705
FXUS66 KEKA 040808
AFDEKA

AREA FORECAST DISCUSSION
National Weather Service Eureka CA
1208 AM PST Tue Nov 4 2025

.SYNOPSIS...The risk for strong and damaging wind gusts will
increase tonight and persists into Wednesday. Widespread heavy
rain will increase the risk for urban and small stream flooding
tonight and Wednesday. Additional moderate to heavy rainfall is
forecast late Thursday through early Friday morning.

&&

KEY MESSAGES:

- Strong Atmospheric River to bring strong and damaging winds,
  widespread heavy rainfall and a risk for urban and small stream
  flooding late Monday through Wednesday.

- Higher than normal tides next week with storm surge and a large swell
  will promote coastal flooding along a large portion of the
  coastal areas Wednesday and Thursday.

- A weaker Atmospheric River will bring additional moderate to
  heavy rainfall and breezy southerly winds Thursday and into
  Friday

&&


.DISCUSSION...A sharp moisture axis with increasing southerly
winds will first take aim over and inland of Cape Mendocino, and
then south through western Mendocino County. The integrated water
vapor transport (IVT) values will begin to sharply increase over
this described area tonight as the winds and precipitable water
values trend upward. Since this axis of persistent moderate to
heavy rainfall will be nearly stationary for nearly 15 hours, a
flood watch has been issued for the described area starting late
tonight.

This axis of moisture and moderate to heavy rainfall will begin
shifting northward Tuesday afternoon as a deep and negatively
tilting trough with an associated strong Pacific cyclone pushes
eastward into the Pacific Northeast. As the cyclone progresses, the
strongest portion of the Atmospheric River will take aim at N CA
Tuesday evening. Persistent moderate rainfall with a period of
widespread heavy rain can be expected late Tuesday and into
Wednesday.

The IVT values continue to trend higher for this system, with peak
values over 1000 kg m/s. This equates to Strong AR criteria, and
there is even a 40% chance for higher values that border Extreme
criteria in localized areas south of Cape Mendocino. These higher
values are likely due to the longer duration of the AR over that
area. HREF depicts a high probability (60-80%) of 6 hour rainfall
greater than 2" for portions of the coast and interior ridges.
Moreover, HREF Probability Matched Mean (PMM) has 1-2 inches in 3
hours for Cape Mendocino and the interior ridges. This system is
more unique in that it coincides with high astronomical tides.
This will contribute to coastal flooding and will likely cause
some urban issues where the heavy precipitation coincides with
high tides near waterways, such as with storm drains. See the
Coastal Flood and Hydrology section below.

Sharp rises to rivers are forecast, but flood stage to the main stem
rivers is not anticipated due to the low antecedent levels. Urban
and small stream/creek flooding is however expected. Additional
Flood Watches are now out for the majority of the forecast area from
Lake County to Del Norte County. The heaviest rainfall will fall
in a shorter duration over southern Mendocino to Lake County
Wednesday, but anticipated heavy rainfall rates may bring some
minor hydrological issues as it passes through. There is a
potential for burn scar debris flows, the main area to watch is
the Smith River Complex in interior Del Norte County where a lower
calculated hourly estimated rainfall rate (1 inch per hour) for a
debris flow has been established. Confidence was not high enough
to move froward with a Flash Flood Watch for that burn scar at
this time, but as high resolution guidance comes into better
focus, this may need to be reevaluated.

As the cold, negatively tilted trough crashes into the N CA Coast
Wednesday, precipitation will become more convective in nature, with
thunderstorms likely (20-25%). Regionally high surface CAPE (400-500
J/kg) will arrive as broad forcing for ascent increases with
steepening lapse rates. This occurrence more rarely coincides with
remaining strong low level southerly winds. The soundings look
impressive with strong shear and a veering wind profile. 925 winds
will also be 40 to 45 kts, so some of the more robust convective
showers or thunderstorm activity would be very capable of strong to
perhaps severe wind gusts over the immediate coast along and north
of Cape Mendocino. A waterspout threat is looking probable in
this environment, and velocity signatures will have to be closely
watched as oceanic storms approach the coast.

Strong winds will begin to develop over the ridges of Southern
Humboldt through Cape Mendocino tonight. These strengthening
winds have trended to arrive earlier, and may require a sooner
start to the wind products. Winds will further strengthen
throughout the day Tuesday, and will be strongest Tuesday
afternoon/night. Strong damaging south winds are probable (60-90%
chance) as surface pressure gradient tightens later Tuesday and a
low level jet at 925mb up to 70 kts develops along the coast. A
well- mixed boundary layer near or along the surface front will
allow for higher momentum air to mix down to the surface. There is
a 40-60% chance of gusts greater than 40 mph for the lowest
elevations. For higher elevations and coastal headlands, NBM
indicates a 60-90% chance for wind gusts greater than 45 mph in
Del Norte and Humboldt counties. ECMWF ensemble mean 6-hourly wind
gusts are over 50 mph for the Del Norte Coast, specifically
Crescent City and over 40 to 50 mph for the greater Humboldt Bay
area, Arcata, Eureka and Fortuna. There are more extreme members
from 60-75 mph in the ECMWF distribution. Stronger wind gusts over
55 mph are highly probable over the coastal mountains. A high
wind watch has been issued as this appears to be a highly
anomalous wind event for this time of year. Would not be surprised
if some the higher elevations venturi- effect RAWS gusts over 70
mph, specifically in SW Humboldt in the King Range. Otherwise,
gusts from 35-55 mph appears more likely for the population
centers. Winds this strong can still rip branches off trees and
may even bring a few weaker trees down. Wind Advisories and High
Wind Warning are out and may require expansion or earlier start
time due to recent trends.

A secondary system with the characteristics of a weak Atmospheric
River will move in Thursday through Friday will take aim on the
Pacific NW Thu-Fri and will bring another round of moderate to
locally heavy rain to NW CA. Once again, there will be a risk for
urban and small stream flooding, as well as rock and mudslides in
steep terrain. Additional Flood Watches may be required due to
the recently saturated ground. A break in the rain is forecast
later on Friday into Saturday as the mid level flow amplifies in
advance of another trough that is forecast to dig over the central
Pacific. The frontal boundary may stall offshore or slow down and
take til Sun or Mon to arrive. There is currently a 60% chance
for a Weak Atmospheric River, but there is notably a 25% chance
for a Moderate Atmospheric River. JJW


&&

.AVIATION...06Z TAFs...The first frontal system continues to move
through the region. Into Tuesday morning, the front will be absorbed
by the incoming larger frontal system. MVFR to IFR conditions are
likely through this TAF period with periods of rain. Rain will pick
up for the region by Tuesday afternoon and spread south by very
early Wednesday morning. Strong wind gusts along with LLWS are
expected as well at most terminals. Gusts to 40 knots could occur at
coastal terminals by Wednesday. KUKI could see gusts to 30 knots by
through Tuesday.

&&

.MARINE...Tuesday, a strong front passing overhead will drastically
increase the threat for mariners. A Storm Warning has been issued
for the coastal waters with storm force gusts very likely. Wind
waves in response to the strong winds will be hazardous, steep, and
large, up to 15-17ft, with winds carrying ocean spray degrading
visibilities.

After this upcoming strongest frontal passage, a large swell will
fill into the coastal waters with peak significant heights up to 18-
19ft @ 14s. A hazardous seas warning will most likely be needed if
the forecast remains consistent. This hazard will be hoisted in time
after the first frontal passage to minimize confusion due to the
many hazards that have been hoisted for the NW CA coastal waters.
DS

&&

.HYDROLOGY...A sharp moisture axis with increasing southerly winds
will first take aim over and inland of Cape Mendocino and south
through western Mendocino County late tonight. This axis of
persistent moderate to heavy rainfall will be nearly stationary
for nearly 15 hours, thus a flood watch has been issued for the
described area starting late Monday night. Widespread heavy rain
expected late Tue into Wed when a risk for urban and small stream
flooding will increase for much of the region. Widespread 0.30
to 0.60 inch per hour rainfall rates are forecast, with up to 1.0
inch per hour over the higher terrain. Thunderstorm activity may
bring higher rates Wednesday.

This system is more unique in that it coincides with high astronomical
high tides. his will contribute to coastal flooding and will
likely cause some urban issues, such as with storm drains around
where heavy precipitation falls during high tides near waterways.
More rain is probable for Del Norte and Humboldt Thu and Fri, but
influx of moisture appears somewhat lower. This will need to be
watched for minor flooding too. All main stem rivers are forecast
to remain below monitor/action stage through at least Wed morning,
but expect some sharp rises. JJW

&&

.COASTAL FLOODING...Starting on Tuesday, astronomical high tides
will approach or exceed 8 ft at the North Spit tide gauge. The
highest tides, at around 8.5 ft will in occur in the late mornings
through midday on Thursday and Friday. Strong southerly winds are
forecast through this time further contributing to a positive
tidal anomaly. Water levels may reach 10 ft at North Spit when
high tides align with the strong southerly winds on Wednesday
morning. A slightly higher tide is forecast Thursday with lighter
southerly winds. A Coastal Flood Advisory has been issued, with
the possibility of a Coastal Flood Warning around Humboldt Bay if
confidences to increase for the southerly wind and tide anomaly.
The combination of high tides, heavy rainfall, large surf, and
strong southerly winds may require an expansion of an advisory
along much of the coast for Thursday. DS

&&

.EKA WATCHES/WARNINGS/ADVISORIES...
CA...
     High Wind Warning from 4 PM Tuesday to 9 AM PST Wednesday
     for CAZ101-103.

     Flood Watch from Tuesday evening through Wednesday morning
     for CAZ101>103-105.

     High Wind Warning from 2 PM Tuesday to 2 PM PST Wednesday
     for CAZ102-104>106.

     Coastal Flood Watch Wednesday morning for CAZ103.

     Flood Watch from 3 AM PST Tuesday through Wednesday morning
     for CAZ104-106-109-110.

     Flood Watch from late Tuesday night through Wednesday
     morning for CAZ108-111>115.

     Wind Advisory from 7 PM Tuesday to 9 AM PST Wednesday for
     CAZ109.

     Wind Advisory from 3 PM Tuesday to 3 PM PST Wednesday for
     CAZ110.

     Wind Advisory from 7 PM Tuesday to noon PST Wednesday for
     CAZ111>115.

NORTHWEST CALIFORNIA COASTAL WATERS...


     Small Craft Advisory from 4 AM to 1 PM PST Tuesday for
     PZZ450-455.

     Storm Warning from 1 PM Tuesday to 7 AM PST Wednesday for
     PZZ450-470-475.

     Storm Warning from 1 PM Tuesday to 4 AM PST Wednesday for
     PZZ455.

&&

$$

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