294 
FXUS66 KPQR 121018
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR 
317 AM PDT Tue Sep 12 2023

.SYNOPSIS...A weak cold front will bring areas of light drizzle to
the coast and southwest WA today, with little to no precipitation
expected elsewhere. Building high pressure will bring a
sharp warming trend Thursday and Friday, with above normal temps
likely continuing into the weekend.

&&

.DISCUSSION...Tuesday through Monday...A front about 100 miles off
the coast is slowly approaching western WA/OR. Radar indicating light
rain over the coastal waters ahead of the front. The front will
continue to move slowly eastward, likely bringing rain to the coast,
especially the north Oregon and south Washington coast, by late
morning or early afternoon. Not much rain east of the coast range as
the front weakens. Best chances expected over southwest WA.
It appears Portland to Salem and Eugene will stay dry. This will
result in slightly warmer temperatures over the Eugene area with
highs near 80, while the Portland metro should see highs around 75-77
degrees. Wednesday forecast to be a dry day as the upper level ridge
rebounds. 

Warming trend begins in earnest on Thursday. Models and their
ensembles continue to suggest high pressure will build aloft Thursday
into Friday while light offshore flow developing
at the surface. This pattern will bring warm and dry conditions to
the region. How warm will it get? Well, the most likely outcome
matches the deterministic NBM with inland high temperatures peaking
in upper 80s on Thursday, lower 90s on Friday, and upper 80s on
Saturday. However, there is still some uncertainty regarding exact
high temperatures with the NBM 90th & 10th percentile range of 
upper 90s to mid 80s. Even if the warmest model solutions verify, not
expecting much in the way of heat-related impacts as this heat event
looks very short-lived with excellent overnight relief as morning
lows in the 50s are expected in most spots. By Sunday, high
temperatures should fall back into the low to mid 80s. Expect the
coast to remain much cooler with highs between 65-75 degrees each day
Tuesday through Monday.    /mh -TK

&&

.FIRE WEATHER...Main fire weather concerns are for the Thursday
through Saturday timeframe. This is when an upper level ridge and
weak offshore flow will bring warm and dry conditions to the area,
except at the coast. Confidence is high that locations to the east
of the coastal mountains will see near critical to critical relative
humidities in the teens to around 25% Thursday afternoon, Friday
afternoon, and Saturday afternoon. The 90th/10th percentile range for
minimum RH is not all that great - with about a 10 percent difference
for the Cascades to near 20 percent difference toward the Coast
Range. 

In addition, a lowering subsidence inversion will likely result in
poor overnight humidity recoveries over the Cascades (mainly for
elevations above 3000 feet) Thursday night and Friday night with max
humidities generally between 30-50%. 

Also expecting a period of light easterly winds Thursday afternoon
through Friday afternoon, but generally less than 10 mph (except 20
mph or less near the mouth of the Columbia River Gorge). While winds
are not overly concerning, will need to watch for potential
instability given the dry conditions that will be in place. As of
right now, it appears instability will be maximized Friday afternoon
when a thermal trough shifts over the Willamette Valley. This also
looks to be the hottest day with inland high temperatures in the low
to mid 90s. Will hold off on any official headlines for the potential
dry and unstable conditions this Friday given the uncertainty
involved with the exact location of the thermal trough. /mh -TK

&&

.AVIATION...12Z TAFs...Westerly flow aloft, with variable mid and
high clouds across the region today. Will see areas of IFR and
lower MVFR CIGS along the coast this am. Meanwhile, will maintain
considerable lower VFR against the Cascades, extending westward
across the interior valleys to the Coast Range. These clouds will
break up somewhat later this am. 

A weakening front will push onshore this afternoon, and further 
inland through early evening. As such, will see CIGS lowering to 
mix of MVFR/low VFR with spotty light rain/drizzle along the 
coast after 20Z. Should remain VFR inland, though will trend to 
more solid lower VFR later this evening into overnight hours. 

KPDX AND APPROACHES...VFR deck around 4000 to 5000 ft over the ops
area this am. This deck will break up a bit later this am as
another front approaches. But, will see variable mid/high clouds
continue through the early evening. Later in the evening, will
have a more solid lower VFR deck form.

&&

.MARINE...Another weak front will push across the waters on today,
but gradients not all that strong. Generally, wind gusts remain 
under 20 kt. Can not rule a few gusts 20 to 25 kt over the far
north outer waters, but these will be few, and not enough to
support any advisories. Seas holding at 4 to 6 ft. 

High pressure will rebuild offshore Wed, and remain through Sat.
Pressure gradient between the high offshore and thermal low 
pressure near far south Oregon coast will tighten a bit for Wed
and Thu. As such, will see gusty northerly winds, along with
choppy wind-driven seas, in the afternoons/evenings. Gradients
will weaken towards the weekend, with winds easing a tad. 

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

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

&&

$$

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