Heat in Oregon, August 17 to 18, 2022

Area Forecast Discussion
National Weather Service Portland OR
1017 AM PDT Wed Aug 17 2022

Amended for forecast update as well as Aviation section

.UPDATE...Viewing satellite, a large area of midlevel moisture is
moving north-northwest from south-central Oregon. Viewing recent
ensemble guidance, increased cloud cover and cooling max temperatures
are being advertised in both HREF and GEFS/ECE. Even the 10th
percentile of the NBM has lowered by about 6 degrees, with values in
the 80s now (had previously been in the 90s). Therefore have trended
the forecast just slightly cooler.  Will still be a very hot day
today, but just lowered temps by 1-3 degrees across the area. In
terms of shower/storm potential, still looks like an isolated day,
with scattered activity closer to the Lane/Linn County Cascades. With
a dry sub-cloud layer, showers will likely be more virga than rain,
which causes some concern for the thunderstorm potential.  However,
additional cloud cover can also limit strength of these storms. Will
keep the PoP/storm coverage forecast the same. May need to expand the
area of thunderstorms west into the Cascade foothills and potentially
the Willamette Valley this evening if CAMs show this trend as well.

This additional cloud cover tonight will allow for even less relief
for overnight temperatures, especially those in the Heat Advisory, as
the clouds act like a blanket to hold in heat. Have increased lows
overnight 1-3 degrees as well. In fact, our forecast lows tonight are
above the record warmest lows for most sites, with some as much as
5-7 degrees warmer.   /Kriederman

&&

.SYNOPSIS...High pressure will foster hot weather today and Thursday,
with temperatures expected to cool considerably by Friday. More
seasonable summertime weather will then continue through the weekend.

&&

.DISCUSSION...

Key Messages:

* Hot weather is in store for today and tomorrow. A Heat Advisory
  remains in effect for the Willamette Valley, south Washington
  lowlands, Columbia River Gorge, Hood River Valley, and Cascades
  below 4,000 ft.

* A thunderstorm threat will exist for the higher Oregon Cascades
  this evening and then expand to the south Washington Cascades
  late Thursday.

.SHORT TERM...(Through Friday)...500hPa heights had climbed to 5863m
by the time the 00z sounding was taken last evening, and likely they
have risen further since. Around 90% of the grand ensemble suggests
that in Salem 500hPa heights will rise to between 590 and 592 dam for
the time period spanning 2-8 PM this evening, so the expectation is
for a much warmer day today compared with yesterday. However, one
wild card is the degree to which surface heating will be impacted by
convective debris clouds streaming northward across the area in the
afternoon. Water vapor satellite at 245 AM showed a compact low
moving northward on the west edge of the upper high, just off the
northern CA and southern OR coast, with mid-level winds becoming
southeasterly across southern Oregon in response. Webcams in the
central Oregon Cascade Foothills show some convective debris clouds
already approaching from the southeast - something high res model
guidance has not handled well. Hence, our confidence in the sky cover
forecast from any model is low at this time, and in fact it appears
there will be more clouds than what any guidance suggests in at least
the Cascades and southern Willamette Valley. As a result,
temperatures there might be a bit lower today than previously
expected. NBM hints at this, and gives an interquartile (IQR) maximum
temperature range from 94 to 101 in Eugene, while the range is 98 to
104 in Portland.

A slight chance for showers has been added to nearly the entire area
for this evening as virtually all of the CAMs depict some simulated
reflectivities not just in the Cascades, but also further west into
the Coast Range. There's precious little QPF in the ensemble
guidance, and this is probably because of the dry low levels evident
in forecast soundings. Virga is more likely than rain, but a few
sprinkles in spots appears plausible enough given the abundant
mid-level monsoon moisture brought north by the aforementioned low
off the coast. If nothing else, these virga showers could increase
low level humidity and make the heat feel worse this evening -
somewhat negating the slightly lower temperatures stemming from the
reduced surface heating associated with the cloud cover. Forecast
soundings suggest the LFC will be 10kft+, which is above nearly all
of the Cascades, so convective initiation from terrain-induced
perturbations will be difficult to achieve. The CAMs suggest
convection will instead fire across northern CA, NV, and southern
Oregon where steep low level lapse rates may allow the convective
temperature to be attained. The slight chance for thunder will thus
not be confined to the Cascades, and has been brought westward to
include even south central Lane County this evening.

Convective debris clouds will again pose a problem for surface
heating on Thursday, and these may inhibit heating a bit more
compared with today as the area will have more moisture in the mid
levels. The NAM often overforecasts moisture, but even so its
depiction of 1500 J/kg of MUCAPE (and an unstable layer extending up
to around 250mb) is eye-catching. The deterministic models are once
again turning flow at 500mb southeasterly Thursday late morning into
mid afternoon, which will bear watching for convective potential. As
with their forecast for today, the CAMs are not showing anything
overly impressive for Thursday at this time, nor are many ensemble
members within either the GEFS or EPS suggesting QPF west of the
Cascades.
However, this is something we'll continue to watch as another round
of thundery activity - which could stretch north and west of the
Oregon Cascades - is not out of the question.

The model spread in temperatures on Thursday is still there because
the model spread in convective activity is still present. NBM's 10th
to 90th percentile range stretches from 91 to 104 in Portland
Thursday, indicating a lot variability among the ensembles in how
much surface heating will be impacted by mid to high level clouds
(and/or showers). Friday, however, looks much cooler; NBM suggests
only around a 20% chance for temperatures of 90 or warmer in the
interior lowlands. For now, highs on Friday are expected to top out
in the low to mid 80s as a stronger marine surge plows across the
area Thursday night and results in stubborn stratus which likely
won't burn off in many locations until midday. Thus, this "heat wave"
will be cut short, and those sensitive to the heat will find some
relief by the end of the work week. -Bumgardner

.LONG TERM...(Saturday through Tuesday)...Near to slightly above
normal temperatures are expected to continue on Saturday, as
southwesterly upper level flow overspreads the PacNW ahead of an
approaching low. Cluster guidance suggests some ensemble variability
in what happens with this low as it moves into the PacNW, but the
Canadian, which is near the middle of the grand ensembles according
to the cluster phase space, suggests it will weaken and pass just to
our north Monday morning. The timing of arrival for the weakening
cool front associated with this feature is uncertain, and could be
anytime between early Sunday and early Monday; with its arrival,
expect another strong marine push which could bring another round of
drizzle to parts of the Coast and Coast Range, along with a thick
layer of stratus which may not burn off until late morning/early
afternoon. The NBM suggests the coolest day in the extended period
will be Monday, with an 85% chance high temperatures will be below 90
at PDX.

While it's still early, it is worth mentioning that another stretch
of hot weather will be possible beginning as early as Tuesday. Model
spread increases by this time, but NBM once again gives around a 30%
chance for triple digit temperatures in the Willamette Valley
Tuesday through Thursday, while days 8-10 of the cluster page
suggest +90 to +150 dam 500hPa height anomalies across NW Oregon/SW
Washington.
-Bumgardner

&&

.AVIATION...18Z TAFs: High pressure over the region will maintain
warm air mass and mostly VFR conditions through the daytime hours
across the area. An upper level low off the north California
coast, is bringing southerly flow from the Sierras northward into
the southern half of Oregon. This flow pattern is favorable for
thunderstorm development across the Casacdes starting around 21Z
Wednesday and could cause some isolated cells to infiltrate into
the Willamette Valley around 00Z Thursday. These clouds and
showers will lift north/northwestward tonight. Overall, there is a
chance of thunderstorms over the Cascades through tonight, with a
non-zero chance of a thunderstorm over the Willamette Valley.

IFR/LIFR conditions are expected to return along the coast later
around 08Z to 10Z Thursday. There also could be some convective
debris that might bring a transient cloud deck around 4000 to 6000
feet into the Willamette Valley after 06Z Thursday, but this will
be very challenging to time out if it happens.

Areas of smoke will continue between Oakridge and Waldo Lake due
to the Cedar Creek wildfire just west of Waldo Lake.

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

KPDX AND APPROACHES...High pressure with clear skies, with the
possibility of scattered mid-level clouds this afternoon and into
tonight. /42

&&

.MARINE...High pressure will remain anchored offshore over the
next several days, with thermal low pressure along the south
Oregon coast into northwest California. This will maintain
moderate northerly pressure gradients over the south Oregon
coastal waters, resulting in a prolonged period of northerly
winds over the coastal waters. Northerly winds will be strongest
during the mid to late afternoon and evening hours each day,
especially to the south of Cascade Head. A Small Craft Advisory
is currently in effect for all of the coastal waters through
Wed evening, for gusts 20 to 25 kt. But, the strongest and most
frequent gusts will be to south of Cascade Head. Seas should
generally range between 4 and 6 ft, with the choppiest seas likely
to occur to the south of Newport.

Not much change on Thursday aside from decreasing northerly winds,
which should fall to around 10 to 15 kt. North winds should
increase again late Friday afternoon and evening as the pressure
gradient restrengthens over the waters, which may bring the return
of marginal small craft wind gusts to 25 kt for the central
waters. -TK

For information about upcoming marine zone changes, go online to:
https://www.weather.gov/pqr/marinezone

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...Heat Advisory until 10 PM PDT Thursday for Cascade Foothills in
     Lane County-Cascades in Lane County-Central Columbia River
     Gorge-Central Willamette Valley-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...Heat Advisory until 10 PM PDT Thursday 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 midnight PDT tonight for coastal
     waters from Cape Shoalwater WA to Florence OR out 60 nm.
&&

www.weather.gov/portland

Interact with us via social media:
www.facebook.com/NWSPortland
www.twitter.com/NWSPortland