Wildfire in Oregon, August 23, 2023

Area Forecast Discussion...UPDATED
National Weather Service Portland OR
1119 AM PDT Wed Aug 23 2023

Updated aviation discussion

.SYNOPSIS...Weak low pressure is moving onshore this morning near
Willapa Bay, pushing a few showers onshore north of Tillamook. This
system will dissipate as a broader upper trough moves east of the
forecast area today. Much warmer and drier conditions are expected to
begin Thursday, with the warm temperatures continuing through the
weekend in southerly flow aloft. The southerly flow will carry
subtropical moisture and weak disturbances northward, increasing the
chance for thunder Thu night through the weekend. Cooler temperatures
return early next week, along with better and more widespread chances
for rain.

&&

.SHORT TERM...Today through Friday...Latest GOES-West satellite
imagery shows weak low pressure moving onshore near Willapa Bay early
this morning. Much of the precipitation has remained north of the
forecast area, and this morning is no exception. Rainfall amounts
fell short of 0.25", but there were a few locations that exceeded
0.10 inch in the Willapa Hills and just outside of Astoria. There may
still be a few showers to come, but the low will continue to weaken
as its upper level support races off to the east.

Lingering morning clouds will tend to diminish this afternoon as high
pressure builds into the Pac NW. Meanwhile, thermal low pressure will
strengthen along the western OR/CA border. This will draw increasing
northerly flow this afternoon and evening, which will usher in much
warmer and drier conditions for Thursday. Despite near-normal temps
in the lower 80s today, the interior lowlands will heat up into the
90s Thursday as 850 mb temps soar from around +10 deg C this
afternoon to +20 deg C Thursday afternoon.

Guidance has been fairly consistent the last couple days in having an
upper low from the Gulf of Alaska dive S-SE. As it does so, an upper
low off the California coast will likely slingshot N-NE across Oregon
Thu night. This would be a favorable setup for nocturnal
thunderstorms, which the NBM hinted at with 10-15% chances of thunder
Thursday night/Friday morning. Given the pattern, we increased these
chances by about 5 percent, resulting in a thunder mention for most
of the forecast area late Thu night through Fri morning. The 00z
deterministic ECMWF lightning flash density output suggests elevated
thunderstorms initiate along a Salem-Newport line, with lightning
peaking around 5 AM Friday morning over the Portland metro area
before moving into SW Washington and decreasing Friday morning. Given
the deep moisture the shortwave taps into Thursday night,
thunderstorms would likely come with plenty of rain. However it is
important to keep in mind that nocturnal, elevated convection tends
to produce much more prolific lightning than our ordinary afternoon
surface-based convection in the Cascades. Temperatures likely come
down a few degrees Friday due to cloud cover, but increased humidity
will likely make it feel similar to Thursday. The increased moisture
and moderately steep theta-e lapse rates could promote some
surface-based convection along the Cascades Friday afternoon.  Weagle

.LONG TERM...Saturday through Tuesday...Models appear set in
forecasting a continuation of temperatures 5-15 deg F above normal
through the weekend, at least inland. Model agreement in the long
term is actually impressive today, considering the disagreement shown
over the past few days. This is especially the case considering there
is a cutoff low involved. This cutoff low is likely to remain outside
130W through the weekend, maintaining the warm southerly flow and
fairly deep moisture over the region. While the increased humidity
should mitigate fire wx concerns for wind/RH, it also maintains a
nonzero chance of lightning on fuels that have only had a very brief
respite from hot and dry conditions. Much will depend on the timing
of individual shortwave rotating around the broader upper low.

Again, model agreement is impressive early next week in pushing the
upper low onshore into the Pac NW during the first half of the week,
leading to cooling trend Monday and Tuesday. Nearly all EC and about
60% of GEFS ensemble members show some measurable precipitation
reaching as far south as Eugene by the middle of next week.  Weagle

&&

.FIRE WEATHER...Benign weather through Wednesday as deep onshore flow
will lead to cooler temperatures and keep RH elevated. Changes begin
Thursday morning as confidence is increasing that the thermal trough
will be pushed west of the Cascade crest, inducing offshore flow.
This will likely yield much drier and hotter conditions west of the
Cascades that may continue through the weekend. The initial offshore
push Thursday may yield enough easterly breezes to push weather
conditions into critical territory. Latest probabilistic guidance
from NBM suggests better than a 70% chance of wind gusts exceeding 25
mph in the general area of the western Columbia Gorge. The switch to
east winds would likely occur in zone 608 Willamette NF as well, but
the magnitude appears less than areas further north. On top of any
uncertainty regarding how dry/breezy it will be Thursday, it remains
a bit of an unknown how much the cool upper trough pattern Tue/Wed
will moisten fuels. With this in mind, we will hold off on issuing
any Fire Weather Watches for the time being, so the day shift can
coordinate with NWCC and other partners regarding the anticipated
fuel status. But for now we are keeping a very close eye on zones
660/605/607 for the potential of critical fire weather Thursday into
Friday.  Weagle

&&

.AVIATION...18Z TAFs: An upper level low will slowly track
southeast offshore which will maintain weak offshore flow inland
throughout the TAF period. Expect VFR with weak northerly winds
for all sites through roughly 08z Thursday. Marine stratus will
move back onshore during the early morning hours bringing MVFR to
IFR stratus along coastal terminals through roughly 18z Thursday.
No impacts expected inland.

Smoke from ongoing wildfires will continue to bring visibility
restrictions to parts of the Cascades across eastern Lane and
Linn Counties.

KPDX AND APPROACHES...VFR conditions expected next 24 hours.

&&

.MARINE...High pressure will build over the northeast Pacific and
thermally induced lower pressure will gradually strengthen over
northern California. This will bring a return to a more typical
north to northwesterly wind pattern to the waters today and
Thursday. Latest guidance indicating northerly wind gusts reaching
Small Craft Advisory (greater and 70% chance) criteria tonight
into Thursday mainly for zones 253, 272, & 273. Small Craft Winds
may extended further north Thursday. Eventually the thermally
induced low pressure shifts inland from the coast. There is some
indication that a southerly wind reversal will spread northward up
the coast late Thursday or Friday morning. Overall, expect seas
to generally hold between 3 ft and 6 ft through at least the
weekend. /mh /Neuman

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...Fire Weather Watch from Thursday morning through Friday morning
     for Mt. Hood National Forest West of Cascade Crest-North
     Oregon Cascade Foothills.

WA...Fire Weather Watch from Thursday morning through Friday morning
     for Extreme South Washington Cascades and Foothills.

PZ...Small Craft Advisory from 5 PM this afternoon to 2 AM PDT
     Thursday for Coastal waters from Cape Foulweather to
     Florence OR out 10 NM.

     Small Craft Advisory from 9 PM this evening to 5 PM PDT Thursday

     for Waters from Cape Falcon to Cape Foulweather OR from 10
     to 60 NM.

     Small Craft Advisory from 5 PM this afternoon to 5 PM PDT
     Thursday for Waters from Cape Foulweather to Florence OR
     from 10 to 60 NM.
&&

&&

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