Wildfire in Oregon, August 6 to 31, 2023

Area Forecast Discussion
National Weather Service Portland OR
339 AM PDT Sun Aug 6 2023

.SYNOPSIS...Isolated to scattered showers continue today, mainly over
southwest Washington and the Cascades. Slight (15-25%) chance of
thunderstorms in the Cascades this afternoon. Dry weather returns
Monday to Tuesday, but a frontal system will increase chances of
precipitation Tuesday night into Wednesday. High pressure builds
Wednesday evening onward, leading to warmer temperartures through the
end of the long-term forecast. Increasing potential for the hottest
temperatures of the year next week.

&&

.SHORT TERM...Sunday to Tuesday...As of early Sunday morning, a
closed upper low remains centered over the Columbia Basin, producing
isolated light showers mainly over the south Washington Cascades. The
latest deterministic models (GFS/Euro/Canadian) and their ensembles
suggest that this upper low will move east and begin to weaken today.
Since we'll still be within the influence of this disturbance, expect
some isolated/scattered shower activity to continue today over
southwest Washington and the Cascades. Therefore, the forecast
includes a 15-40% chance of PoPs for these areas (highest chances
being in the south WA Cascades). With daytime heating, could see more
surface-based initiation of thunderstorms over the Cascades, but
chances are low (15-25%).

Sunday night into Monday, deterministic models and their ensembles
show a negatively-tilted shortwave trough moving into the Pacific
Northwest. Though guidance still shows mainly dry conditions over our
forecast area, HREF does suggest more mid to high level cloud cover
over the region Monday morning. If these clouds persist longer than
mid-morning, then wouldn't be surprised if Monday's high temperatures
struggle to climb above the mid 80s in the Willamette Valley.
However, models show ridging building by Monday afternoon and
lingering through Tuesday, so expect slight warming and more clearing
skies then. Based on NBM 4.1, Tuesday highs have risen a few degrees
from the last couple days, with temperatures now forecast in the mid
to upper 80s for inland valleys and high 60s to low 70s for the coast
and Cascades. This is because models have now pushed the timing of
the next frontal system to late Tuesday night to Wednesday.
                                                        -Alviz

.LONG TERM...Wednesday through Saturday...Deterministic guidance and
their ensembles are in agreement of a frontal system moving into the
Pacific Northwest Tuesday night into Wednesday morning. This will
lead to precipitation chances increasing, with PoPs of 10-35% inland
and 30-60% chance along the coast and Coast Range. The highest
chances are found across southwest Washington and gradually lower to
the southeast toward Lane County. There still remains some
uncertainty with how far inland the front will push and the amount of
rain associated with it. The current NBM 4.1 run shows a 35-50%
chance of a quarter of an inch or greater of rain in the Astoria to
Pacific County area, a 10-20% chance from Salem to Portland, and a
less than 5% chance near Eugene.

Thursday onward, ensemble members are in good agreement that high
pressure will begin to build aloft. This will warm the interior
valleys back up to the upper 80s to low 90s through the end of the
long term forecast. Further into next week, the majority of global
ensembles are suggesting the hottest temperatures of the year so far.
The NWS Climate Prediction Center suggests a 70-80% chance that
temperatures will be above normal between August 13 to August 19.
Normal temperatures are currently in the low to mid 80s for inland
valleys. The main uncertainty in the temperature forecast centers
around the ridge axis and how that will influence how many degrees
above normal we get. NBM 4.1 currently shows the following
probabilities for the Willamette Valley:

Aug 13 probability of exceeding 100 degrees: 25-35%
Aug 13 probability of exceeding 110 degrees: 2-6%

Aug 14 probability of exceeding 100 degrees: 50-65%
Aug 14 probability of exceeding 110 degrees: 5-10%

Aug 15 probability of exceeding 100 degrees: 45-50%
Aug 15 probability of exceeding 110 degrees: 5-15%

With high-impact heat within the realm of possibilities, now is the
time to make a plan for how you and your loved ones will stay safe
and cool during a heat wave. Heat safety tips include: drinking
plenty of water, wearing light clothing, limiting strenuous outdoor
activity, spending time in air conditioned places, and checking-in on
others.      -Alviz

&&

.AVIATION...12z TAFs: Little change over the past 24 hours as
broad and nearly stationary upper low remains over WA/OR. This
system has brought occasional -RA in high-based showers, with
inland areas mostly VFR and expected to remain so through this
evening. The one significant change over the past 24 hours is the
gradual increase of onshore MSLP gradients and thus onshore flow.
As a result, low MVFR cigs are having an easier time spreading up
the Columbia, having reached KKLS by 07z and beginning to round
the bend toward KSPB as of 0930z. Latest hi-res models suggest a
40% chance of low MVFR cigs reaching the PDX metro, mainly east of
the Willamette River. Same models show a 80-90% chance of these
cigs clearing by 18z, with VFR thereafter.

Latest GOES-West imagery is difficult to interpret given the
broken lower and higher ceilings. Based on webcams and surface
observations, it does appear LIFR is fairly widespread along the
Oregon Coast, and there has been occasional -RA/-DZ. Some of this
is from showers associated with the higher cigs, some is being
wrung out of the coastal stratus. Widespread coastal IFR/low MVFR
will be stubborn to burn off again today, but the marine layer
will probably be a little shallower today due weak shortwave
ridging aloft. This should allow the marine layer to back off from
much of the coast for a few hours before returning with the
afternoon seabreeze around 00z or shortly thereafter.

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

KPDX AND APPROACHES...VFR with high-based cigs expected through
early afternoon, then thinning thereafter. Currently seeing about
a 30-40% chance of low MVFR cigs as marine stratus attempts to
push into the PDX metro 12z-18z this morning.  Weagle

&&

.MARINE...A decaying front will cross the waters disrupting the
tight gradient that brought breezy northerly winds over the waters
this evening. Expect winds in the 5 to 10 kt range Sunday into
Monday morning. High pressure will attempt to reestablish Monday
for a brief period before another weak front approaches the waters
Tuesday. Winds will become southerly Wednesday but no impacts are
expected.

Swell remains minuscule over the coastal waters over the next few
days, with seas likely 5 ft or less and primarily wind-driven
until modest W-NW swell associated with Gulf of Alaska low
pressure moves into the Pac NW waters midweek.

Dense fog was apparent on webcams at Newport Harbor and some other
cams along Oregon's central coast. This corroborates well with
the latest obs from Newport airport and latest model expectations,
so we issued a Dense Fog Advisory for PZZ253 until 10 AM this
morning.  Weagle

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...Dense Fog Advisory until 10 AM PDT this morning for Central
     Oregon Coast.

WA...None.

PZ...Dense Fog Advisory until 10 AM PDT this morning for Coastal
     waters from Cape Foulweather to Florence OR out 10 NM.
&&

www.weather.gov/portland

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