Excessive Heat in Washington, August 11 to 12, 2021

Area Forecast Discussion
National Weather Service Portland OR
253 PM PDT Wed Aug 11 2021

.SYNOPSIS...High pressure will continue across the region through
Friday leading to hot and dry weather. An excessive heat warning is
in effect from noon Wednesday to Saturday evening. The upper ridge
will shift east this weekend with a slight cooling trend starting
Saturday and continuing into early next week.

&&

.SHORT TERM...TODAY THROUGH FRIDAY NIGHT...The weather remains quiet
but hot under the influence of a 594 dam 500hPa ridge building into
the area from the eastern Pacific. As of 2p.m. this afternoon,
temperatures have already climbed well into the 90s. With surface
heating continuing over the course of the next few hours, we would
not be surprised if many Willamette Valley locations reach/surpass
100F today. In addition, the overnight recovery is expected to be
less tonight, with temperatures in densely-populated, urbanized areas
struggling to fall below 70 degrees (rural locations still only
dipping into the middle to upper 60s) by daybreak. This positive
feedback cycle continues tomorrow, with temperatures potentially
warming more than today (given the higher overnight lows) provided
smoke and haze advected into our area from fires to our north and
east doesn't trim them back a bit. Confidence is low in the exact
magnitude and duration of the heat, but the models continue to depict
a strong signal that temperatures will be significantly warmer (15+
degrees F) than normal. The ECMWF and GFS still agree that 850mb
temperatures will warm to ~25C by tomorrow afternoon, and the NBM
continues to suggest the probability of exceeding 100F tomorrow at
PDX is around 70%. Friday looks very similar, though perhaps a degree
or two cooler according to both MOS and NBM guidance. Still, the heat
will be oppressive, especially for vulnerable populations such as the
elderly, the very young, and those without air conditioning. In fact,
heat stress is forecast to peak during the Thursday night into Friday
timeframe since we'll be multiple days into the event.

Friday, the mention of thunder in eastern Lane and Linn counties
stands to cover the potential for a few elevated cells that will form
from orographic effects along the spine of the Cascades. This mention
is only slight, since (1) instability doesn't look overly impressive
on this side of the crest, (2) the bottom of the unstable layer
(known as the level of free convection, or LFC) is forecast to be
right around the top of the Cascades, and (3) the mid to upper-level
steering flow has a westerly component that will drive convection
quickly east of the crest (the edge of our area of responsibility).
Still, we believe the mention is warranted given the weak 1.5PV
tropopause fold forecast by the NAM, along with the NBM and SREF's
15-25% forecast chance for thunder in the area. Some concern even
exists in elevated, nocturnal convection tomorrow (Thursday) night in
the Lane County Cascades and foothills, but most high resolution
models - with exception to the more-aggressive HRRR - suggest that
anything moving NW into Lane County overnight will be fizzling, and
hence only capable of dropping a few sprinkles through the dry
boundary layer. -Bumgardner

&&

.LONG TERM...Saturday through Tuesday night...After a few days of
very hot weather, Saturday will initiate a pattern shift that will
bring some relief to the area. On Saturday, vertically stacked
onshore flow will become dominant, which will bring in cooler air
from the North Pacific. The change to onshore flow stems from the
shift of the upper level high to the east, and a weak low forming
aloft over the Pacific. This low is accompanied by elevated
southwesterly winds in the jet stream aloft around 35,000 ft. Because
of the onshore flow, relief from smoke may occur as it may be pushed
out of the area from nearby fires. This of course depends on whether
or not there are new starts that may occur in the previous days. By
Sunday, a broad low that currently sits over the northern Gulf of
Alaska will widen and begin its slow advance southward along the B.C.
coast. As the low aloft moves southeast, a shortwave within the flow
will move over the area beginning Sunday morning. This shortwave will
bring more cool air to the area and even a slight chance for some
rain showers along the northern portions of the forecast area.

Models have varied quite a bit in regards to the timing and location
of the precipitation so confidence is low. There is a chance that if
the parent low stays further north, that the rain could miss our
area completely. In contrast, if the low dips further south,
potentially more of the area could experience rain. Have decided to
keep PoPs and accumulation low to represent the uncertainty. Long
term models show a pretty consistent pattern in regards to the
overall synoptic pattern with the main differences lying in the
intensity of the system. The GFS is currently the stronger and more
southerly pattern, while the ECMWF and Canadian keep a bit more
conservative strength and trends to the north over interior Canada.
Even with these differences, a similar overall outcome is
expected...cooler temperatures, stronger onshore flow, and a slight
chance for rain in the north on Sunday and Monday. -Muessle

&&

.FIRE WEATHER...Strong high pressure over the northeastern Pacific
will gradually drift eastward through Saturday. A subsidence
inversion will lower to 2500-3000 feet Thursday and Friday bringing
poor humidity recovery for the Cascades and Coast Range ridges.
Additionally, cut-off low pressure moves north Thursday and Friday
bringing significant instability and strong ventilation potential.
Friday also brings an elevated thunderstorm threat for the Central
Cascades.

&&

.AVIATION...High pressure over the region will result in
predominantly VFR conditions areawide through 00z Friday. The
main exception to this will be along the coast where a southerly
wind reversal and accompanying fog and low clouds will spread
northward up the coast. This will likely result in a mix of IFR
and LIFR conditions pushing into KONP between 06-12z Thursday.
There is a chance it could push all the way northward and impact
KAST between 12-18z Thursday, but nearly all model guidance
suggests it will fall short and this does not seem unreasonable
given the strength of the current high pressure ridge overhead.

Smoke from the wildfires burning in the Oregon Cascades will
continue to primarily impact eastern Lane, Linn and Marion
Counties over the next 24 hours. The latest HRRR smoke model
does suggest some smoke from BC/WA wildfires will drop
southwestward through the Columbia River Basin and begin to
impact the Columbia River Gorge and northern Willamette Valley
between 18z Thursday and 00z Friday. Do not expect this to bring
IFR visibilities at this point based on limited smoke upstream,
but we might see surface visibilities drop into the 5-9 mile
range in a worst case scenario. Confidence is not terribly high
that these visibilities will come to fruition given
smoke coverage is still somewhat limited across eastern
Washington and southern British Columbia.

For detailed regional Pac NW aviation weather information, go
online to: https://weather.gov/zse

KPDX AND APPROACHES...High pressure aloft will result in light
winds and VFR conditions through 00z Friday. Some smoke from
BC/WA wildfires will begin to spread westward through the
Columbia River Gorge between 18z Thursday and 00z Friday and may
result in surface visibilities falling into the 5-9 mile range.
Do not expect this to bring IFR visibilities at this point based
on limited smoke upstream, but we might see surface visibilities
drop into the 5-9 mile range in a worst case scenario. Confidence
is not terribly high that these visibilities will come to
fruition, though given smoke coverage is still somewhat limited
across eastern Washington and southern British Columbia. /Neuman

&&

.MARINE...High pressure over the northeast Pacific and thermally
induced lower pressure shifting inland today will result in
breezy northerly winds decreasing over the next 24 hours. There
could be a few isolated spots with gusts to 21 kt during the
remainder of this afternoon and early evening, but not expecting
wind gusts this strong to be widespread enough to necessitate a
Small Craft Advisory.

A southerly wind reversal will move northward up the coast
overnight and bring areas of fog and a brief burst of 10 to 20
kt southerly wind gusts, primarily to the waters within 10 to 20
NM of the Oregon coast. It remains unclear if these gustier
southerly winds will make it all the way to the south
Washington coast overnight. Light southerly winds should then
more or less develop across all of the waters Thursday into
Friday and persist through the weekend. These southerly winds
are expected to remain well below Small Craft Advisory wind gust
thresholds of 21-33 kt. Expect seas to climb into the 8 to 10 ft
range over the next 24 hours as an incoming northwesterly swell
pushes into the waters. Seas should then gradually decrease
thereafter and drop below 5 ft for the weekend. Seas may increase
slightly early next week after a weak front drops southeastward
across the waters late Sunday into early Monday. /Neuman

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Excessive Heat Warning until 10 PM PDT Saturday for Cascade
     Foothills in Lane County-Cascades in Lane County-Central
     Coast Range of Western Oregon-Central Columbia River Gorge-
     Central Willamette Valley-Coast Range of Northwest Oregon-
     Greater Portland Metro Area-Lower Columbia-Northern Oregon
     Cascade Foothills-Northern Oregon Cascades-South
     Willamette Valley-Upper Hood River Valley-Western Columbia
     River Gorge.

WA...Excessive Heat Warning until 10 PM PDT Saturday for Central
     Columbia River Gorge-Greater Vancouver Area-I-5 Corridor
     in Cowlitz County-South Washington Cascade Foothills-South
     Washington Cascades-Western Columbia River Gorge.

PZ...Small Craft Advisory until noon PDT today for Waters from
     Cascade Head to Florence OR from 10 to 60 NM.

&&