714
FXUS66 KPDT 170530
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
1030 PM PDT Mon Sep 16 2024

.EVENING UPDATE...Isolated rain showers are ongoing across the
Blue Mountains and their adjoining foothills. Farther south and
east, thunderstorms in Baker, Malheur, and Harney counties are
diminishing as steering winds take them slowly west-southwest into
an environment less conducive to maintenance of convection. Best
synoptic forcing remains south and east of our CWA line, though
Grant, Union, and Wallowa counties do have a slight chance (15%)
to likely (60%) rain showers as well as a slight chance (15%) of
isolated thunderstorms overnight. Elsewhere, a slight chance of
nocturnal activity extends westward across the rest of the Blue
Mountains and along portions of the northern foothills.
Plunkett/86

&&

.AVIATION...06Z TAFs...Isolated rain showers across the Blue
Mountains and adjoining foothills are ongoing this evening,
though the likelihood of rain at terminals is too low (<30%) to
mention. Another round of rain showers may affect ALW/PDT (20-30%
chance) late night into Tuesday morning; chances elsewhere are
very low (<20%). CIGs mostly 080-120 kft through the period. Winds
will increase in magnitude from the west Tuesday. Plunkett/86

&&

PREV DISCUSSION... /issued 304 PM PDT Mon Sep 16 2024/

SHORT TERM...Tonight through Wednesday...With one low swinging
across the region and a second dropping down into Tuesday, the
weather pattern remains unsettled through the short term
period. Ongoing showers and some radar returns are noted mostly
along the foothills of the Blues and and encroaching on the
Columbia Basin, but have limited precipitation to just east of the
Tri-Cities as we`re struggling to find ground truth that these
echoes are creating true rain that is hitting the ground. As the
low continues to shift eastwards, flow will spin this activity to
a southwestward movement which should help to keep it just east of
the region. Overall just not confident enough to bring a slight
chance of precipitation into this area only for it to fizzle out
or dive more southwards before reaching the area.

Otherwise, the low will continue to spin showers and some possible
storms through the overnight hours, with activity shifting from
across Central Oregon through the eastern mountains and even
foothills of the Blues, to primarily the eastern mountains.
Instability is present enough to allow for a few lightning strikes
with this activity, with mesoanalysis showing MUCAPE values of
around 500 J/kg creeping up into Wallowa and Grant Counties, but
the axis of best instability still lies to the southeast where
ongoing storms are already noted. Satellite shows a weak little
cumulus field also popping up, so expect a few lightning strikes
but stronger activity is not anticipated otherwise.

Moving into the overnight hours and into Tuesday, a deep low from
the Gulf of Alaska will drop down and ridge the PacNW coastline,
skimming our area and once again bringing the bulk of it`s energy
to California and just missing the region. As this occurs, breezy
conditions begin to pickup overnight and peak into tomorrow
morning and afternoon. Analysis of 850 mb winds and the pressure
gradient shows that stronger sustained winds and gusts should be
possible, but the majority of which should remain just below
advisory criteria (sustained winds 30-39 mph, gusts 45-57 mph).
Don`t be surprised if we see a few sites hit these criteria
briefly, but sustained for an hour or longer isn`t expected.

Meanwhile, precipitation will continue to shift with rain lining
our CWA from the north to south along both the Cascades and
eastern mountains between the dancing lows. Because of this, rain
along the Cascades should be moving from the northwest to
southeast into the mountains, and rain along the eastern mountains
from the northeast to southwest. Sadly, this means that by
Tuesday afternoon, portions of our interior CWA will likely see
little to any precipitation. As the second low continues to
descend, a low probability (<20%) of rain could make it into the
foothills of the southern Blue Mountains of Oregon, but otherwise
the rain should be limited to our higher terrain through it`s
departure into Wednesday.

On Wednesday as mentioned, precipitation departs and winds calm
as a ridge extending from the Pacific extends into the PacNW. This
will calm the overall weather for the remainder of the day,
allowing for clearing skies and benign, seasonable conditions.
Goatley/87

LONG TERM...Thursday through Monday ...

Key Messages:

1. Rain chances return over the weekend.

2. Slightly cooling temperatures through the period.

The extended period is characterized by a blocking pattern through
the workweek as a closed low slowly tracks across California and
the Desert Southwest ahead of a potential shortwave passing over
the weekend. Ensemble guidance is in good agreement with the
synoptic pattern early in the period, but struggles with the
weekend system. Current consensus leans to zonal west-northwest
flow setting up over the weekend before turning more from the
southwest early next week associated with an incoming shortwave.
This flow would allow for cooling temperatures and slight chances
for showers over mountainous terrain, potentially including the
foothills and east slopes if the shortwave is able to tap into
some Pacific moisture. At this time, that looks to be a low
probability (20-30%) as QPF ensemble clusters are indicating a 65%
chance of drier conditions than what is currently in the
forecast. This coincides with 55% of ensemble members advertising
zonal flow versus a weak shortwave moving onshore on Saturday. At
this time, minimal rain amounts are expected over mountainous
terrain, with the NBM suggesting a 50-70% chance of measurable
(>.01") of rainfall along the Washington Cascades, 30- 50% chance
over the northern Blue and Wallowa Mountains, and 20-30%
elsewhere.

Due to this zonal upper level flow, temperatures will slowly drop
1 to 3 degrees each day and stay below normal for this time of
year. The NBM showcases a 70-90% chance of temperatures staying
below normal through the duration of the week and into the early
part of next week, so confidence is high in these cooling
temperatures persisting. 75

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  51  70  48  72 /  20  10  10   0
ALW  54  72  52  75 /  20  30  10   0
PSC  56  77  56  79 /   0  10   0   0
YKM  47  79  47  78 /   0   0  10   0
HRI  55  75  53  77 /  10  10   0   0
ELN  51  74  52  76 /   0   0  10   0
RDM  43  66  43  69 /  20  10  60   0
LGD  50  66  46  70 /  30  40  20  10
GCD  47  67  44  69 /  40  30  20  10
DLS  55  71  54  75 /   0  20  40   0

&&

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

$$

EVENING UPDATE...86
SHORT TERM...87
LONG TERM....75
AVIATION...86