Excessive Heat in Oregon, August 14 to 16, 2023

Area Forecast Discussion
National Weather Service Portland OR
240 AM PDT Mon Aug 14 2023

.SYNOPSIS...Hot, Hot, Hot. That sums up weather for next several
days, thanks to very strong high pressure over the region. Most
pleasant conditions will be along the coast, with somewhat cooler
temperatures, along with areas of fog Tue into Wed. Will see some
slight onshore flow later Tue and Wed, with temperatures down a tad
across interior. But, will have to wait for Thu for serious cooling
back to more typical mid-August temperatures.
&&

.DISCUSSION...(Today through Wednesday)...

Key messages:
1)  Hot everywhere MON and TUE, except along immediate coast.

2) Warm nights, with overnight lows in the 60s. Larger urban cores
may stay in the 70s for lowest temperatures next three nights.

3) Very dry, somewhat unstable and breezy at times, with elevated
wildfire concerns through Tue.

Very strong high pressure remains anchored over the region, with
warm dry subsidence layer acting like a blanket, keeping very warm
air mass in place across the Pac NW. Temperatures on Sunday
afternoon spiked to 100 to 105 for many interior locations, thanks
to some easterly low level flow. Add to that very low humidity,
mostly 15 to 25%. This made for hot summer day. Not much change in
pattern for today, though temperatures likely to be a few deg higher
than those on Sunday. This mainly due to warm air mass and warm
morning temperatures providing a relatively higher springboard.
Model guidance suggest areas from Salem to Eugene could reach 108 to
110 deg today. Not sold, but yes, could be close. For now, will keep
highs in those areas closer to 106 to 108. Even so, this could be
warmest day ever in the month of August (Salem 108 in 1981, Eugene
108 in 1981, McMinnville 108 in 1927). Will maintain all Heat
Advisories and Excessive Heat Warnings for today.

Over past few runs, models have suggest a slight onshore component
returning to the coast later tonight into Tue as the thermal trough
axis shifts inland a tad. By Tue am, the thermal trough will extend
from Tillamook southeastward over the east slopes of the Coast Range
towards Corvallis. As such, would expect cooler coastal air seeping
through the valleys into the south Willamette Valley. While not all
that strong, may be enough to push shallow marine layer into
interior later tonight/Tue, affecting areas from Corvallis/Albany
southward through Eugene and Cottage Grove. Even though is shallow,
may be enough to bring highs down a tad for Tue. Will lean closer to
MAV guidance, with highs in upper 90s for Eugene and Corvallis for
Tue.
Still warm to the north, with highs in Portland and Salem still at
100 to 103, as well as on to the east through the Columbia River.

Upper low pressure off California Coast will rotate bits of mid/high
level moisture northward into southern Oregon on Tuesday and
Wednesday. That, with some weak instability, will provide for a few
showers/thunderstorms, mainly east of the Cascades. But, far eastern
Lane County is close enough to this area of instability/moisture
that will maintain a small chance (10-20%) along the Cascades crest
in eastern Lane County.

Will gradually see areas of fog and low clouds form along the coast
later tonight along the coast from Lincoln City southward through
Florence, as weak onshore flow strengthens a bit. But, this onshore
flow will strengthen further later Tue and Tue night, with more low
clouds	and fog forming along the coast, and getting a bit further
inland into the Coast Range gaps and up the Columbia River to Kelso
later Tue night into early Wed. This onshore flow will provide a
shallow marine layer for the interior on Wed. Deep enough to provide
some relief inland, but not deep enough to keep temperatures out of
the 90s.  At this point, looks like highs 95 to 100 for much of the
interior, warmest from Portland to Salem.                /Rockey

.LONG TERM...(THURSDAY THROUGH SATURDAY)... Longer range guidance in
NBM finally dials down inland temperatures closer to normal starting
Friday, but there is a lot of uncertainty to the pattern beyond
midweek. This is mainly due to our pattern late week and beyond being
highly dependent on two things models notoriously struggle with - a
cutoff low (remnants of Eugene off CA coast) and potential tropical
activity off the west coast of Mexico.The models that are more
bullish in tropical development are locking the ridge in a more
northern position - which would likely prolong our heat event beyond
midweek. Currently less than 20% of models are suggesting this, so we
are rolling with the NBM deterministic guidance after Tuesday.
&&

.FIRE WEATHER...Various Red Flag warnings are up from the Coast
Range eastward across the Willamette Valley into the Cascades. These
are mainly due to a combination of dry forest fuels, gusty winds,
and low humidity. With the thermal low pressure hugging the coast
through early Tuesday, will see east to northeast winds across the
region, with strongest at night into the mornings with 10 to 15 mph,
though 20 to 30 mph gusts are more likely over the higher exposed
ridges, as well as through the western Columbia Gorge. Minimum
humidity running 15-25% for many areas, with poor overnight RH
recoveries on midslopes/ridges (30-50%). So, potential for active
fire burning continues, along with potential of rapid fire growth
with existing/any new fires. Conditions will slowly improve later
Wed into Thu, as ridge weakens and onshore flow allows for bit
cooler air to filter back across the region.

Will need to keep an eye on increasing moisture and instability
Tuesday, along with a weak upper disturbance expected to brush the
southern periphery of our district. This will at least increase
cumulus buildups in the central/south Oregon Cascades, and latest
NBM guidance suggests a 15-20% chance of thunder along the crest in
the zone 608 Willamette National Forest. At this point, it appears
the lightning threat will remain east of the active fires in eastern
Lane County.  Weagle/Rockey

&&

.AVIATION...12Z TAFs preliminary thoughts...VFR prevails under
mostly clear skies as strong high pressure remains in control
through the period. Latest model guidance has backed off on marine
stratus and fog potential early this morning. There's a 10-30%
chance of cigs less than 3,000 ft and a 10-20% chance of
visibilities less than 5SM south of KONP. Can't rule out localized
shallow fog anywhere between roughly 11-15z. Otherwise, expect
another round of 10-15 kt N-NW winds at most sites developing 18-
21z, with gustier conditions at the coastal terminals.

Southeast winds aloft have pushed smoke from the Lookout Fire
into the central Willamette Valley, with renewed activity on the
Bedrock Fire creating more smoke across eastern Lane County. This
could impact slant-range visibilities in eastern Lane County as
well as for portions of the Willamette Valley between KSLE and
KEUG through the period.

KPDX AND APPROACHES...Clear sky VFR expected through the period,
with 10 kt NW winds redeveloping 18-21z Mon. -HEC

&&

.MARINE...High pressure offshore coupled with a strengthening
thermal trough across the OR/CA border are causing coastal
pressure gradients to tighten. As a result, expect increased
northerlies across the coastal waters with gusts to 20-30 kt to
continue into Monday, strongest in the late afternoon and evening
hours. A similar pattern will continue into the middle of the
week with slightly weaker pressure gradients, indicating slightly
weaker winds.

Seas around 7 to 9 ft continue early this morning and will slowly
build toward 9 to 13 ft through this afternoon as a mid period
northwest swell becomes the dominant swell train. Seas then
gradually subside towards 7 to 9 ft on Tuesday. -HEC

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...Heat Advisory...
       ...until 8 PM today for North Oregon Coast.
       ...until 10 pm Thu for Cascades.

     Excessive Heat Warning...
       ...until 11 pm today for Coast Range & Lower Columbia.
       ...through WED evening for Cascades and foothills,
		      Greater Portland, and Willamette Valley.
       ...through THU evening for Columbia River Gorge and
		      Hood River Valley

     Red Flag Warning through this evening for much of
       northwest Oregon.

WA...Heat Advisory...
       ...until 8 PM today for North Oregon Coast.
       ...until 10 pm Thu for South WA Cascades.

     Excessive Heat Warning...
       ...until 11 pm today for Cowlitz and Lower Columbia
		      Willapa Hills, South WA Coast, and
		      South WA Cascades foothills.
       ...through WED evening for Greater Vancouver.
       ...through THU evening for Columbia River Gorge.

     Red Flag Warning through this evening for much of
       southwest Washington.

PZ...Small Craft Advisory until 3 AM PDT Tuesday for Coastal waters
     from Cape Shoalwater WA to Florence OR out 10 NM.

     Small Craft Advisory until 5 AM PDT Tuesday for Coastal waters
     from Cape Shoalwater WA to Florence OR from 10 to 60 NM.
&&

www.weather.gov/portland

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