717
FXUS66 KPQR 112203
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
303 PM PDT Sun Aug 11 2024

.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 rain, especially for the
coast, coastal mountains and south WA/north OR Cascades. 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...Sunday night through Saturday night...The forecast is
highlighted by a persistent onshore flow pattern as a series of
shortwave troughs move over the Pacific Northwest, bringing below
normal to near normal temperatures to northwest OR and southwest WA
this week. This pattern will also favor persistent morning cloud
cover most days through Saturday, especially on Monday and Tuesday
when onshore flow peaks in strength (+10 to +11.5 mb pressure
gradient from OTH-GEG). This is typically strong enough to push
marine stratus clouds all the way to the Cascade crest, or very near
the crest. As such, cloud cover will likely linger into the afternoon
hours. The HREF supports this, showing cloud cover struggling to
break up even in the mid afternoon. As such, high temps will be
limited to the low to mid 70s Monday and Tuesday across the interior
lowlands. Both days also have anywhere from a 10-40% chance of
morning drizzle, especially at the coast and along the western slopes
of the coastal mountains and Cascades. It appears the marine layer
may be slightly too shallow to support drizzle in the Lane County
Cascades, but to the north the marine layer does appear to be deep
enough. While far from a wetting rain (e.g. 0.25 inches of rain or
more within 12 hours or less), this will help damper ongoing wildfire
activity in the south WA/north OR Cascades.

Warming up slightly on Wednesday due to plenty of afternoon sunshine
and a transient shortwave ridge. Temps on Wednesday will most likely
be right at normal for this time of year according to the
deterministic NBM, suggesting highs in the lower 80s across the
Willamette Valley and greater Portland metro. Models show minimal
change in the weather pattern come Thursday, resulting in conditions
very similar to Wednesday. The one exception to that is in the high
Cascades (particularly towards the Lane County Cascades) where a
negatively tilted 500 mb trough and associated vort max is set to
move northeastward from southwest OR to northeast OR. This will bring
diffluent flow aloft, supporting large scale lift needed for
convective initation to occur. In addition, forecast soundings depict
a fairly moist thermodynamic profile with at least some instability,
albeit instability looks highest to the east of the Cascade crest.
Given the forcing, moisture and instability in place, cannot rule out
wet thunderstorms Thursday afternoon. In fact, NBM PoPs have
increased and now show a 15-25% chance of thunderstorms in the Oregon
Cascades, highest over the Lane County Cascades.

The forecast becomes much more uncertain Friday into Saturday as
models and their ensembles continue to show a large degree of model
spread in regards to QPF. That being said, the vast majority of
ensemble guidance from the ENS/GEFS/GEPS now show at least some rain
falling over northwest OR and southwest WA. Despite significant
timing differences, confidence is increasing for at least some rain
in the area either Friday and/or Saturday. In fact, the NBM is now
showing a 20-30% chance for 24-hr rain amounts in excess of 0.25
inches across the south WA/north OR Cascades and foothills, as well
as the Willapa Hills and north OR Coast Range. Elsewhere,
probabilities range between 5-15%. Keep in mind these probabilities
may be misleadingly low due to timing differences amongst model
ensemble guidance. This helps explain why the 48-hour NBM
probabilities are higher, ranging from 20-40%. Regardless of how much
rain falls, temps will most certainly be cooler than normal and
ongoing wildfire activity will be dampered.  -TK

&&

.AVIATION...Weak westerly flow aloft today while onshore flow
persists in the lower levels. Stronger onshore flow north of KTMK
keeping MVFR cigs this afternoon and not likely to break up. Less
clouds south of KTMK but IFR cigs skirting the coastline. Tonight
into Monday an upper level trough will settle over the region
which will deepen the marine layer again and increase onshore
flow. As a result MVFR cigs return to the coast this evening. Late
tonight drizzle is expected at the coast and could bring areas of
IFR cigs/visibilities. Stratus spreads inland overnight, reaching
inland terminals 10-13Z Mon. Will probably see more mountain
obscuration along with some light precipitation.

Coastal wind northwesterly 10-15 kt with gusts to 20 kt in the
evening before subsiding. Inland northerly winds up to 10 kt
inland this afternoon and evening, except stronger through Coast
Range gaps.

Smoke from wildfires burning across the region could result in
some slant range visibility issues.

PDX AND APPROACHES...VFR expected through about 10-12Z Mon then
50-70% chance for MVFR cigs 2000-3000 ft. Northwest winds
increase up to 8-10 kt this afternoon and evening. /mh /DH

&&

.MARINE...High pressure offshore will maintain north to northwest
winds up to 15 kt across the coastal waters through tonight. Seas
remain choppy and primarily wind driven around 5 to 6 ft. Then,
the surface high retrogrades west as a weak front drops south of
of the Gulf of Alaska on Monday. A broad area of weak low pressure
near Vancouver Island causes winds across the coastal waters to
shift more westerly on Monday, and veering southerly on Tuesday.
Seas expected to subside to around 2 to 4 ft by Tuesday. Fairly
benign conditions continue into Wednesday. /mh /DH

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

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

&&

$$

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