613
FXUS66 KPQR 252149
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
249 PM PDT Sat May 25 2024

.SYNOPSIS...Broad low pressure slowly erodes through Sunday.
High pressure still on deck for Monday, though it is quick
moving as yet another broad low and longwave trough drops down
shifting the ridge eastward. Zonal flow will dominate. Another
round of rain expected on Tuesday through the the remainder of
the week.

&&

.SHORT TERM...Now through Monday Night...The short term forecast
is frankly on the boring side as far as weather is concerned.
The ruminates of the cooler front continue to produce very
light showers over the region with accumulation being minimal
and mainly orographically driven. There are a few weak
shortwaves that are moving through the overall zonal onshore
flow on Sunday which could prolong these light showers through
the afternoon. However, for those looking for a break in the
rain and some sun on this Memorial Day, then we have good news
for you. While clouds will remain, a transient ridge will form
over the state of Oregon. We sit right at the peak of that ridge
so, while conditions will be warmer than the last several days,
not looking at anything exceptional. Higher resolution models
are showing near normal temperatures. The NBM is putting the
mean forecast around 73 degrees F for much of the Willamette
Valley and in the mid-50s for the coast. If we want to look at
what I call the "best case scenario" (aka the warmest
solutions), the 90th percentile is producing temperatures of
75-78 degrees F for the inland lowlands including areas within
the Columbia River Gorge. The Upper Hood River Valley will again
hover in the mid 70s. Looking at the probability spread though
it is narrow which means that there is high confidence in this
"mean" forecast.

This sun will be short-lived though because yet another low
pressure system will drop down from the Gulf of Alaska Monday
night into Tuesday. -Muessle


.LONG TERM...Tuesday through Saturday...Tuesday has seen little
change from this morning`s forecast as models have really hung
their hat on this trough pattern. Ensemble clusters once showed
this pattern as the outlier but now have transitioned it into
being the winning outcome. Cooler temperatures and another round
of rain is on deck for the entirety of northwest Oregon and
southwest Washington on Tuesday through Thursday. Looking at the
deterministic long range models, one would suggest a drier
pattern. But if you look at the culmination of the
probabilities, this will be more of a localized stratiform rain
scenario with orographically driven higher amounts. This high
probability low accumulation scenario spills over into
Wednesday. Thursday will resemble today though in the isolated
nature of the rain and accumulation. Most of these systems will
track to the north as we sit right at the base of the trough vs
being in the thick of it.

One challenge in the long term is temperature forecast -
specifically the high temperature component. The NBM is showing
a wides spread of temperature probabilities. For example, in
Portland the high on Tuesday ranges from 62 deg F (10th
percentile) to 75 deg F (90th percentile). Thursday shows even
more of a spread as the pattern begins to shift between this
trough and a ridge forming on Friday. -Muessle


&&

.AVIATION...Showery conditions persist from KSLE northward with
heaviest rain along the coast. With passing showers expect periods
of gusty winds and possibly reduced visibility. Some IFR CIGs
have been reported around KAST this morning with the passing
showers. Otherwise, VFR CIGs and VIS expected over the next 24
hours. With sun breaks beginning to peek through the overcast
skies, it will allow for more radiational cooling at night and
thus, increased chances for fog. High resolution models and
forecast soundings are showing around a 20-30% chance of fog with
at least MVFR CIGs in the lowlands from KUAO southward including
KCVO and KEUG. However, confidence is low so have only included
the mention of it in the TAF. Will have to continue to watch these
conditions as yet another weak disturbances passes through the
atmosphere after 10Z Sunday. This disturbance will increase
mixing, and could bring another round of non-impactful rain.
Winds will be west-northwest through the forecast.

PDX AND APPROACHES...VFR for the next 24 hours with some lingering
showers through 00Z Sunday. Reduced VIS/CIGs possible in passing
showers but will be short-lived. Otherwise, no weather concerns.
-Muessle

&&

.MARINE...Minimal changes to the forecast as conditions have not
changed significantly. The front that has brought showers and
locally breezy winds is shifting inland which will cause winds to
slowly ease. Seas are hovering right around the 6-8 foot mark at 9
to 10 seconds near Astoria Canyon, but further south seas are much
lower. Likely due to the influence of the onshore flow with the
westerly primary swell. Through the day, showers will come to an
end and high pressure builds on Monday. Another area of low
pressure moves southeastward from the Gulf of Alaska late Monday
night into Tuesday. While this is an area of low pressure, it is
not overly deep and is generally staying more north. Thus, we will
experience very little impact from this incoming system. Will
still see generally onshore flow through the forecast period.

Late in the week there is a chance of more of a southerly wind
which could bring winds to around 15 to 20 kt with gusts to 25 kt.
However, they are not appearing to be widespread. -Muessle


&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...None.
&&

$$

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