300
FXUS66 KPDT 081720
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
1020 AM PDT Sat Jun 8 2024

.Updated for Aviation...
18Z TAFS...VFR conditions are expected through the period.
There is a low chance (<30%) that BDN and RDM could see TSRA late
this afternoon or evening. However, due to the low chances, will
not include any mentions of thunder in the TAFS at this time and
will wait until we see how things develop.

Otherwise, expect gusty winds at DLS later this afternoon and
evening to around 20 kts and again towards morning. Gusty winds
are also expected this afternoon and early evening at BDN and RDM
to around 20 kts, outside of any thunderstorms. All other sites
will generally be 10 kts or less.


&&

.PREV DISCUSSION... /issued 319 AM PDT Sat Jun 8 2024/

SHORT TERM...Today through Monday...

Key Messages:

1. Thunderstorm potential this afternoon through Sunday.

2. Warm, above normal high temperatures that peak today.

3. Breezy winds late Sunday into Monday.

Current radar and infrared satellite imagery showing dry
conditions under clear skies. These conditions are a result of an
upper level ridge that is currently overhead, and slowly tracking
west to east. This eastward progression of the ridge will
continue as an upper level trough approaches the coast later
today before driving through the Pacific Northwest into Sunday.
The upper level trough and associated frontal system is the
primary focus through the period, as the main weather concern
resides with the potential for thunderstorm development beginning
this afternoon as the upper level trough erodes the backside of
the departing upper level ridge. This has lead to portions of our
southern zones being included in the marginal category (1 of 5) of
the Storm Prediction Center`s Convective Outlook product for
today. The best potential for storm development will be along the
east slopes of the Oregon Cascades over western Deschutes County,
as the HREF advertises mean level CAPE (MLCAPE) of 700-1000 J/kg
and low level shear of 35-45 knots through the peak afternoon
hours of 2PM to 7PM. More marginal conditions reside east into
Crook and Grant counties as the HREF suggest MLCAPE of 250-500
J/kg and low level shear of 30-40 knots. However, low level lapse
rates are a bit lacking at 7-8 C/km via the RAP, which will keep
any developing storm cells discrete, isolated, and sub-severe in
nature. Thus, there is low to moderate confidence (20-30%) in
isolated thunderstorms developing across Deschutes, Crook, and
Grant Counties this afternoon and evening, with a low chance
(5-15%) of any storm cell exhibiting severe characteristics. The
main concerns of any developing storms will be frequent lightning,
hail, and periods of moderate to heavy rainfall as precipitable
water (PWAT) amounts of 0.8-1.0" (150-200% of normal) will be
present via ECMWF and GFS guidance.

The main frontal boundary is poised to cross over the region
overnight into Sunday morning, which will extend the potential
for isolated thunderstorms north and east into Jefferson, Wasco,
Sherman, Gilliam, Wheeler, Morrow, Southern Umatilla, and Southern
Union Counties. Confidence in overnight storms does drop
considerably (5-15%) as MLCAPE drops to below 100 J/kg over the
aforementioned areas. However, the crossing cold front will meet
another approaching frontal boundary from the northeast, allowing
instability to persist over the area. Due to the low MLCAPE
values, any storms cells that are able to develop will exhibit
sub-severe characteristics. The upper level trough will pass
through the area on Sunday, keeping the chance for isolated
thunderstorms across eastern Deschutes, Wheeler, Crook, Grant,
Union, Wallowa, southern Morrow, Southern Umatilla, and southern
Columbia Counties through the afternoon before shifting to eastern
Union and Wallowa Counties by late afternoon and evening.
Convective parameters are still moderate as MLCAPE values hover
between 250-450 J/kg with shear of 35-45 knots via the HREF. There
is also a potential for isolated afternoon thunderstorms to occur
over the Kittitas and Yakima Valleys as slightly enhanced MLCAPE
values of 400-550 J/kg are suggested by the HREF. Any developing
cells in these regions will be rather brief, as low level shear is
lacking between 15-25 knots via the HREF. Confidence in isolated
storms developing is low to moderate (15-25%), with low confidence
(5-10%) of any storm exhibiting severe characteristics. Thus, the
same concerns of frequent lighting, hail, and moderate to heavy
rainfall will accompany developing storm cells on Sunday.

The secondary concern over the period resides with warm
temperatures as highs will hover in the upper 80s to low 90s
across lower elevations of the Basin, which is about 10 degrees
above normal. High temperatures are expected to peak today as
mostly clear skies will extend through the morning and southwest
flow aloft is enhanced by the approaching upper level trough. High
confidence (75-95%) resides with portions of the Basin breaking
into the low 90s this afternoon, with the NBM highlighting a 92%
chance in the Tri-Cities, a 79% chance for The Dalles, a 72%
chance for Hermiston, a 57% chance for Yakima, and a 39% chance
for Pendleton. Cloud cover will increase this afternoon as the
cold front and upper level system nears the coast, which will keep
overnight temperatures into Sunday mild and only dropping into the
upper 50s to low 60s across the Basin, foothills, and the
Yakima/Kittitas Valleys. Southwest flow continues on Sunday and
becomes more west-southwest into Monday as high temperatures stay
in the mid-to upper 80s Sunday and in the upper 80s to low 90s
Monday for areas of the Basin, foothills, and the Kittitas/Yakima
Valleys. Temperatures do look to moderate and eventually drop in
the extended period.

The final weather concern is associated with breezy winds across
the Simcoe Highlands, Gorge, Kittitas Valley, and the Lower
Columbia Basin of Oregon Sunday evening into early Monday
morning. This is a result from a pressure gradient developing
along the Cascades in the wake of the departing cold front. The
GFS, NAM, and SREF all advertise a pressure gradient between
Portland and Spokane of 7-9 mb, which coincide with peak gusts of
25-35 mph. Confidence in these wind values is high (80%) as the
NBM suggests a 70-90% chance of wind gusts of 35 mph or greater.
At this time, guidance suggests the highest gusts will occur
between 5 PM Sunday and 5 AM Monday. Confidence is moderate to
high (70-80%) that winds will stay below advisory level. 75

LONG TERM...Tuesday through Saturday...

Key Messages:

1) Zonal flow pattern keeps temperatures generally consistent
during the mid to late week timeframe with periods of breeziness
possible.

2) Deep low approaches and moves into the region by the weekend,
cooling temperatures, increasing winds, and bringing mountain
precipitation.

Ensembles indicate a generally zonal pattern is expected over the
mid to late week timeframe to start the long term, bringing
consistent temperatures with some variation to the winds thanks
to passing systems. For Tuesday, a shortwave trough will skirt the
US in southern Canada, causing the pressure gradient to tighten
while a cold front sweeps across and brings breezy conditions.
There is high confidence (70%) in gusty winds across the
Kittitas/Yakima Valleys, Simcoe Highlands, and Columbia
Basin/Foothills of the Blues, with the NBM indicating a
widespread 30-90% chance of gusts equal to or in excess of 40 mph.
Then Wednesday into Thursday, breeziness subsides under the zonal
flow pattern, while temperatures slightly cool and hover just
above normal in the upper 70`s to low 80`s.

Friday sees flow turn southwesterly with the approach of a deep
low from the Gulf of Alaska. This marks the return of similarly
breezy conditions compared to Tuesday, but extending down into
Central Oregon as well. Confidence is lower (moderate ~50-60%),
but the NBM promotes a 30-85% chance at gusts 40+ mph. Moisture
with this system is limited as well, as ensembles struggle to
produce significant PWAT values outside of the mountains. Any
precipitation that does fall with the system will likely be
shadowed to the Cascades and Blues at best with low confidence
(20-30%). But with the system coming out of the northwest, cooler
temperatures will accompany it. There is moderate confidence
(50-60%) in temperatures in the low to upper 70`s on Saturday,
bringing us to below normal on average. Overall the forecast will
be near or above normal and summer style in temperatures and
breezes, but by the weekend we may see a June cooldown to stave
off stronger summer heat for the time being. Goatley/87

AVIATION...12Z TAFs...VFR conditions expected over the next 24
hours. MVFR or worse conditions could occur at BDN/RDM during
afternoon due to a chance of thunderstorms, but expecting majority
of activity will occur near but not at these sites. Breezy winds
with gusts 15-20 knots at DLS/BDN/RDM this afternoon, but
otherwise expecting winds 10 knots or less elsewhere. Cloud bases
generally mid to high level, around 10-20k as clouds move in
today. Goatley/87

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  58  82  55  83 /  10  10   0   0
ALW  60  84  60  85 /   0  10  10   0
PSC  63  88  60  89 /   0  10   0   0
YKM  61  87  53  88 /  10  20  10   0
HRI  62  88  59  88 /   0  10   0   0
ELN  60  81  55  82 /  10  20  10   0
RDM  56  79  48  81 /  30  30   0   0
LGD  56  75  52  80 /  20  30  10   0
GCD  56  78  50  82 /  30  40  10   0
DLS  62  82  56  84 /  10  10   0   0

&&

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

$$

SHORT TERM...75
LONG TERM....87
AVIATION...77