-- Discussion containing changed information from previous version are highlighted. --
697
FXUS66 KPQR 182233
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
249 PM PDT Sat May 18 2024

.SYNOPSIS...Mild onshore flow will continue through Monday. Could see
a random shower or two in afternoons/evenings, but little if much in
way of moisture. Next decent front will arrive on Tue, with areas of
rain, then showers behind the front in the cooler unstable air mass
for Tue night and Wed. Overall, looks to remain on cool side and
unsettled through end of next week
&&

.SHORT TERM...
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(Sat afternoon through Monday)...Still have rather
modest onshore flow, as evident by the clouds and cool temperatures.
Average high temperatures for mid-May are around 70 deg inland, and
low to mid 60s along the coast. At 1 pm, many areas still hovering
in the upper 50s to lower 60s. With onshore flow, and bit of
overturning of the marine layer, will have some showers about the
area into this evening. But, rainfall amounts will be on the lighter
side, generally a trace up to a tenth of an inch. Suspect those
heavier amounts will be over the higher terrain, such as Coast Range
and into the Cascades.

Onshore continues tonight. Clouds will gradually dissipate during
the evening. But, another weak upper level disturbance, currently
near Vancouver Island, will arrive and move inland over Washington.
Though most areas will remain dry, it may provide just enough lift
to get a renewed potential of showers later tonight into early Sun.
Again, think best chance of this will be over the higher terrain of
the Coast Range/Willapa Hills, as well as the Cascades.

Onshore flow will maintain cool weather again on Sunday. Like today,
will have areas of marine stratus that will be slow to break apart.
Onshore flow weakens on Monday, as high pressure builds over Oregon.
As such, more sunshine for Monday. And, will be a bit warmer, with
afternoon temperatures back to around average for mid-May.

Will need to watch the Upper Hood River Valley for the potential of
frost each morning through Monday, given the chilly air mass. Latest
NBM probabilistic guidance suggests the best chance of temps reaching
35 deg F or cooler will be early Sunday morning, when there is
roughly a 60% chance of Parkdale being that cold. These probabilities
fall off dramatically north of Parkdale, with only a 10% chance shown
for Odell. For now this coverage/probability does not seem sufficient
to warrant a Frost Advisory.      /WeagleRock

.LONG TERM...Tuesday through Friday...No changes. Unsettled weather
looks to return Tuesday as another upper trough dives S-SE from the
BC coast. The 00z WPC clusters show a variety of trajectories for the
upper trough, with members almost evenly split between an "inside
slider" solution with an inland trajectory versus a track closer to
the coast. The latter would pose a better chance for showers across
the forecast area, and it would be colder - potentially cold enough
to bring some snow to the Cascade passes. The latter solution would
also invite a second system to follow later in the week, prolonging
the cool/unsettled weather. Given the uncertainty in track and
timing, our forecast mostly uses the National Blend of Models
deterministic forecast onward from Tuesday. This shows the best
shower chances on Tuesday, with chances generally decreasing after
that. Lowland high temps are forecast to generally be in the upper
50s along the coast and 60s inland, while the higher terrain sees 40s
and 50s. Some of our cooler valleys may not be totally out of the
woods yet when it comes to frost - NBM shows roughly a 10-20% chance
of lows 35 deg F or cooler for Parkdale each morning Wednesday
through Friday. This would require the upper trough to set up far
enough west to bring a chill to the air mass, but not too far west to
where clouds and precipitation would hold temps up during the
overnight hours.       /Weagle
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&&

.AVIATION...Conditions are expected to remain mostly VFR over the
next 24 hours. The fringes of a surface high over the northeast
Pacific remain across the PNW with an upper level disturbance
continuing to traverse the area into tonight. This will support
continued scattered showers, mainly across the northern coast and
inland areas north of Salem. Significant drops in visibilities
and/or cigs are not expected with passing showers (very brief
periods of high- end MVFR cigs at best). Models suggest a low to
moderate chance (30-50%) of some MVFR clouds building along the
Cascade Foothills Sunday morning which could impact some sites
farther east in the valley. Confidence is not high nor is the
model showing widespread cloudcover so have kept it out of the
forecast for now.

PDX AND APPROACHES...Expect VFR flight conditions into Sunday.
Winds will generally remain out of the northwest sustained between
5-10 kt, however gusts should temporarily increase to around 20
kt this afternoon before relaxing overnight. -Batz

&&

.MARINE...The latest buoy observations show occasional gusts to
around 20 knots along with wave highs around 6-7 ft at 8 seconds.
Winds are expected to become less breezy tonight but
northwesterly swell moving into the waters will maintain choppy
seas around 7-8 ft at 7-8 seconds. Winds will pick back up Sunday
afternoon with 40-70% chance of gusts of 21 kts or greater,
mainly across the outer waters. A Small Craft Advisory beginning
at 00Z for the outer waters is mainly for steep seas and
occasional gusts near 20 kts. The advisory then expands into the
inner waters Sunday morning where steep seas will be possible.
Steep seas may persist past 12Z Monday but confidence wanes this
far out.

The next frontal passage of any significance is set to arrive
late Monday night into Tuesday morning, bringing rain and a shift
to westerly winds. However, wind gusts look to generally stay
around or below 20 kt as the pressure gradient associated with
this frontal passage appears rather weak. The exception to that is
over the northern outer waters where isolated small craft
advisory level gusts to 25 kt are possible (20-50% chance).
-Batz/TK

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...None.
WA...None.
PZ...Small Craft Advisory from 11 AM Sunday to 5 AM PDT Monday
   for PZZ251>253.
     Small Craft Advisory from 5 PM this afternoon to 5 AM PDT
   Monday for PZZ271>273.
&&



$$

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