560
FXUS66 KPDT 142222
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
322 PM PDT Sat Sep 14 2024

.SHORT TERM...This afternoon through Monday night...Isolated
showers along the east slopes of the Washington Cascades are
evident in radar and satellite imagery. 12Z HREF soundings
suggest surface-based CAPE of 150-300 J/kg capped around 15 kft
AGL, likely too shallow for lightning. Radar returns paint a more
optimistic picture for lightning chances with returns as high as
20-25 kft, where temperatures are roughly -25 to -30C, perhaps
just sufficient for an isolated strike.

Looking ahead, a couple vorticity lobes over the Pacific will
carve out a deepening upper-level trough just offshore through the
remainder of the day and tonight. Ensemble guidance is in
excellent agreement (>95% chance) that the trough will develop
into a closed low through Sunday with the main circulation center
over southwest OR and northwest CA. This will place our CWA under
southwesterly to southerly flow in the mid- to upper levels, while
low-level flow will become northerly. With sufficient moisture in
place, rain showers are anticipated to develop overnight through
Sunday across the eastern mountains aided by synoptic-scale
lift from an upper-level jet. Sunday afternoon, low (10-15%)
chances of embedded thunderstorms are forecast for southeastern
portions of our CWA, primarily southeastern Deschutes, Crook, and
Grant counties.

Monday through Monday night, the upper-level low will eject
northeastward across the northwestern Great Basin. While the
upper jet will weaken overhead, NWP guidance is suggesting
diffluent flow aloft coupled with a modestly unstable atmosphere
over the Blue Mountains. Afternoon convection is more likely over
the Idaho Rockies, but activity may track into our CWA.
Probabilities of additional upslope showers range from chance
(25%) to likely (60%) across the northern and eastern Blue
Mountains overnight.

While confidence is very high in the overall synoptic pattern,
ensemble clusters suggest some uncertainty remains regarding the
precise track of the low as it comes onshore and tracks across
the region. The most impactful forecast variable that the track
will modulate is QPF. While NBM chances of wetting rain (0.10" or
greater) are medium-high (40-70%) for the 48-hr period tonight
through Monday night across most of the Blue Mountains, individual
global ensemble QPF clusters place a sharp southwest-northeast
gradient across eastern Oregon; the most likely solution (~50% of
members) places the heaviest QPF (cluster mean of 0.10-0.50") over
south- central and southeast Oregon into western Idaho, the
remaining clusters are split (25%/25%) in placing a substantial
(0.5-1.5") or lighter (up to 0.50") amount of QPF over the Blue
Mountains, heaviest over northeast Oregon.

Winds will switch to westerly and become breezy to windy Monday
evening through Monday night. Plunkett/86

&&

.LONG TERM...Tuesday through Saturday...

Key Messages:

1. Breezy winds midweek.

2. Scattered showers Tuesday and Wednesday.

3. Below normal temperatures extend through the period.

The extended period is characterized by an incoming upper level
trough Tuesday and Wednesday, followed by an upper level ridge
building across the area through the remainder of the week. The
digging upper level system and associated cold front will bring
scattered showers to the area through Wednesday before dryer and
warmer conditions return. These synoptic features will also allow
pressure gradients to tighten across the Cascades, leading to breezy
conditions through the Gorge and Kittitas Valley. The most impactful
concerns will primarily be focused on these enhanced winds through
the Gorge, foothills, and the Kittitas Valley Tuesday. Sustained
winds of 25-35 mph will be possible with gusts of 30-40 mph out of
the west-northwest over these areas. Elsewhere, 10-15 mph sustained
winds and gusts of up to 20 mph are expected. Confidence is moderate
to high (60-80%) in these wind values as the NBM advertises a 70-90%
chance of sustained winds of 30 mph and gusts of 35 mph across the
Gorge and Kittitas Valley over Tuesday afternoon. Peak timing for
these increased winds will be late morning through the early
afternoon on Tuesday before winds look to slacken through the
evening into Wednesday morning.

Showers associated with the incoming system on Tuesday will
predominately occur over mountainous terrain across the Cascades and
Blues until the afternoon, before extending into Central Oregon,
Blue Mountain foothills, and portions of the Basin by late
afternoon. This is in response to the trough axis dropping to our
south as the associated cold front passes through the region
overnight into Wednesday morning. Wetting rain (>.10") chances will
be low to moderate (10-35%) for the area, with the exception of
Wallowa County and the east slopes of the Cascades (60-80%). Rain
chances (20-45%) will linger into Wednesday afternoon across the
Cascades and the eastern mountains before upper level ridging begins
to push into the area from the west. The chance for a wetting rain
over mountainous terrain in the Cascades and eastern mountains on
Wednesday is rather lacking, as only 10-30% is showcased by the NBM.

Ensembles and deterministic models are in good agreement with the
overall pattern, but slight differences in system speed is observed
via the 500mb EOF patterns. This would correlate to a slightly
earlier onset of precipitation and elevated winds Tuesday
morning/afternoon. However, this is only depicted by 49% of ensemble
members, so if it does shift earlier it is expected to be slight.
44% of members also hint at a stronger incoming system, versus 34%
suggesting a weaker system, and 22% staying similar to the current
forecast. A stronger system would lead to a better potential for
showers, cooler temperatures, and higher winds, but confidence in a
stronger system is currently low to moderate (20-40%) due to the
variable spread of ensemble members. However, ensembles are in good
agreement with the incoming upper level ridge moving into the region
late Wednesday into Thursday and strengthening into the weekend,
leading to a 80-100% chance of high temperatures staying 5 to 10
below normal through this period. 75

&&

.AVIATION...18Z TAFS...VFR conditions currently at all sites,
which will stay the course through the period. Winds are
increasing for KDLS and KPDT as gusts of 18 to 25 kts will persist
through the afternoon before subsiding this evening. Breezy winds
are also anticipated to occur at KRDM and KBDN later this
afternoon and evening between 20 and 25 kts, but will subside late
tonight and overnight into Sunday morning. These winds are
associated with an approaching system that will also drop ceilings
during the early morning hours on Sunday for most sites. 75

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  50  65  49  72 /  10  30  30  20
ALW  53  67  50  75 /  10  40  40  20
PSC  56  72  55  80 /  10  10  20  10
YKM  46  74  47  82 /  10  10  10   0
HRI  54  71  53  78 /  10  20  20  10
ELN  47  74  48  81 /  10  10  10   0
RDM  44  64  42  69 /  10  20  30  20
LGD  49  64  48  69 /  30  60  60  40
GCD  47  63  48  67 /  30  60  60  40
DLS  52  75  53  80 /   0  10  10   0

&&

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

$$

SHORT TERM...86
LONG TERM....75
AVIATION...75