Small Craft Advisory 172, NWS Portland, OR

Area Forecast Discussion
National Weather Service Portland OR
300 AM PDT Mon Oct 18 2021

.SYNOPSIS...After a brief break in the wet weather later today and
Tuesday, expect a very active weather pattern to bring valley rain
and high mountain snow every 12-24 hours through early next week.

&&

.SHORT TERM...Today through Wednesday...Water vapor satellite imagery
this morning reveals a shortwave trough sliding eastward into the
Great Basin. This storm system dragged a weakening and elongated
front into western Washington and Oregon late yesterday/overnight
that brought some light rain to the region. Surface observations
indicate rainfall is now primarily limited to Lane, Lincoln, Benton
and Linn Counties as of 3 AM. It should be noted that skies are
beginning to clear across our northwestern zones this morning. This
is just beginning to lead to some fog developing along the coast and
lower Columbia River, which will likely expand a bit and last through
~9-10 AM this morning.

Models are in good agreement a shortwave ridge over the northeast
Pacific will slide across the Pacific Northwest today into tonight.
This will lead to what few showers are left early this morning to
gradually dissipate this morning. Aside from a few high clouds
passing overhead tonight, expect enough radiational cooling to allow
some of our typically colder spots to drop down into the 30s
overnight with areas of valley fog developing. With that said, HREF
guidance only puts the probability of fog developing in much of the
Willamette Valley at just 20-30%.

Models and their ensembles are in good agreement a shortwave trough
currently extending southward from the Gulf of Alaska will shift
eastward into the Pacific Northwest late Tuesday into early
Wednesday. This storm system will take on a slight negative tilt and
spread light rain into the region Tuesday evening into Tuesday night.
There may be enough instability over the waters with the trough axis
to generate an isolated thunderstorm or two. In addition, a weak
surface low pressure off Vancouver Island and modeled southwesterly
30-35 kt 925mb winds will create an environment favorable for at
least some weakly rotating showers over the waters off our northern
coastal zones Wednesday morning.

A weak shortwave ridge will then likely slide across the Pacific
Northwest late Wednesday and result in showers decreasing from
southeast to northwest across the area. This will be in response to
what appears likely to be a rapid cyclogenesis event occurring 140W
and 45N. See below for more details...

.LONG TERM...Wednesday night through Sunday...Ensemble guidance is in
very good agreement an active weather pattern will take hold across
the Pacific Northwest late in the work week and continue through the
weekend (and likely beyond).

Model and ensemble guidance is in relatively good agreement the
remnants of Namtheun will merge with a shortwave trough racing across
the north Pacific and lead to a rapid cyclogenesis event across the
northeast Pacific Wednesday. While there are some variations in the
strength and exact track of the resulting low pressure system, and it
will no doubt result in some large seas well offshore and impressive
satellite imagery, it appears it will largely remain far enough
offshore to bring few substantial impacts to the region. Nonetheless,
it should push a cold front and attendant band of rain into the
region late Thursday. A strong wind field will likely accompany the
front well offshore, but between it weakening and pressure gradient
forces remaining largely oriented offshore, thinking we should be
able to get by without any wind headlines for the coast. This is
supported by the GEFS and EPS guidance as well.

A conditionally unstable air mass will then likely spread into the
region Thursday night and result in off and on showers continuing
into Friday. Snow levels appear likely to at least lower to treeline
or slightly below treeline in the Cascades on Friday, but will likely
hold above the Cascade passes.

Model and ensemble guidance is in remarkably good agreement a strong
140-180 kt jet will take aim at the southern Oregon and northern
California coast over the weekend. This will undoubtedly keep the
weather unsettled across southwest Washington and northwest Oregon
with periods of showers and below average temperatures continuing
through the weekend. While rain totals will likely add up over time,
rainfall will be spread out enough to create few problems. The
biggest concern over the weekend, and perhaps early next week if the
pattern does not break down before then, will be our susceptibility
to a windstorm. While there is literally only a member or two in each
of the GEFS, EPS and CMCE systems that at the moment suggest the
possibility out of the 100 total members, the pattern looks ripe for
it with the left exit region of the aforementioned jet positioned to
our southwest and shortwave troughs racing eastward along the jet.
Given the time between now and then, it's just something to monitor
for now... /Neuman

&&

.AVIATION...12Z TAFs: Somewhat challenging forecast today,
especially this morning with a mix of VFR and MVFR across the
area. Generally MOS type guidance and HREF indicating VFR
prevailing at the coast and inland north of about KSLE. But MVFR
along the Cascade foothills to include KTTD. In fact the HREF
shows high chances for MVFR spreading along the lower Columbia
River from KTTD to KKLS and north along I5 corridor in western WA
starting around 15Z and lasting through 21Z. The remainder of
the area shows improvement during that time however. For tonight
light offshore flow develops so may see patchy fog in the valley
later tonight, but generally VFR.

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

KPDX AND APPROACHES...Mix of VFR and MVFR in the area, but as
mentioned above the HREF shows 60-80% chance for MVFR cigs 15-20Z
in the KPDX area. Fortunately HREF is not showing lower than
MVFR. Expect to see VFR after about 20Z through 12Z Tue.

&&

.MARINE...A quiet period before more active pattern sets up. Will
see a ridge of high pressure shift west to east across the waters
today. The next front set to approach the region later Tuesday
into Wednesday. This front will be in the process of weakening as
it enters the waters on Tuesday, but will still result in a
period of stronger southerly winds from Tuesday into the middle
of the week. But it still looking strong enough for gale force
winds over the outer zones. So will issue a Gale Warning from
Tue afternoon through Tue night. The strongest wind expected
west of about 35 NM. Expect periods of enhanced southerlies to
then persist through the end of the week as another front moves
through the waters Thursday into Friday.

Meanwhile, seas will be dominated by a decaying westerly swell
through the beginning of the week as it subsides from around 7
feet this morning to 5-6 feet on Tuesday morning. The next front
will bring an uptick in short period seas Tuesday into
Wednesday, peaking around 12 ft nearer to shore and 17 feet
beyond 35 NM early Wednesday. Seas settle some later Wednesday
as a fresh westerly swell arrives.

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Small Craft Advisory from 11 PM this evening to 11 AM PDT
     Tuesday for coastal waters from Cape Shoalwater WA to Florence
OR from 10 to 60 nm.

     Gale Warning from 11 AM Tuesday to 5 AM PDT Wednesday for
     coastal waters from Cape Shoalwater WA to Florence OR from 10 to
60 nm.

     Small Craft Advisory from 11 PM Tuesday to 11 AM PDT Wednesday
     for coastal waters from Cape Shoalwater WA to Florence OR out 10
nm.

&&