Gale Watch 35, NWS Eureka, CA

AREA FORECAST DISCUSSION
National Weather Service Eureka CA
259 PM PDT Fri Sep 22 2023

.SYNOPSIS...Quiet, seasonable weather will continue through
Saturday. Gusty south wind and rain remains on track to arrive
throughout the area late Sunday through Monday.

&&

.DISCUSSION...Relatively zonal flow aloft has helped bring a period
of quiet, seasonable weather. Most of the interior has reached the
upper 70s today with Coastal highs near 60. Winds are mostly calm,
though low pressure building over the North Pacific has generally
helped turn wind southerly. This has helped push most wildfire smoke
out of the area and into Oregon. Isolated marine stratus has formed
offshore which may push back right along the coast late tonight.
Similar quiet weather will continue through Saturday with only a
very slight cooler trend. By late Saturday afternoon, high clouds
will likely to begin streaming into the area.

A deep trough and associated surface low pressure remains on track
to dip over the Pacific Northwest late Sunday into early next week.
Models show a clear atmospheric river being pulled from the topical
Pacific into the Northern California Coast. The GFS shows integrated
water vapor transport around 800 kg/ms. This puts the event on par
with storms that are much more common later in the season. Between
Sunday evening and through Monday. Much of Humboldt and Del Norte
Counties will most likely receive between 1 to 3 inches of rain.
Uncertainty remains high for the southern half of the area as it
lies along the edge of the moisture band, but generally less than 1
inch is expected for Mendocino and Lake Counties. See the hydro
discussion below for more rain details.

In addition to rain, the strongest pressure gradient along the edge
of the trough will drive gusty south winds late Sunday through
Monday morning. Currently the NBM is placing the best chance at the
high elevations of Humboldt and Del Norte Counties. The NBM show an
80% chance or more of gusts in excess of 30 mph at mid elevation
slopes and higher and along the coastal plain. Chances of gusts over
40 mph vary from 50 to 80% above 3000 ft. The only place with a real
risk of these gusts outside of high elevation ridges are int he
exposed coastal around Crescent City. The only areas with any real
risk of gusts over 50 mph are restrained to the high ridges of the
King Range. The risk of downed trees and power lines is elevated
because this is the first major southerly wind of the season and
because most trees still hold their leaves. While no wind product
was issued this afternoon, the need for one remains likely and will
be watched closely has high resolution guidance becomes available
over the next 24 hours.

As the system begins to move east. There is a threat of isolated
thunderstorms Monday afternoon. At that point, the GFS shows up to
500 J/Kg of most unstable CAPE. Any showers would likely be
restrained offshore and to the immediate coast north of Trinidad
Head. Showers will gradually ease off through Tuesday. In the longer
range, about 20% of cluster ensemble members show wetting rain late
in the week, but most show a dry but generally quiet and seasonable
pattern. /JHW

&&

.AVIATION...Thick layers of wildfire smoke covered most of Del Norte and
Humboldt. The result was MVFR to low VFR reported Cigs/Vis at ACV
and CEC today. A southerly surge of marine stratus by late morning
moved along the North Coast...simultaneously resulting in IFR Cigs
into early afternoon. An eddy off the Point ST. George coast allowed
stratus to keep rotating over CEC after midday. Overnight, various
model guidances are suggesting IFR conditions returning to the Coast
by mid Saturday morning. Winds will remain S-WSW at the coast today;
perhaps with a few breezes. Stratus inundated the Mendocino Coast
near FT Bragg and spilled into local valleys...but the layer was
shallow enough not to overspill into inland mountain valleys.
However, southerly flow up the Russian River resulted in IFR
conditions spreading into UKI this morning. Not convinced at this
time that UKI will see another morning of broken low clouds, but did
add some scattered clouds in the morning. /TA

&&

.MARINE...Winds and seas have continued to slightly diminish this
afternoon. Winds are south to southwest closer to the coast while
the remain northerly well off the coast and these will persist
tonight. Saturday the winds will start to switch to the south in the
north as an unseasonable stronger storm starts to build into the
area. These will gradually spread south into the southern waters
Saturday night. The winds are expected to increase rapidly across
the area Sunday afternoon as the cold front approaches. Confidence
is growing that we will see gales in the northern outer waters so
have added a gale watch to highlight that for Sunday night and
Monday morning. These will likely move closer to the coast, but
confidence is lower so have held off on issuing anything at this
point.

Immediately behind the front Monday night and Tuesday a strong swell
moves in. This is currently expected to build to around 14 to 16
feet at 15 seconds by Tuesday afternoon. Confidence is not
particularly high at this point, however both the ECMWF and GFS wave
models are showing similar solutions. MKK

&&

.HYDROLOGY...The highest rainfall totals are expected to favor
the N portion of the area. The latest QPF totals from late Saturday
through Monday night range from around 3.5 inches over the windward
sides of Del Norte mountains to around 0.2 inches over S Lake
County. Rainfall amounts across the windward faces of the King Range
of SW Humboldt County are expected to be over 2 inches due to
orographic enhancement. Downslope flow along the North Coast will
likely push rainfall later and keep it around 1 inch. Higher amounts
are certainly possible due to the moderate to occasionally strong
water vapor transport, particularly across Del Norte County. Looking
a bit deeper at confidence, the NBM places a 70 to 100% chance of
more than 1 inch of rain throughout Humboldt and Del Norte Counties.
This chance changes to 40 to 70% for more than 2.0 inches of rain.
There is less than a 30% chance of more than 1.0 inch of rain
anywhere south of the Humboldt County line.

Will continue to highlight locally heavy rainfall in Del Norte and
Humboldt counties on Monday, with the potential for rock slides in
the mountainous terrain, especially near recent burns on highways
199 and 96. For the most part, rainfall rates are expected to be
below thresholds for burn scar debris flows, but higher rainfall
rates are possible as precipitation transitions to more convective,
aided by orographic effects. As the first large rain of the season
and with recent burn scar, a flood watch has been issued for Del
Norte and interior Humboldt Counties.

Rivers are forecast to rise in response to the expected rains, but
river levels will remain well below monitor stage, even with the
higher outlier ensemble forecast basin amounts. Most rivers are
likely to rise only 1 to 2 feet. /SEC+JHW

&&

.EKA WATCHES/WARNINGS/ADVISORIES...
CA...
     Flood Watch from Sunday afternoon through Monday evening
     for CAZ101-102-104>106.

NORTHWEST CALIFORNIA COASTAL WATERS...
     Gale Watch from Sunday afternoon through Monday morning for
     PZZ470.

&&

Visit us at https://www.weather.gov/eureka

Follow us on Facebook and Twitter at:
https://www.facebook.com/NWSEureka
https://www.twitter.com/NWSEureka

For forecast zone information
see the forecast zone map online:
https://www.weather.gov/images/eka/zonemap.png