478
FXUS66 KPDT 231639
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
939 AM PDT Sun Jun 23 2024

.MORNING UPDATE...No significant changes were made to the forecast
this morning.

Looking at observations across the PacNW, Day Cloud Phase
Distinction RGB imagery reveals mostly clear skies across our
forecast area. Meanwhile, a marine layer accompanied by stratus
is evident along the Cascade crest westward. Onshore-directed
surface pressure gradients are injecting this moist marine air
through the Cascade gaps eastward into the Columbia Basin and
along the Blue Mountain foothills. The result is surface dew
points in the mid-40s to lower 50s and RHs in the upper 30s to
upper 50s in the aforementioned region.

As the day progresses, drier air from aloft will mix down to the
surface, though 12Z HREF members are in excellent agreement that
the boundary layer will not become sufficiently deep to tap into
the driest air around 700 mb. If HREF members are correct,
afternoon RHs will range from the upper teens to lower 30s across
the Columbia Basin and along the Blue Mountain foothills. However,
if the model depiction of boundary layer depth is too low, even
by 500-1000 ft, afternoon RHs may plummet to the upper single
digits and lower teens area-wide, except for areas most influenced
by the marine layer such as Columbia River Gorge and upper
Kittitas Valley.

Multiple Red Flag Warnings remain in effect today across the lower
elevations. Additionally, the Wind Advisory for the Kittitas
Valley remains in place. Plunkett/86

&&

.PREV DISCUSSION... /issued 436 AM PDT Sun Jun 23 2024/

AVIATION...12z TAFs...VFR conditions persist. The cold front
continues to work its way eastward, and has moved through all
TAF receiving terminals. In the wake of the front, gusty wind
remains the concern through the TAF period. A couple of locations
(KDLS, KPDT) currently have winds greater than 12 kts with this
morning, with others seeing lighter wind. Coverage of the stronger
winds will increase between 16z and 21z today, with most
terminals seeing winds of 10-15 kts with gusts of 20-30 kts. Winds
should weaken to less than 12 kts during the overnight hours,
especially after 04z. Branham/76

PREV DISCUSSION... /issued 224 AM PDT Sun Jun 23 2024/

SHORT TERM...Today through Tuesday night...Red flag conditions
and strong winds will be the primary weather concerns today. The
Red Flag Warning in effect for the Lower Columbia Basin of WA/OR
(fire zones WA691 and OR641) will be expanded to include central
Oregon (fire zone OR640) in effect this afternoon and evening. A
wind advisory is in effect for the Kittitas Valley starting at 5
AM this morning through midnight.

Winds have already increased across the Lower Columbia Basin and
surrounding valleys. These winds are providing mixing of warm and
dry air aloft to the surface, resulting in early morning
temperatures in the lower 70s and RHs in the upper 20s and 30s. To
give a feel for how dry it is aloft, the 00Z UIL sounding has a
dewpoint spread of around 26C at 700mb. However, the depth of the
marine layer at UIL is from the surface to 850mb. GOES nighttime
microphysics shows the marine layer banked up along the west side
of the Cascade Range. The marine layer will deepen as the day
progresses with the passage of a shortwave trough and dry cold
front. There will not be significant winds associated with the
front with the exception of the Kittitas Valley where sustained
winds around 30 mph gusting to 45 mph are forecast. Confidence is
high for these winds to be observed, but gusts at 50+ mph have a
much lower probability (0% from the HREF and 30% from the NBM). As
far as fire weather concerns, the marine layer seeping through
the Cascade gaps will keep RHs high enough along the Kittitas
Valley and eastern CR Gorge despite the gusty winds. Deterministic
models are in good agreement with Min RHs around 30% for these
areas. RHs over the Lower Columbia Basin will be in the teens and
20s. Central Oregon will observe RHs in the teens as well with a
few spots in the single digits. Since the winds are forecast to be
a little stronger in central Oregon, a Red Flag Warning will be
issued to include this zone.

Winds aloft and at the surface will decrease tonight, and Monday`s
winds will be considerably less than today. Afternoon highs on
Monday will be about the same or a tad cooler than today and
closer to seasonal average. No weather concerns are anticipated
for Monday night through Tuesday night. However, the westerly flow
will begin to back to the southwest on Tuesday as an upper level
trough develops offshore. Afternoon temperatures will climb back
into the 80s to mid 90s with the HeatRisk in the minor to moderate
category. There are signs of elevated instability over eastern
Oregon as well, but the threat of any thunderstorms is low...less
than 15%. There is limited mid-level moisture but no indications
of an upper level disturbance to provide the lift needed. However,
I would expect cumulus buildups over the Blues and the John Day-
Ochoco Highlands Tuesday afternoon and evening. Wister/85

LONG TERM...Wednesday through Sunday...Southwest flow expected
over the region as an area of low pressure approaches the PacNW
on Wednesday. Precipitation associated with this system has a
slight to moderate chance (40-60%) of impacting the Cascade
Mountains starting Wednesday. Additionally, temperatures will
decrease on Wednesday by a few degrees, though values will
generally still be around 5 degrees above normal. Wind will be the
other consideration for Wednesday. Wind speeds will increase
through the afternoon and evening hours, with the strongest wind
favored to be across the Columbia Basin, Columbia River Gorge, and
into the Kittitas Valley. In these locations, there is a 60-80%
chance for winds of 25 mph with gusts of 30-35 mph on Wednesday
afternoon and evening. Additional areas of Washington and Central
Oregon will see 70-90% chance for winds around 20 mph with gusts
near 25 mph for the same timeframe. Windy conditions will weaken
slightly overnight.

The area of low pressure continues to move through the area on
Thursday, though models and clusters show variations in placement
and related strength of the system`s influence on Thursday. The
NBM has a return of gusty to breezy wind on Thursday afternoon,
but keeps any threat of shower activity generally confined to the
higher terrain. There are some solutions (GFS mainly) that do hint
at the potential for showers across the lower elevations, though
confidence in that development remains low. Further cooling
temperatures anticipated Thursday with values landing around 5
degrees below normal. While wind will see another increase in the
afternoon, winds are not expected to be as strong. The favored
locations of the Columbia River Gorge and the Kittitas Valley
remain, with most of these areas seeing a 70-90% chance for winds
around 20 mph with stronger gusts. The presence of winds stronger
than that is more isolated.

The area of low pressure will generally move east of the forecast
area by Friday, allowing a drier, northwest flow to influence the
region. A warming trend in temperatures returns with values
reaching near normal values on Friday with widespread dry
conditions. This pattern will keep winds generally light. Similar
conditions expected Saturday with additional warming. An
approaching wave late Saturday into Sunday will turn flow
southwesterly again, with a few showers present over the higher
terrain. Branham/76

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  81  48  81  51 /   0   0   0   0
ALW  85  52  85  54 /   0   0   0   0
PSC  87  52  86  55 /   0   0   0   0
YKM  83  46  83  51 /   0   0   0   0
HRI  85  52  86  55 /   0   0   0   0
ELN  74  47  81  51 /   0   0   0   0
RDM  82  43  83  47 /   0   0   0   0
LGD  83  47  81  51 /   0   0   0   0
GCD  89  47  87  52 /   0   0   0   0
DLS  77  52  84  55 /   0   0   0   0

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Red Flag Warning from 1 PM this afternoon to 8 PM PDT this
     evening for ORZ640-641.

WA...Red Flag Warning from 1 PM this afternoon to 8 PM PDT this
     evening for WAZ691.

     Wind Advisory until midnight PDT tonight for WAZ026.

&&

$$

MORNING UPDATE...86
SHORT TERM...85
LONG TERM....76
AVIATION...76