Strong Wind in Oregon, March 31, 2023

Area Forecast Discussion
National Weather Service Portland OR
251 PM PDT Fri Mar 31 2023

.SYNOPSIS...Potent cold front will move onshore this evening,
bringing blustery winds to the coast, valley rain, and
mountain snow. Unseasonably chilly weather will move in behind the
front, persisting through early next week. Meanwhile, deep and
persistent onshore flow and weather disturbances will bring heavy
snow to elevations above 1500 feet, with snow expected to be measured
in feet in the Cascades over the weekend. Light, spotty accumulations
will be possible as low as 500 feet Sunday morning, but the better
chance for lowland snow comes Monday morning as the core of cold air
aloft moves overhead. Gradual moderation to more normal temperatures
is possible as next week progresses, but confidence in this remains
low.

&&

.DISCUSSION...Today through next Friday...
Key points:

1) Heavy snow will impact the Cascades and Coast Range beginning
around sunset today, spreading south across the forecast
area overnight. South Washington Cascades will start a little
earlier, as snow levels start off around 3000 ft and precip starts
earlier.

2) One to two feet of snow can be expected for elevations above 2000
ft north of Highway 20, and 2500 ft south of Highway 20. Elevations
above 3000 to 4000 feet stand to receive 2 to 4 feet of snow by the
time snow showers taper off Sunday night.

3) Lighter, but still impactful accumulations are likely between 1000
to 2500 ft beginning tonight or Sat morning. Coast Range passes will
be impacted with snow, with the greatest impacts coming during the
cooler night and morning hours.

4) Significant snow impacts are not anticipated below 1000 feet.
Minor accumulations on grassy surfaces are possible as the coldest
air moves through Monday morning.

5) Blustery winds will develop with the cold front today, with gusts
as high as 50 mph possible for beaches and headlands.

Cold front over the coastal waters will moves ashore this evening and
slide south and east tonight. Satellite shows wave clouds forming on
the lee side of the Cascade and coast range indicative of strong,
moist flow crossing the mountains in the warm sector ahead of the
front.	Much colder air is on the back side of the front, and will
cause snow levels to lower from 3000-4000 ft this afternoon to
1000-2000 ft by Sat morning and hover around that through Sunday. But
colder even colder air arrives Sun night/Mon morning as with 850 mb
temperatures -6 to -7C by Mon morning. This is right in that marginal
lowland snow threshold. But by that time showers will be on the
decrease. In the northwest flow, small scale lows develop that models
don't resolve well can develop. These lows can drop the snow levels
to the valley floor.

Snow amounts - All locations in SW Washington and NW Oregon above
1500 ft will be under a Winter Storm Warning for heavy snow - except
the Upper Hood River Valley due to downsloping.

About 1 to 2 feet of snow can be expected above 2500 ft, with 2 to 4
feet likely in areas above 3000-4000 ft that start off as snow this
afternoon and evening. Elevations below 2500 ft will mostly
receive their snow from their post-frontal showers, which will be
numerous, aided by strong onshore flow and steep lapse rates.
Combined with good frontogenesis, especially Saturday, over the
southern half of the forecast give a boost to forecast confidence.

Elevations above 2000 ft can expect 6-12 inches of snow Sat & Sun,
with 3-6 inches possible Sun night. This adds up to more
than a foot of snow. Foothill towns such as Rhododendron,
Detroit, and Oakridge appear likely to see 4 to 10 inches of snow,
with the higher elevations seeing the higher amounts.

Thunderstorms are possible at the coast Saturday through Sunday, and
extending into the interior during the afternoon and evening hours.
With the low freezing levels and steep lapse range into the hail
growth zones expect hail along with thunder. Even some
non-thunderstorm showers could produce small hail.  Monday may also
bring a chance of thunderstorms as well.

Tuesday the PacNW remains in a cool pattern with a broad upper level
trough shifting into the Rockies with upper level ridging developing
offshore. Snow will continue to accumulate in the Cascades Tuesday,
mainly the Lane County zones, but snow rates will be below advisory
criteria. Wednesday into Thursday appears to be a dry period as a
transitory upper level ridge shifts to the region. Will then see
increasing chances for rain again later Thursday into Friday as a
front leans into the region. Current deterministic models vary on the
timing and strength of this system. The ECMWF is slowest/weakest, GFS
faster/stronger with the GEM fastest/strongest. WPC cluster analysis
in good agreement with the upper level ridging offshore Wednesday
into Thursday. There is one cluster still holding on to troughing
however. Clusters are split on the upper pattern Friday. Two with
upper ridging near or over the region, and two with either slight
troughing or a trough approaching. Snow levels will be gradually
lifting Wednesday through Friday from about 1000 ft Wednesday to 4000
ft Friday. But QPF amounts are expected to be low so not expecting
significant impacts at this time.  /26

&&

.AVIATION...00Z TAFS: The front that has been sitting right on the
coast is moving much slower than previously expected. Thus CIGs
have yet to degrade into less favorable conditions. Winds on the
other hand are gusty with gusts near 30 kt along the coast and in
the upper 20s inland through the Willamette Valley in N-S aligned
terrain. Much of the precipitation is showery although is turning
more stratiform as time continues. Should see CIGs become MVFR
over time, but thinking it will occur later than models are
suggesting. As the front shifts east of the Cascades and
northward, the winds will ease and ceilings will begin to lift. As
it stands, still forecasting MVFR but should raise from low end
to higher end at or above 2500 ft AGL. Areas that will struggle to
improve will be within the trapped terrain, along the coast and
the southern Willamette Valley. No concerns for thunderstorms at
this time.

Late in the forecast period, there is around a 20% chance for
thunderstorms after 18Z Saturday. At this time, confidence is low
but all parameters are trending towards a convective day. This
means that we could see thunder, small hail, and gusty erratic
winds. Thunder will transition from the coast inland through the
morning.

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

KPDX AND APPROACHES...VFR over the next few hours until the front
moves inland. Will see a steady decline of CIGs to below 3000 ft
after 02Z but winds will stay fairly consistent with gusts
upwards of 25 kt. Will see improvement to low- end VFR CIGs after
06Z Saturday as the front exits, and showers remain. Could see
isolated gusts with these showers and reduced visibility during
times of heaviest rain. Thunder chances increase after 18Z Sat.

					  -Muessle

&&

.MARINE...A strong frontal system continues to move through the
waters today, although it is moving slower than previously
expected. Southerly winds will increase through the day, with
widespread gale force wind gusts expected to begin late. Have
delayed winds increasing by a few hours as conditions have been
slow to start. Strongest wind gusts to around 40 kt are expected
late this evening, especially across the inner coastal waters.
Winds quickly shift westerly behind the front tonight and will
remain onshore with speeds around 15-20 kt through the weekend.

The primary background swell will persist through the next week
sitting at around 4 to 6 ft at 9 seconds until Sunday when it
builds to near 8 ft. With this front, will have the added
southerly wind wave and southerly secondary fresh swell which will
cause significant wave heights to build to around 11 to 13 ft by
this evening. With post frontal showers, will see increased
chances for thunderstorms with small hail and locally gusty winds.

As we move into Sunday, will see yet another low pressure system
drop down from the northeast Pacific. This low will bring another
round of gusty winds from the northwest and convective potential.
At this time not necessarily seeing as much of an impact on seas,
but if it stalls, as some models are suggesting, could see an
increased westerly wind wave that would build higher seas.

				  -Muessle

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...Winter Storm Warning from 11 PM this evening to 11 PM PDT Sunday

     for Central Coast Range of Western Oregon-Coast Range of
     Northwest Oregon.

     Winter Storm Warning from 9 PM this evening to 11 PM PDT Sunday
     for Northern Oregon Cascade Foothills.

     Winter Storm Warning from 8 PM this evening to 11 PM PDT Sunday
     for Cascades in Lane County-Northern Oregon Cascades.

     Winter Storm Warning from 5 AM Saturday to 11 PM PDT Sunday for
     Cascade Foothills in Lane County.

     Winter Weather Advisory from 11 PM this evening to 11 PM PDT
     Sunday for Upper Hood River Valley.

WA...Winter Storm Warning from 9 PM this evening to 11 PM PDT Sunday
     for South Washington Cascade Foothills.

     Winter Storm Warning until 11 PM PDT Sunday for South Washington

     Cascades.

     Winter Storm Warning from 8 PM this evening to 5 PM PDT Sunday
     for Willapa Hills.

PZ...Gale Warning until 8 PM PDT this evening for Columbia River Bar.

     Small Craft Advisory from 8 PM this evening to 4 AM PDT Saturday

     for Columbia River Bar.

     Gale Warning until 11 PM PDT this evening for Coastal waters
     from Cape Shoalwater WA to Cape Foulweather OR out 10
     NM.Coastal waters from Cape Shoalwater WA to Cape
     Foulweather OR from 10 to 60 NM.

     Small Craft Advisory from 11 PM this evening to 5 AM PDT Sunday
     for Coastal waters from Cape Shoalwater WA to Cape
     Foulweather OR out 10 NM.Coastal waters from Cape
     Shoalwater WA to Cape Foulweather OR from 10 to 60 NM.

     Gale Warning until 2 AM PDT Saturday for Coastal waters from
     Cape Foulweather to Florence OR out 10 NM-Waters from Cape
     Foulweather to Florence OR from 10 to 60 NM.

     Small Craft Advisory from 2 AM Saturday to 5 AM PDT Sunday for
     Coastal waters from Cape Foulweather to Florence OR out 10
     NM-Waters from Cape Foulweather to Florence OR from 10 to
     60 NM.
&&

www.weather.gov/portland

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