High Surf Advisory 4, NWS Portland, OR

Area Forecast Discussion
National Weather Service Portland OR
815 PM PST Mon Jan 11 2021

.SYNOPSIS...A quasi stationary front is expected to produce periods
of moderate to heavy rain through Wednesday. This excessive rain
will likely result in some flooding across the region. Snow levels
will be well above the Cascades passes. Gusty winds are expected
along the beaches tonight and Tuesday night with breezy conditions
for the Willamette Valley possible early Wednesday. Large surf and
high tides will result in hazardous conditions for the beaches and
low-lying coastal areas through Wednesday. Another front is possible
Thursday night and Friday with another very weak front Saturday
night.

&&

.SHORT TERM...No changes since the 409 PM Monday discussion. Tonight
through Wednesday...See the Hydrology section below for additional
information regarding flooding concerns.

A large and broad upper low pressure center continues to funnel a
very moist stream of sub tropical moisture (atmospheric river/AR)
toward the PacNW from south of Japan. The first developing short
wave along the flow is serving to slow the southward progression a
little more than models showed during the previous shift. This
shortwave is also producing impressive vertical motions as evidenced
by transverse banding on the infrared satellite imagery. It's now
more likely that later tonight, the AR and the baroclinic zone will
slide south and phase in with an enhanced PW region to shift the
primary focus of heavy rain south across NW Oregon and SW Washington
from Lincoln City and Mt Hood northward. Will not be surprised to see
total rain exceed 2.50 inches across some Coast Range and Cascade
locations between midnight and daybreak tonight. Snow level will
rise well above passes tonight as well so expect most of the higher
elevations will see that fall as rain as well. Areas to the south
will also see rain rates increase but not as extreme with highest
amounts around 1.30" or less near daybreak.

Additionally, broader surface low pressure with the shortwave is
resulting in a strong offshore component to the pressure fields.
When another shortwave arrives later this evening, do expect a few
hour period where winds along the coast will see the winds better
mix down and bring a few hour period with gusts mainly 40-50 mph and
occasional gusts 50-55 mph. If everything comes together perfectly,
then may see some gusts in the coastal lowlands reaching 55-65 mph.
Do not have enough confidence to issue a high wind warning based on
this scenario at this time.

Trailing shortwaves in the flow will see the heaviest rain focus
vacillate back north through Tuesday under the moderate AR flow.
While rainfall rates will ease a bit, the firehose will remain aimed
across the NW half of the CWA through Tuesday evening. Then, a
modestly larger low pressure wave will start pushing the baroclinic
zone more definitively south. This will then bring the highest rain
rates to the central coast and Cascades. This wave will then further
result in a bit of a pivot late Tuesday night/Wednesday taking the
centerline from more of a SW to NE alignment to then become more SSW
to NNE and along the Cascades/foothills. This will effectively close
the longer pipeline and serve to cut off the high precipitation
rates as the cold front starts to move across the area. Tuesday
evening as this gets underway, expect another burst of wind along
the coast, perhaps a little stronger than tonight or perhaps not as
the surface details remain frustratingly inconsistent. Low strength
and winds have weakened on this second event over the last 24 hours.

Finally, the cold front does move across the region during the day
Wednesday pushing the flow more south to north and limiting the
precip to the Cascades. Snow levels do lower close to 3000 feet by
mid-day Wednesday, however, given the decreased influx of moisture,
only expect a few inches of snow for the higher elevations of the S
WA and N OR Cascades.

All told, expect 1.5 to 3 inches of rain for the interior valleys, 3-
6 inches for the coastal lowlands, and 5-10 inches for the coast
range and northern Cascades ending Wednesday morning.

Beach hazards will also continue through Wednesday, with a
combination of tidal overflow, the potential for sneaker wave
conditions, and possibly high surf on Tuesday. The highest tides are
expected Tue afternoon. The forecast total high tide at Toke Point
is 11.5 feet Monday afternoon, 12 feet Tuesday afternoon, and 11
feet Wednesday afternoon. This will also have impacts on coastal
flooding concerns as addressed in the hydrology section below. /JBonk

.LONG TERM...No changes since the 409 PM Monday discussion.
Wednesday night through Sunday...Wednesday night through Thursday
evening will bring a distinct break in the action as upper ridging
pushes north behind Wednesday's cold front. Another weak front
pushes across the area late Thursday night and early Friday. Models
show highest amounts from the faster moving shortwave and front will
max at 0.50 inches. This should allow river to continue receding.
Another shortwave clips the far northern CWA Saturday night and
brings little rain, especially as compared to the upcoming event.
Then, the ridge stays fully in place for Monday for further help
against any rising flood waters. /JBonk

&&

.HYDROLOGY...No changes since the 409 PM Monday discussion. Given
all the rain mentioned above, do expect more than a few rivers will
rise toward flood levels beginning late tonight and early daylight
hours Tuesday. The flood watch will remain in effect. Main concern
will start for the SW Washington rivers later tonight and then shift
southward with north coast and coast range rivers rivers making
significant rises toward flood Tuesday morning through Tuesday
night. Given the orientation mentioned above, do expect flooding
threats for the east slope coast range and Cascade foothill rivers
as well. With the higher tides mentioned above, do expect this will
exacerbate any flooding which occurs along the coastal rivers,
especially on Tuesday when high tides, rising rivers, and up to 1.5
feet of storm surge will come together around midday Tuesday.
/JBonk

&&

.AVIATION...A very active pattern for this forecast package.
A warm front continues to slowly shift eastward off the Pacific and
is expected to move inland late tonight. A southeast moving cold
front follows, but likely not moving in until late Tuesday. As rain
persists through the forecast period, expect MVFR conditions to
become widespread by 10Z-14Z, with the south Willamette Valley
probably the last to hang on to VFR conditions Monday night.

Southerly winds should widely begin to intensify through the
evening after 06Z Tuesday reaching their peak speeds after 12Z
Tuesday. Highest winds are expected along the coast, especially
near KONP where they will gust around 45 kt or even higher. Inland
areas though will experience gusts around 25 kt, except through the
Columbia River Gorge. Strong pressure gradients will allow for the
gusty winds to continue through the late morning around 18Z Tuesday.
Should mention that winds aloft around 2000-3000 ft will be
stronger with speeds around 40-50 kt from the southwest. Could see
some LLWS in places, especially early on with the warm frontal
passage as the winds at the surface are more southeasterly.

For detailed regional Pac NW weather information, go online to:
http://weather.gov/zse

KPDX AND APPROACHES...VFR conditions will give way to prevailing
MVFR conditions through 08Z, then will likely continue as MVFR
through Tuesday evening as rain continues. There is a chance of IFR
conditions after 09Z when rain may become heavier at times. Could
see some LLWS between 05Z-12Z Tuesday but confidence is low due to
the lack of directional shear.

&&

.MARINE...A warm front will move east across the waters late
tonight, then a cold front is expect to move southeast across the
waters late Tuesday and Tuesday evening. Southerly gales were
already established Monday evening, and are likely to continue into
Tuesday. Tuesday afternoon and evening winds will slowly ease ahead
of the front, before switching to the west and dropping below gale
force behind the cold front Tuesday evening. A few gusts to storm
force are not out of the question, most likely south of Tillamook
Head. Seas will build tonight into Tuesday as the westerly swell
builds by a few feet and periods lengthen. At the same time locally
driven wind waves and fresh southerly swell will build quickly
overnight and continue Tuesday before subsiding again Tuesday night.
This will drive rather chaotic seas to heights ranging from 22 to 26
ft at their peak Tuesday.

Seas and winds quiet down Wednesday into Thursday as high pressure
moves across the waters. Late Thursday into Thursday night winds
pick up again, likely into Small Craft category, as a weaker front
moves across the waters. Expect seas to drop below 10 ft Thursday,
but then are likely to rebuild some Friday as another westerly swell
arrives.

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Flood Watch through Wednesday morning for Cascade Foothills in
     Lane County-Cascades in Lane County-Central Coast Range of
     Western Oregon-Central Columbia River Gorge-Central Oregon
     Coast-Central Willamette Valley-Coast Range of Northwest
     Oregon-Greater Portland Metro Area-Lower Columbia-North
     Oregon Coast-Northern Oregon Cascade Foothills-Northern
     Oregon Cascades-South Willamette Valley-Western Columbia
     River Gorge.

     High Surf Advisory from 4 AM to 5 PM PST Tuesday for Central
     Oregon Coast-North Oregon Coast.

     Coastal Flood Advisory until 4 PM PST Wednesday for Central
     Oregon Coast-North Oregon Coast.

WA...Flood Watch through Wednesday morning for Central Columbia
     River Gorge-Greater Vancouver Area-I-5 Corridor in Cowlitz
     County-South Washington Cascade Foothills-South Washington
     Cascades-South Washington Coast-Western Columbia River
     Gorge-Willapa Hills.

     High Surf Advisory from 4 AM to 5 PM PST Tuesday for South
     Washington Coast.

     Coastal Flood Advisory until 4 PM PST Wednesday for South
     Washington Coast.

PZ...Gale Warning until 4 PM PST Tuesday for coastal waters from
     Cape Shoalwater WA to Cascade Head OR out 60 NM.

     Gale Warning until 10 PM PST Tuesday for coastal waters from
     Cascade Head OR to Florence OR out 60 NM.

     Small Craft Advisory until 10 PM PST Tuesday for Columbia River
     Bar.

&&

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