902
FXUS66 KPDT 260941
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
241 AM PDT Fri Apr 26 2024

.SHORT TERM...Today through Sunday...Overnight water vapor imagery
shows a low pressure circulation centered right along the SW Oregon
coast. This system will gradually work its way towards the Four
Corners region heading into the weekend, leaving us with another
round of shower activity during the day today. With the axis of the
low overhead, however, moisture will be more limited, and any
activity during the day will be more orographic in nature. Guidance
keeps PoPs limited across our population centers at around 15-30%,
with better chances along the Cascades and eastern mountains.

As far as convection is concerned, instability is a bit more
supportive what with the low centered overhead, however shear is
still mild and PWATs are only around 0.5 inches, so any activity
that spawns is expected to be weak and isolated. Best chances would
be over the John Day/Ochoco Highlands and Wallowa County, where
orographic lift and CAPE are strongest, but again, largely expecting
showers rather than thunderstorms today. 06z HRRR suggests best
chance for convective activity will be between 2-10 pm this
afternoon and evening, before daytime heating cuts off support for
shower development, and the ongoing low departs our region.

Concerns shift towards winds this weekend, as the pattern aloft
shifts to more zonal flow in the wake of the departing low. Guidance
depicts another broad low stemming from the Gulf of Alaska beginning
to encroach upon the PacNW, with its SE quadrant inviting an area of
amplified winds aloft right over the forecast area. Winds look to be
moderately breezy on Saturday, with west gusts around 25-30 mph
through the Gorge and Basin, before a jet max really starts to move
in on Sunday. Largely expecting winds below advisory criteria, but
guidance does depict gusts in the 30-35 mph range Sunday afternoon
through the Gorge and Basin, as well as the Simcoe Highlands and
Yakima/Kittitas Valleys. Exposed ridgetops could see gusts eclipse
40 mph. Precip concerns, however, will be limited to mountain crests
under zonal flow, and even then, amounts are light as column
moisture remains low through the weekend. Temps are expected to
remain at seasonal averages, but a cooling trend is anticipated as
this Alaskan low brings colder air with it into the region.
Evans/74

.LONG TERM...Monday through Friday...A period of persistent, but
weak, active weather will be the trend across the mountain zones
through the long term forecast. However, by late Tuesday, ensemble
guidance comes into disagreement on the progression of the
synoptic pattern, with greatest impacts from this uncertainty on
the temperature forecast next week.

Ensemble guidance is in good agreement that the work week will
start off with a broad upper low sliding across southern BC
through early Tuesday, while another low sets up in the northeast
Pacific late Tuesday. The upper low passage to our north will
reinforce increasing surface pressure gradients, resulting in
breezy westerly winds developing across the lower elevations both
Monday and Tuesday. As for precipitation, the trend continues that
this upper low passage will be moisture deprived, resulting in
mostly light precipitation across the mountain areas and little to
no precipitation in the lower elevations including foothill
locations. The Cascade crest will see the best chances for
rain/snow through Tuesday thanks to a zonal component to the flow
aloft, with ensemble cluster guidance showing a 60-80% chance for
0.2 inches of QPF; while precipitation is also anticipated for the
rest of the mountain zones, ensemble cluster guidance only shows
a 40-60% chance for 0.1 inches of QPF through Tuesday. Afternoon
temperatures MOnday and Tuesday are also expected to cool down
into the mid 50s to mid 60s thanks to cool air advection
associated with the trough passage.

Tuesday night into Friday, ensemble guidance comes into
disagreement with the progression of the synoptic pattern
overhead. Overall, there is decent agreement amongst the available
cluster solutions that a brief drying trend will take place into
Wednesday, with precipitation chances increasing mainly across the
mountain zones again Wednesday night through Friday (moderate
confidence 45-60%). Now, on to the synoptic pattern during this
period...Tuesday night ensemble guidance is in agreement that the
aforementioned second low will continue to drop south in the north
east Pacific, however, 45% of members made up from the ECMWF, GFS,
and Canadian ensemble suites depict a transient ridge developing
between the low offshore and the low in the northeast Rockies by
late Wednesday, while the remainder of the solutions bring the
low offshore the PacNW with southwest flow aloft. Late Wednesday
into Thursday, ensemble guidance depicts upper level ridging then
developing over the northern Rockies, with upper level troughing
moving into the PacNW; ~60% of ensemble members favor a deeper,
cooler, and wetter upper trough while the remainder of the
members show a shallow, warm, and drier trough glancing the
region. Then Thursday into Friday, there is agreement amongst
ensemble cluster solutions of troughing continuing to impact the
PacNW; the uncertainty increases in how strong the troughing over
the region will be, but ~60% of the ensemble members favor a
75-90% chance of 0.1 inches of QPF across the Cascade crest and
the eastern mountains. As for temperatures during this period, the
overall confidence is low (20-30%) in the going temperature
forecast. Wednesday through Friday, the difference between the
25th and 75th percentile for afternoon temperatures is generally
9-12 degrees, with temperatures ranging between the 50s and 60s in
the lower elevations. If the stronger troughing solutions
materialize next week, expect cooler temperatures(50s to low 60s);
whereas the weaker troughing solutions, including the ridging
solutions Wednesday, will result in warmer afternoon temperatures
(60s to low 70s). Lawhorn/82

&&

.AVIATION...Previous Discussion...06Z TAFS...VFR conditions
currently at all sites, which will stay the course through the
period. Ceilings will drop to between OVC035-060, with
KDLS/KBDN/KRDM/KALW near the lower bounds into the Friday morning
hours before improving through the afternoon and evening.
KPSC/KPDT should stay between OVC050-060 overnight before
improving Friday afternoon. Winds will stay light and below 10 kts
for all sites except KDLS, which will increase to between 10 -
15kts in the afternoon and extend through the remainder of the
period. 75

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  62  39  61  39 /  20  10   0   0
ALW  65  44  64  42 /  30  10  10   0
PSC  68  47  67  46 /  10  10   0   0
YKM  66  41  63  38 /  20  10   0   0
HRI  68  43  66  41 /  10  10   0   0
ELN  62  39  58  36 /  40  20   0   0
RDM  54  34  55  34 /  30  10   0   0
LGD  57  39  57  39 /  60  20  10  10
GCD  57  36  58  36 /  70  40   0   0
DLS  65  45  61  44 /  30  10   0   0

&&

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

$$

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