High Surf in Oregon, December 7 to 8, 2020

Area Forecast Discussion...UPDATED
National Weather Service Portland OR
946 AM PST Mon Dec 7 2020

Upated Aviation and Marine Sections

.SYNOPSIS...High pressure remains strong over the Inland Northwest
this morning, inducing a bit of offshore flow that will keep the
coast mild and dry today. Meanwhile, valley inversions persist, with
areas of low clouds and fog that will likely be stubborn to burn off
today. High pressure gradually weakens tonight and Tuesday, allowing
a slow-moving cold front to push some light rain southeast across
WA/OR. Additional disturbances will keep a few showers around
Wednesday and Thursday. Longer range forecast models hint at the
possibility of stronger low pressure and/or frontal systems affecting
the Pac NW by the weekend, but details remain sketchy.

&&

.SHORT TERM...Today through Wednesday...It's a quiet morning
weather-wise across SW Washington and NW Oregon as high pressure
prevails over the region. Valley inversions have strengthened
overnight, allowing fog and low clouds to become fairly widespread
across the Willamette Valley (except near the Gorge), Hood River
Valley, and lower reaches of the Columbia River. Webcams and airport
observations show that while fog is certainly dense in some locations
such as Oregon City and Keizer (as of 3 AM), there are also many
areas under the low clouds that have decent visibility. So, thus far,
it's just a typical foggy December morning for our inland valleys -
no Dense Fog Advisory needed. We may reconsider if dense fog appears
to become more widespread over the coming hours.

Meanwhile, east winds have broken through to Troutdale once again as
easterly MSLP gradients ramp up through the Columbia Gorge. KTTD to
KDLS MSLP gradients stood at -4.6 mb as of 3 AM this morning, and
strong gusts of up to 56 mph have already materialized at the Corbett
High School weather station. Suspect similar winds/gusts will persist
through tonight despite the latest GFS/NAM/RAP forecast for weakening
offshore gradients. Models often decrease these easterly gradients
(and their associated winds) too quickly, and this appears to be one
of those cases. The offshore breezes will likely result in another
mild day along the coast, with temperatures pushing 60 degrees in
some areas.

Looking at the bigger picture, a strong but decaying 964 mb Gulf of
Alaska low continues to dominate the circulation pattern over the
north-central and northeast Pacific. An associated frontal zone
extends from the British Columbia southwestward to just NW of Hawaii.
A strong southwesterly jet stream with 150 kt at its core lies nearly
parallel to the frontal zone, with little movement of the front over
the past 24 hours. That said, the jet stream is expected to turn more
westerly tonight and Tuesday, allowing the front to make some
southeastward progress into western WA/northwest OR. As a result rain
will slowly spread down the coast tonight, likely reaching our north
coastal zones by Tuesday morning. The best forcing and deepest
moisture associated with this system will move onshore well to our
north, so QPF should be rather light for our area as the front sags
slowly S-SE across our inland zones. Overall, the coast can expect to
receive 0.25-0.50" from this system... perhaps closer to 1 inch in
the Willapa Hills. While some inland areas may achieve 0.25" from
this system, many will see less than that. With 850 mb temps on the
order of +8 deg C Tuesday, snow levels will be well above the Cascade
passes.

Models then show a weak upper trough swinging eastward across WA/OR
Wednesday, bringing cooler air aloft and perhaps providing enough
instability for a few showers. We kept PoPs fairly low Wednesday
afternoon, as it appears this particular system won't have much
moisture to work with. The cooler air will probably cause snow levels
to dip back down to the Cascade passes Wednesday, but we are not
expecting much accumulation due to the limited moisture.

Large energetic swell originating from a storm south of the Aleutian
Islands should begin to be felt in our coastal waters within the next
few hours. This long period west-northwesterly swell can be expected
to first order breakers between 25 and 30 ft, with the largest waves
expected to arrive later this morning through this evening. Waves
will be slow to decrease tonight and Tuesday. A High Surf Advisory
remains in effect for all our coastal zones from 7 AM this morning
until 7 AM Tuesday morning. Fortunately, astronomical high tides
will be near their lowest of the month, which should mitigate
coastal flood concerns.  Weagle

.LONG TERM...Wednesday night through Sunday...Transient ridge will
bring a brief period of dry weather Wednesday evening. A shortwave
will bring another round of light rain overnight Wednesday into
Thursday. Beyond that, models and their ensembles begin to diverge in
their solutions, but agreement seems to be improving that we will not
be stuck under a blocking ridge as had been suggested by earlier
runs. Much will depend on the chaotic motion of the upper low that
moved through our forecast area Saturday night. This low is moving
into the western Great Basin this morning, and is becoming detached
from the main flow pattern aloft. As such, confidence is lower than
usual in its motion, but models show reasonable agreement that it
will turn south-southwestward and move off the southern CA,
eventually parking itself somewhere near 30N/120W tonight through
Wednesday. The 00z/06z GFS, 00z GEM, and 00z ECMWF all agree that
this will be far enough north that it will eventually be kicked
eastward by the Pacific jet sometime Wed/Thu. This would eliminate
the prospect of a prolonged Rex Block that would likely have kept our
region well drier than normal for mid-December.

Additionally, the Madden-Julian Oscillation is getting into its
active phase over the western tropical Pacific. This often suggests
more active weather for our region about a week later. Thus it is
unsurprising that, with the Rex Block eliminated, models are
increasingly forecasting stronger low pressure and frontal systems
affecting our forecast area this weekend. We are not totally
convinced this will occur yet, but it does appear increasingly likely
that weather will become more unsettled by this weekend, perhaps
continuing into early next week.  Weagle

&&

.AVIATION...Morning satellite imagery showed considerable mid and
high-level clouds streaming across the area. However, areas of
LIFR to VLIFR existed in the central and south interior valleys,
with MVFR conditions in the Gorge around K4S2. Expect the valley
fog to dissipate by early afternoon. IFR or lower conditions
return to much of the central and south Willamette Valley after
03Z Tue and will likely persist through 18Z Tue. There is the
potential for LIFR or VLIFR fog coverage to decrease overnight
with light southeast to south surface flow.

Coastal areas to remain VFR through 08Z Tue, then lower into
MVFR. IFR to low-end MVFR develops along coast after 12Z. The
PDX Metro terminals will continue to show VFR through Tue
morning. Easterly gusts at KTTD will continue 25-30 kts through
at least 12Z Tue, then slowly decrease.

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

KPDX AND APPROACHES...Gorge outflow will help to maintain VFR
conditions at the terminal and vicinity through Tue morning.
Cannot rule out fog or low stratus west of KPDX along the river
to KSPB late tonight through Tue morning. Weishaar

&&

.MARINE...No changes to current forecast. A large upper and surface
low will move south of Anchorage in the Gulf of Alaska today as
it slowly weakens. WW3 generated 43 foot seas which it has sent
toward Haida Gwaii. Seas to 28 feet reached buoy 46005 just
before 15Z. However, forerunners to 26 feet were noted at the
buoy around 12Z. These waves decay to around 22 feet at 20 seconds
at buoy 46029 around 20-22Z. Seas look to hover around 20 ft
through midnight and then continue to slowly subside. Assuming
current model runs are correct, seas would fall below 10 ft sometime
Wednesday evening. Swell and periods of this magnitude present
beach hazards, mainly in the form of high surf and potential sneaker
waves.

Monday night through Tuesday morning, a weakening cold front
moves southeast across the coastal waters. This will bring wind
gusts ranging from 20 to 30 kt, with the highest winds over the
northwest waters. An active pattern continues past mid week, with
another weaker front moving east across the waters Wednesday,
then another slightly stronger front Thursday. Weishaar/JBonk

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...High Surf Advisory until 7 AM PST Tuesday for Central Oregon
     Coast-North Oregon Coast.

     Air Stagnation Advisory until 7 PM PST Tuesday for Central
     Columbia River Gorge-Upper Hood River Valley.

WA...High Surf Advisory until 7 AM PST Tuesday for South Washington
     Coast.

     Air Stagnation Advisory until 7 PM PST Tuesday for Central
     Columbia River Gorge.

PZ...Small Craft Advisory until 6 PM PST Tuesday for coastal waters
     from Cape Shoalwater WA to Florence OR out 60 NM.

     Small Craft Advisory until 4 PM PST Tuesday for Columbia River
     Bar.

&&

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