896
FXUS66 KPDT 121817 AAA
AFDPDT

Area Forecast Discussion...UPDATED
National Weather Service Pendleton OR
1117 AM PDT Mon May 12 2025

Updated aviation discussion

.AVIATION...18Z TAFS...MVFR ceilings linger at KRDM and KBDN but
should lift to VFR in the next few hours. VFR conditions are
expected to be predominant for the next 24 hours though showers
and isolated thunderstorms will make brief periods of MVFR cigs
and vsby possible this afternoon and early evening. Did not
mention CB, VCTS or TSRA at any TAF site due to the low
probability but will be possible at any TAF site through 04Z.
Showers return after 12Z with MVFR cigs at KRDM and KBDN. KDLS
will have northwest winds of 10 to 20 kts gusting to 30 kts from
20Z-06Z and again after 13Z. Other TAF sites will have generally
westerly winds of 10 to 15 kts with gusts to 20 to 25 kts from
21Z-05Z then begin increasing again after 14Z. Perry/83

&&

.PREV DISCUSSION... /issued 429 AM PDT Mon May 12 2025/

SHORT TERM...Today through Wednesday...

1. Widespread rain through Wednesday morning becoming dry

2. Afternoon thunderstorms today and tomorrow

Current radar shows showers over central OR and moving north.
Current ground observations show that 0.02-0.08 inches of rain has
fallen over central OR in the last hour. Rain will continue through
central OR and steadily spread east across the eastern mountains.

Models show the upper level low to be just off shore of OR right now
and steadily moving Southeast. The low will continue to push onshore
and begin to undercut the region. Southerly flow aloft will continue
to surge precipitation into the region becoming widespread later
this morning. All but the Basin, Yakima and Kittitas Valleys will be
mostly free from any heavier precipitation with 30-50% probabilities
of 0.05 inches of rain in the Basin and 10-20% for the lower
Kittitas and Yakima Valleys. 50-80% of the 24 hour raw ensembles
show the heaviest rainfall amounts to be over the OR Cascades and
across the eastern mountains and the Blues with 0.10-0.25 inches.
Models show the rain to linger through the day before the
precipitation dwindles overnight. Models Tuesday show the axis of
the upper level to be mostly off to the least with the backside of
the trough bringing some lingering showers to the Cascades and
eastern mountains. 24 hour raw ensembles show the precipitation
amounts to be slightly less with 60-80% probabilities of 0.10-0.15
inches of rainfall along the eastern mountains and the OR Cascades,
while the Basin and foothills have a 10-40% chance of 0.05-0.10
inches of rainfall. Models show the leading edge of an upper level
dirty ridge to begin sliding in over the PacNW by Wednesday early
morning which will keep some linger showers over the Cascades and
Blues until the afternoon when conditions dry out.

Along with the precipitation, CAMs models show the upper level low
will also bring along with it some instability. Looking at the CAMs
models all the ingredients needed for thunderstorms are there.
MUCAPE values between 305-450 J/kg, surface to 3 km lapse rates of
8.8 C/km and mid level lapse rates of 7.8 C/km, Lifted index of -3
and SHIP (significant hail perimeter) of 0.1 which could lead to
small hail accompanying these storms. While effective bulk shear is
not significantly high, orographic lift will aid these storms in
development. Raw ensembles show 30-40% probabilities over central/n
central OR, the lower Columbia Basin of OR & WA, the eastern
mountains, foothills of the Blues and across the Wallowas beginning
around 11 AM and continuing through 8 PM. Tuesday, not as much
instability will be over the region as the low pulls farther to the
east. However, raw ensembles still show a 10-15% probability along
the eastern mountains. MUCAPE values over 150-250 J/kg, surface
lapse rates of 8.3 C/km, mid level lapse rates of 7.3 C/km, lifted
index of 2 and SHIP of 0.1. This coupled with the orographic lift, we
cannot rule out the probabilities of thunderstorms over the eastern
mountains on Tuesday afternoon. Bennese/90

LONG TERM...Thursday through Monday...Sensible weather concerns
through the extended forecast will center around the passage of a
shortwave trough over the weekend, resulting in periods of breezy
winds, rain showers, and afternoon thunderstorm chances.

Thursday starts with a flattening upper level ridge moving across
the PacNW with dry conditions and locally breezy winds developing
in the afternoon. The flattening upper ridge will allow a
shortwave to slide through the PacNW late Thursday into Friday
afternoon, resulting in light rain shower chances(20-35%) across
the Cascade crest and the eastern mountains.

Late Friday through Saturday, ensemble cluster guidance is in good
agreement that an upper shortwave trough will dive into the PacNW
from the northeastern Pacific. This will bring increased shower
activity across the forecast area, with the bulk of the
precipitation occurring over the Cascades and eastern mountains.
Ensemble guidance also indicates increasing instability across the
eastern mountains Saturday afternoon, with a 15-20% chance of
isolated thunderstorms developing over this area. The upper trough
passage will also produce a tightening surface pressure gradient
Saturday, resulting in breezy to locally gusty conditions through
the Cascade gaps, and breezy conditions in portions of the Lower
Columbia Basin and central OR. The upper trough will continue to
push east Sunday, however ensemble guidance becomes split on how
far east the trough will travel during the day. Regardless, there
is a moderate confidence (40-50%) that light rain showers will
continue to develop across the Cascade crest and the eastern
mountains through Sunday, with shower activity diminishing Monday
as the trough pushes east of the Rockies.

With persistent upper troughs/shortwaves impacting the forecast
area through the extended forecast, confidence is moderate
(50-70%) that afternoon temperatures in the lower elevations will
generally be in the 60s to lower 70s, with mid 40s to 50s
mountains.


&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  62  44  64  46 /  70  50  50  20
ALW  62  46  63  48 /  60  50  60  30
PSC  69  45  72  46 /  30  30  30  10
YKM  69  45  73  45 /  20  10  10   0
HRI  67  45  70  46 /  50  40  30  10
ELN  65  46  69  46 /  20  20  20   0
RDM  58  36  60  36 /  80  40  20   0
LGD  55  41  56  43 /  90  70  60  50
GCD  56  39  55  40 /  90  80  60  50
DLS  65  47  67  48 /  60  20  10   0

&&

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

$$

SHORT TERM...90
LONG TERM....82
AVIATION...83