NWS Portland, OR wrote its last area forecast discussion before the first event at Sep 7, 9:02 AM PST, 2 hours 43 minutes earlier.
Area forecast discussion, Sep 7, 9:02 AM PST, 2 hours 43 minutes before
Area Forecast Discussion...UPDATE
National Weather Service Portland OR
1002 AM PDT Mon Sep 7 2020
Updated aviation discussion
.SYNOPSIS...A unusually potent east wind event will develop today
lasting through at least Tuesday. Winds likely peak later tonight
and Tuesday morning. Extreme fire danger and locally damaging winds
are likely. Warm and dry weather appears likely through the end of
the week.
&&
.SHORT TERM...Today through Wednesday...Forecast appears to be very
well on track this morning, with strong east to northeast winds
expected to develop across much of the forecast area later today. As
such, we only made minor changes on this shift. The synoptic features
models have been portraying for days are becoming evident on
satellite and surface observations. High pressure is indeed strong
over northern British Columbia at nearly 1040 mb. Meanwhile, 3.9 um
thermal satellite imagery clearly shows very warm conditions
along/near the Oregon Coast south of Cape Blanco, maintaining a
thermal trough of low pressure there. North Bend to Spokane MSLP
gradients remain weakly onshore, but that will be changing soon - and
rapidly - as the strong B.C. high pushes south into the Interior
Northwest. In other words, all the ingredients are coming together as
expected for a potent and likely historic September East Wind event
in our forecast area.
The strongest east winds still appear to focus through the gap in the
high Cascades between Mt Adams and Mt Hood (sort of the broader
Columbia Gorge area) across western portions of the Columbia Gorge,
Portland/Vancouver metro area, north Oregon Coast and Coast Range.
While high resolution guidance has generally backed off ever so
slightly with the magnitude of this event, 00z NAEFS and EPS means
continue to show the type of event that only occurs a couple times a
century in September. 00z NAM/GFS are still forecasting Spokane to
North Bend MSLP gradients of 21 to 23 mb late tonight and early
Tuesday morning, with 900 mb east winds of 65-75 kt over Portland.
Any weakening in the models' projections for this event has only put
the expected wind speeds down into the ranges we're forecasting - if
they had remained in the 23 to 26 mb range with 75-85 kt 900 mb winds
over Portland, we would have had to increase our forecast. As such,
confidence is only increasing that 40 to 60 mph east wind gusts will
develop through the corridor mentioned at the beginning of this
paragraph, with gusts 60 to 80 mph expected on exposed ridges. In
other words, the High Wind Warnings currently in place look to be in
great shape, and no changes were made on this shift.
The existing Red Flag Warnings also look to be in good shape. We are
on track for an extremely dangerous period of fire weather beginning
this afternoon, as discussed below by the evening shift. The rare
combination of an easterly downslope windstorm and extremely low
relative humidities have led the Storm Prediction Center to put the
area mentioned above into a rare "Extremely Critical" category in
their Day 1 Fire Weather Outlook.
We will be issuing a Heat Advisory starting 4 PM this afternoon until
10 PM Tuesday for the coast Tillamook southward and the central Coast
Range of Oregon, where high temperatures in the 90s will be common
once the east winds arrive. The timing of this advisory is a bit
tricky, as some portions of Oregon's Central Coast may remain cool
and cloudy today until the east winds arrive sometime this afternoon
or evening. It's entirely possible many coastal locations south of
Lincoln City will see their temperatures rise tonight as offshore
winds develop. Offshore winds will persist through the day Tuesday,
holding off any sea breeze which could cool off the immediate coast.
The remainder of the short and long term discussions are below and
are unchanged from this evening. Weagle
-------
Little change in the overall forecast for the next 48-72 hours. High
fire danger will impact much of northwest Oregon and southwest
Washington beginning Monday afternoon and continue through Wednesday.
The period of most extreme fire weather conditions will likely begin
Monday evening and persist through Tuesday morning across the Coast
Range, portions of the Portland/Vancouver metro and the Cascades.
After that point, winds should begin to relax and while fire danger
will be high, it will not be as dire.
The main update this evening was to upgrade the wind advisory for the
Portland/Vancouver metro to a high wind warning. The NAM, GFS and UW
WRF soundings support a well mixed atmosphere persisting into the
evening hours with wind speeds at the top of the boundary layer
pushing 50kt or more. This certainly suggests stronger wind gusts of
50 kt could mix down in the form of sporadic, but big powerful gusts
near the West Hills and across portions of the eastern
Portland/Vancouver metro Monday evening/night. In addition, most
models keep the lower atmosphere well mixed with just a hint at the
surface trying to decouple, but struggling to do so. This appears to
be in part due to the breezy to windy conditions expected. In
addition, the urban heat island will act to keep the metro at the
surface warmer than it would naturally, particularly since we are
dealing with a much higher sun angle than many of our east wind
scenarios in Nov/Dec/Jan. When coupled with 900mb temperatures
steadily cooling through the night as cooler air is advected through
the Columbia River Gorge, the lower atmosphere should stay mixed
enough to allow some of the stronger winds aloft to surface at
times.
Finally, we have had a couple quiet years wind wise. In other words,
many of our trees haven't really undergone a good pruning in a while.
When coupled with the fact deciduous trees are fully leafed, tree
limb damage should be higher than it would be during many of our
winter east wind events. Even wind events that produced widespread
45 to 55 mph wind gusts during the winter months several years ago
produced a lot tree damage and ample power outages. For example, the
strongest downslope wind event in the last decade, resulted in over
60K losing power across Clark County on Veterans Day 2014.
So while confidence is relatively high significant chunks of the
metro will experience wind gusts of 40 to 55 mph Monday evening into
Tuesday morning, similar to the Veterans Day 2014 event, confidence
is not as high stronger wind gusts will materialize given the rarity
of such wind speeds. Nonetheless, feel the odds of 60 to 65 mph wind
gusts to surface across at least pockets of the metro is high enough
to warrant the warning given model soundings and theta/wind cross
sections. Either way, winds should be strong enough to produce down
tree limbs that will likely result in at least some power outages
across the metro. /Neuman
.LONG TERM...Wednesday night through Sunday...It will be still be
very warm on Thu with the thermal trough shifting to the interior.
This means cooler weather at the coast as a southerly wind reversal
develops later Tue or Wed. More cooling looks likely Fri into Sat
with weak onshore flow and lowering 500 mb heights along with weak
embedded shortwaves. Shallow morning inversions Thu and Fri may lead
to lower somewhat stagnant air for the interior valley and may result
in decreased air quality. /mh
&&
.AVIATION...The 18Z TAF period is highlighted by mainly clear
skies across the area, along with an impressive east wind event
beginning early this evening and continuing into tomorrow.
Offshore flow will begin to strengthen this afternoon with
easterly to northeasterly wind gusts between 20 and 35 kt
impacting most terminals this evening. Locally stronger wind
gusts between 40 and 50 kt will be possible across the
Portland/Vancouver metro after 00Z Tuesday. In addition, expect
very strong low level wind speed shear on the order of 30 to 60
kt at around 2000 ft across portions of NW Oregon and SW
Washington.
The easterly winds will bring some smoke and haze into the
central and southern Willamette Valley from nearby fires. In
addition, blowing dust from the Columbia Basin could produce
hazy conditions across the northern Valley and Columbia River
Gorge Mon evening. Visibility reductions are not expected at the
terminals at this time.
Refer to http://weather.gov/zse for detailed regional aviation
weather and hazard information.
KPDX and APPROACHES...Clear skies will continue over the next 24
hours, with increasing offshore flow this afternoon and evening.
This offshore wind event will bring very gusty east winds to the
terminal beginning late Monday afternoon, with occasional gusts
to around 50 kt expected late this evening through tonight. Very
strong low level wind shear on the order of 30-50 kt over
1000-2000 feet will likely be present over the terminal for much
of the night. -TK/DDH
&&
.MARINE...High pressure will persist across the northeast
Pacific today. Winds will decrease somewhat Mon morning,
especially for the inner waters, but square seas are still
expected over the outer waters through Mon evening. Therefore,
will extend the small craft advisory over the outer waters
through Monday afternoon for steep and choppy seas.
An unusually strong and impressive offshore wind event is
expected to unfold Monday evening through Monday night. Easterly
wind gusts between 25 and 35 kt should be fairly widespread
across the waters, with pockets of stronger Gale Force wind gusts
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, especially over the
outer waters. Have issued a small craft advisory for the inner
waters beginning 00Z Tue, and continued for it for the outside
waters, through 12Z. Later shifts may determine that a Gale
watch might be needed for Tues morning.
Offshore flow should weaken Tuesday night into Wednesday as the
tight surface pressure gradient begins to subside. 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 into Friday. DDH/Neuman
&&
.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 from 5 PM this afternoon to 1 PM PDT Tuesday
for Coast Range of Northwest Oregon-Greater Portland Metro
Area-North Oregon Coast-Northern Oregon Cascade Foothills-
Northern Oregon Cascades.
Heat Advisory from 4 PM this afternoon to 11 PM PDT Tuesday 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 from 5 PM this afternoon to 1 PM PDT Tuesday
for Greater Vancouver Area-South Washington Cascade
Foothills-South Washington Cascades.
PZ...Small Craft Advisory until 5 AM PDT Tuesday for coastal waters
from Cape Shoalwater WA to Florence OR from 10 to 60 NM.
Small Craft Advisory from 5 PM this afternoon to 5 AM PDT
Tuesday for coastal waters from Cape Shoalwater WA to
Florence OR out 10 NM.
&&
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