-- Discussion containing changed information from previous version are highlighted. --
544
FXUS66 KPQR 082223
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
322 PM PDT Sat Jun 8 2024

.SYNOPSIS...
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Overall the upper-level weather pattern stays
fairly progressive through next week although weather distances
through this period are largely shunted to our north. Thus
expect high pressure of varying amplitude over the region to
dictate continued near to above normal temperatures and mostly
dry conditions through the middle of next week. Little to no
appreciable weather impacts to speak of going forward as a
result outside of slight chances(15-20%) for T-storms over the
Cascades tonight. Any limited chances for precipitation(15-35%)
peak across the Cascades into early Sunday morning and again
over far northwest Oregon and southwest Washington on Tuesday
morning - high confidence in dry conditions for the Willamette
Valley through this period. Slight better chances (15-40%) for
precipitation return late next week.


 .SHORT TERM...Today through Sunday night...This afternoon
water vapor satellite imagery depicts an upper-level trough
continuing to march towards the Pacific Northwest coastline with
SW-SSW flow aloft ahead of this feature streaming mid to high
based cloud cover over the region. The radar has shown some high
based returns this afternoon as well however a dry layer near
the surface has ensured this has just remained virga and hasnt
reached the ground - surface observations under these radar
returns have all shown continued dry conditions. As the upper-
level wave progresses overhead this evening and overnight, it
likely provides enough forcing to tap into accompanying mid-
level moisture/instability leading to shower and isolated
thunderstorm development over the Cascades. At least locally the
extent of any such activity appears fairly limited.

Jumping further into mid to high based shower/T-storm chances,
deterministic guidance like the NAM/GFS/Canadian shows
increasing MUCAPE values into the 300-800j/kg range between
700-300mb tonight over the Cascades collocated with decent
moisture within this same layer. Its worth noting model
soundings confirm this instability would have to be initiated
aloft between roughly 700-600mb and wont be surface based, but
this is typically the case for most nocturnal thunderstorm
formation. At least the aforementioned shortwave should be
robust enough to provide the upper-level forcing needed and
itll just come down the placement of the moisture/instability
which guidance keeps pinned near the Cascade crests and
eastward. Exploring high resolution guidance like the HREF and UW-
WRF does add confidence to the development and placement of high
based convection this evening/overnight near the Cascade crests with
the best chances(15-25%) in eastern Lane/Linn County. Steering winds
aloft will push any activity further into eastern Oregon - sorry
Willamette Valley lightning enthusiasts. Once the axis of the upper-
level trough moves directly overhead, winds aloft switch
northwesterly and the field of mid-level moisture shifts out of the
area ending any chance for high based shower/T-storm activity over
the western side of the Cascades by mid Sunday morning.

From there temps will continue their gradual cooling trend
moving through the rest of Sunday with highs forecast in the mid
to upper 70s for inland valleys and 60s along the coast. The
latest NBM guidance still suggests a 25-50% chance that the
Willamette Valley will meet or exceed 80 degrees; highest in the
Portland/Vancouver metro and near the base of the Cascade
foothills. By Sunday night, high pressure will re-build behind
the exiting trough now moving into Rockies which will lead to
generally decreasing cloud cover outside of coastal stratus.
-Schuldt

.LONG TERM...Monday through Saturday...Ensemble systems are in
agreement of 500mb heights remaining above normal through
Wednesday/Thursday of next week supporting near to above normal
temperatures and generally dry conditions over the region. After
an upper-level ridge briefly rebuilds for Monday the ridge
begins to flatten Tuesday-Wednesday as a trough near the Gulf of
Alaska pushes lower 500mb heights into British Columbia and
northern Washington. This would result in a slight cooling
trend and a slight chance (15-25%) for light precipitation over
far NW Oregon/SW Washington Tuesday morning as the trough axis
grazes us to the north.

By Thursday, most ensemble members (75%)suggest that ridging
will persist over the Pacific Northwest. However, 25% of members
suggest that the aforementioned trough near Alaska will begin
to dip southward and bring more southwest flow over us. It still
appears by Friday, the majority of ensemble members have the
trough dropping close enough to the Pacific Northwest to return
rain chances to our area. Meanwhile, 20% of the ensemble members
keep the ridging on Friday with drier conditions persisting. By
Saturday almost every ensemble member has a trough in the
vicinity of the Pacific Northwest with higher confidence in a
wetter weather pattern by this point. -Schuldt/Alviz
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&&

.AVIATION...A weak upper level disturbance was moving northward
over far northwest OR and southwest WA at 21Z Saturday, bringing
scattered high clouds clouds with cigs generally at or over 15-20
kft. Areas of virga were continuing to show up on the KRTX radar
reflectivity with some of these high clouds, meaning
precipitation is evaporating before reaching the ground. As such,
VFR conditions will continue across the area Saturday afternoon
through Saturday night. Probabilities are very high (90% or
higher) for the return of a marine stratus deck along the coast
towards sunrise on Sunday, with cigs most likely ranging between
500-1000 ft. While this marine push looks to be weak with limited
inland extent, there is a 15-25% chance the marine stratus deck
will end up extending as far inland as KHIO/KPDX/KTTD (however for
a short period of time between 15-18Z Sunday). Even if the
stratus deck is able to push that far inland, conditions would be
MVFR at worst and would quickly improve back to VFR after 18Z
Sunday. Surface winds will generally be out of the northwest
Saturday afternoon, under 10 kt.

PDX AND APPROACHES...VFR with typical northwesterly flow over the
next 24 hours with periods of broken high clouds (cigs over 20
kft). It still appears northwesterly winds just off the surface
Saturday evening will be higher (around 15-20 kt) when compared
to surface winds which will likely stay under 10 kt. There is
around a 25% chance for a brief period of 2000-3000 foot cigs
between 15-18Z Sunday.  -TK

&&

.MARINE...Skies were clearing out a bit over the coastal waters
Saturday afternoon in the wake of a decaying cool front. Buoy
observations continued showing seas around 4-5 feet at 11 to 15
seconds with northerly winds between 10-20 kt. Still expecting
seas to rise a few feet from the current conditions as guidance
continues to suggest seas will peak around 8-9 ft at 12 to 14
seconds late Saturday night due to a combination of increased
northerly wind waves and westerly swell.

Conditions will settle again late Sunday into Monday as high
pressure rebuilds over the waters. Another weak cool front moves
over the waters on Tuesday, bringing a brief shift to south winds.
Northerly winds return mid week and ramp up late in the week as a
thermal trough deepens along the south Oregon coast. As usual
for this time of year, northerly winds will be strongest over the
central and southern waters during the afternoon and evening
hours. The strongest northerly winds are likely to occur on
Wednesday and Thursday when there is a 50-70% chance for small
craft advisory level wind gusts up to 25 kt. -TK
&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Small Craft Advisory from 5 AM to 11 AM PDT Sunday for PZZ210.
&&

$$

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