188
FXUS66 KPQR 211816
AFDPQR

Area Forecast Discussion...Updated Aviation
National Weather Service Portland OR
1116 AM PDT Tue May 21 2024

.SYNOPSIS...Widespread rain returns later this morning,
spreading through the region through the day, which could
lead to a wet evening commute for most areas. Scattered showers
tonight into Wednesday with a thunderstorm chance over the
Cascade foothills. Dry and warmer Thursday before becoming
unsettled for Friday into the weekend.

&&

.SHORT TERM...Tuesday through Thursday...Satellite imagery
around 1 AM Tuesday morning shows mid-level clouds beginning to
push into SW Washington and NW Oregon from the northwest ahead
of an incoming frontal system. The onset of the widespread
stratiform rain band associated with this frontal system has
slowed significantly from previous guidance. Comparing latest
hi-res surface pressure guidance to Monday`s 12z runs, the
amplitude of the surface low has become wider in current run,
which in turn slows the progression of incoming clouds and
rainfall. Rain is now not expected to reach the far NW corner of
the region (generally Pacific County and far northern Oregon
coast) until around 8-10 am, slowly spreading southwest into the
Portland/Vancouver metro area by early afternoon, and reaching
Lane County around late afternoon/early evening. Still expecting
to see quite a bit of rain shadowing in the western Willamette
Valley stretching north into western Columbia County (generally
east of the Coast Range and west of cities such as Portland and
Salem) due to the northwesterly wind trajectory. Also, hi-res
guidance is beginning to indicate the strength of the
precipitation may fizzle as it moves south, so the lowlands and
Coast Range of Lane County may not see as much rain, either.
HREF indicates only a 5-20% chance that these areas could
receive 0.1 inch of rain through 5 AM Wednesday. Elsewhere in
the Willamette Valley has 60-90% probability of reaching 0.25
inch of rain for the same time period, increasing to 90+% for
the lowlands north of Portland, the coast, Coast Range, and
Cascades. The westerly flow will allow for enhanced upslope flow
over the mountains with these areas receiving anywhere from
0.75-1.5 inches of rain. Snow levels will remain above 6000-7000
feet ahead of the front, and with the majority of precipitation
also occurring ahead of or along the front, little to no
mountain snow is expected. Additionally, daytime temperatures
are not expected to fall as much for locations south of a line
from southern Tillamook County to southern Clackamas county.
Interior lowlands north of here will see temperatures in the
upper 50s to low 60s. South of here, temperatures will remain in
the mid to upper 60s with even a 20-40% chance of reaching 70
degrees around and south of Eugene.

Rain turns showery this evening, becoming isolated showers for
areas west of the Cascades overnight. Quickly behind this front
will be an upper closed low moving south from western British
Columbia into western Washington by 5 AM Wednesday then pushing
southeast through northeast Oregon through the day. Latest
ensemble guidance indicates the core of this low will just miss
our forecast area to the north and east, as will the majority of
the forcing for widespread precipitation. With westerly flow
continuing, the majority of precipitation will be over the
Cascades due to upslope flow, with lighter amounts over the
Coast Range. Showers will be very isolated over the lowlands.
Although colder air aloft will funnel in behind today`s front
and Wednesday`s closed low, there doesn`t seem to be quite
enough instability building for thunderstorm chances except over
the western Cascade foothills on Wednesday. However, stronger
showers are possible with the limited instability elsewhere with
the potential for small hail. The high Cascades will also
receive some snow with the colder air, though snow levels are
expected to remain mainly above mountain passes.

On Thursday, the closed low moves east over the northern Rockies
with guidance continuing to show a weak shortwave developing on
the backside of the low over the PacNW as ridging builds over
the eastern Pacific. Temperatures will warm back into the upper
60s for the interior lowlands under sunny skies. No rain is
expected with this wave, but it will cause winds aloft and at
the surface to turn north to northwesterly for a brief period
Thursday. Winds are expected to remain relatively light with the
only impact to potentially help enhance the warming
temperatures. -HEC

.LONG TERM...Friday through Monday...An unsettled weather
pattern is expected to continue through at least Saturday. WPC`s
500 mb cluster analysis indicates a week shortwave could bring
light showers Friday followed by a an upper level closed low
bringing slightly more showers to the region Saturday. Specific
details on timing and amounts of rain continue to remain
uncertain, though temperatures are expected to remain near or
just below seasonal normals.

The pattern becomes a bit more uncertain Sunday on whether
troughing will still around to continue showers. By Monday, dry
conditions are expected. However, about 60% of the 500 mb
clusters indicate strong ridging building right over the Pacific
NW on Monday, about 27% of the clusters indicate this ridging
building more east over the Intermountain West, and 15% of the
clusters indicate a weak trough approaching the Pacific NW. This
brings significant uncertainty in temperatures with NBM 25th to
75th percentiles for inland valleys ranging anywhere from upper
60s to low 80s. -HEC

&&

.AVIATION...The first bits of rain from the next front are
beginning to move in, with rain showers starting to begin at
northern areas as of 18z Tue. KAST is already seeing IFR cigs,
with ceilings having dropped below 1000 ft, though overnight fog
is starting to rise. By 22z Tue to 00Z Wed, the front pushes in
and the rest of the area should drop to high end MVFR cigs (70%
confidence). Coastal areas see at least a 80% chance for IFR/LIFR
conditions at that time. Southern terminals may remain low-end
VFR, as precipitation mostly passes to the north and southern
terminals may not be affected.

Between 06-10z Wed, conditions transition to post-frontal, and
most terminals will trend back to VFR, with the valley
transitioning faster. By 09z Wed, even the coast only sees 30-40%
chance of MVFR or lower ceilings. West/southwesterly winds will
also begin to pick up around that time, with gusts for most areas
up to 20 kts (25-30 kt at KAST).

PDX AND APPROACHES...VFR conditions look to degrade to MVFR around
23z Tue (80% confidence in MVFR or lower cigs). Winds gradually
turn W as the front passes through, though top gusts should only
be around 10-12 kt. Conditions trend back to VFR around 08z, with
less than 10% chance of MVFR conditions continuing past that time.
Winds will pick up slightly, with top gusts up to closer to
15-16 kt from the west at that time. /JLiu

&&

.MARINE...The morning begins in a brief lull between weather
disturbances before the next frontal system arrives as the day
progresses bringing rain and Small Craft Advisory Winds ahead and
behind the front. Confidence is very high (80-90%) wind gusts
reach into the 20-25 knot range for the outer waters north of Cape
Foulweather later Tuesday morning spreading southward to PZZ273
and persisting at times through Tuesday night and Wednesday. With
the persistent northwest fetch in the post-frontal environment,
also expected seas to become fairly steep Tuesday night into
Wednesday with greatest impacts to PZZ251 as seas rise to 10 to 13
ft with a period 9 to 10 seconds.

As we move into Thursday, calmer winds/seas return for a day followed
by yet another weather disturbance moving into the region from the
NNW late Friday/Saturday continuing the rather progressive and
active weather pattern. Seas likely rise to 7-10 feet at 8-9
seconds on Saturday before decreasing Sunday into early next
week - wave steepness may again be a concern Saturday.
-Schuldt/mh

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...None.
WA...None.
PZ...Small Craft Advisory until 5 PM PDT Wednesday for PZZ210.

     Small Craft Advisory until 5 PM PDT Wednesday for
PZZ251-252-271-
     272.

     Small Craft Advisory from 8 PM this evening to 5 PM PDT
     Wednesday for PZZ273.
&&

$$

www.weather.gov/portland

Interact with us via social media:
www.facebook.com/NWSPortland
www.twitter.com/NWSPortland