Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 122243

Area Forecast Discussion...UPDATED
National Weather Service Portland OR
947 AM PDT Sat Jun 12 2021

.SYNOPSIS...Low pressure just east of 40N/140W will turn northward
tonight and Sunday, pushing another slow moving front onshore. Given
the atmospheric river involved with the front, rain may be locally
heavy nearer the coast tonight and Sunday with thunderstorms
possible along the Cascades Sunday afternoon. Showers continue into
Monday as the front lingers, but the front will eventually be pushed
eastward with one final round of showers and possible thunderstorms
Tuesday afternoon. Warmer and drier weather should take hold later
in the week.


.SHORT TERM...Tonight through Monday Night...Finally, a 24 hour
period where the models have not changed the larger scale features
all that much since 24 hours ago. A large longwave trough has
remained off the PacNW coast today. At this hour, a shortwave which
rotated around the sound end of the primary low center has developed
into a small scale closed low about 400 miles off the Port Orford,
Oregon coast. This secondary low is starting to take the upper flow
over the region to a more south to north direction. Embedded
subtropical moisture is also starting to near the area with radar
returns now appearing off the southern Oregon coast. Associated
upstream Band-8 upper level water vapor imagery is perhaps not as
impressively loaded as one would expect which perhaps is a signal
that overall lift through this feature is underperforming the model
intentions. Regardless, there is plenty of moisture appearing on the
band 9 and 10 (mid and lower level) data along with 1.50" of PW
appearing on the blended TPW data feed.

The incoming rain band associated with the shortwave/front will
reach the central coast in the next couple hours. Rain over the next
24 hours will primarily be focused west of I-5. a couple of
additional embedded waves in the flow will shift the core rain band
slightly east and west and enhance rates after midnight tonight and
again sunday afternoon. Otherwise, expect a relatively static
scenario of stratiform rain to bring around an inch of rain broadly
west of I-5 with some local south facing coast range spots pushing
up to 2 inches. East of I-5 am generally expecting 1/2 inch or less
ending Sunday afternoon.

However, continue to expect instability to increase along the
Cascades Sunday afternoon and evening as the second shortwave
arrives with thunderstorms most likely to occur over the high
Cascades. Cannot completely rule out storms occasionally pushing
further west to cover the foothills either. Main concern will be for
heavy rainfall over the burned areas but also from the potential of
storms getting organized and potentially moderate outflow gusts up
to 45 mph. The main limiting factor against the heavy rain affecting
the burned areas will be storms moving fairly quickly at 25-35 mph
in the southerly flow. That will, however, provide a bit on a boost
to the winds as mentioned above.

Storms will likely push slightly east of the Cascade crest Sunday
evening leaving showers over the Cascade west slopes. To the west,
rain rates over the coast range will also drop off as the moisture
feed begins to get severed as high pressure builds to its west.

Stratiform rain then gradually turns to showers across the region by
daybreak Monday. The upper low transitions to a negatively tilted
trough which may bring another afternoon and very early evening set
of thunderstorms to the Cascades. These should be somewhat less
active storms that should move east of the crest fairly quickly in
the evening and a little later for the south central Washington
Cascades. /JBonk

.LONG TERM...Tuesday through Friday...The frontal zone remains
stalled over the Pac NW Monday night and very early Tuesday as the
jet stream remains from the S-SW and parallel to the front. By
Tuesday afternoon, a strong and more zonal push of Pacific jet
energy will plow through Washington and NW Oregon Tuesday, finally
pushing the frontal zone east of our forecast area. This appears to
come with a decent chance of thunder Tuesday afternoon, but mostly
for areas Portland northward. Strengthening shear could lead to some
stronger storms Tuesday afternoon if today`s models pan out, but
much of this activity appears to focus north of our forecast area
except perhaps in Pacific County.

Model/ensemble agreement remains strong that flat ridging will begin
to build into the region as the Four Corners upper ridge strengthens
and expands. Confidence is increasing that temperatures will climb
above normal by Thursday, with highs in the 80s or possibly even
lower 90s by the end of the week. Given our forecasted rainfall
prior to this heat, residual ground moisture may keep dew points
elevated to the point where it will feel more humid than our typical
warm weather.  JBonk/Weagle

.AVIATION...VFR conditions across the area will dominate through
00Z Sunday. A warm front is expected to start impacting the coast
between 00Z Sunday and 03Z Sunday, bringing a mixture of
MVFR/IFR conditions to the coast. KONP could also experience
LIFR conditions after 09Z Sunday. These deteriorating conditions
will likely spread into the Willamette Valley between 03Z Sunday
and 09Z Sunday, likely producing low end VFR/high end MVFR
conditions. As this warm front pushes northward, expect
widespread rain and breezy southerly winds, with gusts along the
coast up to 25 kt. In addition there is a possibility of
thunderstorms Sunday afternoon through the evening along the
Cascades and associated foothills.

For detailed regional Pac NW aviation weather information, go
online to:

KPDX AND APPROACHES...Predominantly VFR flight conditions
through 06Z Sunday. A quick moving warm front may result in
low end VFR/ high end MVFR conditions, along with precipitation
returning after 03Z Sunday. In addition there is a possibility
of thunderstorms Sunday afternoon through the evening along the
Cascades and associated foothills. /42


.MARINE...A broad area of low pressure will continue to drop
slowly southward across the northeast Pacific. In addition,
satellite observations show a distinct secondary low near
43N/134W. This secondary area of low pressure is expected to
lift northward just east of 130W tonight. This will likely result
in gusty southerly winds that will spread northward across the
waters this evening. There still remains some disagreement as to
the track that this secondary low will take. If it comes closer
to the Oregon Coast some marginal gale force winds up to 35 kt
could be experienced beyond 30 nm. The opposite is also true, if
this low stays further west the likelihood of 30 kt gusts
decreases. Given that ensemble guidance has this low traveling
closer to 130W than the coast have chosen to maintain the current
small craft advisories that are currently out for all waters.

Expect seas to become steep and choppy tonight as wave heights
peak around 8 to 10 feet with periods around 8 to 10 seconds.
This storm system will be quick moving with winds most likely
dropping below 20 kt and seas subsiding towards 4 to 6 ft through

The broad area of low pressure mentioned previously will
gradually give way to surface high pressure. This high pressure
will build over the northeast Pacific through the upcoming week.
This will result in winds gradually shifting and becoming
northerly by Wednesday. Seas will also continue subsiding further
towards 3 to 5 ft through the remainder of the week. /42


PZ...Small Craft Advisory from 6 PM this evening to 11 AM PDT Sunday
     for coastal waters from Cape Shoalwater WA to Florence OR
     from 10 to 60 NM.

     Small Craft Advisory from 10 PM this evening to 11 AM PDT
     Sunday for coastal waters from Cape Shoalwater WA to
     Florence OR out 10 NM.



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