Arcata, California Weather Forecast Discussion

999
FXUS66 KEKA 170902
AFDEKA
AREA FORECAST DISCUSSION
National Weather Service Eureka CA
202 AM PDT Wed Sep 17 2025
.SYNOPSIS...Hot and dry weather will persist through Wednesday with
gusty nearshore winds. Cooler weather, scattered showers, and a
slight chance for thunderstorms will return Thursday into the
weekend.
&&
.DISCUSSION...The returning stratus overnight at the coast,
also advecting into the Eel River valley as well as low lying
coastal areas, is likely to erode significantly by the late morning
or afternoon Wednesday as the last few days of ridging return
similar predictable patterns to the CWA. This will soon be
interrupted by the approaching cyclonic low pressure system, seen
from water vapor satellite imagery with a distinct frontal boundary,
sure to bring moisture lobes and unsettled weather by the weekend if
not sooner.
A quickly amplifying ridge is bringing some late season heat.
Interior valley highs will reach the upper 90s with some isolated
low 100s forecast. This is creating some isolated Moderate HeatRisk
through Wednesday.
A remnant tropical system will begin pushing in moisture from the
south by late Wednesday. Elevated instability will accompany this
moisture, which will initially be focused in the southern zones from
Lake county to southern Mendocino counties. There is wide
uncertainty in the exact trajectory of the initial moisture surge.
Showers and a slight chance for thundertorms may begin just off or
along the coast of Mendocino County late Wednesday/early Thursday
morning, but the Thursday period has the greatest uncertainty with
it. Soundings showed a good deal of convective inhibition (CIN)
Thursday, but southern zones such as Lake County and Mendocino
counties may see some thunderstorm development (10%).
Precipital water values will be high (1 to 1.6 inches), but there
will initially be less precipitation reaching the ground from any
showers or thunderstorms before the dryness at the lowest levels
begins to saturate. Coverage may be highly limited depending on
where the best moisture axis sets up.
The moisture and instability will to continue to spread in Friday
through Saturday when precipitation totals will increase more with
deeper moisture. A surface low will form along the North Coast
Friday, and this will increase forcing and work to wrap the moisture
farther west. This will continue to bring the chance for showers and
isolated thunderstorms through Saturday. The exact timing and
location of this activity will have to be further resolved in coming
higher resolution guidance.
&&
.AVIATION...(06zTAFs)...Coastal stratus will continue along the
North Coast into early Wednesday morning. By Wednesday afternoon,
winds will begin to increase along the coast, leading to increased
mixing at the surface and a faster break up of stratus by 16-18Z.
Inland areas can expect VFR conditions through the TAF period.
Wednesday, the influence from tropical storm Mario will begin to
impact NW California leading to a change in the weather pattern.
&&
.MARINE...A change to our synoptic weather setup over the next
few days changes the sea state through the end of the week. High
pressure offshore interacting with the low pressure inland will
drastically enhance the pressure gradient creating strong, gale
force sustained winds with gusts over 45kts possible over the outer
waters Wednesday night through Thursday night. Wind waves of 15ft or
more are probable across the northern waters and southern outer
waters.
Hazards have been issued for all marine forecast zones. Gale
Warnings have been issued for the outer waters. Inner water zones
out 10nm can expect near gale gusts with large wind waves
propagating into the waters. Outer water zones 10-60nm can be
subject to gale force winds south of Cape Blanco and Cape Mendocino
and large, very steep, wind waves with blowing spray.
Winds begin to ease slightly Thursday night through Friday morning
with wind waves beginning to ease Friday afternoon through Saturday
morning. This weekend, a NW swell of 5ft@11-13s is expected with
winds between 10-20kts across all the waters. Global deterministic
models are not aligned as to what the synoptic set up will be, yet
cluster analysis indicates relatively zonal flow aloft with a cutoff
low off the coast of Central CA. This will keep winds breezy, wind
waves mild, and the NW swell decaying. DS
&&
.FIRE WEATHER...Unseasonably warm and dry conditions will persist
through Wednesday and will aid in quickly drying out fuels. Interior
valley highs will be in the mid 90s with humidity as low as the
upper teens. Strong thermal belts will maintain poor overnight
recoveries at mid and upper elevations. Winds will be mostly gentle
and terrain driven. Northeast winds will be breezing for interior
Del Norte County Wednesday night through Thursday. A surge of breezy
southwest winds will briefly develop in Lake County Thursday
afternoon with minimum RH values in the low to mid 20s. Daytime
temperatures will lower Thursday with more moisture and cloud cover
entering the area. There will be a slight chance (10%) of semi-dry
thunderstorms Thursday afternoon mostly focused around the Yolla
Bollys and over Mendocino and Lake Counties with some slight
potential further north. A slight chance (20%) for additional more
rainfall productive showers and thunderstorms will continue Friday
and Saturday. Storms would have moderate storm movement and would be
relatively wet. There is currently high uncertainty in the exact
details for showers or isolated thunderstorms.
&&
.EKA WATCHES/WARNINGS/ADVISORIES...
CA...
None.
NORTHWEST CALIFORNIA COASTAL WATERS...
     Small Craft Advisory from 11 PM this evening to 5 AM PDT
     Thursday for PZZ450.
Hazardous Seas Watch from late tonight through Thursday
     evening for PZZ450.
Small Craft Advisory from 5 PM this afternoon to 5 AM PDT
     Thursday for PZZ455.
Small Craft Advisory from 5 PM this afternoon to 11 PM PDT
     this evening for PZZ470.
Gale Warning from 11 PM this evening to 5 PM PDT Thursday
     for PZZ470.
Small Craft Advisory from 5 AM early this morning to 5 PM
     PDT this afternoon for PZZ475.
Gale Warning from 5 PM this afternoon to 11 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

NWS EKA Office Area Forecast Discussion