082
FXUS66 KPDT 181001
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
301 AM PDT Fri Oct 18 2024

.SHORT TERM...Today through Sunday night...Temperatures are about
5-10 degrees colder from this time Thursday morning under mostly
clear skies and light winds. Freeze warnings for many of our lower
elevation zones have already verified, although the northern Blue
Mtn Foothills are in the lower 40s and dewpoints are in the
mid-30s. High clouds spreading eastward will help insulate this
area around sunrise, so these zones will likely not observe
freezing temperatures--but close. Will cancel the Freeze Warnings
for WAZ29 and ORZ507 in the morning package.

Mid and high clouds are spreading across WA/OR this morning, and
it will be mostly cloudy during the afternoon. This will keep
temperatures unseasonably cool. A warm front spreading precipitation
across western WA and northwest OR will spread rain and high
elevation snow as far east as the Kittitas Valley, western Yakima
Valley, the Columbia Gorge west of Mosier, and the Simcoe
Highlands this afternoon, but there is a slight chance for rain to
develop about 20 miles east of these areas. Snow levels will rise
to above 9k feet with the warm front, therefore only the higher
peaks of the WA Cascades will receive snow. The warm front will
lift northward tonight and Saturday, cutting off precipitation
with the exception of the WA Cascade east slopes where stratiform
precipitation will continue into Saturday. Looking at the HREF
mean and NBM probability graphics, the most likely rainfall
amounts along the WA Cascade east slopes will be 0.25-0.5". There
is a 40% chance that areas northwest of Cle Elum such as Bumping
Lake and Lake Keechelus will receive 0.5-0.75" of rain. The front
will gradually lift north Saturday afternoon and night, and
precipitation will temporarily shut off along the central and
southern WA Cascades before the next front arrives on Sunday.

The rest of the forecast area will remain dry through Sunday
morning. A ridge will strengthen and temperatures will climb to
above seasonal averages in the afternoon. On Sunday, highs will be
around 65-75...or 10-15 degrees above seasonal average. Overnight
lows will be relatively mild for this time of the season,
although it will be below freezing in our typical colder mountain
valleys such as Ukiah, Seneca, and La Pine.

The next system to arrive on Sunday will be a sagging cold front
in a southwest flow aloft. Because the front parallels the upper
flow, it will be a slow moving cold front. Therefore, there will
not be a dramatic drop in temperatures and there will will not be
the isentropic descent for strong winds. Models are showing a weak
AR from the the Columbia Basin and Blue Mtn Sunday night. The
Cascades will once again observe 0.1-0.3" rain (confidence 80%),
and there is a good chance (50%) that the northern Blue Mtns will
observe the same. The WSW flow and the stretched out front lowers
the confidence that the Columbia Basin down to Central Oregon will
observe significant rain, but a tenth of an inch is likely for
many areas. Wister/85

.LONG TERM...Monday through Friday...

Bottom Line Up Front

1. Wet and breezy start to the workweek Monday

2. Dry conditions and seasonal temperatures return midweek

3. Rain returns late in the week

Models are in decent agreement with an upper level shortwave making
its way across the region beginning Sunday night into Monday.
Clusters show the main variance within the models being the
amplitude of the trough. However, the signal in the variance is
weak. Models also show a weak atmospheric river being entrained
into the wave bringing with it increased chances of rain across the
region. 24 hour precipitation shows 80-100% of the crests of the
Cascades seeing at or above 0.10 inches of rain, 40-60% at or near
0.10 inches of rain along the Blue Mountains and 40-60% of the
remaining locations seeing at or near 0.05 inches of rain on Monday.
Not only will the rain increase on Monday, but due to the passage of
the leading edge of the upper level wave there will be a increase in
the Cascade Gaps. While models are not picking up on anything out of
the ordinary, winds will be elevated with 60-80% of the raw
ensembles placing a large portion of the region with 15 mph
sustained winds, especially through the Gaps, along the foothills
and through the lower Columbia Basin.

Monday night the models show the upper level wave to begin to
flatten out with a more westerly flow aloft taking over ahead of an
upper level ridge. Clusters show a bit of uncertainty with the timing
and or position of the upper level flow. Regardless, all models and
ensembles point to dry conditions returning Tuesday and as the ridge
continues to build in, remaining in place through Thursday morning.
With the upper level ridge moving in over the region, temperatures
will steadily increase through the midweek. EFI shows temperatures
to remain at or slightly above average through Thursday. NBM shows
temperatures to be in the upper 50s to low 60s slowly increasing
through the workweek leveling off with the warmest temperatures
across southern ans southeastern portions of the CWA where the NBM
puts that area in the upper 60s to low 70s while the rest of us see
low to mid 60s.

Models begin to diverge quite a bit by days six and seven however,
all show some form of an upper level trough making its way into the
region. Clusters show the biggest variance within the models to be
primarily with the timing of the incoming wave. However, all models
do show precipitation moving into the area with much of it locked
along the Cascades. NBM QPF put amounts nearing 0.15 near the crests
with 71% of the raw ensembles in agreement. With the variance in the
models, confidence in the timing of the event is low/moderate (30-
50%). Bennese/90


&&

.AVIATION...12Z TAFS...VFR conditions expected through the forecast
period. Mostly high clouds through the early morning hours ahead of
some mid level clouds moving in tonight. CIGs will be mostly 25kft
through the day until dropping to 5 to 10kft by this afternoon.
Winds will be mostly terrain driven and below 10 kts. Bennese/90


&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  59  41  72  44 /  10  10   0   0
ALW  61  45  74  48 /  10  10  10   0
PSC  61  49  73  49 /  10  10  10   0
YKM  55  39  69  42 /  30  30  10  10
HRI  61  44  74  46 /  10  10   0   0
ELN  54  39  68  44 /  30  60  30  30
RDM  59  35  72  40 /  10   0   0   0
LGD  56  38  68  43 /  10  10   0   0
GCD  57  35  70  40 /   0  10   0   0
DLS  59  44  72  47 /  30  30  10  10

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Freeze Warning until 9 AM PDT this morning for ORZ505-508-510.

WA...Freeze Warning until 9 AM PDT this morning for WAZ026-027-521.

&&

$$

SHORT TERM...85
LONG TERM....90
AVIATION...74