961
FXUS66 KPDT 281752
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
1052 AM PDT Sun Apr 28 2024

.UPDATED AVIATION...18Z TAFs...VFR conditions expected despite the
increase in cloud cover incoming with the shortwave trough.
Precipitation is expected to crest the Cascades and reach
BDN/RDM/DLS between 12-15Z with 60-80% probabilities of rain in
DLS and a 50-60% for RASN mix at BDN/RDM. CIGs will continue to to
decrease through the period to 0VC050 and winds will continue to
be breezy with sustained winds between 9-15 kts with gusts of
15-22 kts at all until this evening. Bennese/90


&&

.PREV DISCUSSION... /issued 247 AM PDT Sun Apr 28 2024/

SHORT TERM...Today through Tuesday...Satellite imagery tonight
shows areas of low to mid level cloud cover scattered across the
mountain areas with high level cloud cover pushing east of the
Cascades. Meanwhile, radar imagery shows scattered showers
developing along the windward side of the Cascades and across
portions of the interior norther Blues.

Today, a weak frontal system west of the Cascades will push east,
while under a westerly flow aloft. Precipitation will mainly be
confined to the Cascades as this system moves across the
intermountain PacNW, with rain and snow showers above 4kft-4.5kft
along the Cascades, and rain showers across the eastern mountains.
In the lower elevations, the frontal system passage will tighten
surface pressure gradients, resulting in breezy west to southwest
winds across the lower elevations. The strongest winds are
expected along the southern Blue mountain foothills, Simcoe
Highlands, and the the exposed higher terrain throughout the
Columbia Basin; NBM probabilities show at least 60-75% of gusts
reaching 40 mph in the Columbia Basin and adjacent foothills,
while the Simcoe highlands are closer to a 90% chance. Winds are
expected to decrease overnight, though locally breezy conditions
will still be possible as pressure gradients are slow to weaken.

By late tonight into Monday morning, an upper low offshore
northern BC will be pulled south towards the PacNW by a
strengthening shortwave rounding it`s western periphery. The
shortwave trough is progged to reach the PacNW early Monday
morning, then exiting into ID and western MT late in the evening.
The shortwave arrival will result in snow levels dipping to
between 2.5kft to 3.5kt across the forecast area, allowing snow
showers to develop across most of the mountain areas. The heaviest
snow accumulations are expected to occur Monday morning for the
Cascades, and by the early afternoon for the northern Blues. Snow
amounts will generally be 5-8 inches along the Cascade crest,
where winter weather advisories have been issued between 11pm
tonight through 11am Monday morning; NBM probabilities do show
a 60-70% chance of at least 5 inches of snow accumulations by 11am
in along the Cascade crest. In the northern Blues, snow
accumulations will be just shy of advisory criteria with 4-6
inches of snow expected, and up to 3 inches across the remainder
of the eastern mountains. Light rain showers putting down a couple
hundreths of an inch can be expected across the lower elevations
Monday, though rain-shadowing on the leeward side of the Cascades
will result in only a trace of precip in the Kittitas/Yakima
valleys and central OR. Weak instability and a cold core aloft
associated with the shortwave passage will also result in isolated
chances of thunderstorms developing across the Cascade crest,
Blues, and Wallowa county in the afternoon. The shortwave trough
passage overhead will also result in a strengthening of surface
pressure gradients, with westerly winds between 20-30mph
developing during the day Monday. However, current deterministic
guidance and hires short- term guidance show a lack of any jet
support overhead with winds only between 30-35kts at 850mb and
700mb, resulting in a low confidence (30%) for strong gusts to
develop. That said, NBM guidance is depicting 60-80% chances of
wind gusts meeting or exceeding 45mph in portions of the Simcoe
Highlands, the OR Columbia Basin, and the Kittitas valley.

By Monday evening, the shortwave trough will have exit to the east
while the remnants of the parent low swings into the PacNW by
Tuesday morning. In the Monday overnight hours, winds will taper
off in the lower elevations, while light precipitation continues
across the mountains. The first half of Tuesday, the remnants of
the parent low will move across the forecast area, resulting in a
brief increase in rain/snow chances across the mountains, with
light rain showers across portions of the Blue mountain foothills,
central OR, and north central OR. Isolated thunderstorms will
once again be possible over the Blues and Wallowa county thanks to
the cold core aloft associated with the upper low. Locally breezy
west to southwest winds up to 15-20mph will redevelop across the
lower elevations Tuesday afternoon, but quickly diminish into the
evening hours.

A cooling trend Monday and Tuesday will result in afternoon
temperatures in the 50s to lower 60s both days. As for morning
lows, expect a return to the mid 30s to lower 40s across many
lower elevation locations, except upper 20s to mid 30s across
central OR. Lawhorn/82

LONG TERM...Wednesday through Sunday...While ensembles still
exhibit a bit of disagreement with regards to the synoptic pattern
during the latter half of the work week onward, deterministic models
are in better agreement, increasing forecast confidence for at least
the first half of the period.

The general picture painted by the deterministic GFS and ECMWF is
this: A broad Arctic low departs to our southeast Wednesday, with
northwesterly flow aloft ushering in a chance for some light
mountain precip, followed by a weak embedded shortwave riding the NW
flow on Thursday. Transitory ridging then builds in on Friday,
before models hint at a period of amplified SW to S flow heading
into next weekend. The GFS hints more at a closed low centered
offshore, while the ECMWF suggests more of an atmospheric river
pattern. Ensemble guidance is in good agreement on the midweek
Arctic low moving out of our region, but discrepancies arise on what
exactly occurs behind the low.

Clustering solutions are more or less in agreement on the NW flow
behind the departing low, but after that, Friday onward, some
ensemble members lean toward a more stubborn transitory ridge, while
others suggest the embedded shortwave in the NW flow on Thursday may
deepen out to a broader trough. Ridging appears to be the more
favorable solution, however with some ensembles keeping the ridge
lingering about through the weekend, this runs into conflict with
what deterministic guidance depicts. This is at the tail end of the
forecast period, where uncertainty is to be expected, but for now
will say forecast confidence is high (70%) Wednesday and Thursday,
moderate for Friday (50-60%, as ridging does appear to materialize
across the majority of guidance), and low Saturday onward (20-30%).

So what does the overall weather look like for the period? Expect
cool conditions lingering about under NW flow Wednesday into
Thursday, with overnight lows potentially dropping into the 30s even
in the lower Basin, while light PoPs of about 15-30% occur across
our mountain and foothill zones with snow levels in the 4000-5000 ft
range. Friday looks to be the most pleasant day of the week with dry
conditions and seasonable temps of highs in the 60s and 70s, while
next weekend is shrouded in uncertainty. Perhaps ridging persists,
and nice weather prevails, or deterministic guidance verifies and we
see area-wide shower chances and potentially storms should strong
SW/S flow aloft occur. Need better consensus across guidance to
determine for sure. Evans/74

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  61  37  54  34 /  10  10  70  10
ALW  64  42  56  38 /  20  20  80  30
PSC  67  44  61  41 /  10  10  40  10
YKM  63  35  58  32 /  10  10  40   0
HRI  66  40  60  38 /  10  10  50  10
ELN  58  34  54  32 /  10  20  30   0
RDM  58  32  49  27 /   0  10  50   0
LGD  59  36  49  31 /  30  20  80  30
GCD  61  35  49  29 /  10  10  70  10
DLS  62  41  57  38 /  20  40  60  10

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory from 11 PM this evening to 11 AM PDT
     Monday for ORZ509.

WA...Winter Weather Advisory from 11 PM this evening to 11 AM PDT
     Monday for WAZ522.

&&

$$

SHORT TERM...82
LONG TERM....74
AVIATION...90