Wildfire in Washington, September 8 to 30, 2020

Area Forecast Discussion
National Weather Service Portland OR
230 AM PDT Tue Sep 8 2020

.SYNOPSIS...East winds will continue but gradually weaken through
Wednesday for much of the region as low pressure remains near the
coast and high pressure over the northern Rockies weakens. Warm and
dry weather appears likely to continue through the week as a ridge
of high pressure remains over the Pacific Northwest.

&&

.SHORT TERM...Today through Thursday...The Pacific Northwest was
under a high amplitude upper air pattern early Tuesday morning as a
strong upper low continued to move south over northern Utah and
Nevada, and strong ridge of high pressure extended north offshore.
At the surface strong high pressure with a central pressure near
1038 mb was over the Montana Rockies and thermal induced low
pressure extended north along the Oregon and Washington coast. A
strong offshore pressure gradient measured near -22 mb between KOTH
and KGEG. Models generally indicating gradients peaking this
morning, so for now will leave the high wind warnings alone, but do
expect to see winds begin to diminish some later today. With the
offshore winds, a little cooler air will continue to affect the
northeast part of the forecast area with 850 mb temperatures are
expected to run from 14C to 16C this afternoon. Virtually all of
numerical guidance suggests the central Oregon coast limited to the
80s or cooler today, but with 850 mb temperatures over the southwest
part of the forecast area remaining quite warm, near 22C, and
offshore flow adding an element of diabatic warming, will continue
with the heat advisory today along the southern half of the coast
and into the adjacent Coast Range where some 90s still appear likely.
The air is also extremely dry this morning with dewpoints in the 20s
most areas, and with the continuation of offshore flow, it will
remain extremely dry today.

Surface offshore pressure gradients expected to slowly decrease
overnight and Wednesday as the surface high pressure over the
Rockies weakens. As the upper low settles down over southern Utah,
the east Pacific upper ridge will press east into the Pacific
Northwest. Offshore breezes with subsidence will keep the air mass
very dry tonight and Wednesday, while warming 850 mb temperatures
suggest the interior valleys west of the Cascade Crest will be right
back into 90s Wednesday. A slight migration inland of the surface
trough Wednesday promises to bring onshore winds to the coast in the
afternoon for some local cooling there.

Models generally show the surface pressure pattern becomes much more
diffuse Wednesday night and Thursday while the upper ridge remain
over the region. This will keep temperatures well above normal with
very low humidity levels through Thursday, although a narrow surface
ridge developing along the coast should bring more cooling and
increased humidity levels to the coast.

.LONG TERM...Thursday night through Monday...Thursday night through
Saturday models show a gradual weakening of the upper ridge along
with a shift of the ridge axis inland. Resulting onshore flow will
bring modest cooling Friday and Saturday. Sunday and Monday sees the
approach of a low pressure system from the west. The details on the
strength and orientation of the trough are not well agreed upon
though, so although the 00Z EC operational run suggests chances for
rain by late Monday, will stick with the drier National Blend solution.
Temperatures will tend to moderate further, but still appear likely
to run a little warmer than normal.

&&

.AVIATION...Strong east to northeast winds along with smoky skies
will challenge aviation operations for the next 24 hours. Gusts
of 20 to 40 kt will continue to be likely into Tue evening.
Locally stronger wind gusts of up to 50 kt will be possible. In
addition, expect very strong low level wind speed shear on the
order of 30 to 50 kt at around 2000 ft across portions of NW
Oregon and SW Washington through around 19Z Tuesday.

The east winds are also transporting smoke from nearby wildfires.
Visibilities of around 2 to 4 sm are being reported as of 09Z
Tues in the interior valley as well as along the coast. The
visibility should gradually improve through today, especially
north of KSLE. Latest smoke model guidance suggests smoke may
return late Tue night into Wed morning.

Refer to http://weather.gov/zse for detailed regional aviation
weather and hazard information.

KPDX and APPROACHES...Strong east winds can be expected through
at least Tue morning, and likely continuing into Tue evening,
with gusts up to 50 kt possible. Strong low level wind shear of
30 to 50 kt is possible as well. Smoke from nearby fires is being
transported by the easterly winds and has reduced visibility to 2
to 4 miles at times. Recent observations show visibilities have
improved to VFR, but smoke and haze will likely stick around for
the next 24 hours. /DDH

&&

.MARINE...An unusually strong offshore wind event is expected to
continue through Tuesday night. Easterly wind gusts between 25
and 30 kt should be widespread across the waters, with a few
gusts approaching 35 kt. Pockets of stronger Gale Force wind
gusts will be possible downwind of the coastal river gaps such as
the mouth of the Columbia River, Tillamook Bay, and Yaquina Bay.
This will bring another round of steep, choppy seas as wave
heights rise to 7 to 9 ft with a period around 8 seconds.
Therefore, a Small Craft Advisory is in effect for all waters
this morning through Tuesday night.

Offshore flow should weaken considerably on Wednesday as the
surface pressure gradient begins to relax over the waters. This
should result in winds falling below advisory thresholds and seas
dropping below 5 ft. By late Wednesday, low pressure is forecast
to drift off the Oregon coast and bring southerly winds across
the waters Thursday and Friday. DDH/TK

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Red Flag Warning until 8 PM PDT Wednesday for Central Oregon
     Cascade Foothills-Central Oregon Coast-East Slopes of the
     Central Oregon Coast Range-Mt. Hood National Forest West
     of Cascade Crest-North Oregon Cascade Foothills-North
     Oregon Coast-North Oregon Coast Range-Willamette National
     Forest-Willamette Valley.

     High Wind Warning until 1 PM PDT this afternoon for Coast Range
     of Northwest Oregon-Greater Portland Metro Area-North
     Oregon Coast-Northern Oregon Cascade Foothills-Northern
     Oregon Cascades.

     Heat Advisory until 11 PM PDT this evening for Central Coast
     Range of Western Oregon-Central Oregon Coast-North Oregon
     Coast.

WA...Red Flag Warning until 8 PM PDT Wednesday 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.

     High Wind Warning until 1 PM PDT this afternoon for Greater
     Vancouver Area-South Washington Cascade Foothills-South
     Washington Cascades.

PZ...Small Craft Advisory until 5 AM PDT Wednesday for coastal
     waters from Cape Shoalwater WA to Florence OR out 60 NM.

&&

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