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000
FXUS66 KPDT 211153
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
453 AM PDT Thu Sep 21 2023

.AVIATION...12Z TAFs...TAF sites have observed prevailing VFR
conditions during the overnight hours with BKN-OVC low- to mid-
level clouds over all but DLS/YKM where FEW-SCT mid-level clouds
are apparent on satellite imagery. -RA at PDT/ALW/PSC this
morning is not forecast to reduce VSBYs to below 6SM, though NBM
probabilities of MVFR VSBYs are 10-20% for the aforementioned
sites through mid-morning. Probabilities of MVFR or lower CIGS at
PDT/ALW/BDN/RDM are highest between 14-20Z at 20-60% chance and
peak at 16-17Z. Otherwise, some gusty north-northeast (BDN/RDM) to
northeast-easterly (DLS/YKM) winds are forecast at western sites
through the afternoon. Rain chances diminish this evening with dry
conditions forecast overnight through Friday morning. Plunkett/86

&&

.PREV DISCUSSION... /issued 230 AM PDT Thu Sep 21 2023/

SHORT TERM...Today through Saturday night...Current radar is
showing some wrap around moisture bringing in some showers across
the Blue Mountains and the foothills. Ground obs are showing that
within the last hour roughly 0.10 inches has fallen in the
aforementioned area. Showers will continue along the eastern
region through the morning before becoming widespread this
afternoon. The short term will be characterized by showers through
today and then into a brief drying and warming period Friday and
Saturday.

Short term models are in firm agreement with the positioning of
the upper level low that is creating the wrap around moisture over
the eastern portion of the region. The center of the low is
sitting just southeast of Grant county with winds aloft being
northeasterly along the eastern portion of the area and northerly
long the foothills of the Cascades. As the low continues to spin
through the region moving southeast, moisture will continue to
wrap around bringing precipitation to the eastern portion of the
region. By this afternoon the low will be to the south of the
region and retrograding to the west bringing more wrap around
moisture that will become more widespread although guidance does
show the amounts to be light with up to 0.10 inches of rain
(30-60%). CAMs models as well as the HREF are showing that there
is some instability over the eastern region with MUCAPE hovering
near 250J/kg and lapse rates nearing 7C/km with LIs of -1, this
could lead to a few stray pop up thunderstorms this morning
through this afternoon (>10%). Lows this morning for the peaks and
higher terrains will be in the mid 30s with 40s elsewhere. EFI
and ensembles are showing the eastern portion of the area to be
well below the seasonal average (10-15 degrees) with highs barely
reaching the mid to upper 60s while the northwestern portion of
the region will see the the high 60s to low 70s.

Models continue to remain in firm agreement with the upper level
low initially continuing to retrograde west before quickly exiting
the region Friday night and an upper level ridge moves over. The
center of the low will be far enough south that no wrap around
moisture will enter the region and dry conditions will prevail
through Saturday. EFI shows the below seasonal temperatures to be
farther to the east with mostly the eastern mountains seeing below
average temperatures. NBM and ensembles show the temperatures
along the eastern portion of the area increasing a few degrees,
but still remaining in the upper 50s to low 60s. The western
portion of the area will see temperatures reaching the low 70s and
by Saturday the temperatures will return to seasonal normal with
most of the area back in the low 70s. Bennese/90

LONG TERM...Sunday through Thursday...Sensible weather concerns
through the first week of Fall will revolve around an active
weather pattern that will bring persistent rain shower chances to
most, if not all, of the forecast area through next Thursday.

Sunday through Tuesday, ensemble guidance in excellent agreement
that an upper low originating from the Gulf of Alaska will
gradually transition into an upper trough as it swings into and
over the PacNW through this period. Rain chances will initially
stay along the Cascade crest Sunday, but will spread east late
Sunday through Monday morning, then persist across the forecast
area through early Tuesday. As the axis of the now trough pushes
inland and through the PacNW, chance of rain showers will
momentarily diminish across the Columbia Basin, but persist
elsewhere. Wednesday and Thursday, ensemble guidance continues to
indicate a general troughing pattern along the BC coast into the
PacNW, however differences arise in how far south the trough dips
into the PacNW as well as how low 500mb heights reach. While 50%
of members made up from the Canadian, ECMWF, GFS ensemble suites
favor a deeper trough pattern, 25% favor troughing with higher
500mb heights, and the remaining 25% favor a shallower trough with
zonal flow in the PacNW. While each solution does favor continued
chances of rain showers continuing, the deeper trough solution
would favor better rain chances and greater precipitation
accumulations. While under a troughing pattern for the better part
of next week, wind concerns remain generally low as light to
locally breezy winds will expected nearly every day. There is,
however, a signal for breezy south winds across central OR on
Monday, then breezy west to southwest winds across the higher
ridges along the east slopes of the Cascades and western portion
of the WA Columbia Basin. Lawhorn/82

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  56  40  67  42 /  70  20   0   0
ALW  61  43  69  45 /  70  20   0   0
PSC  66  49  72  49 /  40  20   0   0
YKM  71  44  73  45 /  10  20   0   0
HRI  63  44  73  45 /  60  20   0   0
ELN  72  46  74  46 /   0  10   0   0
RDM  57  34  70  39 /  60  20   0   0
LGD  57  39  66  40 /  70  20  10   0
GCD  55  36  67  39 /  80  30   0   0
DLS  75  48  79  50 /  20  20   0   0

&&

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

$$

SHORT TERM...90
LONG TERM....82
AVIATION...86/82