Wildfire in Washington, September 22 to 30, 2022

Area Forecast Discussion
National Weather Service Portland OR
229 PM PDT Thu Sep 22 2022

.SYNOPSIS...Onshore flow will produce seasonable temperatures through
Friday. High pressure to build over the weekend leading to an
increase in temperatures. Low pressure out of the Aleutians will drop
into the Pacific NW midweek bringing a slight chance for light rain
across parts of the area.

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.SHORT TERM...Tonight through Saturday...Onshore flow has brought a
deep marine layer inland with areas west of the I5 corridor finally
starting to breakup as of 2 PM. Forecasted high temperatures have
been brought down a few degrees. A stray shower or two is
possible anywhere in our CWA as daytime mixing turns over this deep
marine layer, but most areas west of the Cascades should stay dry.

Weak ridging aloft will lead to slightly drier boundary layer and
less cloud cover Friday morning relative to today. High temperatures
should be able to climb into the mid 70s across the Willamette Valley
and mid 60s along the coast. The remnants of a decaying front will
sag down along the BC coastline and into the Washington coast Friday
morning. Quite a few ensemble members from both the 12Z GEFS and EPS
show this rain making its way far enough south to produce light
accumulations along the coast Friday. It's unlikely that any of this
precipitation will make its way east of the Coast Range.

A broad area of high pressure will start to build across the West
Coast Saturday and remain into early next week. An upper level
disturbance will make its way through the ridge and over Oregon
Sunday into Monday but no thunder activity is expected. If anything
it'll just produce some upper level clouds as it passes.

.LONG TERM...Sunday through Thursday...Ridging will persist producing
unseasonably warm and dry weather into early next week. All clusters
continue to show significant positive 500 mb height anomalies Sunday
with nearly all members showing 500 mb
heights in the 580-590 decameter range over the Pac NW. Weak
shortwaves lifting northeastward over the NE Pacific look to begin
eroding the ridge Monday or Tuesday, which should eventually deepen
the marine layer and bring a decent southwesterly marine push into
our CWA by midweek. But until then, temps will be several degrees
above seasonal norms, with 80s appearing likely for our inland
valleys Sunday through Tuesday before the stronger onshore flow
begins to return temps to more seasonable levels later in the week.

Confidence is high that no significant, widespread rain event is
expected through early next week. In fact, roughly 40 percent of the
00z ECMWF ensembles and about half of the 00z GEFS members suggest we
may have to wait until October for our first widespread frontal rain
event. That said, the Madden-Julian Oscillation appears to be getting
more into the active phase over Indonesia per the latest CPC 5-day
running mean cloud cover plot. This can suggest more active weather
as we head into Week 2, which means there may be an end in sight to
our dry spell west of the Cascades. Stay tuned though, as tropical
activity can certainly complicate this relationship.  Weagle

&&

.AVIATION...Generally northwest flow aloft with onshore flow in
the lower levels. VFR conditions inland expected to continue
through the remainder of the day with skies clearing later this
afternoon. Another round of marine stratus becomes more likely
after 12Z Friday, while HREF guidance suggests anywhere from a
40-80% chance of MVFR cigs briefly developing. The marine stratus
along the coast will likely hang around through much of the day,
though with some breaks into this evening, with a mix of MVFR and
VFR conditions. Cigs should lower to MVFR along the coast by 00Z
this evening. Guidance suggests a 60% chance that IFR conditions
develop at KONP by 03Z Friday with possible LIFR cigs along the
central Oregon coast overnight. Widespread VFR with skies clearing
is expected by late Friday morning into the afternoon.

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

KPDX AND APPROACHES...VFR expected through this evening. Latest
guidance suggests a 40% chance for MVFR cigs to redevelop around
12-14Z Friday, but would only last a few hours at most. Southwest
wind becoming northwest this evening around 5-7 kt. /DH

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.MARINE...A weak cold front dropping from the Gulf of Alaska will
approach the northern waters on Friday while surface high pressure
builds in from the southwest. Southerly winds around 10-15 kt will
gradually through this evening, especially farther south across
the central waters. Winds turn more northerly through Friday night
and then are expected to increase on Saturday as surface high
pressure offshore strengthens and the surface thermal trough
redevelops along the southern Oregon coast. Northerlies with gusts
up to 25 kt may be possible by late Saturday afternoon. Expect
seas to generally remain in the 3-5 ft range through the remainder
of the week and into the weekend. The main exceptions to this will
be Sunday when seas will likely build towards the 5-7 ft range
driven mainly by a northwest swell with a period of around 11-12
seconds. /DH

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

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.PQR WATCHES/WARNINGS/ADVISORIES...

OR...None.
WA...None.
PZ...None.

&&

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www.weather.gov/portland

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