Gale Watch 32, NWS Eureka, CA

AREA FORECAST DISCUSSION
National Weather Service Eureka CA
537 AM PST Wed Nov 20 2024

.SYNOPSIS...A strong atmospheric river storm system will deliver
strong to damaging winds and prolonged moderate to heavy
rainfall, which could lead to life threatening flash flooding,
along with heavy mountain snow. Rain and winds will only briefly
lessen today before ramping up again Thursday when a second
strengthening cyclone brushes our coast, with impacts lasting
through Friday.

&&

.DISCUSSION...Strong southerly winds sweep south with a
progressive boundary this morning, and wind gusts have dropped below
40 mph for most of Humboldt and Del Norte counties as this boundary
has moved SE toward Mendocino, Lake, and Trinity counties.  Latest
radar imagery is showing widespread moderate to heavy precipitation
this morning. Most exposed sites in Humboldt and Del Norte saw 45 to
65 mph gusts overnight, and the highest RAWS reports were from:
Kneeland with a gust to 78 mph, Crescent City with a gust to 80 mph,
and Mattole Rd with a gust to 94 mph. Strong gusts of 45 to 55 mph
continue across Mendocino and Lake counties this morning, then this
boundary is forecast to stall over Lake county later today. This
will support wind gusts of around 45 mph at elevations above 1500 ft
in Lake county through the day today, and have extended the wind
advisory.

Latest radar imagery and local precip observations are indicating
widespread moderate to heavy rainfall this morning, with around a
half to an inch and a half of rain having fallen already this
morning in Humboldt and Del Norte. This is a little below what was
expected with this feature, however given the strength of the winds
overnight, precip amounts could be under-reported due to rain
missing the bucket and/or downsloping effects drying out the lower
levels as SE flow moved over the mountains. In any case, lighter
precip rates are expected in the lee of this morning's boundary in
these two counties, while heavier rain rates are forecast to
continue this morning for Mendocino, Lake, and Trinity as it moves
SE.

Guidance continues to hold steady on the general pattern, with with
nearly unanimous agreement depicting an atmospheric river tracking
southward today then stalling over Mendocino this afternoon. A
second cyclone is forecast to take shape and quickly strengthen this
afternoon far offshore as it is drawn in around the parent deep
trough, previously stalled off the Pacific Northwest. The two
surface lows will likely interact with each other complexly, winding
around each other in a Fujiwhara Effect. This places some question
as to how close this strengthening cyclone is to our coast as it
passes northward Thursday, which would have implications to the wind
strength. What is more certain is a reinvigorated atmospheric river
with even higher moisture flux and IVT values. A stream of
persistent, high rainfall rates is forecast to occur in South
Humboldt along Cape Mendocino and through portions of coastal
Mendocino County with this storm Thursday into Friday, with steady
moderate and embedded heavy rainfall elsewhere.

WPC has included our area in a rare High Risk on Thursday through
the described locations, with a wide swath of Moderate risk
elsewhere. An additional 4 to 6 inches with isolated 8 inches is
forecast Wednesday night through Friday, with the bulk falling
Thursday through Thursday Night. This risk category is warranted
with this magnitude of rainfall expected to fall over already
saturated grounds. Meanwhile, mainstem rivers will have continued
upward, with the Eel River at Fernbridge to touch into Moderate
Flood Stage on Friday. Please see the Hydrological Section below for
more details and watch for changes in the river forecasts as models
ingest rainfall verification.

Snow levels out in NE Trinity county have been slow to drop
overnight, however local Caltrans CCTV webcams show snow on highway
299 at Buckhorn (2650 ft) and CHP reports multiple spin-outs on
Highway 36 east of Highway 3 (above 3000 ft). Observations across
the county indicate the freezing level at varying elevations,
however it appears that for the most part they are between 2200 and
3000 ft. Snow levels quickly rise for areas SW of South Fork Mtn.
The heaviest precipitation is a little later than forecast, and just
moving into Trinity County early this morning. Thus heavy snow can be
expected to continue for areas where it is already falling, and snow
levels are expected to rise through the day, first around South Fork
then for areas in NE Trinity. Snow should switch to rain at some
point today for areas below 3000 ft, although when exactly this will
happen remains unclear, and mixed precip within heavier showers is
possible through the day.

After the passage of the secondary cyclone Thursday, most ensemble
guidance shifts the parent trough westward on Friday, which would
put NWRN CA in the cold sector and shut off our subtropical moisture
tap, marking a change from persistent moderate rainfall to showery
rainfall. Rain will likely continue during this period through the
weekend, however be generally less, and with more sparse coverage.

TDJ/JJW

&&

.AVIATION...The strongest winds have diminished at the coastal
terminals while moderate to heavy rain generates periods of MVFR/IFR
visbility's across the area. Winds have kept ceilings fairly high (3-
5K feet) but will likely descend further as rain intensifies. LLWS
is also likely this morning as winds remain elevated aloft while
weakening at the surface. As an occluded front stalls offshore, UKI
will experience similar wind gusts exceeding 30 to 40 mph and heavy
rain through the morning. Widespread, impactful rainfall and gusty
southerly winds 15 to 25 knots will continue through Thursday as
this strong AR makes landfall in northern California. The strongest
LLJ winds will shift south of Cape Mendocino on Thursday and
generate >30 knot gusts across Mendocino and Lake counties.

&&

.MARINE...Seas peaked overnight at around 24 to 26 feet at 13 seconds
with storm force gusts 50 to 65 knots. Winds will continue to
diminish as an occluded front stalls offshore, but seas will remain
steep and hazardous (14 to 16 feet) as a large nearshore NW swell
fills into the waters today. Marginally high surf conditions are
being observed north of Cape Mendocino, but breakers are not
expected to exceed 20 feet with winds largely parallel to the
shoreline. Confidence is increasing for an additional upper
shortwave to produce a fast moving surface low Thursday afternoon
into Friday. Gale force gusts 35 to 40 knots are likely across all
zones as a strong frontal boundary passes through the waters. Brief
but significant southerly wind wave response will help maintain 14
to 16 foot wave heights through the end of the week even as the NW
swell diminishes. Conditions are expected to fall below advisory
levels by this weekend.

&&

.HYDROLOGY...A strong atmospheric river has moved onshore and is
forecast to persist through the rest of this week. NAEFS mean IVT is
showing a prolonged period of anomalously high values (500 to 800
kg/m/s) from the SW, perpendicular to the local terrain. This will
likely lead to a prolonged period of persistent rainfall this week,
further enhanced by the local orography. Latest QPF guidance is
showing 4 to 10 inches of precip falling across the CWA in the next
36 hours, with lighter rain possibly continuing into the weekend.

Latest MRMS soil moisture modeling is indicating 50% to 70%
saturated soils at the moment in Humboldt and Del Norte, with 20% to
30% in Mendocino and Lake Counties. Thus the initial band of precip
today is likely to be mostly absorbed, however with the prolonged
nature of this system, saturation is likely, leading to enhanced
runoff.

Latest ensemble guidance is indicating there may be a second pulse
of intensity Thursday afternoon and Friday, and confidence has
increased on this scenario with the return of some steady heavy and
widespread moderate rainfall. Assuming the first pulse today
saturates soils across the region, this second pulse could lead to
more intense flooding concerns later this week, including main stem
river responses and some life threatening flash flooding. At the
moment the RFC is forecasting the Eel River at Fernbridge to reach
moderate flood stage, and the Russian River at Hopland to reach
Flood Stage on Friday, and the other CNRFC forecast points are
currently forecast to see rapid rises Wednesday and Thursday but
remain below flood stage. Scotts Creek at Lakeport is forecast to
reach 16 feet Thursday and up to 19 feet Friday, and this may have
significant impacts along the creek. TDJ/JJW

&&

.EKA WATCHES/WARNINGS/ADVISORIES...
CA...
     Flood Watch through Friday evening for CAZ101>106-108>115.

     High Wind Warning until 6 AM PST early this morning for
     CAZ104.

     Winter Storm Warning until 10 PM PST this evening for
     CAZ107.

     Winter Storm Warning until 6 AM PST early this morning for
     CAZ108.

     Wind Advisory until 8 AM PST this morning for CAZ109>113.

     Wind Advisory until 10 PM PST this evening for CAZ114-115.

NORTHWEST CALIFORNIA COASTAL WATERS...
     Hazardous Seas Warning until 9 PM PST this evening for
     PZZ450-470-475.

     Storm Warning until 7 AM PST this morning for PZZ455.

     Hazardous Seas Warning until 9 PM PST this evening for
     PZZ455.

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Preliminary Local Storm Report
National Weather Service Eureka CA
1102 AM PST Wed Nov 20 2024

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0948 AM     Non-Tstm Wnd Dmg 1 NNW Fort Bragg        39.45N 123.81W
11/20/2024                   Mendocino          CA   Utility Company

            1251 customers without power on the
            Mendocino coast, including Fort Bragg and
            Westport.

&&

Preliminary Local Storm Report
National Weather Service Eureka CA
1118 AM PST Wed Nov 20 2024

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

1117 AM     Landslide        11 W Navarro            39.15N 123.73W
11/20/2024                   Mendocino          CA   Law Enforcement

            Rockslide reported on Highway 1.

&&

Preliminary Local Storm Report
National Weather Service Eureka CA
1200 PM PST Wed Nov 20 2024

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

1159 AM     Landslide        3 ENE Bridgeville       40.48N 123.75W
11/20/2024                   Humboldt           CA   Law Enforcement

            Water and rocks reported on Highway 36.

&&

Preliminary Local Storm Report
National Weather Service Eureka CA
149 PM PST Wed Nov 20 2024

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0149 PM     Landslide        6 SW Gasquet            41.79N 124.06W
11/20/2024                   Del Norte          CA   Law Enforcement

            Rocks in the northbound lane of highway 199.

&&

Preliminary Local Storm Report
National Weather Service Eureka CA
222 PM PST Wed Nov 20 2024

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0150 PM     Snow             10 NE Weaverville       40.86N 122.80W
11/20/2024  M13.0 Inch       Trinity            CA   Trained Spotter

            13 inches of snow reported as of 1:50 PM.
            Snow has transitioned to rain.

&&

Preliminary Local Storm Report
National Weather Service Eureka CA
239 PM PST Wed Nov 20 2024

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0238 PM     Non-Tstm Wnd Dmg 3 W Briceland           40.09N 123.95W
11/20/2024                   Humboldt           CA   Public

            Trees down in both lanes of Ettersburg Rd.

&&