693
FXUS66 KPDT 161736
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
1036 AM PDT Mon Sep 16 2024

.Updated Aviation Discussion.


.AVIATION...18Z TAFS...VFR conditions currently at all sites, which
will stay the course through the period. An upper level system is
tracking to our south, leading to extensive cloud cover across the
region that will extend through the period with BKN-OVC ceilings of
7-12 kft. Breezy afternoon winds will occur for KRDM/KBDN as gusts
of 15-20 kts are expected. Elsewhere, winds will stay below 10 kts
through the period, with the exception of 20-25 kt gusts for KDLS
toward the end of the period. There is a slight chance (<20%) of a
light shower this morning into the afternoon hours, but did not
mention any rain at any TAF site due to the low probability. 75


&&

.PREV DISCUSSION... /issued 248 AM PDT Mon Sep 16 2024/

SHORT TERM...Today through Wednesday...Radar returns tonight show
light showers developing under a shortwave impulse pushing north
across the eastern mountains. Precipitation amounts the last 6
hours have generally be under 0.1 inches. Otherwise, low to mid
level cloud layers cover much of the forecast area, with some
breaks in cloud cover across the Columbia Basin and north central
OR.

Today through Tuesday morning, an upper low over northern CA will
swing across the Great Basin and into southern ID. While the track
of the low will keep the best precipitation chances well to our
south, deterministic and short-term high res ensemble guidance
indicate another shortwave impulse rounding the low and across
northeast OR and far southeast WA. That said, there is only
moderate confidence (40-55%) in light showers reaching as far
west as central OR and OR Cascade east slopes through Tuesday
morning. While HREF members show a 50-60% chance of at least
measurable precip, the NBM only depicts a 10-20% percent chance
in those areas. Otherwise, the eastern periphery of the forecast
area, including the Blues, will see rain showers through Tuesday
morning. A slight chance of isolated thunderstorms will also be
present along the Blues and Wallowas this afternoon through the
evening, as CAPE values increase to around 300-400J/kg.

Tuesday, the forecast area will be between two upper closed lows.
The aforementioned low will continue to swing across the
intermountain west, while a second low dives down the southern BC
and PacNW coast. The eastern low will continue to support
wrap around moisture into far northeast OR and southeast WA, while
the western low will increase precipitation chances (30-60%)
across the Cascade crest, central OR, and the eastern mountains.
Precipitation chances will diminish throughout Wednesday as
northerly flow develops aloft as both lows move out of the region.

Only other concern through the next several days will be breezy to
windy conditions developing late this evening through Tuesday.
Surface pressure gradients across the Cascades will begin to
tighten as an upper low descends along the BC coastline and the
other upper low swings through the Great Basin. Winds will
increase to 20-30mph with winds gusts up to 40mph through the
Kittitas valley, eastern Gorge, and portions of the OR Columbia
Basin; winds will generally be 15-25mph with gusts 30-35mph
elsewhere. Confidence in wind gusts meeting or exceeding 45mph in
the Gorge and the Kittitas valley is low(20-30%) owing to the
lack of low level jet support in these areas. Lawhorn/82

LONG TERM...Thursday through Monday...Ensemble guidance does not
have a strong grasp on the synoptic pattern in the long term, but
does generally point toward weak NW flow trailing a blocking pattern
that will settle in place over CONUS to kick off next weekend. Some
ensemble members try to bring in a trough over the weekend, but
deterministic guidance does not favor this approach. The beginning
of next work week then looks benign, with weak zonal flow to perhaps
ridging, before the synoptic pattern really falls apart across
guidance beyond day 6.

The highest forecast confidence lies in the first two days of the
period, where models are in good agreement in a low cutting off and
remaining stagnant over California, with another low centered over
the eastern CONUS leading to an omega block that leaves us under
weak NW to N flow. Expecting a dry forecast as a result Thursday and
most of Friday, before the NW flow becomes more defined during the
latter half of Friday and introduces shower chances to the WA
Cascades. Beyond that, the pattern becomes murky as the
deterministic GFS and ECMWF suggest weak NW flow with perhaps an
embedded shortwave that may produce shower chances across the
entirety of the forecast area, while the Canadian tries to bring in
a full on NW low into the PacNW. NBM does suggest slight chance PoPs
across most of the CWA, tapping into at least the potential for some
kind of impulse, so will leave rain chances in for Saturday. The
shift toward a zonal pattern Sunday onward would produce light PoPs
across our high mountains, but otherwise Saturday looks to be the
most impactful day of the period as far as precip is concerned.

Expect generally mild temps with highs in the 70s and low in the
40s. Overall forecast confidence is medium (40-50%). Should note
that both the deterministic GFS and ECMWF hint at the potential for
an atmospheric river middle of next week, but confidence in this
occurring is low (<20%) given how far out that is. Evans/74


&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  71  49  69  49 /  20  10  10  20
ALW  74  53  72  53 /  10  20  20  10
PSC  80  56  76  58 /  10  10   0  10
YKM  81  50  77  50 /   0   0   0  20
HRI  77  54  74  54 /  10  10   0  10
ELN  82  50  72  51 /   0   0   0  10
RDM  67  44  65  45 /  20  10  20  70
LGD  70  48  65  47 /  40  40  40  20
GCD  69  45  67  46 /  40  40  20  40
DLS  82  55  71  55 /  10  10  30  50

&&

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

$$

SHORT TERM...82
LONG TERM....74
AVIATION...75