971
FXUS66 KPDT 061109
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
409 AM PDT Sun Apr 6 2025


.UPDATED AVIATION...12Z TAFs...VFR conditions to begin all TAF
sites. However, an area of troughing will make its way into the
region later today, which in turn will increase POP chances. Rain
is expected at all sites with the precip coming in from the
Cascades and continue east until it impacts the entire CWA. Rain
will first impact sites in the western part of the region
(DLS/RDM/BDN/YKM) at 01Z to 02Z then makes its way toward the more
eastern sites (PDT/ALW/PSC) later in the evening at 07z to 08z.
CIGs are expected to decrease to MVFR with some of these rain
showers at RDM/BDN/YKM/PSC. Winds remain light and variable up to
12 knots sustained at all sites.

&&

.PREV DISCUSSION... /issued 331 AM PDT Sun Apr 6 2025/

SHORT TERM...Sunday through Tuesday Night...After fair weather
this morning, a more active weather pattern is expected through
the rest of the short term period. The Extreme Forecast Index
highlights a couple of aspects. Temperatures today and tonight
have values of 0.79 and 0.88 respectively and are spread over the
entire area, It highlights CAPE values of 0.78 on Monday over the
eastern half of the area which shows the potential for
thunderstorms. QPF values are at 0.86 on Monday with a Shift of
Tails over the Columbia Basin suggesting that a subset of model
ensemble members have higher rainfall values than what other
members are indicating, probably related to thunderstorm
activity.

Currently, an upper level ridge is centered over our area this
morning though it is moving east as an upper trough approaches the
coast. Satellite imagery shows a warm front with increasing
clouds moving ashore and radar is showing rain moving ashore along
the Washington/Oregon coast. Having the ridge overhead and a
warmer southwest flow aloft will give us warmer temperatures today
with highs in the upper 60s to mid 70s in the lower elevations
and in the mid to upper 60s in the mountains, which is about 10-15
degrees above normal. A chance of rain will begin crossing the
Cascades in the afternoon and will spread across the area this
evening giving all areas likely rain overnight. Rain amounts
through tonight will be moderate with the Cascade crest getting up
to an inch, away from the crest a half to three quarters of an
inch. a quarter to a third of an inch in western portions of the
area and up to 10 to 15 hundredths of an inch from the Tri-Cities
and Hermiston eastward. Snow levels will be at 7000-8000 feet so
only the highest peaks will see snow.

As we get into Monday, the trailing cold front will move into the
area and this will keep a chance of showers across the area in the
morning. Colder air moving overhead will help destabilize the
atmosphere. Models are highlighting negative Lifted Indices, CAPE
values of 100-300 J/kg over the eastern mountains and Blue
Mountain Foothills and a 20-30 percent chance of thunderstorms.
Have a slight chance of thunderstorms over those areas Monday
afternoon and early evening. The thunderstorms are expected to
give the eastern mountains and Cascades up to three quarters of
an inch of rain and the Blue Mountain Foothills a quarter to a
third of an inch. Snow levels will be at about 6000 feet, so only
the highest peaks will see any snow. The colder air will drop
temperatures 6 to 10 degrees Monday to the upper 50s to mid 60s in
the lower elevations and to the mid 40s to mid 50s in the
mountains.

After a brief break Tuesday morning, another wave will a chance
of rain Tuesday afternoon and evening before tapering off
overnight. The atmosphere looks more stable and models keep a
slight chance of thunderstorms confined to Wallowa county. Rain
amounts look lighter though the Cascades look to get up to a half
inch of rain, the northern Blue Mountains up to a quarter inch of
rain and the rest of the area will have less than a tenth of an
inch. Temperatures cool a couple more degrees.

One concern with the warmer temperatures today and rain Monday and
Tuesday will be rising water levels in streams and rivers. At this
time, only the John Day River at Service Creek is expected to
reach minor flood stage Tuesday afternoon and night. The Grande
Ronde River at Troy and Naches River at Cliffdell are expected to
reach action stage on Tuesday, but are not expected to reach minor
flood stage. Perry/83

LONG TERM...Wednesday through Sunday Night...

Key messages:

1. Dry, warm start to the long term.
2. Another round of precip in the back half of the period.
3. Uncertainty late this week through next week.

A ridge emerging from the California/Nevada/Arizona region will
extend into the PacNW region, starting the long term dry and warmer
for the first few days of the period. Temperatures will rise to 5 to
15 degrees above the season average (>90% confidence). High temps
across the Basin/Foothills towards Central OR will run as high into
the mid-70s, while higher elevations will rise to the low to mid
60s. Temperatures will then start to cool off by 5 to 10 degrees
heading into Friday as precip chances increase as the synoptic
pattern changes into a more trough set-up.

Clusters come into agreement by Thursday night, rain will begin to
enter the CWA from the Cascade Mountains, but quickly begin to
diverge in agreement on the set-up heading into late Thursday as 24%
members considerably delay the precipitation until Friday evening. By
Friday morning, 45% of clusters present widespread rain precip,
while the remaining keeps rain to more isolated areas. Heading into
Friday night into Saturday night, the clusters become a bit
unreliable with the forecast becoming considerably less confident
with 58% giving a stronger trough, enhancing low level rain and
light mountain snow. The other 42% give off a much weaker trough
with less moisture advection making its way into the CWA. What we
can take from guidance at this point is that it`s more than likely
going to be another wet pattern by the end of the week, but to what
extent is unknown at this time. There is some hinting of drying out
by the beginning of next week with 42% members suggest drying out by
Sunday Night, with the other 58% depicting a stronger trough with a
more southwest/west-southwest surface level wind and more precip
associated.

Mountain snow totals do not look to be impressive through the long
term with NBM displaying only up to 2 inches in the Cascades from
Saturday afternoon through Sunday Night. Snow levels remain fairly
elevated above 5000 feet to prevent any accumulations near advisory
level, but with guidance not being reliable this far out, it`s one
thing to monitor as we get closer to the event to watch for any
additional changes in the forecast.


&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  73  49  64  41 /   0  70  80  50
ALW  73  50  61  42 /   0  70  90  60
PSC  73  50  67  43 /   0  70  60  20
YKM  69  46  63  37 /  20  80  30  10
HRI  74  49  67  40 /   0  80  60  30
ELN  67  46  61  37 /  20  80  40  20
RDM  72  42  60  32 /  20  80  50  20
LGD  68  46  58  36 /   0  50 100  80
GCD  70  46  60  35 /   0  70  90  60
DLS  69  49  61  42 /  30  90  70  40

&&

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

$$

SHORT TERM...83
LONG TERM....95
AVIATION...95