148
FXUS66 KPQR 130528
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
1020 PM PDT Mon Aug 12 2024

...Updated aviation discussion...

.SYNOPSIS...Westerly onshore flow will continue over the next
seven days, bringing a cooling trend along with increasing
morning clouds, and chances for drizzle or light through Tuesday,
especially for the coast, coastal mountains and south WA/north
OR Cascades. Scattered showers and thunderstorms possible over
the central Oregon Cascades Thursday and Friday. More widespread
rain is possible Friday into Saturday as an upper level low
settles near the area. However, exact timing and amounts remain
uncertain.

&&

.DISCUSSION...Tonight through Sunday...Troughing will remain in
place across the PNW through Tuesday evening before drifting
east of the region. This will maintain onshore flow and
persistent cloud cover through much of Tuesday. Broken to
overcast cloud deck around 4-5 kft currently resides over the
CWA which is expected to break up but not completely clear this
evening. A repeat of widespread stratus is expected tonight
into Tuesday morning with saturated low levels up to around 4-5
kft. Cloud cover is expected to persist through much of Tuesday
morning throughout the Willamette Valley but will gradually
clear out from south to north as transient shortwave ridging
builds into the area during the afternoon. The coast will follow
a similar pattern but could hang on to clouds into the
afternoon, mainly around the Astoria area. Cloud cover and
continued onshore flow will support another day of temps in the
mid to upper 70s inland and in the 60s around the coast.

Wednesday will see slightly warmer temps as the aforementioned
ridge crosses the area. Temperatures inland will be closer to
normal with low 80s expected and mid to upper 60s along the
coast. Thursday will see a slightly negatively tilted shortwave
trough moving into the area, bringing increasing cloud cover and
the potential for some precipitation, mainly along the central
Oregon Cascades. Chances for showers are around 20-30% with a
10-20% chance for thunderstorms Thursday afternoon. Friday will
see increasing chances areawide but remain highest along the
central Oregon Cascades. Shower chances are generally 10-20% for
areas outside the Cascades and 25-30% in the Cascades. The
thunderstorm chances are slightly lower for Friday around 15%
along the central Oregon Cascades.

A more substantial upper level trough is expected to drop
southward from the Gulf of Alaska toward the PNW coast where it
could linger for several days beginning late Friday into the
weekend. However, significant uncertainty remains in the WPC
500mb cluster analysis with a large degree of model spread in
the exact timing and location of the troughing. The ensembles
indicate that this pattern could bring the return of rain to the
area, but exactly when and where the rain will be is the
question. NBM indicates a 20-35% chance of rain over the
Cascades Friday and Saturday, slightly lower on Sunday, with a
15-25% chance of rain across the rest of the area on Saturday.
Additionally, NBM shows 20-30% chance of 0.1 inch of rain in 24
hours for all areas except a 30-50% chance over the northern
coast and Cascades. NBM also indicates a 20-30% chance of 0.25
inch of rain for the northern coast and Cascades in 24 hours.
Keep in mind these probabilities may be misleadingly low due to
timing differences amongst model ensemble guidance. Temperatures
will also fall back to below normal under this pattern, which
in combination with cloud cover, higher humidities, and rain
chances could help dampen ongoing wildfire activity. /Batz/HEC


&&


.AVIATION...Deep onshore flow will produce mainly light winds
and predominantly VFR conditions through 06z Wednesday across most
of northwest Oregon and southwest Washington. With that
said, there is a 30-50% chance (highest near the Cascade foothills)
for MVFR ceilings to develop across the northern Willamette Valley
and a 10-30% chance (lowest farthest south) across the central and
southern Willamette Valley per the latest model guidance. With that
said, do worry that the current 5-6kft ceilings will limit the
ability for an MVFR stratus deck to develop overnight across the
northern Willamette Valley and perhaps the lack of ceilings
across the southern Willamette Valley will actually make the
probability higher that MVFR ceilings form across the southern
Willamette Valley overnight. The bottom line is that confidence
in whether or not MVFR ceilings will form in the Willamette
Valley, primarily between 12-18z Tuesday is low.

Meanwhile, the coast is providing another tricky forecast.
Ceilings are holding around 4kft along the coast currently with
satellite imagery suggesting similar cloud heights offshore.
Nonetheless, model guidance was suggesting 50-60% probabilities
for MVFR and IFR conditions to develop along the coast this
evening that have not materialized. Given they have not, do not
have a lot of confidence that`s going to change. As a result, pushed
back the start times on flight deteriorations, but not terribly
confident that even those will materialize in the end. /Neuman

PDX AND APPROACHES...Deep onshore flow will produce light winds
and predominantly VFR conditions through 06z Wednesday. With that
said, there is a 30-50% chance for MVFR ceilings primarily between
12-18z Tuesdsay. With that said, do worry that the current 5-6kft
ceiling will limit the ability for an MVFR stratus deck to develop
overnight so confidence for that time period is relatively low.
/Neuman


&&


.MARINE...Expect benign conditions over the waters this week. A
weak trough will push through the waters tonight into tomorrow
(Tuesday), shifting winds more southerly to southwesterly. On
Wednesday, high pressure will re-build and return northerly to
northwesterly winds. Wind gusts will remain around 10 kt or less
this week across all waters. Expect predominately wind-driven
seas of 2 to 4 feet through the week.     -Alviz


&&


.PQR WATCHES/WARNINGS/ADVISORIES...

OR...None.
WA...None.
PZ...None.


&&

$$

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