138
FXUS66 KPDT 170946
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
246 AM PDT Tue Sep 17 2024

.SHORT TERM...Today through Thursday...Water vapor satellite
imagery tonight shows an upper low swinging into southern ID at
this time, with radar showing light showers developing along the
northwest side of the low over Grant, Union, and far southern
Umatilla county. Meanwhile, another upper low is diving south
along the BC coastline, and is approximately 400 miles off the
coast of Vancouver Island.

Today will the tale of two lows, as two upper level low pressure
systems bring precipitation and wind impacts to the forecast area.
First, the upper low that is currently entering southern ID will
continue to swing northeast throughout today and exit into
the northern Rockies late this evening. This low will be the main
driver of shower activity across far eastern OR, with rain shower
chances (25-45%) mainly over the Strawberrys, Blues, and across
Wallowa county. The upper low currently west of Vancouver Island
will continue to dive south and glance the PacNW coastline by this
afternoon, after which it will continue to push south of the
OR/CA border. This low will briefly provide shower chances across
the Cascade crest and down into portions of central OR late this
afternoon and early evening, when it is at it`s closest approach
to the forecast area. The rest of the area will see little to no
precipitation throughout today. Secondly, surface pressure
gradients across the Cascades and east will tighten in response to
the second upper low approaching the region. This will result in
west to northwest winds of 20-30mph and gusts around 40mph
developing through the Cascade gaps and into portions of the OR
Columbia Basin this morning through this evening. Elsewhere, winds
will be westerly (southwest along the Blue mountain foothills)
15-25mph with gusts 30-35mph.

Wednesday through Thursday, the PacNW will see a northwest flow
develop aloft as upper level ridging amplifies over the northeast
Pacific. The flow aloft will be fairly dry, and with the lack of
the shortwaves/troughs clipping the region, precipitation chances
will be less than 10% area-wide. The cross-Cascade surface
pressure gradient will only weaken slightly during this period.
While light winds will be anticipated across much of the forecast
area each day, locally breezy afternoon winds (around 15mph with
occasional gusts to 25mph) will develop in the Columbia Basin,
while breezy afternoon winds (around 20-25mph with gusts to 35mph
and locally 40mph) will develop through the Eastern Columbia
River Gorge and the Kittitas valley. Lawhorn/82


.LONG TERM...Friday through Tuesday...Models are coming into better
agreement on the synoptic pattern in the long term, depicting
continuous NW flow over the PacNW, before differences arise at the
start of next work week, lowering forecast confidence significantly
by the tail end of the period. Until then, expect mild temps and
persistent chances for showers, especially in the mountains. The
pattern being depicted as is, it should be noted, would yield only
light precip outside of our high mountain zones.

This continuous NW flow pattern looks to be the result of an upper-
level low cutting off and diving down toward the southwestern CONUS,
which will then be slow to move out over the weekend. Looking at
ensemble clusters, models still differ a bit as to the extent and
magnitude of the NW flow, with some members even suggesting weak
zonal flow, but either way, such a pattern supports slight chance to
chance PoPs across our high mountain zones. Confidence is moderate
to high (50-60%) in our mountains seeing wetting rains over the
weekend, but much lower (10-20%) on the Basin and central Oregon
seeing any meaningful rainfall at the same time.

Ensembles fall out of phase Monday onward, with solutions varying
tremendously across guidance. Everything from the onset of ridging
to another NW trough moving in is being depicted across the GEFS and
EPS, therefore forecast confidence is very low (<20%) after the
upcoming weekend. Further hindering confidence is the deterministic
GFS and ECMWF falling completely out of phase starting Monday, with
ridging and troughing being depicted around the same time,
respectively. Evans/74

&&

.AVIATION...12z TAFs...VFR conditions expected. Windy conditions are
expected to impact most sites, with W to NW wind gusts potentially
reaching up to and over 30 kts at times in the afternoon,
particularly at DLS and PDT. Wind gusts closer to 20 kts expected at
all other sites. Sites outside of PDT, ALW, and PSC generally
expected to see clear skies to light mid-level clouds, while bkn-ovc
cigs around 10 kft are expected elsewhere, with similar cigs
building at sites along the Cascades closer to the evening. Light
rain may impact the central Oregon sites at the tail-end of the
period as well, but impacts at this time are expected to be minimal.
Evans/74


&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  71  48  73  45 /  20  10   0   0
ALW  74  52  76  50 /  30  10   0   0
PSC  77  56  79  53 /  10  10   0   0
YKM  79  49  79  46 /   0  10   0   0
HRI  75  53  78  50 /  10  10   0   0
ELN  74  51  76  48 /   0  10   0   0
RDM  67  43  70  40 /  10  30   0   0
LGD  67  45  71  44 /  40  10   0   0
GCD  67  43  71  42 /  40  20   0   0
DLS  72  55  77  51 /  10  30   0   0

&&

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

$$

SHORT TERM...82
LONG TERM....74
AVIATION...74