipated to give way to FEW-BKN 050-090 this afternoon and
evening. Additional clearing is forecast overnight. Plunkett/86

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  54  31  56  31 /   0  10   0   0
ALW  57  36  58  34 /   0  10  10   0
PSC  61  38  63  37 /   0   0   0   0
YKM  57  31  61  33 /   0   0   0   0
HRI  60  34  62  33 /   0  10   0   0
ELN  52  33  59  33 /   0   0   0   0
RDM  54  24  53  25 /   0   0   0   0
LGD  52  28  51  28 /   0  20  10   0
GCD  55  28  53  27 /   0   0   0   0
DLS  57  37  62  38 /   0   0   0   0

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...Wind Advisory from 1 PM this afternoon to 8 PM PDT this evening
     for WAZ026.

&&

$$

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