782
FXUS66 KPQR 141833 AAA
AFDPQR

Area Forecast Discussion...UPDATED
National Weather Service Portland OR
1133 AM PDT Wed Aug 14 2024

Updated Aviation Discussion

.SYNOPSIS...Westerly onshore flow will continue over the next
seven days, bringing near to below normal temperatures.
Scattered showers and thunderstorms possible over the central
Oregon Cascades Thursday and Friday. It is looking increasingly
likely that widespread rain is possible Friday into Saturday,
with chances for thunderstorms on Saturday. Slight precipitation
chances continue into early next week as an upper level low
settles near the area. However, exact timing and amounts remain
uncertain.

&&

.DISCUSSION...Wednesday through Monday...Observations indicate
mid level cloud cover never fully dissipated for locations
generally north of McMinnville on Tuesday, and IR satellite
imagery shows these clouds beginning to build back to the south
early this morning in the Willamette Valley as northerly winds
returned Tuesday evening. Model guidance is not picking up this
cloud cover well, but as winds around and south of Salem are
more from the northwesterly direction and expected to remain
this direction in addition to low-level and mid-level water
vapor satellite imagery showing a drier slot moving into the
area south of McMinnville, not expecting this stratus to move
much farther south than Salem. The upper trough from earlier
today continues to slowly move through eastern Oregon, and as it
moves east of Oregon by around daybreak, clouds are expected to
dissipate from south to north as shortwave transient ridging
builds over NW Oregon and SW Washington. Temperatures today will
bounce back to near normal under mostly clear skies by the
afternoon, generally in the low 80s for the Willamette Valley
and mid to upper 60s along the coast.

Model guidance is in good agreement of a negatively tilted low
moving through southern and central Oregon tonight through
Thursday. This will bring a 30-50% chance of scattered showers
along the Lane and Linn county Cascades with a 15-20% chance
for the Cascades north of Linn County Thursday. Latest guidance
is honing in on the best chances for thunderstorms being east
of the central Oregon Cascade crest, but can`t rule out enough
orographic forcing for a 15% chance of thunderstorms right along
the Lane County Cascade crest. Widespread cloud cover will also
return to the majority of NW Oregon and SW Washington, allowing
temperatures to drop back below normal into the mid to upper 70s
across the Willamette Valley and low 60s across the coast.

Latest GFS, Euro, and Canadian ensemble guidance is coming into
better agreement in the general pattern for Friday into the
weekend, though some uncertainty remains in specific details.
All 500 mb WPC clusters indicate a longwave trough deepening
over the eastern Pacific Friday into Saturday, lingering in this
area into early next week, with the closed low generally
lingering somewhere west of the Oregon coast. The uncertainty
comes in the exact location of the low, whether it will be
closer to the coast to the east or remain farther west over the
Pacific as well as whether it will be closer to the
Oregon/Washington border or the Oregon/California border. These
seemingly small differences in position will lead to
differences in rain amounts, timing, and thunderstorm chances as
shortwaves move along the low. Clusters continue to indicate
Friday will be dry. Precipitation chances increase Friday night
with NBM indicating a 30-50% chance of widespread rain pushing
into the region from the south to the north early Saturday
morning through Saturday afternoon as a shortwave moves through
the region. As this will bring south to southwesterly upper
level flow along with instability and moisture, there`s a 10-20%
chance of thunderstorms east of the Coast Range and a 20-25%
chance over the Oregon Cascades Saturday afternoon and evening.
However, some ensemble members show this shortwave bringing rain
later on Saturday into Sunday, so shower chances continue into
Sunday. Shower chances linger into Monday and Tuesday, mainly
for the coast, as ensembles depict the closed low more offshore
after the shortwave passes. Temperatures through early next week
will remain very similar to Thursday`s temperatures. -HEC

&&

.AVIATION...Northwesterly flow will maintain predominantly VFR
conditions across most of northwest Oregon and southwestern
Washington through the TAF period. Inland cloud cover around
4000-6000 ft continues to dissipate this morning and should leave
mostly clear skies for the afternoon. An upper level disturbance
will move in from the southwest, bringing the chance for marine
stratus along the coast after 00-06Z this evening. Model guidance
also suggests some scattered MVFR cloud cover developing inland
closer to 10-12Z Thursday morning. Winds remain N/NW around
between 5-10 kts.

PDX AND APPROACHES...Onshore flow will produce predominantly VFR
conditions through 18Z Thursday. There is a 30-50% chance MVFR
CIGs develop after 12Z Thursday morning. -Batz

&&

.MARINE...Expect benign conditions over the waters this week.
This afternoon, 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, with wind-driven seas of 2 of
4 feet. -Alviz/Sala

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...None.
&&

$$

www.weather.gov/portland

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