Arcata, California Weather Forecast Discussion

904
FXUS66 KEKA 262001
AFDEKA
AREA FORECAST DISCUSSION
National Weather Service Eureka CA
101 PM PDT Fri Sep 26 2025
.SYNOPSIS...Warm and dry weather will persist this week. Dry
northerly flow will clear out most of the coastal stratus this week,
allowing for better duration of coastal sunshine. Rain chances
arrive late Sunday, with increasing probability for widespread
rainfall and periods of breezy winds through early next week. The
opportunity for additional rainfall may continue through mid next
week.
&&
Key Point:
A succession of storm systems will bring a wet pattern
with periods of moderate to heavy rainfall along with breezy to
strong southerly winds.
.DISCUSSION...A cutoff upper low has been pushed south over southern
CA by a weak front. Offshore winds will increase tonight and into
Saturday as the pressure gradient squeezes between the ridge and
the cutoff low.
The ridge will quickly begin to breakdown going into the weekend as
a NE Pacific trough takes shape and aims towards California.
Temperatures will turn much lower Sunday with the arrival of
cloudcover and chances for light rainfall as the trough nears.
The trough and associated cold front will then move in late
Sunday/Monday. The trough will become negatively tilted upon
arrival. The system will contain a band of high precipital water
values and likely a somewhat strong southerly jet. That said, the
IVT (integrated water vapor transport) values will be 350 to up to
500 kg m-1 s-1, supporting a weak atmospheric river type system.
This will allow for some periods of heavier rainfall rates. The US
Drought Monitor now has a good portion of Humboldt and Trinity
County in the `Abnormally Dry` category, with Del Norte entering
`Moderate Drought`, so this rainfall will be welcomed and
beneficial.
There will be an up to 40 kt 925 mb southerly jet and some
instability with this first trough. Breezy southerly winds will be
possible with 20-30 mph gusts, and locally 40 mph gusts on the
higher ridges. Ensembles still show a large spread in rainfall
totals. NBM holds a general 60% chance for 24 hour totals over 1
inch. Greater precipitation totals would be likely over the higher
more southerly to southwest oriented terrain.
A cyclone and associated cold front will then form in the NE Pacific
region. Models continue to struggle with the depth and southern
track of the of the low. The GFS model suite depict a much deeper
low, down to 976 mb, and on a more southern track before swinging
north into the Pacific Northwest. The system will also bring
chances for moderate to heavy rainfall with probability for IVT
values over 250 70-80%. Chances for 24 hour rainfall totals over
an inch are around 55%, and greater precipitation totals would
again be likely over the higher more southerly to southwest
oriented terrain.
The dynamics of this system will be more favorable for stronger
southerly winds, but the strength will be dependent on the depth
and track of the low. NBM 90% shows a larger percentage of the
area to see the 30 to 35 mph winds, with locally 40 mph gusts,
but if models trend closer to the deterministic GFS, these winds
may be much stronger. A small percentage of the ensemble members
show gusts of 40 to 45 mph for some coastal regions, so this
aspect of the forecast will be watched closely. Unsettled weather
with additional light rainfall will be possible mid to late next
week. JJW
&&
.AVIATION...(18Z TAFs)...VFR conditions are being observed at all
the terminals late this morning. Gusty northwesterly winds are
likely at the coastal terminals, especially CEC, this afternoon and
evening. CEC could see gusts approaching 30 kts, while ACV and other
coastal areas may approach 20 kts. Hazy skies are possible with the
strongest winds. Winds ease this evening and overnight and turn
light and southerly. The marine inversion is forecast to be strong,
but offshore winds aloft, while decreasing, may continue to inhibit
stratus formation tonight. HREF is modeling some stratus forming
around Humboldt Bay, which may get carried northward to ACV and CEC,
but confidence is low. The most likely timing for impacts would be
early Saturday morning around sunrise, and impacts would likely be
IFR to LIFR ceilings and reduced visibilities. VFR conditions are
expected for interior areas, including UKI, for the next 24 hours.
JB
&&
.MARINE...Northerly winds remain elevated today as the pressure
gradient remains steep. Gale warnings remain in effect for the outer
waters through early morning Saturday. Gusts up to 45 knots will
also make for hazardous sea conditions as wind waves combined with
swell will make a choppy and confused sea state lasting through
Saturday afternoon. Expect combined seas of 12-14 ft consisting of
steep wind waves of 11-12 ft along with a building long-period NW
swell peaking at 6-8 ft by Saturday evening.
Winds and seas begin to ease late Saturday as a trough approaches
the Pacific Northwest and eases the pressure gradient. Winds turn
southerly and increase again to 25-30 kts ahead of a cold front.
Brief, isolated gales are possible ahead of the cold frontal passage
Sunday night into Monday, but confidence is low (less than 30%).
Periods of moderate to heavy rain are possible. Stronger southerly
winds are possible by Tuesday ahead of another frontal passage.
JB/EYS
&&
.FIRE WEATHER...This afternoon, RH is expected to drop into the
teens in many areas across the interior with coastal clouds
eroding. Northerly winds will surge south through the Sacramento
Valley and over the higher terrain of eastern Lake County. This
direction is favorable to be terrain enhanced, and will create
some locally critical fire weather conditions given the dry
minimum RH values. The critical conditions will be localized and
mainly over the higher terrain. Tonight poor RH recoveries are
expected over the higher terrain with moderate recoveries in the
valleys. Winds will mainly be light and terrain driven limiting
the impact of the dry air. Saturday night into Sunday RH is
expected to increase ahead of a strong cold front. At the same
time southerly winds are expected to increase. The tricky part
will be which one increases first. There could be some periods and
locations of increased fire danger if the winds come up before
the RH does. Confidence is fairly high that most areas will see a
wetting rain starting Sunday late evening in the north and
spreading south into Lake county before midday Monday. Rainfall
amounts are expected to range from a 0.75 inch to 1.5 inches with
the heaviest amounts in the north and in the higher terrain.
Additional moderate to heavy rain is expected on Tuesday before
drier conditions return Wednesday. Southerly winds may be stronger
with the second front Tuesday.
&&
.EKA WATCHES/WARNINGS/ADVISORIES...
CA...
None.
NORTHWEST CALIFORNIA COASTAL WATERS...
     Small Craft Advisory until 3 PM PDT Saturday for PZZ450.
Small Craft Advisory until 11 PM PDT Saturday for PZZ455.
Gale Warning until 3 AM PDT Saturday for PZZ470-475.
Hazardous Seas Warning from 3 AM to 11 AM PDT Saturday for
     PZZ470-475.
&&
$$
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