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000
FXUS66 KPDT 251114
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
414 AM PDT Thu Apr 25 2024

Updated Aviation Discussion

.SHORT TERM...Today through Saturday...Overnight satellite
imagery shows cloud cover building into the forecast area as an
oncoming trough moves in to encompass the PacNW over the next
couple of days. Radar already showing shower activity over most of
the west side of the Cascades, and as the trough continues its
onshore progression, those showers will move in to our neck of the
woods by the afternoon hours at the latest. PoPs were drawn
across our area based on the latest HREF, which puts relatively
light amounts across our population centers of less than a tenth
of an inch through early Friday morning, with higher amounts over
the foothills of the Blues. Our mountain zones look to see closer
to 0.2 to 0.5 inches, with heavier amounts over the Cascades,
however guidance does paint quite the bullseye over the Oregon
Cascades over the next 24 hours, due to a favorable orographic
lift pattern aloft. WPC QPF is anywhere from 1 to 2 inches as a
result. Snow levels will remain elevated at generally above 4500
feet, but if model QPF verifies, areas around the Three Sisters
and Mt Bachelor could see over a foot of snow. Did make an effort
to trim down snow amounts from the last forecast, however, by
tuning down SLR`s, as it is a bit late in the season to see
widespread 10:1`s and higher.

This ongoing trough will begin to dig down into the western CONUS by
Friday, leaving us under its axis during the daytime hours. Light
orographic showers may spawn intermittently throughout the day, but
with us under the dry slot of the low, not expecting anything too
impactful. NBM does produce some slight chances for thunderstorms
across primarily the Strawberries and Wallowa County, with MUCAPEs
around 400 J/Kg and moisture just ample enough at the mid-levels,
however shear is pretty weak and overall column moisture is lacking,
so not too enthusiastic about storm chances Friday for these areas.

Forecast looks drier heading into Saturday as the low dips down into
the south central CONUS, leaving zonal flow in its wake. Light
orographic showers are expected across our mountain zones under this
pattern, with the best chances in the Cascades, but guidance QPF is
light at only a couple hundredths at best. Expect generally
seasonable temps for the period. Evans/74

.LONG TERM...Sunday through Thursday...There remains decent
agreement amongst ensemble members that a progressive weather
pattern will bring daily chances of mountain rain/snow showers
through the long term period. However, rain/snow amounts will be
light as current ensemble products indicate little moisture
support with each system. Locally breezy conditions will also
develop with each system passage early next week.

Ensemble guidance is in great agreement that Sunday will start
the week off with an upper low centered off the BC coast ejecting
a shortwave trough and cold front across the PacNW. In the lower
elevations and through the Cascade gaps, breezy winds of 15 to 25
mph with gusts up to 25-40mph will develop with the cold front
passage in the afternoon Sunday, and will continue into Monday as
surface pressure gradients will be slow to weaken post cold front
passage. As for precipitation, ensemble members of the GFS, ECMWF,
and Canadian all favor at least 0.25 inches of QPF in the WA
Cascade crest Sunday, with at least 0.1 inches along the east
slopes of the Cascades and OR Cascade crest. However, only about
55% of cluster members favor around 0.1 inches in the northern
Blues with a more southerly passage of the shortwave trough; the
rest of the eastern mountains will see QPF values less than 0.1
inches. By Monday morning, the shortwave trough is advertised to
exit into the Northern Rockies, leaving a low off the BC coast and
a westerly flow aloft. The westerly flow aloft will result in the
heaviest rain/snow occurring along the Cascade crest with good
agreement amongst ensemble members that at least 0.25 inches of
QPF will fall in this area. As for the rest of the mountain areas,
ensemble members keep QPF below 0.1 inches throughout Monday. The
lower elevations Sunday/Monday will remain fairly dry, though a
light rain shower or two may impact the immediate Blue mountain
foothills.

Tuesday into Thursday, ensemble guidance begins to come into
disagreement with the progression of the upper low off the BC
coast. There is general agreement amongst ensemble members that
the low off the BC coast will deepen, but open into a trough in
the northeast Pacific by Wednesday morning, and begin a gradual
push towards the PacNW the second half of Wednesday into Thursday
(moderate confidence 60-70%). However, there is disagreement
amongst these members as to how far offshore and how deep the
trough develops, with cluster solutions (~20% of ensemble members)
even depicting a transient ridge developing over the PacNW Tuesday
and early Wednesday. The closer the trough develops to the PacNW
and the deeper the trough becomes results in a wetter solution
across the mountains, whereas the eastern mountains will see drier
conditions with a shallow and more offshore trough. Currently,
there is mod-high confidence (70-80%) that the Cascade crest will
continue to see rain/snow showers develop during the middle of
the week, but low-mod confidence (30-40%) in rain/snow showers
continuing in the eastern mountains Tuesday through early
Wednesday. While ensemble members are in general agreement that
afternoon temperatures will remain near to below normal (60s to
low 70s...50s mountains) through the beginning of next week, the
uncertainty in the trough/ridge development results in wider
temperature ranges, with the ridge favoring afternoon highs in the
upper 60s to lower 70s, the shallow, more offshore trough
resulting in mid to upper 60s, and the deeper/closer trough
resulting in temperatures in the upper 50s to lower 60s. Current
confidence in the going temperature forecast is low (25-30%).
Lawhorn/82

.AVIATION...12Z TAFs...VFR conditions to prevail through the
period. Rain showers are expected to impact all sites today, with
rain developing at western sites between 17Z-20Z and persisting
into the afternoon, while rain will impact eastern sites between
23Z-04Z. Stronger showers may bring brief MVFR vsby conditions at
impacted sites, but confidence is low (15-20%) that vsby will drop
lower than 6SM. CIGS will become bkn-ovc AOA 3.5kft to 6kft AGL as
showers impact sites. Winds will be light, less than 12kts,
through most of the period, except at sites BDN/PSC where winds of
12-15kts will develop between 16Z-23Z. Lawhorn/82

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  62  45  62  42 /  60  70  30  10
ALW  65  47  65  47 /  60  80  40  10
PSC  65  49  68  46 /  40  40  10   0
YKM  61  42  65  39 /  60  20  10   0
HRI  65  48  67  45 /  50  40  10   0
ELN  58  41  62  40 /  70  30  20  10
RDM  56  40  54  34 /  60  50  20   0
LGD  61  43  57  41 /  50  90  50  20
GCD  61  41  57  38 /  70  80  70  30
DLS  61  48  62  44 /  90  70  20  10

&&

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

$$

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