Small Craft Advisory 174, NWS Portland, OR

Area Forecast Discussion
National Weather Service Portland OR
300 AM PDT Wed Oct 20 2021

.SYNOPSIS...Expect a very active weather pattern to bring valley rain
and high mountain snow every 12-24 hours through early next week. A
brief burst in wind is likely early this morning with additional
stronger bouts of wind possible over the weekend and/or early next
week.

&&

.SHORT TERM...Today through Friday...Water vapor satellite imagery
early this morning shows a negatively tilted shortwave trough
lifting northeastward across Oregon. Strong mid level lift ahead of
this feature has already resulted in one band of light to moderate
rain lifting northeastward across the area overnight with rainfall
amounts generally ranging between 0.05-0.20". The next band of rain
tied to the low level front is lifting northeastward across western
Oregon. Models suggest this front will weaken and do not see any
obvious signs via satellite or radar imagery to suggest otherwise.
Nonetheless, as the low level airmass destabilizes behind frontal
passage early this morning, stronger winds aloft should mix down and
produce some gusty winds along the coast and in the Willamette
Valley. The period of highest concern will be between 5-8am where
HREF guidance suggests wind gusts of 30-40 mph. The lower resolution
global model ensemble systems such as the GEFS and EPS suggest wind
gusts will top out more in the 20-30 mph range. Given it will likely
be a blend between these two scenarios, made few changes to the
inherited wind forecast this morning. Otherwise, expect off and on
showers today with shower coverage generally decreasing from south
to north this afternoon and evening. A rather quickly lowering
subsidence inversion should limit thunder potential.

Models remain are in good agreement a bomb cyclogenesis event is just
beginning to unfold near 140W and 40N. Most available guidance
suggests an ill defined surface low pressure currently near ~1000mb
will undergo rapid intensification and drop below 960mb by this time
Thursday as it nears 137W and 48N. While the strongest winds will
remain well offshore, it will push an occluding front towards the
region Thursday. While very strong southerly winds will exist ahead
of this front over the waters off the Oregon and Washington coast,
global ensemble guidance continues to suggest these winds will have a
difficult time pushing inland. However, there are at least a few
members of the GEFS and slightly more in the EPS system that like
HREF guidance, suggest 35-45 mph wind gusts could surface in the
central, and particularly, the southern Willamette Valley Thursday
midday/afternoon. Some of this variability is likely due to
variation in ensemble guidance on just how strong this low pressure
gets by this time tomorrow as some solutions suggest it will drop
near 950mb.

Finally, a south-north oriented band of rain will shift eastward
across the area Thursday afternoon and evening. There is some hint in
model guidance that a narrow, but intense burst of rain may arrive
with frontal passage, particularly along the coast. Given its
orientation do not think it will create too many issues, but
depending on the exact timing, it may hit the I5 corridor around the
time of the Thursday evening commute. Given its expected south-north
orientation, not too much concern that any waves that develop along
the front will result in the particularly intense rain training for
30+ minutes over one of the burn areas in the Cascades so hydro
concerns remain limited.

Otherwise, expect cooler air to spread into the region Thursday night
and precipitation to turn more showery in nature Friday. /Neuman

.LONG TERM...Friday night through Tuesday...Confidence remains high
that we will remain in a very active weather pattern that will result
in an extended stretch of wet weather and likely beneficial rains.
Expect several stretches of 3-9 hours of steadier rain followed by
12-24 hours of off and on showers over the weekend and early next
week. This is in response to a strong zonally oriented jet streaming
across the north Pacific and into the western US. A series of
shortwave troughs will race eastward along the jet and help to
initiate cyclogenesis over the northeast Pacific this weekend and
early next week.

The first storm system will generate a low pressure over the
northeast Pacific that will approach Haida Gwaii Friday night. This
will result in an occluding front pushing into the region late Friday
night and Saturday morning. This storm system appears likely to bring
a solid shot of rain to the region, but rainfall amounts should
remain well below amounts that bring flooding concerns. Some strong
winds may push onto the coast, but winds appear unlikely to reach
high wind warning criteria along the coast per the latest GEFS and
EPS guidance.

After a brief break in the wet weather late Saturday, the next
developing low pressure will form across the northeast Pacific. There
remains quite a bit of spread among the ensemble members on where
this low pressure will track and exactly how strong it will be, but a
significant number of the EPS, GEFS and CMCE ensemble members suggest
it will bomb out west of 130W. This scenario should limit wind
impacts across the region Sunday. However, there are certainly a
decent number of members that bring the low pressure east of 130W,
and thus open the door for significant wind impacts spreading onto
our coast and even inland. Interestingly, a large number of these
members are found in the EPS system relative to the GEFS and CMCE.
In addition, some of the farther north low pressure solutions that
would result in less wind concerns for our area, do result in the
attendant WSW-ENE oriented front and atmospheric river lifting a bit
farther north and at least clipping our southern zones instead of
remaining farther south over the northern California and southern
Oregon coast. The bottom line is there remains quite a bit of
uncertainty around Sunday, but there are multiple scenarios that
could bring significant impacts to the region so stay tuned.

Uncertainty in where and to what degree another low pressure will
develop over the northeast Pacific grows considerably as we move into
Monday and Tuesday with ensemble solutions literally all over the
place. Bottom line...expect unsettled weather to continue...it just
remains highly uncertain whether or not we see significant impacts.
/Neuman

&&

.AVIATION...12Z TAFS: Precipitation has spread over the area
early this morning. However, conditions as of 09Z are still
primarily VFR. There have been areas of MVFR vis, mainly in the
interior valleys. The frontal boundary reaches the central coast
of Oregon at or slightly after 12Z Wed and then should be along a
KHQM to KEUG line by 15Z. Guidance suggests low-end VFR to
prevail ahead and along the frontal boundary, but would expect
areas of MVFR, especially along the coast. Gusty south wind is
expected with the frontal passage, strongest between 12-15Z when
brief gusts between 25-35 kt will be possible. Some of the higher
resolution models show south to southwest wind 40-50 kt at FL020
through 18Z. LLWS will be a concern and will continue to include
in the 12Z TAFS. The wind direction does not look to change very
much between the surface and 2000 ft, so expect predominantly
speed shear rather than directional shear.

Precipitation diminishes after 18Z, with predominant VFR through
12Z Thu. There is a chance for local IFR to MVFR vis in the
interior valleys after 06Z Thu as wind speeds diminish.

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

KPDX AND APPROACHES...Low-end VFR conditions to continue through
18Z this morning. Frontal boundary forecast to move across the
terminal and vicinity around 15Z. East to Southeast wind around
10 kt will shift to Southeast to South with the frontal passage
with gusts to 25 kt. LLWS will be a concern through 18Z with
surface and low-level East to Southeast wind and South to
Southwest wind 35-45 kt at FL020. Shower activity diminishes
after 18Z, with VFR then prevailing through 12Z Thu. Weishaar

&&

.MARINE...A strong front was moving through the coastal waters
early this morning as seen in infra-red satellite imagery. The
strongest wind as of 09Z Wed was over the far outer waters,
generally beyond 50 nm. Buoy 46089 reported gusts to 31 kt as of
08Z. The NAM, GFS, Hi-Res FV3 and ECMWF valid 12Z appear to be
too strong with boundary layer wind speeds. Will maintain the
current suite of gale warnings. Not as confident with the inner
water gale warning. The 00Z GEFS valid 12Z Wed indicates only a
20 to 30 percent chance of wind speeds 30 kt or greater near
shore. The majority of models agree wind speeds slowly diminish
after 18Z today. Will likely need to replace the gales with a
short-term small craft advisory ahead of the next potentially
strong system.

This next system is forecast to impact the waters Wednesday
night and Thursday. Models continue to show differences with the
central low pressure strength and corresponding boundary layer
wind speeds. Models are in decent agreement depicting a rapidly
deepening surface low out near 45N 135W Wed night/early Thu
morning. A few 00Z ECMWF ensemble members show a 950 mb central
low pres near 45N 135W valid 12Z Thu. Confidence is high that
another round of gale force gusts will occur over the outer
waters with this system, with a few models showing 50-55 kt
boundary layer gusts Thu morning out beyond 50 nm. Not confident
enough to upgrade the gale watch to a storm watch. Confidence is
lower for gale force gusts over the inner waters. There will be a
considerable east component to the low-level wind over the inner
waters, which is not favorable for gales. Will continue with the
current gale watches. Would go ahead and issue the gale warning
for the outer waters, but do not want to create potential
confusion with the current gale. The active pattern looks to
continue through the weekend with additional gale force wind
periods likely.

Total wave heights are running about 4-5 ft higher than the
spectral guidance. Buoy 46050 was at 19 ft earlier tonight, but
has eased to 15 ft as of 09Z. The Astoria Canyon buoy reported
17.7 ft at 08Z, but was at 16.3 ft at 09Z. The peak wave heights
may have just occurred and should continue a slow decline through
the morning. Seas will be at or just above 10 ft Wed evening. The
second round of gale force winds expected Wednesday night and
Thursday will allow seas to rise back into the 15 to 20 ft range,
highest over the outer waters. Seas 20-25 ft are possible late
Wed night through Thu morning out near 60 nm. Seas then subside
beginning Friday afternoon, but will remain above 10 ft through
the weekend. Weishaar

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Gale Watch from this evening through Thursday evening for
     coastal waters from Cape Shoalwater WA to Florence OR from 10
    to 60 nm.

     Gale Warning until 11 AM PDT this morning for coastal waters
    from Cape Shoalwater WA to Florence OR from 10 to 60 nm.

     Gale Watch from Thursday morning through Thursday evening for
     coastal waters from Cape Shoalwater WA to Florence OR out 10 nm.

     Small Craft Advisory until 2 AM PDT early this morning for
     coastal waters from Cape Shoalwater WA to Florence OR out 10 nm.

     Gale Warning until 11 AM PDT this morning for coastal waters
    from Cape Shoalwater WA to Florence OR out 10 nm.

     Small Craft Advisory from 5 AM early this morning to 5 AM PDT
     Thursday for Columbia River Bar.

&&