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859
FXUS66 KPQR 210450
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
950 PM PDT Fri Sep 20 2024

.SYNOPSIS...
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High pressure offshore will maintain pleasant days
and cool nights this weekend. A weak front will push across
southwest WA late Sunday into Monday morning. Becoming warm and
dry on Tuesday as offshore flow develops. Flow turns back
onshore on Wednesday, bringing relatively cooler temps and
increasing chances for light rain showers late in the week.
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&&

.SHORT TERM...
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Friday night through Sunday night...The forecast
this weekend is highlighted by pleasant daytime temps and cool
nights as high pressure maintains dry weather and mostly clear
skies. Models show a very weak front brushing southwest WA late
Sunday into early Monday morning, bringing a 15-30% chance for
light rain showers. Conditions should stay dry across northwest
OR, aside from the Astoria area where there is also a chance for
light showers (20%). Locations that do experience rain showers
likely won`t receive much more than 0.01-0.03 inches of rain.

How warm will it get during the afternoon each day? Confidence is
high for temps of at least 75 degrees Saturday and Sunday
(80-90% chance each day). While unlikely to occur, cannot rule out
high temps of 80 degrees or warmer each day (10-25% chance, except
20-40% on Sunday which looks to be the warmest day). Conditions will
warm even more early next week as offshore flow develops; this is
discussed below in the long term discussion. -TK

.LONG TERM...Monday through Friday...Next week begins with models in
excellent agreement on an upper level ridge building over the Pacific
Northwest, producing warm and dry conditions through the first half
of the week. Monday looks to be another in the recent string of
pleasant and seasonable September days as the ridge builds overhead,
yielding mostly sunny skies and inland temperatures in the mid to
upper 70s. Tuesday will be the hottest day next week as the ridge
axis shifts inland and an upper trough digs over the Rockies,
bringing about a short lived period of modest offshore flow.
Temperature guidance has nudged slightly upward but remains in
excellent agreement, producing temperatures mainly in the upper 80s
across the interior valleys with little spread noted within the
forecast envelope. To further demonstrate the relatively high degree
of confidence in the temperatures, NBM probabilistic guidance still
only indicates around a 10 percent chance to surpass 90 degrees in
the Portland area and around a 25 percent chance from Salem to
Eugene, even as NBM mean temperature guidance resides in the upper
80s. Offshore flow will also lead to a warm day on the coast on
Tuesday, with most coastal communities likely to warm into the mid to
upper 70s and probabilistic guidance indicating a roughly 10-30
percent chance to reach 80 degrees depending upon the location.

Regarding the potential for east winds as flow turns offshore on
Tuesday, expectations remain tempered by a few factors. The synoptic
pattern is sub-optimal for a stronger wind event as guidance shifts
the ridge axis inland relatively quickly and has trended farther east
with the digging trough. As a result, forecast offshore pressure
gradients remain rather weak (0 to -1 TTD-DLS/-4 to -5 OTH-GEG).
As such, ensemble members are mostly clustered around maximum wind
gusts in the 25 mph range through the Gorge and over higher Cascade
slopes, with the a few of the higher end members maxing out around
35-40 mph in those locations. Will certainly still be worth keeping
an eye on for those in the fire weather community, but the bulk of
the guidance at this time points to a lower end and short lived
offshore flow event.

The forecast becomes more uncertain for Wednesday and Thursday.
This uncertainty is clearly evident via a cluster analysis as roughly
half of the GEFS/ENS/GEPS ensemble space favors weak ridging to
relatively stronger ridging, while the other half favors troughing or
zonal flow. This means there is roughly a 50% chance conditions will
become cooler and wetter and a 50% chance conditions will remain warm
and dry. This uncertainty is also highlighted well by the model
spread of high temps; the NBM 10th-90th percentile ranges from 70 to
90 degrees on Wednesday and mid 60s to the mid 80s on Thursday. Since
neither outcome is favored over the other at this time, have
maintained the deterministic NBM for the official forecast. -CB/TK
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.AVIATION...
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High pressure over the airspace through the weekend.
This is resulting in widespread VFR under predominately clear
skies. North winds around 10 knots will become light and variable.

Clearing skies could result in some IFR/LIFR conditions developing
for locations within the Willamette Valley. However, with the
general warming trend, will have a 20-40% probability of the
IFR/LIFR conditions manifesting with a likely start time around
12Z-14Z Saturday. Any lowered flight conditions that do develop
are expected to be shallow and should scour out shortly after
sunrise Saturday.

PDX APPROACHES...VFR mostly clear skies. 20-40% probability of
IFR/LIFR conditions at or near the terminal starting around 12Z
Saturday. Light north winds around 5 knots. /42-Batz
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&&

.MARINE...
Minimal changes in the marine environment as high
pressure lingers over the area. North/northwesterly winds 5 to 15
kts through the weekend with the gustier conditions through early
Saturday morning.

On Sunday, a weak front will move over the waters. This front is
not overly strong so it will have minimal impact other than
showers north of Tillamook.

The full moon on will produce stronger tidal currents during the
evening ebb cycles. Those moving in and out of harbors and
crossing coastal bars should use caution and be aware of any bar
restrictions in place. /42 -Muessle

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
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OR...None.
WA...None.
PZ...Small Craft Advisory from 5 PM to 9 PM PDT Saturday for PZZ210.
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$$

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