Small Craft Advisory 45, NWS Portland, OR

Area Forecast Discussion...UPDATE
National Weather Service Portland OR
940 PM PDT Wed Mar 17 2021

Updated Aviation discussion

.SYNOPSIS...High pressure will keep dry weather through much of the
night. The next system will arrive early Thursday bringing rain
across the area, with continued wet weather through the weekend.

&&

.SHORT TERM...Tonight through Friday...Clear skies across NW Oregon,
SW Washington will continue through this evening. Overnight lows will
be warmer than the last two nights as southwesterly flow brings a
warmer airmass into the region. According to GOES water vapor, there
is currently a low pressure near 45N, 140W. This low will slowly
progress towards Vancouver Island over the next couple of days.
Convective allowing models are showing multiple
disturbances splitting off ahead of the parent low throughout the day
Thursday. These shortwaves will disperse moisture over much of the
Pacific Northwest with the first round of rain moving onshore
Thursday morning. Winds along the coast will ramp up to around 25 mph
Thursday as the shortwaves offshore move north. There is a chance
that thunderstorms could pop-up along the coast early Thursday, but
confidence is low for this time. Confidence in thunderstorms
increases slightly for Thursday evening into early Friday morning
over the coastal waters and near the beaches along the coast.
Thunderstorms are not expected to move east of the Coast Range. The
cold front of the parent low is expected to push onshore Friday
afternoon. Winds along the coast will be strong but should remain
below warning criteria Friday.

Snow levels will drop to Cascade pass levels Thu afternoon then down
to around 3000 ft by Fri morning and 2500-3000 ft Fri afternoon.
Snow accumulations will not be very impressive as the orographics
aren't favorable.

.LONG TERM...Friday night through Tuesday...The upper trough and
surface low finally moves inland late Fri/early Sat with west to
northwest flow developing to provide more favorable orographics for
the Cascades with snow levels in the 2000-2500 ft range. Will have
better snowfall rates then, but QPF is decreasing Sat so will
probably be sub-advisory criteria. Precipitation tapers off slightly
Saturday night but quickly returns Sunday afternoon as the next
front moves towards the coast. The front associated with this system
looks to be moving at a pretty good pace so the rain should taper by
Monday afternoon but not quite end, especially for the mountains, as
the area will be under cyclonic northwest flow aloft which is a good
pattern to keep showers going. Showers look to end Tue as the upper
flow turns northerly as an upper ridge builds offshore. /mh

&&

.AVIATION...Mostly clear skies tonight will eventually become mostly
cloudy as a Pacific front approaches the region. Expect VFR
conditions to persist overnight as cigs gradually lower. Rain
associated with the frontal system will likely reach the coast
around 12Z while surface winds become more southeasterly. By around
16Z to 18Z coastal southerly winds increase with gusts of 25 to 30
kt likely, especially along the central Oregon coast. Precipitation
spread inland Thursday as the weakening front pushes onshore. Model
guidance continues to suggest cigs will remain predominately VFR,
but could see periods of MVFR cigs though with the passing rain.
Rain will likely transition to showers after 00Z Friday, before
another front approaches after 06Z Friday.

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

KPDX AND APPROACHES...VFR conditions will persist although
increasing clouds are expected as a front moves through Thu. Chances
for rain begin around 16Z Thu with southeasterly winds expected to
increase to around 12 kt. /DDH

&&

.MARINE...High pressure over the waters is beginning to erode as
large broad low pressure system moves into the eastern Pacific
this evening. Along the leading edge of the low is a cold front
which will push over the waters late tonight/early Thursday
morning. This front will cause winds to increase but likely will
remain around 20 to 25 kt with gusts up to 30 kt. There is a
chance that there could be isolated gusts up to 35 kt or even 40
kt, especially in the outer waters but more widespread Small
Craft Advisory levels. Seas will be composed of a westerly swell
around 4 to 6 ft at 12 seconds with a 10 ft wind wave. Combined
seas expected to be around 12 ft at 14 seconds.

As the initial front moves inland, a second disturbance will
fill in behind it. This system has a lot more energy and upper
level support which will likely cause it to be a bit more active.
Expecting winds to increase leading up to this system's onset so
have issued a Gale Watch for Thursday afternoon and evening. The
challenge at that point is that there appears to be an equal
chance for small craft winds and seas, and gale force winds.
Because confidence is lower than what would be needed for a
warning, the Gale Watch will remain in place through late Friday.
Seas too will begin to build up to 14 ft at 13 seconds.

By Friday morning, this next system will be in full swing with a
weak westerly background swell and a strong wind wave. Winds will
be 25 to 35 kt with gusts to 40 kt increasing to 45 kt by Friday
morning. Seas too build build drastically with much of the
significant wave heights comprised of the wind wave. Expecting
seas around 16 to 19 ft through the outer northern waters north
of Cascade Head, and up to 21 ft in the central outer waters.
Expecting gusty winds through the inner waters but seas will be
slightly lower up to 18 ft. Periods in all locations will be
around 15 seconds. Should also mention that there is a chance
for thunderstorms over all of the waters on Friday as the system
moves inland.

In the post frontal environment conditions will ease and become
more settled. Conditions will persist through early next week.

-Muessle

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Gale Watch from late tonight through Friday morning for Waters
     from Cape Shoalwater WA to Cascade Head OR from 10 to 60
     NM.

     Small Craft Advisory from 8 PM this evening to 5 AM PDT
     Thursday for Waters from Cape Shoalwater WA to Cascade
     Head OR from 10 to 60 NM.

     Small Craft Advisory from 5 AM Thursday to 11 AM PDT Friday for
     coastal waters from Cape Shoalwater WA to Florence OR out
     10 NM.

     Gale Watch from late tonight through Friday morning for Waters
     from Cascade Head to Florence OR from 10 to 60 NM.

     Small Craft Advisory from 8 PM this evening to 2 AM PDT
     Thursday for Waters from Cascade Head to Florence OR from
     10 to 60 NM.

&&

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