717
FXUS66 KPDT 301546
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
846 AM PDT Sun Jun 30 2024

.MORNING UPDATE...Upper trough continues to swing across the
region today, with precipitation expected to exit through the day.
HREF seems to be in pretty good agreement with the morning
forecast package including the chance of showers, possibly an
isolated thunderstorm, through the afternoon across the eastern
mountains of Oregon, but nothing in the way of strong activity is
expected. Winds meanwhile breezy winds expected, but the NBM
struggling to place significant wind gusts in the region as
chances of gusts 45+ mph limited to the higher terrain near the
Kittitas Valley. Overall morning forecast remains good with
minimal changes needed. Goatley/87


&&

.PREV DISCUSSION... /issued 503 AM PDT Sun Jun 30 2024/

AVIATION...12Z TAFs...VFR conditions are very likely through the
period (90-99% chance) for all sites. Light rain showers are
tapering off this morning, and precipitation chances through the
remainder of the period will consist of very low chances (2-8%)
of isolated showers. Some diurnally driven stratocumulus
(050-080) is forecast today, with otherwise mostly FEW-SCT mid-
and high-level clouds. Winds will increase this morning and
afternoon such that all sites have 10-15 kts sustained winds with
gusts of 15-25 kts. Plunkett/86

PREV DISCUSSION... /issued 250 AM PDT Sun Jun 30 2024/

SHORT TERM...Today through Tuesday...Current nighttime satellite
imagery and radar returns show a narrow band of showers exiting
into ID, with scattered cloud cover developing behind this band.
Breezy winds also developed late yesterday afternoon with the
shortwave passage, and have persisted slightly through the
Kittitas valley tonight.

The band of showers across far eastern WA/OR will exit into ID
ahead of an upper trough passage across the region today. An upper
trough and surface cold front passage today will continue to bring
isolated/scattered showers across the WA Cascade crest as well as
Wallowa county through the afternoon. Weak instability and modest
low level lapse rates will also allow for slight chances (15-25%)
of isolated thunderstorms across eastern Wallowa county this
afternoon. Breezy wind will also develop through the Cascade gaps
and into the Columbia Basin, with generally 15-25 mph sustained
winds and gusts up to 40 mph. A localized gust up to 45 mph may
be possible through the Simcoe Highlands and portions of the
Kittitas valley, with NBM probabilities of 55-75% through today. A
marine push associated with the incoming trough/frontal passage
will keep RHs above critical levels, though the anticipated breezy
conditions will continue to bring elevated fire weather concerns.

A secondary shortwave behind the upper trough axis exit will
support breezy winds through the lower elevations Monday, with
another day of 15-25 mph sustained winds and gusts up to 40 mph.
That said, probabilities of 45 mph or greater gusts through
Monday will increase to 70-85% through the Kittitas valley, Simcoe
Highlands, eastern Gorge, and the Columbia Basin. While fire
weather concerns will continue to be elevated through Monday, RHs
are anticipated to remain above critical thresholds in the
afternoon (confidence 70-80%). Upper level ridging will amplify
across the eastern Pacific Tuesday, resulting in a dry northwest
flow aloft over the PacNW. Breezy winds will continue through the
Cascade gaps, with light to locally breezy winds elsewhere.
Lawhorn/82

LONG TERM...Wednesday through Sunday...Warm to hot temperatures
and little to no precipitation is forecast as an upper-level
ridge dominates the region`s weather (95% chance through Saturday,
and 75% chance Sunday). The main weather concern will be
increasingly hot temperatures and Moderate to locally High
HeatRisk for the lower elevations Friday through Sunday.

On Wednesday, the 500-mb pattern will be characterized by
downstream troughing over the Northern Plains and Northern
Rockies, with an anomalously strong upper-level ridge in place
upstream in the Pacific Ocean. This will place the forecast area
under a dry northwest flow aloft. Deterministic guidance still
advertises a shortwave sliding southeast across eastern WA and
northeast OR during the day, but the dry air mass should preclude
any significant chances of precipitation (<5% area-wide) as the
shortwave traverses the region. Breezy to locally windy gap winds
are likely (>90% chance) through the Kittitas Valley and eastern
Columbia River Gorge. Currently, no Red Flag conditions are
forecast, and NBM 24-hr probabilities of advisory-level wind gusts
are 30-70% through the Kittitas Valley so wind highlights are not
anticipated at this time.

For the 4th of July, over 80% of ensemble members keep the
offshore ridge in place. The remainder advertise a more
progressive longwave pattern with the ridge axis shifting inland.
The latter solution would result in the warmest temperatures east
of the Cascades, though the difference would only be several
degrees relative to the favored solution. Afternoon high
temperatures are currently forecast to be 5-10 degrees above
normal, resulting in some areas of Moderate HeatRisk for the lower
elevations on the 4th of July.

Friday and Saturday, ensemble clusters reveal a broader range in
member solutions, though the main differences are in how members
handle an upstream closed low/open wave and exactly where the axis
of the ridge will be placed. Per 00Z ensembles, the upper ridge
axis  will likely (78% of members) remain offshore, though 17% of
members place the ridge overhead, and a minority of members (4%)
break the ridge down altogether and bring a trough into the PacNW.
The latter solution would result in cooler temperatures, while
the overhead ridge would result in the warmest weather.

Sunday, guidance shows the most uncertainty in the longwave
pattern. The multi-model ensemble mean places the upper-level
ridge axis overhead, though individual clusters range from an
anomalously strong ridge overhead (18%) to a trough passing to the
north resulting in a suppressed ridge (27%) to an offshore ridge
(55%).

Regarding uncertainty Friday through Sunday, ~50% of ensemble
variance is due to differences in the location of the upper ridge,
while an additional ~25% is associated with the magnitude of the
500-mb height field over the PacNW.

Despite persistent low afternoon relative humidity (upper single
digits to upper teens for the lower elevations), Red Flag
conditions are not anticipated to be widespread enough to
warrant highlights. However, fire weather concerns will still be
elevated.

Precipitation chances are extremely low (<5% chance) through the
period. Plunkett/86

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  83  55  82  53 /   0   0   0   0
ALW  86  58  85  57 /  10  10   0   0
PSC  89  60  88  58 /  10   0   0   0
YKM  86  55  85  53 /   0   0   0   0
HRI  88  59  87  56 /   0   0   0   0
ELN  78  55  78  53 /  10   0   0   0
RDM  80  49  79  45 /   0   0   0   0
LGD  79  50  78  49 /  10  10  10   0
GCD  81  50  80  47 /   0   0   0   0
DLS  84  60  83  56 /   0   0   0   0

&&

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

$$

MORNING UPDATE...87
SHORT TERM...82
LONG TERM....86
AVIATION...86