Area Forecast Discussion
Issued by NWS Pendleton, OR

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FXUS66 KPDT 230229
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
729 PM PDT Fri Mar 22 2024

.EVENING UPDATE...Satellite imagery shows a well-developed closed
low slowly pushing into the Oregon coast this evening. The SW flow
leading this low has allowed for some shower activity and even
some convective development to occur across the Cascades and
eastern mountains over the last couple of hours, however rain
activity outside of the mountains thus far has been very light.
00z HRRR suggests more widespread activity will begin by 06z
(11pm) tonight, spreading to the rest of the forecast area
overnight. Confidence in convection outside of the mountains
remains low (<10%), however cannot completely rule out a stray
rumble of thunder, especially along the eastern foothills of the
Cascades. Guidance then has precip activity dwindling down by
daybreak tomorrow, before the low overhead leads to more potential
convective activity in the afternoon.

Changes to the forecast package this evening were focused
primarily on touching up PoPs based on latest trends. Did add
mention of thunder in the northern Blues, where lightning had been
detected on satellite earlier this evening. Evans/74

&&

.PREV DISCUSSION... /issued 502 PM PDT Fri Mar 22 2024/

AVIATION...00Z TAFS...Light precipitation is afoot during this
period as low to mid clouds start coming in (BKN to OVC 030-080) for
most sites through the overnight period. Breezy conditions are
expected for KRDM and KBDN this evening and again tomorrow morning,
bringing winds above 10kts. For others, winds will be around 10kts.
Feaster/97

SHORT TERM...Tonight through Sunday...Satellite and radar imagery
this afternoon shows a band of light precipitation pushing
northeast across WA, while light echoes over northeast OR and
southeast WA have only resulted in virga so far. Meanwhile, partly
cloudy skies have developed in areas behind this main band, though
current visible satellite shows little indication of vertical
cloud development in these areas. Through the rest of today and
tonight, a deep low offshore will gradually push a cold front east
across the eastern half of the PacNW while shortwave impulses
periodically pass overhead. During this afternoon and into the
evening, the arrival of the cold front/shortwave impulses as well
as weak instability at the surface is expected to result in
increasing shower activity. Increasing instability at the surface
(about 100-300 J/kg of CAPE) along the northern Blues and the east
slopes of the OR Cascades is expected to aid in the shower
development through the evening, with even an isolated clap of
thunder possible in these areas.

Tonight into tomorrow morning, the center of the low offshore
will continue to approach the PacNW while the cold front boundary
pushes east and north across the region. This will result in snow
showers developing across the higher mountain terrain, while rain
showers will be possible elsewhere overnight. The upper low will
transition into an open trough throughout tomorrow as it pushes
ashore near the OR/CA border, while the cooler airmass associated
with this low feature will filter into the region. There will be
another round of shower activity across the forecast area, and
with snow levels dropping to between 4kft to 4.5kft along the
Cascades, there will be snow chances at the pass levels tomorrow.
Also, modest low to mid-level lapse rates and increasing surface
based instability will allow for isolated thunderstorm development
from central OR to northeast OR and far southeast WA tomorrow
afternoon into the evening. Shower activity will retreat to the
mountains tomorrow night as the now upper trough continues to push
into the Desert Southwest.

Sunday, a quick moving shortwave will dive down the back of the
trough and bring another chance of mountain snow showers and light
rain showers to the lower elevations. Breezy westerly winds will
also develop in the lower elevations, with sustained winds
15-25mph expected through the Gorge and the Kittitas valley. The
shortwave is expected to exit by the afternoon, allowing a
transient ridge in the northeast Pacific to slide across the
PacNW, with only light snow flurries continuing along the Cascade
crest, northern Blues, and the Wallowas.

A cooling trend starting tomorrow will result in afternoon
temperatures dropping to near to below normal this weekend, with
afternoon temperatures mainly in the 50s. As for morning lows,
they will remain near normal, with upper 20s to lower 30s in the
mid elevation and mountain valleys, and mid to upper 30s across
the rest of the lower elevation zones. Lawhorn/82

LONG TERM...Monday through Friday...An active, progressive pattern
will dominate the region for the period, with persistent chances of
precip and largely cooler-than-average temps extending to even
beyond the long term.

Ensembles are largely in agreement on the spirit of the pattern,
being that of a train of systems largely W/NW in origin, however the
progressive nature of the pattern causes the ensembles to fall out
of sync by the midweek next week. As a result, can speak with good
confidence (60%) on the nature of the synoptic pattern, but as far
as the exact timing on when the axes of troughs/ridges lie over the
forecast area, confidence is low (30%).

The low expected to envelop the region for most of this weekend will
depart to the east by the start of next work week, leaving us under
relatively benign NW flow. GEFS and EPS produce light precip for
portions of our lower elevations, namely the foothills of the Blues
and Grande Ronde Valley, while the NBM largely keeps precip in our
high mountain zones. Even then, precip looks to be light, with even
the wetter LREF solution suggesting about a tenth to two tenths
during the day Monday over the mountains. Snow levels look to be
around 3000 to 4000 feet.

NW flow turns into transitory ridging by Tuesday, before a more
organized low arrives around the midweek. This low is the main
source of discrepancy across the ensembles, as some clusters, namely
from the GEFS, are much more progressive in arrival time, while the
EPS keeps ridging around for a bit longer before bringing the low in
closer to Thursday. The primary effect of this low, however, will be
widespread breezy winds, PoPs, and cool conditions, as ensembles do
agree that the low will stem from the WNW rather than a more mild
westerly or SW origin. Snow levels start to trend more towards 2500-
3000 feet, leaving pass-level impacts across the WA Cascades
uncertain, but confidence is moderate (40%) in the forecast area
seeing a wetting rain from this midweek system. Other specifics,
including potential convective threats and more precise snow levels,
are too difficult to determine at this time, considering how far out
into the long term this system will occur.

With this midweek low looking to envelop the PacNW beyond the long
term, expect cool and wet conditions to persist into next weekend, a
stark departure from the dry and warm conditions we saw under the
ridge earlier this week. Evans/74

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  42  58  38  53 /  40  40  30  70
ALW  44  60  41  53 /  40  70  40  80
PSC  47  64  42  60 /  40  30  30  50
YKM  42  60  37  60 /  80  60  20  20
HRI  45  64  41  58 /  50  40  30  60
ELN  38  55  35  57 /  80  80  20  20
RDM  35  54  33  50 /  80  30  30  20
LGD  39  53  34  48 /  50  80  50  70
GCD  38  55  33  50 /  60  60  50  60
DLS  43  60  41  58 / 100  40  30  30

&&

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

$$

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


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