Small Craft Advisory 152, NWS Portland, OR

Area Forecast Discussion
National Weather Service Portland OR
240 PM PDT Tue Oct 5 2021

.SYNOPSIS...A cold front continues to push east today. Rainfall
will persist through the night, becoming showery on Wednesday. Expecting
showers through Wednesday until a weak high pressure ridge
develops on Thursday. More active weather to round out the week.

&&

.SHORT TERM /THROUGH THURSDAY NIGHT/...Satellite shows the
frontal boundary stretching from southern Oregon northeast into
Washington, with the low centered over the northeastern Pacific.
As the low shift northeast into Canada, the front will weaken
and advect eastward. In it's wake will be lingering showers and
quite a bit of cold air. Snow levels will begin to drop on
Wednesday reaching as low as around 4000 ft by Wednesday
afternoon. The instability of the post frontal environment in
combination with cooler air aloft may create an atmosphere
susceptible to convection. However, a limiting factor will be a
lack of daytime heating and the fact that sea surface
temperatures are about as warm as the inland surface air.
However, it is a non-zero chance so have put thunderstorms
stretching from around Tillamook, OR to the south Washington
Coast Range. That threat will dissipate by Wednesday night.

An area of weak high pressure will develop over the eastern
Pacific on Thursday bringing a break from the rain, wind and
clouds. Expecting slightly warmer and drier conditions and a
northerly flow. May see periods of fog in the southern
Willamette Valley on Thursday morning due to the residual
moisture and northerly winds, however a bit too early to tell.
Something to keep in mind though. Expecting settled and mild
fall weather on Thursday.   -Muessle

&&

.LONG TERM /FRIDAY THROUGH TUESDAY/...Confidence begins to wane
moving into the long term forecast as the overall synoptic
pattern is 'messy' for a lack of better words. In previous runs,
a cold front was expected to make landfall on Friday. Now in
more recent numerical outputs this front appears to have
weakened and split which will likely keep precipitation amounts
lower than previously expected. Still expecting some rain but
likely will stay to the northern portions of the forecast area
and should have little accumulation. Saturday will be yet
another transitional day comprised of onshore flow and lingering
showers.

Sunday through Monday appears to be the chance for the next
more impactful system. As it stands, there is fairly good
agreement in a longwave trough dropping down from a closed low
over the eastern Gulf of Alaska on Sunday afternoon. Most recent
outputs are trending this system slightly slower than past
runs, however the changes are minimal. What is not minimal is
just how much cold air will be behind this front. Expecting
temperatures to drop around 10F from Saturday to Sunday's highs,
and even lower on Monday into Tuesday. Temperatures around 850
mb will be around -2 C. Looking aloft with this event, the jet
stream at 250 mb (around 35,000 ft) is meridional and has two
distinct jet streaks. The first will pass over Sunday morning
and has wind speeds around 120 mph. The next will move over the
region on Monday. Now this streak is significantly stronger and
almost completely vertical with wind speeds around 150 mph.
These two features will help to enhance the already approaching
front bringing ample rainfall and maybe even some convective
activity on Monday afternoon. Will have to continue to see if
this system maintains the same strength over the next few days.

    -Muessle

&&

.AVIATION...Southwest flow aloft will continue as the air mass
remains relatively moist. The rain band associated with a cold
front will slide south and east before exiting the region around
06Z to 08Z this evening. Predominately VFR conditions expected
this evening although there still remains a mix of MVFR/IFR cigs
and visbys within some of the heavier showers. Mid to high level
clouds expected to exit the region along with the front around
08Z to 10Z Wednesday. Broken to scattered lower level clouds
then expected through the remainder of the TAF period likely
remaining in the VFR category. A weak front will approach the
coast later this evening with showers likely increasing after
12Z. Brief MVFR cigs will be possible with heavier showers
through Wednesday morning.

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

KPDX AND APPROACHES...Predominately VFR conditions expected
through the period although heavier showers may briefly reduce
cigs and/or vis to MVFR categories. Steady rain should end around
03Z with isolated showers possible overnight. Broken to scattered
low VFR cigs likely through Wed afternoon. Southwest winds
expected to remain under 10 kt.    /DH

&&

.MARINE...A weak front will move across the coastal waters
tonight into Wednesday morning. Southwest winds will increase to
around 20 kt with gusts to 25 kt across the northern zones
overnight so have extended the Small Craft Advisory there through
the night. Behind the front a westerly swell will build seas up
to around 10 to 11 ft through Wednesday morning thus have
extended the small craft advisory for the outer waters through
noon on Wednesday. High pressure will then build across the
waters through Wed night. Winds will shift to the north and fall
to around 10 to 15 kt through Thursday morning. Northerly winds
will likely increase to around 15-20 kt Thu afternoon and
evening especially across the southern waters. Another front is
expected to approach the coastal waters later this week but does
not look particularly strong at this time.

Seas continue to range around 9-10 ft with a period of 10 to 11
seconds. The northwesterly swell will gradually subside to around
7 ft before the aforementioned westerly swell train builds in
overnight. Seas are expected to fall below 10 ft by Wed
afternoon and gradually subside to around 4-6 ft Thursday into
Friday.    /DH

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Small Craft Advisory until noon PDT Wednesday for PZZ270.
     Small Craft Advisory until 2 AM PDT Wednesday for PZZ250.
     Small Craft Advisory until 5 PM PDT this afternoon for PZZ255-
     275.
     Small Craft Advisory from 2 AM to noon PDT Wednesday for
     PZZ275.

&&