High Surf Advisory 5, NWS Portland, OR

Area Forecast Discussion
National Weather Service Portland OR
242 AM PST Thu Feb 25 2021

.SYNOPSIS...The next Pacific system and associated front will bring
valley rain and heavy mountain snow through at least Friday night.
Precipitation will gradually diminish over the weekend with most
locations getting a break from precipitation as ridging develops late
in the weekend. Precipitation is likely to return by early next week.

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.SHORT TERM...Today through Saturday...A Pacific trough and
associated front continues to move on shore through the daytime
hours. The initial frontal band of precipitation will bring heavy
snow to the Cascades and rain to the Valleys, followed by an extended
period of moist, cold onshore flow. 850mb temperatures associated
with this primary front look to be between -4C to -6C which will
result in lowering snow levels to around 2000-3000 feet.
Colder 850mb temperatures around -5C to -7C along with slugs of
moisture will also move across the area through Friday and result in
periods of moderate to heavy snowfall along the Cascades. Short term
guidance combined with QPF trends seen in the GEFS plumes show an
increasing trend in QPF. Using these tools along with pattern
recognition suggests that areas above 2500 feet for the south
Washington Cascades and 3000 feet for the northern and central Oregon
Cascades could see upwards of 3 feet of snow accumulation by Saturday
morning. The frontal system will sag south on Friday towards the Lane
County Cascades, resulting in the heaviest accumulations for the
south Washington Cascades occurring today into Friday and for the
Oregon Cascades tonight through Friday night. The bulk of the
precipitation will be over by Saturday with the trend continuing that
way through Saturday night. /42

Tonight expect a large west swell around 20 ft with a period of
16 seconds will reach the coast which will continue through
Friday afternoon. As a result a High Surf Advisory has been issued
for this event. /mh

.LONG TERM...Saturday night through Thursday...Onshore flow will
continue Saturday night and is expected to gradually diminish through
Sunday brining a brief drier period to most of the CWA. After Sunday
night confidence becomes very low as the GFS, Canadian, ECWMF in
addition to the GEFS Plumes differ significantly in model outputs.
Therefore, will keep with the NBM solution, which brings
precipitation back into the forecast by Monday with a momentary break
Monday night into Tuesday. Precipitation returns towards the middle
of the upcoming week and will linger through Thursday. /42

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.AVIATION...12Z TAFS: Radar showing rain reaching the central WA
coast with MVFR conditions at KHQM. Rain and lowering conditions
expected to the KAST the area by 12Z with MVFR to possibly IFR
conditions at the coast. After about 18Z should see some
improvement but likely be a mix of VFR and MVFR in post frontal
showers. Also gusty westerly winds will follow the front with
gust 25 to 30 kt expected into the evening hours.

For inland areas: MVFR conditions to inland TAF sites 15-18Z as
the front moves through. Inland areas should see an improvement
to a mix of VFR and MVFR towards 00Z Fri as westerly flow
increases across the coast range, providing some rainshadowing
across the valley. Higher terrain becomes obscured this morning
with frontal passage and then will be frequently obscured in the
post-frontal air mass due to strong westerly orographic flow. /mh

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

KPDX AND APPROACHES...VFR through 14Z Thu or so, then mainly
MVFR likely from 14Z through 20Z, followed by a mix of MVFR and
VFR with showers for the remainder of the TAF period. Gusty
westerly winds develop 18-20Z in the area then gradually subside
during the evening. /mh

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.MARINE...Winds will turn more southerly and increase this
morning as a quick moving front crosses the waters this morning.
The front still expected to push across the coastal waters
between by 10 am Thu, with an abrupt change to more westerly
winds. Decent post-frontal pressure gradients will support gusts
to 30 kt. Winds just aloft will be stronger, and with more
convective pattern should provide stronger gusts to surface from
aloft so it is reasonable to see gust 30 to 40 kt today. Wind not
quite a strong further south so will maintain small craft
Advisory for areas south of Cascade Head, as think most peak
gusts stay closer to 30 kt.

With this system will see large west to northwest swell Thu
afternoon and Thu night. Latest models have toned down the the
sea heights a bit with the swell around 20-22 ft at 16 seconds.
The peak should be late tonight and early Friday morning, then
subside in the afternoon, but remaining around 15-18 ft Friday
night then to the lower teens Saturday.

Longer range indicates more quiet marine weather pattern for the
coming weekend with building high pressure. The next front
approaches the region later Mon, but this front is weakening and
looks not to be much of a wind maker. /Rockey

FOR SHORT TERM: Large west swell around 20 ft with a period of
16 seconds will reach the coast tonight and continue through
Friday afternoon. A High Surf Advisory has been issued for this
event.

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...High Surf Advisory from 1 AM to 4 PM PST Friday for Central
     Oregon Coast-North Oregon Coast.

     Winter Storm Warning from 4 AM early this morning to 4 AM PST
     Saturday for Northern Oregon Cascades.

     Winter Storm Warning from noon today to 10 AM PST Saturday for
     Cascades in Lane County.

WA...High Surf Advisory from 1 AM to 4 PM PST Friday for South
     Washington Coast.

     Winter Storm Warning from 4 AM early this morning to 4 AM PST
     Saturday for South Washington Cascades.

PZ...Small Craft Advisory until noon PST today for coastal waters
     from Cape Shoalwater WA to Cascade Head OR out 60 NM.

     Gale Warning from noon today to noon PST Friday for coastal
     waters from Cape Shoalwater WA to Cascade Head OR out 60
     NM.

     Small Craft Advisory until 4 AM PST Saturday for coastal waters
     from Cascade Head OR to Florence OR out 60 NM.

     Small Craft Advisory until 6 AM PST Friday for Columbia River
     Bar.

&&

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Storm Warning from 4 AM Thursday to midnight PST Friday
     night for Northern Oregon Cascades.

     Winter Storm Warning from noon Thursday to midnight PST Friday
     night for Cascades in Lane County.

WA...Winter Storm Warning from 4 AM Thursday to midnight PST Friday
     night for South Washington Cascades.

PZ...Small Craft Advisory until noon PST Thursday for coastal waters
     from Cape Shoalwater WA to Cascade Head OR out 60 NM.

     Gale Warning from noon Thursday to noon PST Friday for coastal
     waters from Cape Shoalwater WA to Cascade Head OR out 60
     NM.

     Small Craft Advisory until noon PST Friday for coastal waters
     from Cascade Head OR to Florence OR out 60 NM.

     Small Craft Advisory until 6 AM PST Friday for Columbia River
     Bar.

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