129
FXUS66 KPQR 142331
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
431 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...High pressure continues, and VFR conditions with
clear skies remain in place for most of Tuesday. Around 05z
Wed, some lower level stratus clouds move inland, bringing 90%
chance of MVFR cigs at coastal terminals. Slightly more cloud
cover is expected at northern areas (such as KAST); around a 80%
chance of IFR cigs can be expected there during this time.
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 40% probability of backbuilding stratus
along the Cascade foothills in eastern Clark, Multnomah, and
Clackamas counties dropping ceilings below VFR levels around
12-17z Wed. This would mainly affect KTTD.

Winds will be northerly throughout the period, with somewhat
breezy/gusty winds at times throughout the rest of Tuesday
afternoon and evening. Coastal terminals could see gusts up to
25-30 kt up until around 08z Wed, after which winds will begin to
weaken. At all inland terminals, winds should peak around 20-25 kt
during this period. No LLWS expected.   /JLiu/HEC

PDX AND APPROACHES...VFR with relatively clear skies throughout
most of the TAF period, other than some light high cloud cover.
Winds will become a bit gustier going into the evening, with gusts
up to 20kt possible beginning 22z Tue. These calm again going into
Wednesday morning, dropping to 5-10 kt around 07z Wed. Some cloud
cover backbuilding from the Cascade foothills will approach KTTD
and KPDX, but model guidance points towards it not reaching
quite far enough. 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

Interact with us via social media:
www.facebook.com/NWSPortland
www.twitter.com/NWSPortland