830
FXUS66 KPQR 292245
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
345 PM PDT Wed May 29 2024

.SYNOPSIS...Diminishing showers this evening gives way to dry
weather and a slight warming trend to near 80 degrees through
Friday. A weakening front pushes across the area on Saturday,
pushing temperatures back down to the lower 70s, generally remaining
dry. Then, a much wetter system associated with tropical moisture is
expected Sunday through Monday.

&&

.SHORT TERM...Tonight through Friday night...The axis of an upper
shortwave trough has shifted east of the Cascades this afternoon,
with light rain showers gradually decreasing through early this
evening. Then, high pressure gradually builds across the region
bringing dry weather and northerly flow to the area. A transient
upper ridge moves across the Pacific NW on Friday. Expect this to
bring a warming trend through the end of the work week, with inland
temperatures warming to around 70 on Thursday and upper 70s to near
80 degrees on Friday. Onshore north-northwesterly flow will keep
temperatures at the coast near normal for late May. /DH

.LONG TERM...Saturday through Wednesday...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, but still in the lower 70s. 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, with the general trend toward a mostly dry day on Saturday.

Uncertainty in the overall pattern begins to grow on Sunday as the
remnants of Typhoon Ewiniar are ingested into the westerlies
crossing the Pacific. 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.

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 65-75% chance for
above average temperatures between June 6-12th. 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/DH

&&

.AVIATION...As of 20Z Wednesday, showers continued moving over
the region, primarily northward of a line extending from KTMK to
KSLE. Cigs were mainly VFR across the area aside from brief drops
to MVFR cigs with passing showers, affecting mainly KAST, KPDX,
and KTTD. Have handled this with TEMPO groups with a start/stop
time that reflects the most likely time frame for showers to move
through. This trend will continue through Wednesday afternoon
before showers diminish Wednesday evening, mainly after 00-03z
Thursday.

Northwesterly winds will increase to 10-15 kts around 21-23Z
Wednesday to 04-06Z Thursday for all terminals throughout the
region as the pressure gradients tightens. With possible clouds
breaks, could see a brief/isolated thunderstorm or two over or
near the Portland/Vancouver metro and points north until 21Z
Wednesday (15% chance).

PDX AND APPROACHES...Expect VFR conditions through the TAF period
aside from very brief drops to MVFR cigs with passing showers
through 21z Wednesday. There is a 15% chance for a brief/weak
thunderstorm to impact the terminal until 21z Wednesday. Any
intense shower or thunderstorm would almost certainly bring a
brief dip into MVFR thresholds during that period as well.
Otherwise, northwesterly winds will increase to 10-15 kt between
00Z-05Z Thursday. -TK/JH

&&

.MARINE...Weak high pressure will continue building across the
northeast Pacific into Thursday, leading 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 likely outcome for central/southern
waters is gusts up to 25 kt). Most model guidance suggests winds
will back off a bit for Friday, but there is still a high
probability (>75% chance) wind gusts will peak above 20 kt across
the waters off the central coast of Oregon. As a result, a Small
Craft Advisory remains in effect for zones PZZ273, PZZ272, PZZ253,
and PZZ252 Thursday into Friday morning.

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 (>90% 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 40-70%
chance for at least a brief period of unusually late season Gale
Force southerly wind gusts of 35 kt developing sometime late
Sunday into Sunday night. Probabilities of this happening are
highest off the far north Oregon and south Washington coasts than
areas farther south towards Lane County. -TK/JH

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...None.
WA...None.
PZ...Small Craft Advisory from 3 PM Thursday to 5 AM PDT Friday for
     PZZ252-272.

     Small Craft Advisory from 11 AM Thursday to 11 AM PDT Friday for
     PZZ253-273.
&&

$$

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