Excessive Heat in Oregon, August 1 to 4, 2017

Area Forecast Discussion
National Weather Service Portland OR
1011 AM PDT Tue Aug  1 2017

.SYNOPSIS...The longest heatwave since at least 2009 is expected for
much of the Pacific Northwest this week. The hottest period will be
today through Friday when afternoon high temperatures will be near
or above 100 degrees every day in the interior. Many sites will be
close to all-time record high temperatures Wednesday and Thursday.
Some relief is expected for the Coast Thursday or Friday and inland
over the weekend. However, high temperatures are expected to remain
well above average through at least early next week.

&&

.SHORT TERM...Today through Thursday...Today is the start of our
stretch of hot weather this week. Temperatures in the interior this
morning are currently in the 70s, with temperatures in the interior
lowlands expected to reach the century mark this afternoon. This will
likely set a few daily high temperatures records in the interior.
Higher elevations in the interior won't see much relief today, with
temperatures expected to be around 90F this afternoon. It will also
be a warm day at the coast today, with temperatures in the 70s at the
beaches and in the 80s in the coastal communities further inland.
Wednesday will be even hotter as the thermal trough builds over the
Willamette Valley and 850mb temperatures remain in the mid 20s. The
last three times PDX hit 107F, the 850mb temperatures were around
25-26C, so all time records are certainly within reach. Expect daily
high temperatures records to be shattered at several interior
locations. In addition, as the thermal trough sits overhead on
Wednesday night, temperatures will remain in the upper 60s to low 70s
in the Portland metro area. This will exacerbate heat sensitivities,
especially for those without air conditioning who rely heavily on
cooler overnight temperatures to cool their home. Warm low
temperature records may also be set as most records for this week are
in the mid 60s. The mid shift made some minor adjustments to low
temperatures the next couple of nights to provide for a better
contrast with the warmer temperatures expected in the higher terrain
which is expected to be noticeably warmer. These adjustments remain
on track based on recent guidance so no changes were made.

Thursday will be very similar to Wednesday with light winds and
extreme heat as the thermal trough remains overhead. Some models,
notably the GFS, are quick to try to shift the thermal trough east
of the Cascades as early as Thursday afternoon, but do not expect the
thermal trough to fully shift east of the Cascades until sometime on
Friday. The fcst models are also showing a surge of low-level
southerly winds along the coast starting Thu morning. It is unclear
just how far north this surge will extend during the day Thu, but it
is usually faster than models indicate. It looks likely that it will
affect the central OR coast, cooling temps off considerably. The
north Oregon and south Washington coast are a bigger question mark.
There is some model support for offshore winds continuing for
portions of these zones well into Thu, which could allow for another
warm day. Confidence in this is not high at this time.

Excessive Heat Warnings remain in in place across the CWA, except at
the coast. Decided to keep the Excessive Heat Watch along the south
Washington and north Oregon Coast for Wednesday as there is still a
question as to how warm it will get. Some guidance suggest that
temperatures will climb into the lower 90s as you get away from the
coast, but the coast is always a tough forecast in these scenarios
because the difference between temps in the 70s and temps climbing
into well into the 90s is only a few miles. Will wait until after the
full 12Z guidance comes in before deciding whether or not to upgrade
the Excessive Heat Watch to a Heat Advisory, but based on our current
thinking, unless things change drastically, the watch is likely to be
upgraded to an advisory with the afternoon package.
/64/mh/Pyle/Bentley

&&

.LONG TERM...No Changes. Previous discussion follows...Thursday Night
through Monday...The thermal trough is expected to ease its grip over
western WA/OR Friday. This will lead to temps beginning to moderate
over the interior. However, highs are still expected to be around 100
on Friday. Coastal locations look like a sure bet to see much cooler
temperatures Friday, if they haven't already cooled off on Thursday
as onshore flow increases. Many times, heat waves will come to an
abrupt end as a weak wave deepens the marine layer and filters marine
air into most of the area. Unfortunately, without a strong wave, it
does not appear that will be the case. In fact, the ECMWF keeps
temperatures in the upper 90s to near 100 through next Monday. Even
the cooler models have temperatures at or above 90 through the
weekend.

In addition, thunderstorms are possible next weekend as the ridge
breaks down. For now, have only added a slight chance for the
Cascades for a few periods, but will need to monitor this through
time. This will be especially important for fire weather concerns
after a period of very hot, dry weather. Pyle/Bentley

&&

.FIRE WEATHER...No significant changes to current thinking with dry
and windy Red Flag conditions expected today in the central and
south Willamette Valley and dry and significantly unstable
conditions everywhere except for the coast. Morning high resolution
models have indicated a slight increase in wind speeds in the valley
this afternoon. This increases confidence that red flag conditions
will be met for at least the south and central Willamette Valley. Do
have some concern that red flag conditions may be met in the north
Willamette Valley and Clark county. Winds last evening were quite
strong in the Portland metro area, and with lower humidities today,
if similar winds materialize, criteria will likely be met for
several hours. Will not make any changes to the hazards at this
time, but will continue to monitor into the afternoon.

Morning KSLE sounding had a mid-level Haines 5 and high-level Haines
6, however mid-level haines is close to a 6 and will likely be there
by this afternoon. Any fires at elevation should easily realize
these Haines 6 conditions today, but valley locations will not be
quite as unstable due to a weak capping inversion centered around
3000 feet. On Wednesday and Thursday, this inversion will be gone
and the unstable conditions will be felt by fires at all elevations.
Specifically, mixing heights will reach ~15000 feet at the
Whitewater fire the next 3 afternoons.

The most extreme dry and unstable conditions are expected on
Wednesday and Thursday as the thermal trough sets up somewhere
between the Coast Range and Cascades. High temperatures in the
valley will shatter daily records, and will be approaching all time
records. There won't be much relief from the heat at higher
elevations with temperatures between 4000 and 5000 feet along the
Cascade crests rising well into the 90s and even approaching 100
degrees. In addition to the extreme heat during the day, a thermal
belt will lead to poor overnight recoveries in areas between 2000
and 4000 feet in the Coast Range and Cascades. These locations will
likely remain in the 70s at night with some areas struggling to drop
below 80.

Although it will be hot, dry, and unstable, there is not much
offshore flow with this event. There will be some light easterly
flow in the Cascades and Cascade foothills at times, but stronger
widespread easterly flow does not appear likely in this pattern.
There is still some uncertainty as to exactly when the thermal
trough will shift east of the Cascades. Today's forecast models have
trended later with the onshore flow, so our area may remain under
the influence of the thermal trough through Friday. While the return
to onshore flow will bring some relief from the extreme heat, the
models have been consistent indicating thunderstorm activity
associated with the upper trough which initiates the pattern change.
Primary focus at this time will be on the critical conditions this
week, but it is worth noting that thunderstorms are possible at the
end of this hot streak. /Bentley

&&

.AVIATION...High pressure aloft will keep VFR conditions areawide
through 18Z Wednesday. With winds turning east this afternoon, could
see some smoke from the fire in the Cascades east of Salem drift
over SLE and maybe even EUG.

KPDX AND APPROACHES...VFR through 18Z Wednesday. -McCoy

&&

.MARINE...No changes. Previous discussion follows...
Expect high pressure to continue over the northeast Pacific for
much of the next week. This will bring a fairly typical
summertime northerly wind pattern to the waters for much of this
time. Winds will generally be strongest during the afternoon and
evening hours across the waters...especially near shore. The next
24 hours will likely be a near repeat of the past 24 hours with
seas likely becoming especially steep and hazardous across the
waters off the central Oregon coast. A southerly wind reversal
may impact the waters Thursday and/or Friday, but it appears weak
at this time. Otherwise, a return to northerly flow should ensue
over the weekend and winds may creep above advisory thresholds
towards early next week. /Neuman

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Excessive Heat Warning from noon today to 11 PM PDT Friday 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.

     Red Flag Warning until 11 PM PDT Thursday for Central Oregon
     Cascade Foothills-Mt. Hood National Forest West of Cascade
     Crest-North Oregon Cascade Foothills-Willamette National
     Forest-Willamette Valley.

     Red Flag Warning until 11 PM PDT Thursday for East Slopes of
     the Central Oregon Coast Range-North Oregon Coast Range.

     Excessive Heat Watch from Wednesday morning through Wednesday
     evening for North Oregon Coast.

WA...Excessive Heat Warning from noon today to 11 PM PDT Friday 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-Willapa Hills.

     Red Flag Warning until 11 PM PDT Thursday for Clark County
     Lowlands-East Willapa Hills-Eastern Gifford Pinchot
     National Forest Mt Adams Ranger District-Extreme South
     Washington Cascades and Foothills-South Washington Coast
     and West Willapa Hills.

     Excessive Heat Watch from Wednesday morning through Wednesday
     evening for South Washington Coast.

PZ...Small Craft Advisory for winds until 3 AM PDT Wednesday for
     Coastal Waters from Cape Shoalwater WA to Florence OR out
     60 nm.

     Small Craft Advisory for hazardous seas until 3 AM PDT
     Wednesday for Coastal Waters from Cascade Head OR to
     Florence OR out 60 nm.

&&

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This discussion is for Northwest Oregon and Southwest Washington
from the Cascade crest to 60 nautical miles offshore. This area is
commonly referred to as the forecast area.