569
FXUS66 KPDT 170526
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
1026 PM PDT Fri Aug 16 2024

.AVIATION...06Z TAFs...Quiet weather and VFR conditions,
consisting of winds of 10 kts or less coupled with sky cover
ranging from SKC to FEW-SCT mid-high cloud will persist overnight
through Saturday morning.

Saturday afternoon, confidence is very high (>90% chance) that
thunderstorms will initiate over the Cascades and propagate
northward through evening. Along the east slopes, confidence is
lower in shower and thunderstorm coverage...generally 30% or less
for central Oregon. By evening, showers and thunderstorms will
likely expand northward to north-central Oregon and south-central
Washington, impacting DLS and YKM (>50% chance of precipitation).
Afternoon thunderstorms along the Cascades are expected (>75%
chance) to produce outflow winds of 40-50 mph, with lower chances
(<20%) of 60 mph. These outflow winds will manifest as a stark
wind shift at DLS/PDT, and outflow may also reach the remainder of
TAF sites, though confidence in magnitude/timing is lower.
Showers and thunderstorms will be capable of producing temporary
MVFR or lower conditions. Plunkett/86

&&

.PREV DISCUSSION... /issued 413 PM PDT Fri Aug 16 2024/

SHORT TERM...Tonight through Sunday night...Latest visible
satellite shows a moderate cumulus field over Wallowa County where
their is a slim chance (10-15%) for a stray shower or brief TSTM
through about 5 PM. Otherwise attention focuses on Saturday and
Saturday night with the developing upper low off the coast
impinging on the Cascades. Increasing southerly flow and large
scale lift will lead to the development of numerous showers and
thunderstorms along the east slopes...first in Oregon during the
mid to late afternoon and then in Washington during the evening.
Moderate to heavy QPF is expected with this activity near the
crest with rainfall amounts of .25 to .50 inches common and
locally heavier amounts in the heavier storms. WPC has the
Washington Cascade Crest in a Marginal Risk for excessive rainfall
on Saturday and this matches very well with our current Flash
FLood Watch for burn scars.

With the copious rainfall expected will not issue fire weather
highlights for areas near the east slopes though the threat for
abundant lightning is high (75-85%). Did issue fire weather
watches for the western portions of the Columbia Basin and
Kittitas Valley where storms are more apt to be dry. Latest SPC
fire outlook has an isolated dry tstm outlook in this area.
Further, the SPC upgraded the Marginal Risk to slight risk along
the Cascades for Saturday and this seems reasonable given the
late afternoon/evening timing of a mid level shortwave and strong
deep layer shear.

Also on Saturday humidities will be near or below 10% in Oregon
Fire Zone 642 with moderate southerly winds. A red flag warning
is in effect from 1 pm-8 pm. Haines will also increase to 5
locally 6 during the day creating at least a moderate potential
for plume dominated fires.

The weather quiets down on Sunday and Sunday night with dry
conditions and cooler temperatures. Highs on Sunday will be
mostly in the 80s with 70s in the mountains. 78

Long Term...Wednesday through Sunday...

Bottom Line Up Front

1. Dry pattern to start

2. Mountains showers and thunderstorms Wednesday

3. Seasonably cooler temperatures through the period

Models are in relatively firm agreement with the upper level low
creeping into the PacNW. Clusters do show a slight variance in the
position with some models showing the low a bit farther off the
coast while others have it closer. This will however, not affect the
weather outcome through the beginning of the period. Dry and benign
weather will prevail through Tuesday before a shift in the pattern
will change things up a bit.

Wednesday the models show the upper level low lifting off to the
northeast and grazing the WA Cascades as it moves over the area.
This will shift the flow to a southwest component allowing some mid
level moisture to peruse over the WA Cascades. Models derived
soundings are showing a bit of instability over the Cascades with
values creating 500 J/kg, LIs of -4 and mid level lapse rates of
over 7.5 C/km. However, bulk shear is very low and PWATs are below
0.65 inches. Orographic lift will assist these thunderstorms along
the crests of the Cascades on Wednesday. With this, and looking at
the raw ensembles, 10-15% show there to be thunderstorms over the
Cascades so have left thunderstorm chances in the forecast for
Wednesday. After Wednesday night, models show another upper level
low making its way towards the PacNW and parking off the coast
through the remainder of the period bringing with it southwest flow
and continued chances of rain showers along the Cascade crests
through Friday.

The upper level lows will keep temperatures cooler over the region
with the southwest flow coming off the coast and ushering continued
cool air into the region. EFI is showing at or just below seasonal
averages for this time of year with the NBM showing the average
temperatures across the region to be in the mid to upper 80s for the
lower elevations with some isolated 90s through the Basin to start
the week before steadily decreasing into the low to mid 80s through
the remainder of the week. Raw ensembles are a bit more widespread
when it comes to the temperatures through the Basin with 53% saying
the Basin will see over 90 degrees while the remainder of the region
will see mid 80s on Monday. By Wednesday over 60% of the raw
ensembles show the Basin and surrounding areas in the low to mid 80s
and it remains as such through Friday. Bennese/90

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  54  93  56  84 /   0  10  20   0
ALW  57  99  62  88 /   0   0  20   0
PSC  60  99  65  89 /   0   0  30   0
YKM  55  93  58  84 /   0  10  80  10
HRI  58  98  62  88 /   0   0  30   0
ELN  55  93  60  84 /   0  10  80  10
RDM  49  87  47  82 /   0  40  50   0
LGD  51  94  53  89 /   0   0  10   0
GCD  54  93  52  88 /   0   0  10   0
DLS  60  95  61  83 /   0  20  80  10

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Fire Weather Watch from Saturday afternoon through Saturday
     evening for ORZ641.

     Red Flag Warning from 1 PM to 8 PM PDT Saturday for ORZ642.

WA...Fire Weather Watch from Saturday afternoon through late Saturday
     night for WAZ691.

     Fire Weather Watch from Saturday evening through late Saturday
     night for WAZ690.

     Flash Flood Watch from Saturday afternoon through late Saturday
     night for WAZ522-523.

&&

$$

SHORT TERM...78
LONG TERM....90
AVIATION...86