660
FXUS66 KPDT 141626
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
926 AM PDT Sat Sep 14 2024

.MORNING UPDATE...Expanded PoPs to a slight chance (15%) across
the northern Blue Mountains and their foothills as well as far
northern areas of the Columbia Basin of WA where some light, weak
showers are evident on radar and satellite imagery. Otherwise,
added some patchy smoke and definite haze to portions of central
Oregon and the John Day Basin through morning where air quality
remains reduced due to smoke from regional wildfires. Plunkett/86

&&

.PREV DISCUSSION... /issued 409 AM PDT Sat Sep 14 2024/

UPDATED AVIATION...

PREV DISCUSSION... /issued 248 AM PDT Sat Sep 14 2024/

SHORT TERM...Today through Monday...A shortwave passing to the
north of the forecast area tonight is pushing a narrow band of
light rain showers across eastern WA, with mid to upper level
cloud cover spreading across the rest of the forecast area.

The shortwave clipping the region at this time will exit into
southeast BC and southern Alberta this morning, while an upper
trough slowly slides down the BC coastline placing the forecast
area in a near zonal flow aloft today. Shortwave impulses
rounding the upper trough will lead to light scattered showers
developing along the WA Cascade crest through the evening hours.
Breezy winds of 15-25 mph with gusts up to 35 mph will develop
through the Cascade gaps as cross Cascade pressure gradients
tighten this afternoon.

Tonight through Sunday, the upper trough will slide down the PacNW
coast into northern CA thanks to a reinforcing low arriving from
the Gulf of Alaska. The trough will gradually develop into a broad
upper closed low as it moves down the west coast. This will allow
shortwave impulses to round the low and impact the eastern
mountains throughout Sunday. Rain shower chances (30-55%) will
mainly be confined to the eastern mountains, central OR, and the
central OR Cascades throughout Sunday, with drier conditions
elsewhere. NBM 24-HR probabilities for 0.1 inches of QPF ending
Sunday night will generally be 45-65% across the Blues, western
portions of the Ochoco-John Day Highlands, and Wallowa County;
60-70% across eastern portions Ochoco-John Day Highlands; and only
20-40% across central OR and central OR Cascades.

Monday, the upper low will swing through the Great Basin and south
central ID. This will lead to shower chances (20-40%) retreating
east and being confined mainly to the Strawberrys, Wallowas, and
northern Blues. Through Monday night, probabilities of 0.1 inches
of QPF will be 40-65% in these areas. Otherwise, an approaching
shortwave trough late Monday night will lead to the cross Cascade
pressure gradient strengthening in the late evening, with breezy
winds 15-20mph and gusts up to 35mph developing through the
Cascade gaps.

Northerly flow on Sunday will lead to afternoon temperatures
dropping into the mid 60s to lower 70s, while today and Monday
afternoon temperatures will generally be in the 70s. Lawhorn/82

LONG TERM...Tuesday through Saturday...

Bottom Line Up Front

1. Rain along the crests of the mountains, central OR and Wallowas

2. Winds through the Cascade Gaps, foothills and Basin

3. Below average seasonal temperatures

Models are in firm agreement with the upper level cutoff low
snaking its way towards the northeast. Another upper level low
will be making its way down the Canadian coast. Clusters show the
main variance between models will be the amplitude of the incoming
system to the west which can play a part in the amount of
rainfall the area receives. However, the low to the east will send
some wrap around moisture into the eastern portion of the CWA
across the Wallowas while the next will bring precipitation to the
crests and east slopes of the Cascades. Not only will these
systems bring rain, but they will also bring enhanced winds
through the Cascade Gaps, Basin and foothills of the Blues as well
as at or below seasonal average temperatures.

NBM QPF shows that the region will see rainfall amounts along the
Wallowas due to the wrap around moisture from the previous upper
level cutoff low while the low moving down the Canadian border
will bring rainfall to the crest of the Cascades and east
slopes. Amounts along the crests and east slopes will range
between 0.2-0.40 inches of rain with 60-80% of the raw ensembles
in agreement, 20-40% of the raw ensembles show the eastern slopes
and portions of central OR seeing amounts between 0.05-0.15
inches. As we move to the eastern mountains, 30-50% of the raw
ensembles show the Wallowas could receive between 0.05-0.017
inches of rain from the wrap around. This is roughly a 36 hour
interval for the rainfall amounts. Models then show the first
upper level low moving to the far northeast while the second
follows the same track undercutting the region and dragging the
precipitation to the south and allowing the majority of the region
to remain under dry conditions.

Models show the second upper level low swing to the far south
causing the gradients to turn northerly aloft. Models show a
tightening of the surface pressure gradients across the Cascades
with more models coming into agreement of the cross Cascade
gradients tightening to 9 to 11 mb. This will lead to increased
breezy winds through the Cascade Gaps. 50-70% of the raw ensembles
clock the Gorge with 35 mph or greater winds Tuesday afternoon as
well as through the Kittitas Valley. 80-100% of the raw ensembles
show the winds to stretch across the lower Columbia Basin and
foothills of the Blues with winds 20 mph or greater and gusts to
30 mph. Once the gradient passes the Cascades, models show the
winds to return to near normal with sustained winds of 10 mph
across much of the region.

Lastly, between these two systems and the northerly flow aloft,
temperatures across the region will become a bit more fall like.
The EFI continues to signal portions of the area to be below
seasonal average through a large portion of the work week.
Especially in the central OR mountain region, while the remainder
of the region sees at or near normal. NBM shows temperatures
through the Gorge, foothills of the Blues, central OR, Basin and
adjacent valleys to see temperatures in the low to mid 70s through
much of the work week while the eastern mountains and Cascade
crests will see temperatures in the mid to upper 50s. Bennese/90

AVIATION...12z TAFs...VFR conditions are expected through the TAF
period. A weak disturbance will make its way across the area
bringing vicinity showers to PSC before moving out. Winds will
become breezy 10-15 kts at DLS/PDT/BDN/RDM between 17-21Z
respectively with gusts to 25 kts likely. CIGs will remain BKN250
across all TAF sites with BDN seeing HZ in the early morning hours
that will bring VIS to near 4SM. Bennese/90

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  74  49  66  49 /  10  10  30  30
ALW  77  52  68  50 /  20  10  30  30
PSC  79  55  72  56 /  10  10  10  20
YKM  77  44  73  47 /  10   0  10   0
HRI  79  52  71  52 /   0  10  10  20
ELN  74  47  73  48 /  10  10   0   0
RDM  74  42  64  41 /   0  10  20  30
LGD  74  48  63  48 /   0  20  50  50
GCD  76  47  65  47 /   0  20  50  60
DLS  77  52  74  54 /  10   0  10  10

&&

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

$$

MORNING UPDATE...86
SHORT TERM...82
LONG TERM....90
AVIATION...90