289
FXUS66 KEKA 090737
AFDEKA

AREA FORECAST DISCUSSION
National Weather Service Eureka CA
1237 AM PDT Thu Oct 9 2025

.SYNOPSIS...An approaching upper low is cooling temperatures.
Precipitation chances increase through Friday with light to
moderate rainfall forecast. Thunderstorm capable of strong winds
will accompany this storm system Friday over the coastal waters
and likely through portions of the North Coast. Overnight low
temperatures will continue to trend colder over the week and
through the weekend, bringing the potential for frost and some
freezing interior temperatures. A colder storm systems is forecast
early next, and will bring additional rainfall and the potential
for some mountain snow.


&&

Key Points:

-A cold upper low will bring light to moderate rain over the
 region for the next few days.

-The instability and dynamics of this low will be favorable for
 oceanic to coastal thunderstorms capable of strong wind gusts of
 40 to 50 mph and potentially some waterspouts.

-Colder trending overnight lows will bring chances for frost and
 freezing temperatures for the interior.

-A colder frontal system is forecast early next week which will
bring additional rainfall and a chance for mountain snowfall


.DISCUSSION...Daytime highs are 7 to 10 degrees cooler than what
has been observed over the last couple of days as the influence of
a cold upper trough is beginning to increase. The low can clearly
be identified on IR and mid level water vapor imagery. The
convective instability is beginning to also take shape with the
congregation of cold cloud tops also observed around the
circulation of the low.

Increasing southerly flow and some weak instability will begin to
produce some convective showers over Trinity County tonight
through Thursday ahead low. Though precipitable water values will
be on the lower side, high resolution guidance shows a persistent
moisture axis of low level convergence over far northwest Shasta
and into far NE Trinity County. If productive training showers
with a possible thunderstorm or two do develop as depicted, some
localized hydrological issue may arise. Prefrontal light, warm
sector showers or light isentropic upglide rainfall will be
possible Thursday for the remainder of the CWA.

The upper dynamics associated with the low continue to look
impressive. 500mb temperatures as low as -25C within the core of
the low will sweep through Friday. A 100kt 250mb jet streak will
develop Thursday and linger through Friday while forcing increases
with the approaching low. An elongated vorticity maximum will
locally enhance forcing over the north coastal ocean waters and
toward the coast Friday. Soundings show mainly strong
unidirectional shear with weak veering over most of the waters. A
healthy zone of more pronounced veering flow will develop over the
nearshore waters where the coastal barrier jet of surface
southerly winds forms, especially around and south of Cape
Mendocino. Surface CAPE will be on the high side for this region,
at 200 to 300 J/kg over the North Coast, with higher amounts over
400 J/kg over the ocean waters.

The sounding profiles show the potential for some more organized
possibly bowing storm structures (QLCS) over the ocean and pushing
towards the coast. The variable shear profiles will also promote
some isolated thunderstorm cells favorable for waterspout
formation. High resolution guidance shows enhanced 925 mph winds
with pockets of 40 to 45 kts speeds. With this magnitude of
instability and forcing, isolated wind gusts of 40 to 50 mph will
be possible down to the surface. This will be mainly over the
coastal waters, but instability will encompass portions of the
coast allowing for this activity to potentially impact coastal
areas around Cape Mendocino and northward.

Rainfall will be mainly light to moderate outside of thunderstorm
activity. There is a high probability (80-90%) for over 0.5 inch
over a 24 hour period through Friday over most of the region apart
from far south through Lake County. There are also more localized
high probabilities (60 to 80 percent) for over 1 inch over 24
hours for the ridges and some coastal areas from Cape Mendocino
and northward.


Overnight temperatures will trend much lower through the weekend
beyond the passage of a cold front. Frost and freezing
temperatures are currently forecast for the portions of the
interior. How cold it gets will be highly dependent on the amount
of clearing/drying that happens. The upper low will be slow to
depart the region before the next low begins its quick descent.
Cloudcover and lingering showery weather would moderate some of
the cold subfreezing overnight lows, but Saturday and particularly
Sunday have the potential to clear out.


There is increasing confidence for a colder area of low pressure
to dive straight over our region from the north early next week.
Additional rainfall and some mountain snow look probable based on
ensemble and cluster analysis. There is currently a 30-40 percent
chance for 24 rainfall over an inch through Monday. Given the
pattern of the setup, snow levels will need to be watched for the
mountains as the forecast period nears. JJW


&&

.AVIATION...(06z TAFs)...A low pressure system is continuing down
the West Coast. A cold front is expected to move into the region by
early Friday. Throughout the day on Thursday, cloudiness will build
with conditions mostly VFR to MVFR pre-frontal. Into early Thursday
morning, MVFR level ceilings will blanket Mendocino, interior
Humboldt,and Lake counties with VFR conditions returning into
Thursday afternoon. Light pre-frontal showers could occur in higher
terrain interior regions with shower potential drifting generally
west before the main frontal passage early Friday morning.


&&

.MARINE...Northerlies and steep waves are expected to continue
into Thursday morning, especially over the southern outer waters.
Northerlies will diminish and eventually turn southerly for the
northern waters on Thursday. Southerlies will strengthen in advance
of a front on Friday. Approaching cold pool aloft and steep lapse
rates will make convection increasingly favorable over the coastal
waters Thursday night and into Friday. Convective allowing model
simulations and 925mb winds from all models indicate potential for
strong wind gusts to 45 kt or more late Thursday night into Friday.
Timing of strongest wind gusts remains elusive, but strong wind
gusts may occur with linear convection and perhaps with strong
single cell storms. We issued a marine weather statement to call
attention to this threat for wind gusts > 45 kt. Also, lack of a
long southerly fetch may limit the short period wave generation to
only 4 to 5 ft.

Northerly-northwesterly winds will build behind the front on
Saturday through Sunday. Strongest north winds look to occur south
of Cape Mendo. Another trough is expected to drop down from the NW
Sunday night into Monday. Larger NW swell will be possible with this
northerly next cold surge.

&&

.COASTAL FLOODING...The perigee moon cycle is generating above
normal high tides. The tides are forecast to be higher Thursday and
Friday, reaching 8.03 ft MLLW around 1:30 PM Thursday and 7.94 ft
MLLW around 2:15 PM Friday. Southerly winds ahead of the system may
increase the anomaly which could lead to minor coastal flooding. If
the 0.85 to 1.0 positive tidal anomaly persists, the North Spit high
tide will reach 9.0 feet. Minor coastal flooding occurs around
Humboldt Bay at tides of 8.8 ft at locations such as King Salmon,
Jackson Ranch Road, and the Arcata Bottoms. The large tidal swings
will also create stronger than normal currents through channeled
waterways.


&&

.EKA WATCHES/WARNINGS/ADVISORIES...
CA...
     Coastal Flood Advisory from noon to 3 PM PDT Thursday for
     CAZ103.

     Coastal Flood Advisory from noon to 3 PM PDT Friday for
     CAZ103.

NORTHWEST CALIFORNIA COASTAL WATERS...
     Coastal Flood Advisory from noon to 3 PM PDT Thursday for
     PZZ415.

     Coastal Flood Advisory from noon to 3 PM PDT Friday for
     PZZ415.

     Small Craft Advisory until 6 AM PDT Thursday for PZZ475.

&&

$$

NOTE: The full forecast discussion is now regularly issued between
10 am/pm and 1 pm/am. Additional updates will be issued as needed.

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