571
FXUS66 KPDT 020959
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
259 AM PDT Thu May 2 2024

.SHORT TERM...Today through Saturday...The region is under a
northwest flow with a shortwave passing over the forecast area this
morning then exiting into Idaho late this afternoon and evening.
Most of the precipitation (.10-.20") associated with this shortwave
will be confined to central Oregon which tapers off to showers in
the afternoon then ending in the evening. Snow levels will be 4000
to 5000 feet with the Oregon Cascade crest and passes most likely to
be impacted by some snow accumulations for which a Winter Weather
Advisory is in effect until 11am.

The departing system will be replaced by a weak ridge of high
pressure overnight and into Friday but then moves off to the east as
a deep closed low in the Gulf of Alaska takes up position off the
PacNW coast. This will begin to spread precipitation into the
Cascades Friday afternoon and then slowly across the western portion
of the forecast area overnight under a moist southerly flow with
snow levels around 6000 feet. The center of the low settles over SW
Oregon late Saturday providing a continued very moisture laden S-SE
flow across the forecast area. Saturday night the low center moves
east along the southern Oregon border with the continued moisture
wrapping around the low coming from Idaho back into the forecast
area. Overall models are showing that this system will provide most
of the forecast area with some substantial precipitation amounts on
the order of half an inch to an inch with some locally higher
amounts of 1 to 2 inches over the mountains. This in turn will cause
area streams and rivers to rises over the weekend with some expected
to approach bankfull Sunday and Monday. Snow levels over southern
and central Oregon will lower Saturday night to 3500 to 4500 feet
but remain 5000 to 6000 feet over the rest of the forecast area.




.LONG TERM...Sunday through Thursday...

Key Messages:

1. Some rivers are forecast to approach or reach into action stage.

2. Breezy winds Sunday through Tuesday.

3. Thunderstorm potential Monday and Tuesday.


The extended period is characterized by a slow moving upper level
low pressure departing our region early in the workweek, ahead of a
couple shortwaves passing through the area embedded in northwest
flow aloft. These synoptic features will keep daily precipitation
chances in the forecast for much of the region as afternoon snow
levels drop from 5000-6000 feet on Sunday to 4000-5000 feet Monday
onward. Temperatures will also stay below normal for this time of
year, as the coolest day will occur on Sunday following a slow warm
up through the remainder of the week. High temperatures on Sunday
are expected to range from the upper 40s across Central Oregon into
the mid-50s to low 60s for lower elevations of the Basin and through
Yakima/Kittitas Valleys.

The persistent weekend rainfall associated with a deep, slow moving
upper level low will lead to rises on area rivers. The ECWMF EFI
suggests not only an above normal precipitation event for this time
of year, but a rather significant event due to a 1-2 shift of tails
(SOT) highlighting areas across the Basin and along the foothills of
the northern Blue Mountains. Current river forecasts project that
the Naches River near Cliffdell and John Day River at Service Creek
will reach into action stage during the early morning hours on
Sunday and late Monday respectively. Other river reaches that look
to approach action stage via the Northwest River Forecast Center`s
current forecast include the Naches River near Naches Sunday
morning, Umatilla River near Gibbon Sunday afternoon, Yakima River
at Umtanum Sunday afternoon, Grande Ronde River at Troy late Sunday,
and Imnaha River at Imnaha early Monday. There does exist some
uncertainty as there is 53% variance with ensembles regarding the
depth of the slowly tracking upper level low as 53% of ensemble
members show a slower progression and slightly more southerly track.
However, the most south solution only amounts to 28% of ensemble
members, which is the only solution that suggests a slightly drier
forecast than the NBM. Probabilities of 0.20 of an inch of rainfall
on Sunday via the NBM range from a 25% chance in the Tri-Cities
area, to a 40-60% chance along the northern Blue Mountain foothills,
to a 70-80% chance over the northern Blue Mountains and Elkhorns on
Sunday. These rain amounts may lead to localized flooding and will
continue to be monitored as the event nears.

A transient upper level ridge will sneak onshore in the wake of the
slowly departing upper low pressure to allow a pressure gradient to
develop along the Cascades, producing breezy winds through the
Simcoe Highlands, northern Blue Mountain foothills, Eastern Gorge,
and the Kittitas Valley late Sunday afternoon through the evening.
Wind gusts of 30-40 mph out of the west will be possible across the
aforementioned areas. Confidence in these values is moderate to high
(60-80%) as the GFS showcases a 10.5mb pressure gradient between
Portland and Spokane setting up Sunday evening, which is shy of the
advisory threshold of 12mb - but significant. The NBM also
highlights an 85-100% chance of wind gusts reaching 39 mph or
greater across the earlier mentioned areas, with the highest chances
residing over the Simcoe Highlands. When bumping up to 47 mph or
greater, the probabilities drop to 75-90%. These values near
advisory criteria, which will also be further analyzed as additional
short-term guidance becomes available (SREF/NAM). A passing cold
front associated with a dropping upper level shortwave on Monday
will again elevate winds across the Basin, Simcoe Highlands, and the
Kittitas/Yakima Valleys, with gusts of 20-30 mph possible out of the
west. Another transient upper level ridge attempts to build in from
offshore in the wake of the departing shortwave on Tuesday, allowing
another pressure gradient to develop along the Cascades. The GFS
shows a more substantial pressure difference between Portland and
Spokane of around 12.4mb Tuesday afternoon/evening, which is above
the earlier mentioned 12mb advisory threshold. However, this is due
to a weaker shortwave and resulting stronger and more efficient
infiltration of the transient ridge (ie. stronger gradient setting
up along the Cascades). This solution is not currently favored, and
only represents 23% of all members, with half of those members being
that of the GFS.

The passing cold front on Monday and shortwave on Tuesday will allow
for instability to be present, as these features are bookended by
transient ridging. Guidance currently suggests surface CAPE of 50-
100 j/kg each day primarily along the John-Day Basin and the
northern Blue Mountains and foothills. Monday currently looks to be
the more favorable day as the transient ridge is eroded by the
incoming shortwave and associated cold front. The NBM highlights a
15-20% chance of thunderstorms over the aforementioned areas Monday
afternoon, dropping to a 10-15% chance of thunderstorms Tuesday
afternoon. These forcings elude to the presence of discrete storms
cells forming as temperatures peak in the late afternoon, but these
values are lacking as also indicated by the absence in any Storm
Prediction Center`s convective outlook product. Thus, there is low
to moderate confidence (30-50%) in thunderstorms developing either
day, with a low chance (10%) of any exhibiting severe
characteristics. 75



&&

.AVIATION...06Z TAFs...A weather system from the Pacific will
bring increasing chances of MVFR or lower conditions (CIGS and/or
VSBYs) overnight to KBDN/KRDM (80-95% chance of MVFR, 30-40% chance
of IFR, and <20% chance of LIFR) as ample cloud cover and
precipitation spreads over Oregon. Precipitation type will be a
challenge with this system, leading to lower-than-average confidence
in CIGs/VSBYs as precipitation may (20-40% chance) turn over to snow
or a rain/snow mix by late night and early morning at KRDM/KBDN.
Elsewhere, VFR conditions are very likely (85% chance or greater).
Winds of less than 10 kts are forecast. Plunkett/86


&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  61  35  71  47 /  20   0   0  10
ALW  66  41  74  53 /  10   0   0  10
PSC  69  43  74  53 /   0   0   0  10
YKM  67  38  69  46 /   0   0   0  60
HRI  68  39  76  49 /  10   0   0  20
ELN  65  37  66  45 /   0   0   0  50
RDM  52  30  63  41 /  90   0  10  90
LGD  55  30  65  47 /  40  20   0  10
GCD  52  32  63  45 /  90  30   0  40
DLS  66  42  71  49 /  60   0  10  80

&&

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

WA...None.
&&

$$

SHORT TERM...91
LONG TERM....75
AVIATION...75