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000
FXUS66 KPDT 191643 AAA
AFDPDT

Area Forecast Discussion...Updated
National Weather Service Pendleton OR
943 AM PDT Fri Apr 19 2024

.UPDATE...Current radar and visible satellite imagery showing dry
conditions and clear skies across the region. This is in response
to the northwest flow aloft advecting cooler and drier air into
the area as an upper level ridge slowly moves onshore through the
day today. The two concerns in the short term consist of the
potential for freezing temperatures across the Kittitas Valley
tonight into Saturday morning, and breezy winds nearing Advisory
criteria overnight Saturday into Sunday morning as an upper level
trough and associated cold front passes through the Pacific
Northwest. Winds will again be light tonight as the upper level
trough axis passes through the area, but southwest flow aloft and
a shift from offshore to onshore winds will slightly increase
cloud cover - which may be enough to inhibit the efficiency of
radiational cooling overnight. Confidence in freezing temperatures
across the Kittitas Valley tonight is currently low to moderate
(30-50%), as the NBM suggests a 45% chance of Saturday morning low
temperatures dropping to 32 degrees or below at Ellensburg.

The other concern resides with the potential for Advisory level
winds across the Basin and east slopes of the Washington Cascades
Saturday night into Sunday morning associated with the passing
trough and associated cold front. Pressure gradients are expected
to tighten along the Cascades as a result, with the GFS, NAM, and
SREF all highlighting a 10-12.5mb difference between Portland and
Spokane. However, the GFS is alone in anticipating the gradient
will exceed 11.5 mb, which is below the normal advisory threshold
of 12 mb. The NBM aligns with this uncertainty, showing a 20-50%
chance of wind gusts of 47 mph or greater across the Blue Mountain
foothills and through north-central Oregon. A better chance
resides in the Simcoe Highlands, as the NBM presents a 50-70%
chance of gusts of 47 mph or greater. These two weather concerns
will be analyzed further as updated guidance is released over the
next several hours. 75

&&

.AVIATION...18Z TAFs...VFR conditions will prevail through the
period. Some mid/high clouds will stream into the area overnight.
Winds will be 8-14 kts with higher afternoon/early evening gusts.
Winds will decrease overnight to 8 kts or less. Earle/81

&&

.PREV DISCUSSION... /issued 457 AM PDT Fri Apr 19 2024/

SHORT TERM...Today through Sunday night...Nighttime microphysics
RGB imagery reveals mostly clear skies across the entire forecast
area this morning. On the synoptic scale, the forecast area is
between an offshore upper-level ridge and a trough downstream over
the Northern Plains. This is translating to a dry northwesterly
flow aloft with a well-defined West Coast thermal trough (WCTT)
along the OR coast resulting in low-level east-northeasterly
winds. These winds are keeping breezy conditions along ridges and
preventing a widespread freeze for the lower elevations. That
said, have opted to issue Freeze Warnings for the foothills of the
northern Blue Mountains of OR/WA due to near- to sub-freezing
temperatures that have developed in areas that have decoupled.
Sub-freezing temperatures for these zones are not expected to be
ubiquitous (elevated ridges that do not decouple will stay
warmer), but enough of our population centers are being impacted
to warrant the highlights. Elsewhere, have also opted to issue a
Freeze Warning for the lower Columbia Basin of OR where
temperatures are hovering near freezing. The Yakima Valley is
observing a wide range of temperatures - lower 30s to lower 40s
depending on the influence of the previously mentioned northeast
winds. Have opted to not issue a Freeze Warning here as coverage
of sub-freezing temperatures this morning is expected to remain
patchy.

Offshore winds will persist through the day as very little changes
on the synoptic scale. Expect locally breezy east-northeast winds,
especially for ridges and elevated plateaus with low afternoon RH.

Tonight into Saturday morning, the offshore upper-level ridge is
expected to push onshore, followed by a trough passage Saturday
afternoon and evening. This will lead to a reversal in wind
direction. Ensemble guidance is still exhibiting some uncertainty
with regard to the exact track of the trough, and sustained winds
and gusts associated with it. Confidence is low-medium (20-50%
chance) in reaching Wind Advisory criteria Saturday afternoon and
evening across the Columbia Plateau region. Otherwise, some light
precipitation is forecast for the mountains, primarily the
Cascades, with mostly dry conditions for the lower elevations.

Sunday, precipitation chances taper off as the trough exits to the
east and an upper-level ridge moves along the coastline. Breezy
west winds are forecast through Sunday, but are not expected to
reach advisory levels (<20% chance). Plunkett/86

LONG TERM...Monday through Friday...Starting Monday morning to
Tuesday night, an upper-level ridge moves over the PacNW as the
previous trough passes through North Rockies. Late Tuesday night,
another trough will occur onshore that will dominate over PacNW as
it deepens throughout Thursday. Dry conditions for Monday and
Tuesday will stick around due to the ridge building over the
region. Rain showers are expected in the low elevations Wednesday
into Thursday night with low possible chance (<30%) of snow within
the mountain areas. Snow levels will be around 2000-4000ft Monday
but increases up to 4000-6000ft Tuesday into Thursday night. As
the deepening trough continues over PacNW, chances of
precipitation increases while snow levels remain high (>70%
confidence). Breezy conditions will occur with wind gusts up to
about 20-30 kts across the Simcoe Highlands and Kittitas Valley
Wednesday but might come and go until the end of these next few
days (50-70% confidence). Temperatures will moderate between the
upper 60s to low 70s through this period (50-70% confidence).
Feaster/97

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  63  35  66  38 /   0   0   0  20
ALW  67  37  69  42 /   0   0   0  40
PSC  68  40  71  44 /   0   0   0  10
YKM  65  36  67  36 /   0   0   0  10
HRI  67  37  70  42 /   0   0   0  20
ELN  61  33  64  37 /   0   0   0  20
RDM  61  33  70  30 /   0   0   0   0
LGD  60  34  69  36 /   0   0   0  20
GCD  63  36  73  35 /   0   0   0  10
DLS  68  42  68  43 /   0   0   0  10

&&

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

$$

UPDATE...75
SHORT TERM...86
LONG TERM....97
AVIATION...81