805
FXUS66 KEKA 012051
AFDEKA

AREA FORECAST DISCUSSION
National Weather Service Eureka CA
151 PM PDT Mon Sep 1 2025

.SYNOPSIS...Hot temperatures likely through Wednesday for the
interior. Stratus likely for the coastal areas with little clearing
in the afternoons for the coming days. Isolated dry thunderstorms
possible in the interior Tuesday afternoon and evening.


&&

.DISCUSSION...The upper level low off the Pacific Northwest is
pushing north towards the Gulf of Alaska, allowing high pressure to
build into northern CA. This is expected to bring warmer
temperatures this afternoon with most areas seeing 3 to 5 degrees of
warming and high temperatures in the mid to high 90`s. This is also
expected to strengthen the marine inversion and the immediate coast
is not expected to see too much clearing this afternoon. Stratus is
likely to push inland again tonight. Smoke and haze impacts are
mostly confined to Siskiyou County and in the immediate vicinity of
the fires. Some pooling in the Klamath River Valley occurred this
morning with unhealthy to poor air qualities reported. Air quality
will improve in the afternoon, but smoke is likely to return
overnight.

A shortwave moving up from the southwest arrives Tuesday, bringing
moisture, instability, and breezy winds. Both moisture and
instability are marginal and there is a weak capping inversion that
may prevent storms from forming. Dry low levels and DCAPE over 1000
J/kg support gusty outflow winds of up to 40-60 mph for any storms
that do form. CAMs are starting to resolve some activity, but
confidence still remains low. The highest chances remain in Trinity
County, but some chances exist in interior Del Norte, interior
Humboldt, interior Mendocino, and perhaps far northern Lake.

Winds are likely to be breezy overall in the interior, especially in
the higher terrain and in Trinity County. Greater instability is
likely to mix down stronger winds. 925 mb winds are peaking around
20 kts, so southeast gusts up to 25 to 30 mph are possible.
Thunderstorms may bring even more elevated gusts.

Wednesday the shortwave moves off to the north and the weather
quiets down. An approaching upper level trough is expected to
gradually approach the area from the west pushing the ridge off to
the east. Temperatures are expected to start to diminish, Wednesday
this is expected to be only a degree or two cooler in most areas and
another few degrees Thursday. There is some uncertainty on what the
marine layer will do, there is some weak offshore flow Wednesday and
Thursday mornings, but it doesn`t really look like it will be enough
to clear the coast completely. It may help push clouds back to the
coast in the afternoon. Friday and Saturday more significant cooling
is expected with interior highs in the high 80`s to low 90`s Friday
and the 80s mainly on Saturday. The inversion will be fairly weak by
this point and it may allow the coast to clear out. MKK/JB


&&

.AVIATION...An upper level trough lies just off the coast, with
still a positive height anomaly along the coast. Surface high
pressure is still dominant in the northeast Pacific. A strong
marine inversion has allowed a dense deck of stubborn stratus to
move onshore, impacting coastal terminals, while inland locations
remain clear. This pattern will persist for the next 24 hours.

For the coastal terminals at Arcata (KACV) and Crescent City (KCEC),
the primary forecast challenge is the persistence and depth of the
marine stratus. Observations confirm that both terminals were
reporting IFR and LIFR conditions. The latest NBM probabilistic
guidance suggests only a 30-40% chance of MVFR ceilings or worse
this afternoon, but the strength of the inversion suggests that this
will be a persistent stratus event, and any scattering or clearing
of ceilings will be limited from much diurnal improvement.
Confidence is high (70-80%) that ceilings will lower back into the
LIFR category (below 500 ft) again tonight. This will occur as
strong radiational cooling at the top of the stratus layer occurs,
lowering the inversion height, and re-establish a very low cloud
deck. Conditions are expected to improve by late Tuesday morning.

For the inland terminal at Ukiah (KUKI), VFR conditions are expected
to prevail through the entire forecast period with clear skies. The
NBM shows a less than 10% chance of any ceilings impacting the
terminal. The main forecast concern for Ukiah will be a period of
gusty west to southwest winds this afternoon and evening. There is
about a 25% chance that wind gusts will exceed 20KTS between 21Z and
04Z, which is typical for a strong onshore gradient day. Mean 24
hour maximum wind gusts are 18 KTS. Wind will decouple and become
light and variable overnight. /MH



&&

.MARINE...

Synopsis -

Steep seas, driven primarily by short to medium period northwest
wind waves, will continue to be the primary marine hazard today.
Surface high pressure remains in control of our weather, resulting
in a persistent, strong northwesterly wind field, particularly over
our southern outer waters near and just south of Cape Mendocino.
These conditions will lead to hazardous conditions for small craft,
with Small Craft Advisories (SCA) in effect for southern zones
today, joined by northern outer zone later tonight into Tuesday.

Today and Tonight -

An analysis of local buoy observations confirms northwest flow
(robust in ares) along the entire coast today. High pressure
centered over the eastern Pacific is driving this pattern.

The most significant hazard remains the steep seas, generated
primarily by locally generated wind waves from the northwest,
although there is a prominent low-amplitude long period south swell
(14-16 seconds). Observations indicate combined seas around 3-6 feet
today. Probabilistic guidance from the NBM shows only a about a 10%
chance of combined seas above about 5 feet through this evening for
the northern outer zone and about a 10% chance of above 6-8 feet for
the southern outer zone. The steepness of these shorter period waved
are the main concern, despite the lower overall combined heights.

Winds are the other main story. Latest NBM probabilistic guidance
shows a 70-90% chance of frequent gusts exceeding 25 knots in our
outer waters, especially south, and in a fan south of Cape
Mendocino. This gives high confidence that SCA conditions will be
met. Furthermore, there is even a 20-30% chance of frequent gusts
reaching Gale force (34 knots) in the immediate vicinity south of
the Cape this evening, a risk that warrants close monitoring.

Tuesday through Sunday night -

The surface pressure gradient will modestly ease by Tuesday
afternoon and evening. This will allow northerly winds to ease
slightly. The probability of SCA-level winds (sustained 21+ KTs) in
southern waters drops from near 100% today to about 50-60% during
the afternoon hours on Tuesday, suggesting conditions will be more
borderline. Seas will also begin to slowly subside.

By late week, ensemble guidance shows high agreement on a more
benign pattern with no significant storms on the horizon. The
primary marine concerns will transition to more localized and
diurnal wind patterns. The probability of widespread, advisory-level
winds falls below 20% for all zones from Friday afternoon onward. /MH


&&

.FIRE WEATHER...This afternoon RH is expected to be in the single
digits to teen`s once again. Poor overnight RH recoveries around 25
to 35 percent are forecast over the higher terrain, while good to
moderate recoveries for areas below the inversion (<2000 feet).
Warmer temperatures are expected today through mid week.

A weak upper level trough will bring some moisture and moderate
instability across the area on Tuesday. This will bring isolated dry
thunderstorms, primarily to Trinity County. The Yolla Bollys,
northern Lake County, and far eastern Humboldt and Del Norte
Counties may see a storm or two, but models are generally trending
downward in those areas. Storms that do form will have a strong
potential for downburst bringing gusts of 40 mph or possibly
stronger. Even if no storms form, fire danger is expected to
increase. The instability will increase ventilation and allow for
more fire growth potential. This will also help to mix down some
gusty winds and gusts of 25 to 30 mph are possible in the higher
terrain and exposed areas.

Winds trend downward Wednesday, but warm temperatures and very low
afternoon RH with poor overnight recoveries in the high elevations
are forecast to continue through Thursday. MKK/JB


&&

.EKA WATCHES/WARNINGS/ADVISORIES...
CA...
     Red Flag Warning from 1 PM to 8 PM PDT Tuesday for CAZ204-
     283.

NORTHWEST CALIFORNIA COASTAL WATERS...
     Small Craft Advisory until 3 PM PDT Tuesday for PZZ455-475.

     Small Craft Advisory from 11 PM this evening to 3 PM PDT
     Tuesday for PZZ470.

&&

$$

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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