272
FXUS66 KPDT 291801
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
1101 AM PDT Mon Apr 29 2024

Updated Aviation Discussion

.AVIATION...18Z TAFS...VFR currently at all sites with cloudy skies
(FEW to OVC, 040-120 kft) for this period. Wind gusts will be the
primary concern for all sites as the frontal boundary passes through
PacNW, inducing strong surface pressure gradient. KDLS/KALW are
currently having rain showers until tonight while rain showers at
KPDT might occur around late morning into afternoon. However, there
is a 20-30% chance of thunderstorms happening at the Blue Mountains
late Monday morning into night, but was excluded from the TAFs due
to low confidence (20%). Expect light precip for the remaining TAF
period (80% confidence). Winds will be moderate at 10-20 kts for
most sites during morning into tonight for KDLS/KRDM/KYKM/KALW/KPSC
with wind gusts at 20-30 kts, but wind gusts vary for KPDT/KBDN
until Tuesday morning. Feaster/97

&&

.UPDATE...Latest water vapor imagery shows a well defined upper
trough moving into the area at the present time. This feature will
be responsible for scattered to numerous showers and mountain snow
showers today. Cold temperatures aloft and 700-500 mb lapse rates
of 8 C/KM will contribute to sufficient instability (200-500 J/KG)
for isolated tstms from the eastern part of the Columbia Basin to
the eastern Mountains (SPC HREF Probabilities this afternoon
15-30%). Otherwise Winter weather advisories continue for the
east slopes of the Cascades until 1100 AM. Snotels and CAMS
suggest that only a few inches of snow have fallen in these areas
thus far. Winds will pick up this afternoon across the lower
elevations with sustained west winds 20-25 mph and gusts 30-38
mph. Made a few minor adjustments to winds and sky cover for this
mornings forecast update but otherwise the current forecast is on
track. 78

&&

.PREV DISCUSSION... /issued 236 AM PDT Mon Apr 29 2024/

SHORT TERM...Today through Wednesday...Water vapor satellite
imagery tonight shows a shortwave trough approaching the PacNW,
while nighttime imagery shows areas of low to mid level clouds
over the mountains, and mid to upper level cloud decks developing
across the Columbia Basin. Meanwhile, while the radar shows little
to nothing east of the Cascades, precipitation bands are currently
impacting western WA and northwest OR. Today, the shortwave trough
offshore will swing across the PacNW, increasing chances of
mountain snow early this morning, with light precipitation across
the lower elevations developing late in the morning through the
mid afternoon hours. The main focus of this system will be the
lowering snow levels facilitating moderate to locally heavy
snowfall across the Cascade crest. NBM still indicates great
chances of reaching 6 inches of snowfall through 11pm tonight
along the Cascade crest, with a 70-80% chance above 4kft, while
below this elevation chances fall to only 40-45%. The US97
corridor through central OR will see very little of this snow
fall, with up to 2 inches in the Sunriver/LaPine area possible. An
isolated thunderstorm or two will be possible this afternoon
across the eastern mountains, supported by steep low level lapse
rates and afternoon SBCAPE increasing up to 250-300 J/kg. The
shortwave trough passage will also result in breezy to locally
windy westerly winds developing this morning through the afternoon
hours. Current guidance indicates that the surface pressure
gradient between PDX- GEG will only peak around 9mb, and while
this will result in breezy sustained winds 20-30mph with locally
35mph, this gradient is not conducive for widespread advisory-
levels winds. Also, guidance continues to show very little jet
support in the lower to mid levels, with 850mb and 700mb winds
only peaking around 40mph; confidence is low (20%) that wind gusts
will exceed 45mph at the surface.

Tonight through Tuesday afternoon, a weakening stacked low will
make a pass across the PacNW, renewing chances of light snow in
the mountains and light rain across the lower elevations. While
confidence is only moderate (45-55%) for up to 1-1.5 inches of
snow in the most mountain areas, the OR Cascade crest looks to be
favored again, with NBM probabilities indicating a 80-95% chance
of 2 to 3 inches of snowfall. With a cold core aloft and
increasing surface based CAPE, a few isolated thunderstorms will
be possible in the afternoon across portions of northeast OR and
far southwest WA. Breezy westerly winds will continue Tuesday,
with only 15-25mph sustained winds and gusts up to 35-40mph
expected.

Wednesday, the remnants of the upper low will move east of the
region, with a weak northerly flow over the intermountain PacNW.
While most of the forecast area will remain dry, wrap-around
moisture from a low over the far northern Great Plains will result
in continued light snow chances across the northern Blues and the
Wallowas, with light rain across the lower portions of Union and
Wallowa counties. Light to locally breezy winds will develop by
the afternoon in the lower elevations, though a decent cross
Cascade surface pressure gradient will result in breezy northwest
winds through the Kittitas valley. Lawhorn/82

LONG TERM...Thursday through Monday...Forecasting the long term
period continues to be a frustrating endeavor as ensemble guidance
remains out of sync, especially next weekend onward. As a result,
forecast confidence after Friday is low (30-40%), but at least the
remainder of the work week looks a bit more solid across guidance,
with higher confidence in the 60-70% range.

The PacNW will find itself under NW flow as a broad area of low
pressure develops over the northern Plains states. There is
disagreement amongst guidance on whether or not a shortwave embeds
itself into this NW flow, which would yield greater chances for
precip outside of the mountains, but QPF ensembles do suggest at
least a slight chance of precip occurring across the Basin on
Thursday, with PoPs in the 20-30% range. Such a NW flow pattern
would favor better precip chances over the central Washington
Cascades and the eastern mountains, while PoPs fall off heading
south towards central Oregon and the Oregon Cascades.

Guidance then depicts transitory ridging building in sometime around
Friday, however the upper-level pattern is progressive enough such
that pinning down timing is difficult. This is because, across
deterministic guidance over the weekend, the ECMWF depicts a low
coming in from the northwest, while the GFS suggests a system will
arrive from the southwest. Ensemble guidance suggests that a low of
some kind will impact the forecast area next weekend, but the origin
of this system and where it ultimately centers itself remains
uncertain. A more northwesterly system would mean colder air and
potentially mountain snow, while a southwesterly system would advect
in warmer, more moist air and potentially spawn thunderstorms in the
forecast area. So based on guidance, we can say confidence is
moderate to high (50-60%) that a low and thus precip of some kind
will impact us next weekend, but details are elusive until guidance
gets a grip on where exactly this low comes from.

Despite the continuous model discrepancies over the past several
days, ensemble clusters are strangely in agreement around the tail-
end of the period, suggesting yet another, broader low will usher in
SW flow aloft into the forecast area. Can`t speak with too much
confidence this far out, but the first week of May does look to be
semi-active as we head into the beginnings of the warm season.
Evans/74

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  54  34  54  34 /  60  10  30  20
ALW  56  37  57  37 /  70  30  40  20
PSC  61  42  61  40 /  40  10  20  10
YKM  58  35  56  34 /  50  10  20   0
HRI  60  39  59  37 /  40  10  20  10
ELN  54  34  54  35 /  50  20  20   0
RDM  49  27  48  27 /  50  10  30  10
LGD  49  30  49  32 /  80  20  50  40
GCD  49  27  50  30 /  80  10  40  30
DLS  57  38  57  39 /  80  20  30  10

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory until 11 AM PDT this morning for ORZ509.

WA...Winter Weather Advisory until 11 AM PDT this morning for WAZ522.

&&

$$

SHORT TERM...78
LONG TERM....74
AVIATION...97