878
FXUS66 KPQR 211703
AFDPQR

Area Forecast Discussion...Updated
National Weather Service Portland OR
1003 AM PDT Sat Sep 21 2024

.SYNOPSIS...High pressure offshore will maintain pleasant days
and cool nights through MON. A weak front will push across the region
later SUN into early MON, but little if any rain expected.
Afterwards, high pressure rebuilds over inland Pac NW, bringing much
warmer temperatures with offshore flow for TUE. Then, back to more
typical late September temperatures for WED through FRI as onshore
flow strengthens.
&&

Updated Aviation Discussion

.SHORT TERM...(Today through Monday)...
Light offshore flow this morning across the region, giving clear
skies and cool temperatures. Air mass relatively dry, so only patchy
fog or low clouds expected into early this am, mainly along the coast
and inland from Portland northward. Otherwise, not much change today,
as high pressure maintains mostly sunny skies and mild temperatures
with 60s along the coast, and 70s inland.

Onshore flow will increase a tad for Sunday, as a weakening front
approaches. So, will see increasing mid/high clouds through the day.
Lower clouds will gradually increasing along the coast during the
afternoon, with those clouds spreading farther inland Sunday night.
Overall, little chance in temperatures along the coast. But, a few
deg warmer on Sunday as compared to Saturday, given slight increase
in the 850 mb temperatures. With that, highs on Sunday across the
interior mostly in the mid to upper 70s.

Now, a weakening front will push across the region later Sunday
night. But, not much in way of precipitation expected. Will maintain
small (20%) chance of showers for the coast and Willapa Hills, with
any precipitation being quite light (less than 0.05 inch). But, will
have to watch, as may have to push a similar chance on the east
across the Cowlitz Valley to the south Washington Cascades for Sunday
night into early Monday. Otherwise, gradually clearing on Monday,
with light onshore flow. Does appear that temperatures will be
similar to Sunday, with inland highs 75 to 80 for most areas, though
could reach into the lower 80s closer to Corvallis and Eugene. This
due to the low/mid level flow becoming more northerly in the
afternoon, and bit warmer 850 mb temperatures./Rockey

.LONG TERM...(Tuesday through Friday)...
No changes as overall trends look good. Prior discussion as follows:
Next week begins with models in good agreement on an upper level
ridge building over the Pacific Northwest, producing warm and dry
conditions through the first half  of the week.. 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

&&

.AVIATION...VFR conditions with prevail through this evening.
North winds will increase slightly this afternoon for a brief
period with gusts around 25 kt. The greatest chance for these wind
gusts are along the coast near Florence and Newport, and in the
Willamette Valley south of Salem.

A weak front will support stratus returning to the coast late
tonight into Sunday morning. There is a 60% chance for MVFR
ceilings along the coast around 12z Sunday. Light onshore winds
will help the stratus moving up the coastal valleys a bit, but do
not expect much intrusion into the Willamette Valley.

PDX APPROACHES...VFR conditions expected through tonight. There is
a 5-10% chance for MVFR CIGs 15-17Z Sunday.
~TJ

&&

.MARINE...High pressure lingers over the region which will
maintain persistent conditions. Seas are around 6-8 ft at 11-13
seconds and will do so for the next several days. Winds are a bit
more tricky as they are flirting with marginal Small Craft
Advisory levels. Based on current observations have issued a Small
Craft Advisory for the south-central outer waters which will be in
place through the afternoon. Conditions are supported further by
the afternoon northerly wind surge which will increase chances for
gusty winds up to 25 kt in the central outer waters. Will continue
to watch conditions through the day for improvement.

On Sunday through Monday, a weak front will move in but it will
have minimal impact on the marine environment. Clouds will move in
but winds and seas are not expected to respond.
-Muessle

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...None.
WA...None.
PZ...Small Craft Advisory from 5 PM this afternoon to 9 PM PDT this
     evening for PZZ210.

     Small Craft Advisory from 2 PM this afternoon to 10 PM PDT this
     evening for PZZ272.

     Small Craft Advisory until 10 PM PDT this evening for PZZ273.
&&

$$

www.weather.gov/portland

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