Arcata Post Office, California Weather Forecast Discussion

672
FXUS66 KEKA 240751
AFDEKA
AREA FORECAST DISCUSSION
National Weather Service Eureka CA
1251 AM PDT Wed Sep 24 2025
.SYNOPSIS...Isolated light showers will be possible for southern
Lake and southern Mendocino Counties Wednesday. Otherwise, dry and
stable weather with an increase in coastal fog and stratus is
expected for the rest of this week. Additional troughs may yield
some rain late in the weekend, with increasing chances through
early next week.
&&
.DISCUSSION...A closed low is slowly moving northward tonight along
the CEN CA coast. The low is evident on water vapor imagery, and a good
deal of lightning is also observed along the CEN CA coastal waters.
The low will nudge slightly farther north Wednesday. This will
increase the chances for shower activity for the far southern zones
of southern Lake and perhaps into souther Mendocino counties. The
associated instability is forecast to fade significantly as the
low moves into a more dry and stable environment. Latest HREF and
NBM probabilistic guidance continues to indicate low potential (5%
or less) for thunderstorms along the border with Napa and Sonoma
County Wed evening. High resolution guidance has been subtly
shifting this activity more northward. Have increased the
precipitation chances for southern Lake and Mendocino. The lower
levels will be quite dry, inhibiting accumulating precipitation.
Gusty outflow winds would occur with stronger showers that can
make it that far north.
Conditions are favorable for areas of radiational fog along the
coastal areas tonight, mainly where greater sources of surface
moisture exists such as along the Eel River delta and Humboldt
Bay. The Lower Loleta areas have observed solid radiational fog
recently, with likely low level moisture contribution from mature
corn crop transpiration or "corn sweat".
Generally above normal temperatures and greater cloud cover are
forecast for Wednesday. The stratus will begin to return as near-
coast southerly flow taps into the moisture associated with the area
of low pressure. The inversion will initially be shallow, so areas of
lower visibility and fog will be possible during the day Wednesday
as the stratus surges north over the coastal waters and over
portions of the near coast.
A weak front will push the upper low back south on Thursday. High
pressure ridging will then attempt to nose back in from the west
Friday. There is growing evidence that NE Pacific troughing will
then suppress the ridge through the weekend. Models handle trough
advancement poorly this time of year. There will not be a strong
and dominant high pressure ridge ahead of it, giving better
chances at stronger weather systems punching in.
Rain chances increase late Sunday and especially Monday. NBM
currently has moderate chances (up to 60%) for 24 hour rainfall
over 0.5 inch through Monday. A closer look at ensembles and
ensemble members reveals much uncertainty in spread. Members
of nothing-0.5 inches (29% of members) represent the most likely
scenario. Higher end members range from over 0.5 inch to over 2
inches (45% of the members) in a 24 hour period. The ensembles
maintain more chances for precipitation next week, so there may be
a pattern change into a wetter regime. JJW
&&
.AVIATION...Calm conditions are expected across the entire forecast
area tonight. Radiation fog (MIFG) is expected intermittently at
KCEC and possibly even KACV tonight, yet less so at the latter.
Tomorrow, southerly winds could potentially bring stratus/fog to
the north coast, however as of the 06Z TAFS, the closest fog
observed on satellite is in Monterrey Bay. At KACV and KCEC,
stratus would likely arrive sometime in the afternoon Wednesday.
Any daytime heating will likely keep this stratus elevated and off
the coast until the evening, but this depends on when it arrives.
Inland areas are likely to remain VFR with generally light winds
for the next 24 hours.
&&
.MARINE...Northerly winds continue to  move further offshore as
southerlies along the coast fill in overnight through the morning.
Seas are also starting to subside as these winds diminish leaving
decaying wind waves and a mid-to-short NW swell of 5ft@10s. The
aforementioned southerly winds nearshore may be breezy around Cape
Mendocino, and will potential carry a slug of stratus and fog
northward with it. Confidence on the transportation of fog is low
because the closest coastal fog is currently shown to be near
Monterey Bay as of 9:30 PM PDT 9/23.
High pressure will then noses back from the west in Thursday as a
center of low pressure off the CA central coast moves inland. The
synoptic movement will strengthen the surface pressure gradient
against a cutoff upper low over Central CA and the High pressure
building from the NE Pacific. As a result of this pressure
gradient, northerly winds strengthen Thursday through Friday. Gale
conditions are looking more likely in the outer zones. NBM now
holds a 40 to 70% chance for gales late Thursday and Friday. A
Gale Watch has been issued for the outer waters for this period,
and is subject to be updated as higher resolution model data
becomes available on the timing and orientation of the pressure
gradient. Steep seas are currently forecast to build to 10-12 ft
in the outer waters through that period, creating conditions
hazardous to small crafts in the inner waters.
&&
.FIRE WEATHER...Above normal high temperatures and low daytime
RH`s are expected through Friday and perhaps into the weekend.
Southerlies and upslope breezes should generally prevail as we head
into Wed as a cutoff low drifts closer from the south. Potential for
lightning and deep moist convection with this low is looking quite
low (5%) for our southern interior, particularly southern Lake on
the border with Napa and Sonoma. RH`s should increase slightly with
the southerlies in some areas near the coast and areas susceptible
to marine air intrusions from the south and eventually the west. A
westerly wind surge will generate gusts over 25 mph in Lake County
Wednesday afternoon. Northerlies ramp up over the coastal ranges
toward the end of the week, Thu- Fri, after a cut- off low moves
back toward south across southern California.
A band of mid level moisture is forecast to advance northward
through the day on Wed. A few high based showers or sprinkles will
be possible for southern Lake and far southern Mendocino in the
afternoon and evening. Latest guidance has subtly nudges this
activity farther north. Most if not all model sounding depict a
very dry lower atmosphere and any drops from mid level clouds will
probably evaporate. There is slim chance for stronger west-
southwest gusts if evaporatively cooled downdrafts couple with
surface winds, especially in southern Lake County. Only one model
(HRRR) from the HREF shows convective cells >40dBZ. Right now it
appears the consensus is for the band to dissipate upon moving
into a dry and stable air mass.
&&
.EKA WATCHES/WARNINGS/ADVISORIES...
CA...
None.
NORTHWEST CALIFORNIA COASTAL WATERS...
     Small Craft Advisory from 3 PM Thursday to 11 AM PDT Friday
     for PZZ450-455.
Gale Watch from Thursday morning through Friday morning for
     PZZ470.
Gale Watch from Thursday afternoon through Friday morning
     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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NWS EKA Office Area Forecast Discussion