339
FXUS66 KPQR 150506
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
1006 PM PDT Tue May 14 2024

Updated aviation discussion.

.SYNOPSIS...High pressure will maintain mostly clear, dry, and
warm conditions through tomorrow (Wednesday). Expect the marine
layer to return to the coast tonight. On Thursday, the ridge
begins to break down as a trough pushes southeast into BC from
the Gulf of Alaska. This will return slight chances for
precipitation for higher terrain as well as cloud cover. The
long term forecast looks mostly dry, however, weak troughing
could bring slight chances of precipitation.

&&

.SHORT TERM...Tonight through Thursday...Satellite imagery as
of 3 PM PDT depicts mostly clear skies across northwest Oregon
and southwest Washington as upper level high pressure builds
over the Pacific Northwest. High temperatures this afternoon are
on track to reach the mid 70s for interior valleys and upper
50s to low 60s along the coast and higher terrain. As we cool
down tonight, we`ll likely see a re-development of the marine
layer along the coast. Onshore flow doesn`t look strong enough
for the marine stratus to filter into the Willamette Valley via
Coast Range gaps and the Lower Columbia River Valley. However,
there could be some back-building stratus against the Cascades,
particularly in the east Portland Metro Area. Locations in this
area could start tomorrow morning with some cloud cover, but
will quickly dissipate as daytime heating progresses.

Clear and dry conditions prevail tomorrow (Wednesday) as high
pressure remains over our area. Tomorrow is expected to be the
warmest day of the week as models suggest the upper level ridge
building to its maximum amplitude. Afternoon highs tomorrow are
forecast in the upper 70s for interior valleys and upper 50s to
60s along the coast and higher terrain. NBM suggests a 30-50%
chance of exceeding 80 degrees tomorrow (highest in the Portland
Metro Area) within interior valleys. With this summertime
pattern in place, we`ll also maintain northerly to northwesterly
surface winds.

On Thursday, ensemble guidance shows an upper level trough
from the Gulf of Alaska progressing southeastward toward British
Columbia. This weak trough will begin breaking down the ridge
into more zonal upper level flow over the Pacific Northwest.
Additional cloud cover will begin filtering in to the area,
leading to partly sunny skies. Thus, we`ll see cooler afternoon
highs for interior valleys (about 4-8 degrees cooler than
Wednesday). By evening time, precipitation chances (15-35%) over
higher terrain return as the upper trough quickly brushes the
northern most areas of the Pacific Northwest.     -Alviz


.LONG TERM...Thursday Night through Monday...As the weak upper
trough quickly progresses through the Pacific Northwest, the
Cascades will maintain a 15-35% chance of precipitation. Snow
levels will be above 6500-7500, so any snow shower activity
would be well-above the Cascade passes. Friday to Saturday, WPC
cluster analyses depict mostly zonal flow or weak troughing over
the Pacific Northwest. This suggests that we`ll be mostly dry
and maintain relatively cooler high temperatures.

On Sunday, the majority of clusters suggest another trough
dipping down into the Pacific Northwest from Canada. This
scenario could also return a slight (15-24%) chance of
precipitation over higher terrain. By early next week (Monday),
some uncertainty remains with the progression of the
aforementioned trough. About 60% of ensemble members suggest
that the trough will continue progressing southward into the
Great Basin. In this case, we would maintain more cloud cover
and precipitation chances. Meanwhile, 40% of the members suggest
a return to either zonal flow or ridging. If this pans out, we
would return to clearer and drier conditions. -Alviz

&&

.AVIATION...Predominately VFR conditions with clear skies remain
in place for most locations under continuing high pressure. An
exception is along the coast where low-level stratus will push in
from north to south around 05-10z Wed, bringing 90% chance of
MVFR cigs at coastal terminals. Slightly more cloud cover is
expected at northern areas (such as KAST) with around a 60-80%
chance of IFR cigs with a 40-70% chance of IFR cigs at other
coastal terminals. Stratus clouds along the coast dissipate going
into Wednesday morning, with 90% confidence in return to VFR
conditions by 20z Wed. There is also a 30-40% probability of
backbuilding stratus along the Cascade foothills in eastern Clark,
central Multnomah, and northern Clackamas counties dropping
ceilings below VFR levels around 12-17z Wed. This would mainly
affect KTTD.

Northerly winds continue through the TAF period. Gusts 20-25 kts
along the coast will be decreasing by 08z Wed, then increasing
back to 20-25 kts after 18z Wed, mainly for the central Oregon
coast. Elsewhere, sustained winds remain less than 10 kts, briefly
increasing to 10-15 kts around 21z Wed. -JLiu/HEC

PDX AND APPROACHES...VFR with relatively clear skies throughout
most of the TAF period, other than some light high cloud cover.
North winds with gusts up to 20-25 kts will decrease around 07z
Wed, remaining less than 10 kts through the TAF period. Some
cloud cover backbuilding from the Cascade foothills could
approach KTTD and KPDX around 12-17z Wed, but model guidance
points towards it not quite reaching KPDX. Most likely outcome is
continued clear skies through the night. -JLiu/HEC

&&

.MARINE...Typical summertime pattern of northerly winds as high
pressure builds through mid week. Winds for the rest of this
afternoon and evening will see gusts up to 30 kt possible. A few
geographically isolated pockets may see one-off gusts to 35 kt,
but due to how isolated this would be, a Gale Warning has not
been issued. Winds will continue to barely meet Small Craft
Advisory criteria each afternoon and evening until at least the
weekend, as diurnal driven changes in wind continue to be the
main determining factor.

In regards to seas, nothing overly remarkable as the primary
swell is from the northwest, and the wind wave from the north.
Wind waves will range from around 4 to 6 feet. Combined seas
will be 6 to 8 ft through Tuesday, though conditions ramp up
mid-week. -Muessle/JLiu

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Small Craft Advisory until 11 AM PDT Wednesday for PZZ251>253-
     271>273.
&&

$$

www.weather.gov/portland

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