679
FXUS66 KPDT 091017
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
317 AM PDT Sun Jun 9 2024

.SHORT TERM...Today through Tuesday...

Key Messages:

1. Thunderstorm potential this morning and afternoon.

2. Breezy winds this afternoon through Monday morning, returning
Tuesday.

3. Warm, above normal temperatures extend through Tuesday.

Current radar and infrared satellite imagery showing pockets of
light returns across the Lower Columbia Basin, Central Oregon,
and through the John Day-Ochoco Basin under cloud skies. This is a
result from the upper level trough and associated cold front
moving onshore, before passing through the area later this
afternoon. These synoptic features will allow for enhanced lift
and moisture to occur, leading to the development of scattered
showers (20-50%) and isolated thunderstorms (5-15%) across our
Oregon zones this morning before extending north into south-
central Washington this afternoon as the frontal system slowly
crosses the area. The Storm Prediction Center does include our
area in the General Thunderstorm (0 of 5) category of their
Convective Outlook for today, as the ECMWF EFI focuses above
normal CAPE and low level shear values over our southern zones,
Simcoe Highlands, and the Yakima/Kittitas Valleys with 60-80% of
ensembles in agreement. The best chances (20-30%) for developing
thunderstorms will occur this afternoon across Southern
Deschutes, Southern Crook, Grant, Southern Union, and Wallowa
Counties, with slight chances (10-20%) across the Yakima and
Kittitas Valleys. Convective parameters are much more conducive
for storm development over our southern and eastern zones as the
HREF advertises surface CAPE (SBCAPE) of 250-450 J/kg and low
level shear of 40-55 knots, compared to SBCAPE of 400-550 J/kg and
low level shear of 15-25 knots over the Yakima and Kittitas
Valleys. The RAP suggests stout low level lapse rates of 8-9C/km
over the Yakima/Kittitas Valley compared to 6-7.5 C/km over our
southern and eastern zones. These parameters correlate to a
better chance of longer duration storms across our southern and
eastern zones, with more brief and discrete storm cells expected
over the Yakima and Kittitas Valleys. Showers and thunderstorms
will track east after 5 PM as chances (10-20%) linger across
Grant, Wallowa, Union, and eastern Crook, eventually exiting to
our east by 11 PM. The main concern for any developing cells will
be gusty winds, frequent lightning, and moderate to heavy
rainfall. However, precipitable water (PWAT) values will be
dropping substantially from 1.0" to 0.6" (180% to 100% of normal)
through the afternoon as drier air advects into the area, so the
threat of heavy rainfall will diminish through the afternoon and
evening. Rain amounts will stay between 0.01-0.05" for most of the
region, with up to 0.15" over areas in and near developing storms.
Lower elevations of the Basin are expected to stay dry, with
potentially a trace along the northern Blue Mountain foothills.

Today`s passing cold front will also allow for a pressure gradient
to develop along the Cascades, enhancing winds this afternoon
through Monday morning across the Gorge, Simcoe Highlands, and the
Kittitas Valley. Sustained winds of 15-25 mph and gusts of 30-40
mph are expected in these areas, peaking between 8 PM today
through 5 AM Monday. Confidence in these wind values is high
(80-90%) as the NBM suggests a 60-80% chance of wind gusts of 40
mph or greater over the aforementioned areas, as the GFS, NAM, and
SREF all highlight a pressure gradient between Portland and
Spokane of 6-8 mb. Winds will slacken through the day on Monday as
an upper level ridge builds over the region in the wake of the
exiting system. This will be short-lived as a quickly moving
upper level shortwave passes to our north and associated cold
front passes through the area on Tuesday, tightening isobars to
allow another pressure gradient to develop along the Cascades as
it suppresses the incumbent ridge. The GFS and SREF both
highlight a 8-11.5mb gradient between Portland and Spokane, with
the GFS hinting at the potential of advisory-level winds (11 mb
threshold) over the Simcoe Highlands, Lower Columbia Basin of
Oregon, and the Kittitas Valley. Thus, gusts of 35-45 mph will be
possible over these areas, with the NBM showcasing a 70-90% chance
of 45 mph gusts or greater over these areas - which is advisory-
level. Moderate to high confidence (60-80%) resides with wind
advisories needing to be issued for Tuesday afternoon and/or
evening.

High temperatures peaked at 90 degrees yesterday in the Tri-Cities
and Hermiston, but after a short reprieve will again approach
these values by Tuesday. After a mild night with morning lows only
dropping into the upper 50s to low 60s across the Basin and
foothills due to cloud cover, afternoon temperatures will drop 4-8
degrees from yesterday. Highs across the Basin and foothills will
reach into the low to mid-80s, as a response to cloud cover and
a brief stint of west-northwest flow aloft. These features will
not last long as an upper level ridge begins to build over the
area Monday morning, clearing skies and bringing southwest flow
aloft back as highs increase 1-4 degrees on Monday with values in
the mid-to upper 80s for lower elevations of the Basin. Highs bump
up another 1-3 degrees Tuesday as the approaching shortwave
enhances southwest flow aloft to see low-90s again return to the
Tri-Cities area. Confidence in the return to low 90s over portions
of the Basin is moderate (50-60%) as the NBM suggests a 58%
chance for the Tri-Cities and a 21% chance for Hermiston reaching
high temperatures of 90 degrees or greater on Tuesday. 75


.LONG TERM...Wednesday through Sunday...

Key Messages:

1) Conditions remain largely similar Wednesday and Thursday under
zonal flow.

2) Low pressure approaches the PacNW on Friday bringing breezy
winds, cooler than normal temperatures, and gusty winds.

The long term will begin fairly benign but by next weekend we
should see the return of active weather including spring-like
temperatures helping to stave off summer heat. Model ensembles
remain largely in good agreement on the first few days of the
forecast period, indicating we`ll remain in a zonal flow pattern
through at least early Thursday before flow turns more
southwesterly due to an approaching system from the Gulf of
Alaska. Under this pattern, Wednesday and Thursday look largely
uneventful for sensible weather concerns, with highs Wednesday in
the upper 70`s to mid 80`s in a post-frontal environment, and
temperatures rising 2-6 degrees on Thursday due to a weak,
transient ridge passing through. There is high confidence (70-80%)
in the forecast conditions for these days.

Over the weekend is when we will see more significant weather as a
deep low from the Gulf of Alaska drops down into the region.
Ensembles begin to lose consistency during this timeframe leading
to a bit more uncertainty overall. Ensemble clusters show the low
will largely be just offshore to the northwest Friday morning,
then expected to move inland by Saturday but with around 13% of
members expecting this system to weaken notably at that time. Then
on Sunday, clusters are largely in disagreement due to issues
resolving the speed and strength of the system. There is moderate
confidence in the forecast on Friday and Saturday (50-60%), with
low confidence on Sunday (30%).

The NBM expects temperatures to already begin dropping on Friday
for everywhere outside of the Columbia Basin and Foothills of the
Blues as the front associated with this Alaskan low progresses
quickly across the region. Then by Saturday we should see highs
notably below normal for this time of year by 5 to 10 degrees,
continuing into Sunday, ranging in the upper 60`s to mid 70`s.
Overnight lows will similarly decline, dropping into the upper
30`s for Central Oregon and up to the mid to upper 40`s elsewhere
for our population centers. Meanwhile our usual breezy winds make
their return on Friday as the pressure gradients tighten with this
system. The NBM has a widespread 40-100% chance of gusts of 40+
mph, with a 40-80% chance of gusts 45+ mph for the lower Columbia
Basin/Foothills of the Blues in Oregon, Simcoe Highlands, and
Kittitas Valley. Probabilities go down around 10-30% on Saturday,
but breezy winds should persist for the day as well, decaying much
further by Sunday. Finally, although the Cascades may receive
mountain precipitation through the duration of this event, low
PWAT values and rain shadowing are anticipated, preventing notable
rainfall for the majority of our region. Overall look for a cool
and breezy spring-like weekend as we move into the middle of June.
Goatley/87


&&

.AVIATION...12Z TAFs...VFR conditions expected over the next 24
hours. Widespread scattered showers possible with VCSH all sites
this morning into the early afternoon except for PSC/DLS - these
sites have probabilities too low (< 15%) to include, but a rogue
shower moving nearby cannot be ruled out. Breezy winds with gusts
20-25 knots on average possible for DLS/RDM/BDN this afternoon -
winds generally 10 knots or less for all other sites. Low to mid-
level cloud decks this morning through the beginning of the
afternoon (around 5-10k feet), rising and then clearing thereafter
as the system that brought the precipitation and breezy winds
exits. Goatley/87


&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  80  54  83  55 /  20  10   0   0
ALW  83  57  85  59 /  30  10   0   0
PSC  87  59  89  59 /  20  10   0   0
YKM  86  53  87  55 /  30  10   0   0
HRI  87  58  88  58 /  20   0   0   0
ELN  81  54  80  56 /  30  10   0   0
RDM  80  47  82  51 /  30   0   0   0
LGD  76  50  79  52 /  30  10   0   0
GCD  78  49  82  52 /  40  10   0   0
DLS  83  56  84  58 /  10   0   0   0

&&

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

$$

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