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743
FXUS66 KPQR 282141
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
230 PM PDT Tue May 28 2024


.SYNOPSIS...After some light rain today and a few showers on
Wednesday, drier weather returns Thursday and Friday. A wetter
weather pattern returns over the weekend into early next week.


&&


.SHORT TERM...
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Through Friday...Water vapor satellite imagery reveals
a shortwave trough sliding into the Pacific Northwest this afternoon.
This has resulted in a weak front approaching the I-5 corridor as of
2 PM. There has been just enough surface heating ahead of the front
and frontal forcing to generate a few heavier showers along the
front. With additional surface heating behind the front and
temperatures continuing to cool aloft, a few more showers will
develop across the area this evening. An embedded shortwave trough
sliding down the backside of the parent shortwave trough will slide
southeastward, but mainly bypass the region by staying to our north
and east. Nonetheless, it should bring a reinforcing shot of showers
pushing onto the coast later tonight into Wednesday.

Ensembles systems are in good agreement shortwave ridging will build
into the Pacific Northwest Thursday into Friday. This will bring
drier weather and near to above average temperatures during this
period. /Neuman


.LONG TERM...Saturday and beyond...WPC Cluster Analysis shows there
is strong agreement that a weak shortwave trough will begin to take
hold across the Pacific Northwest on Saturday. There is good
agreement this will result in temperatures cooling on Saturday when
compared to Friday. The main uncertainty in the forecast on Saturday
revolves around the amplitude of the shortwave trough and
precipitation chances tied to a weakening front sliding towards the
region. Either way, any precipitation would be light.

Uncertainty in the overall pattern begins to grow on Sunday as the
remnants of Typhoon Ewiniar are ingested into the westerlies crossing
the Pacific. More on that in a minute. The majority of ensemble
guidance keeps us in zonal flow on Sunday, but our proximity to a
shortwave trough offshore does vary quite a bit between ensemble
members, which influences the amount of rain expected across the
region on Sunday and how far south it extends. Even then, there are
still 10-20% of the ensemble members that build a shortwave ridge
across the Pacific Northwest on Sunday and dry the area out entirely.

Farther out in time, 70-80% of the WPC cluster membership suggests
the shortwave trough digging over the northeast Pacific will remain
close enough to northwest Oregon and southwestern Washington to leave
the region susceptible to a moderate to strong atmospheric river,
particularly across the northern half of the CWA late Sunday into
Monday. While that may sound ominous, we see many moderate to strong
atmospheric rivers over the course of a winter season that bring only
beneficial rains to the region. This appears most likely to do the
same. Nonetheless, there are ~5% of the ensemble members that produce
enough QPF at places like Astoria or Portland that we are not 100%
out of the woods for potential impacts, particularly given many
reservoirs are near their summertime full pool levels and do not have
the flood control capacity that they do in the winter.

Finally, the CPC 8-14 day outlook does suggest a 60-70% chance for
above average temperatures between June 5-11th. While the map looks
ominously hot, average high temperatures are still only in the low
70s right now so a stretch of days in the 80s would count as above
average. There are still fewer than 20% of the ensemble members that
suggest temperatures climbing to near 90F or warmer late next week.
/Neuman
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&&


.AVIATION...
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A weak cold front is currently pushing eastward across
southwest Washington and northwest Oregon this afternoon. As it does
so, rain and low clouds will keep probabilities of MVFR
conditions remaining near or above 50% at any given hour through
~22-23z Tuesday for most TAF sites across northwest Oregon.
Thereafter, daytime heating coupled with low level moisture will
create a more showery weather pattern, which should result in most
locations turning predominantly VFR. Statistical guidance does
suggest a 15% chance for a short lived thunderstorm for KPDX and
KTTD between 22z Tuesday and 02z Wednesday, but high resolution
statistical model output has probabilities less than 10% during
that same time. Given the pattern and high probability of false
alarming, will continue to keep a mention of thunder out of the
official TAFs for now. Nonetheless, any intense shower or
thunderstorm would almost certainly bring a brief dip into MVFR
thresholds as well. High resolution model guidance does suggest
probabilities for MVFR conditions temporarily climbs to 50-60% around
17z Wednesday as daytime heating and low level moisture produce an
increased likelihood of brief MVFR ceilings forming late Wednesday
morning.

PDX AND APPROACHES...Expect a mixture of MVFR and VFR conditions
through ~22-23z Tuesday as a weak front and rain traverse the
region. Conditions should trend to predominantly VFR thereafter.
There is a ~10-15% chance for a brief thunderstorm to impact the
terminal between 22z Tuesday and 02z Wednesday. Any intense shower or
thunderstorm would almost certainly bring a brief dip into MVFR
thresholds during that period as well. /Neuman
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&&


.MARINE...
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A weak front crossed the waters this morning, but has
produced few impacts. Weak high pressure will build across the
northeast Pacific Wednesday into Thursday and lead to increasing
northerly winds across the waters. The strongest winds will be off
the central coast of Oregon. For example, there is a >90% chance for
wind gusts in excess of 20 kt across those waters by Thursday
afternoon and evening. Most model guidance suggests winds will back
off a bit for Friday, but there is still a high probability (>75%
chance) wind gusts will climb above 20 kt across the waters off the
central coast of Oregon on Friday.

There is increasing agreement a more organized and stronger storm
system will move into the northeast Pacific over the weekend.
This will bring a high probability (>80% chance) for winds to turn
southerly and wind gusts to at least climb into Small Craft
Advisory thresholds of 25-30 kt. In addition, there is a 30-40%
chance for at least a brief period of unusually late season Gale
Force southerly wind gusts of 35 kt developing sometime Sunday into
Monday across the waters. Probabilities of this happening are
highest off the far north Oregon and south Washington coasts than
areas farther south towards Lane County. /Neuman
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&&


.PQR WATCHES/WARNINGS/ADVISORIES...

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

&&


$$

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