444
FXUS66 KEKA 221340
AFDEKA

AREA FORECAST DISCUSSION
National Weather Service Eureka CA
540 AM PST Fri Nov 22 2024

.SYNOPSIS...A strong atmospheric river storm system will continue
to bring widespread moderate to heavy rainfall through today,
resulting in life threatening flooding. Moderate to locally heavy
rain, mountain snow and isolated thunderstorms will follow this
evening through Saturday afternoon.


&&

.DISCUSSION...A strong atmospheric river storm will continue
impacting the Northwest California with moderate to heavy rainfall
through late morning/early afternoon. Key messages are:

*Life-threatening and dangerous flooding today.

Satellite imagery depicts a strong cyclone (area of low pressure)
located around 300mi offshore the OR/WA border, while a strong
atmospheric river continue streaming across the area this morning.
Very active weather continued overnight across Northwest
California with moderate to heavy rainfall and strong southerly
winds. Flash flooding, rock and mudslides, down trees, and rivers
reaching flood stage were observed. Strong gusty winds from 40 to
60 mph were observed along the coastal headlands and exposed
ridges overnight. Although winds has been diminishing, a tight
pressure gradient combined with a 40-60 kts jet streak around
925-850 mb continue promoting gusty southerly winds early this
morning. Gusts of around 40 to 55 MPH prevail over the higher
terrain in Del Norte and Humboldt counties. Gusty winds are
expected to continue over the highest elevations above 2,000 feet
with gusts between 30 to 50 MPH through late morning.

Moderate to heavy rainfall rates are forecast to continue through
at least this morning. The widespread heavy rain rates will wind
down during the afternoon, but flood waters will be slow to drain.
Additional rain and shower activity is still expected during the
afternoon and some showers may be locally heavy as cooler air
aloft arrives with an upper trough.

A colder air aloft will subsequently spread across the West Coast
this afternoon and evening. As result, instability will increase
to around 100- 300 J/kg of CAPE. Instability could allow for low-
topped thunderstorms (15 to 40% chance) over the waters and right
along shore. Another round of periods of moderate to locally heavy
rainfall are anticipated. HRRR model indicated max hourly rain
rates around 0.15 to 0.30 inches. Additional rainfall amounts from
this system are forecast to range from 0.75 to 2.0 inches from
today through late Saturday afternoon. The highest amounts are
expected across the southwest windward facing terrain with total
rainfall up to 3 inches.

Snow levels across NE Trinity has rises to around 8000 to 9000
feet overnight, resultant in stratiform precipitation. In the
wake of the cold front, snow levels are expected to lower between
5000 to 4000 feet this evening into Saturday morning. Thus, up to
4 inches will be possible above 4000 feet and could impact travel
on Highway 3 at Scott Mountain Pass late this afternoon through
Saturday afternoon. More significant snowfall is expected for the
Trinty Alps. A Winter Storm Advisory is now in effect from 4 PM
this afternoon to 4 PM Saturday for elevations above 4000 feet in
portions of NE Trinity County.

A brief break will be possible over the weekend, before another
series of fronts impacts the area Sunday through Tuesday. These
fronts will be progressive and have less moisture. 6-hourly
rainfall and snowfall rates and storm total rain and snow amounts
are forecast to be much less than our current storm with less
potential for major impacts. In addition, strong east

Upper ridging is forecast to restore more peaceful and dry weather
mid to late next week, though after multiple days of rain, valleys
will no doubt fog up during the night and morning hours. ZVS



&&

.AVIATION...Moderate to heavy rainfall continues this morning as a
strong atmospheric river provides a steady stream of moisture to
NorCal. Periods of MVFR/IFR conditions will continue intermittently
as locally heavy rainfall reduces visibility`s and lowers ceilings.
With the core of the LLJ having shifted south, elevated southerly
winds (15 to 2 5 knots) will continue at UKI while the coastal
pressure gradient briefly weakens at the northern coastal terminals.
Light southerly flow will rapidly increase this evening and
overnight with the arrival of an additional surface low and strong
frontal boundary. LLWS is possible for a few hours this evening
before winds mix down to surface levels. Gusts exceeding 50 knots
are possible at CEC in the few hours before and after midnight.


&&

.MARINE...Elevated wave heights from a NW swell have finally
diminished to 12 to 13 feet after a brief resurgence Wednesday
evening. Small craft conditions and light southerly flow will
continue through this afternoon in all but the southern inner
waters, where gale gusts 35 to 40 knots persisted through the night
in the core of a LLJ near Point Arena. Gale conditions will rapidly
spread north into the remaining zones this evening with the arrival
of an additional low pressure system and strong surface front. High
end gusts may exceed 50 knots in the outer waters in the hours
before and after midnight. This fast moving system will generate
rapid wind wave response, driving wave heights back up to 15 to 17
feet early Friday morning. Winds and seas are expected to quickly
diminish this weekend, potentially falling below advisory levels
as the NW swell continues to decay. Unsettled conditions are
likely to develop late this weekend into early next week with
additional upper shortwaves moving through the PNW waters.


&&

.HYDROLOGY...Life-threatening flooding through today. Main stem
rivers, smaller streams and creeks continued to rise sharply today
in response to widespread moderate to heavy rainfall last night
through today. Moderate to heavy rainfall rates will continue
through this morning as a long duration and high magnitude
atmospheric river intersects the coastal terrain. Even after the
widespread heavy rain subsides through the day, flood waters will
be slow to recede. Do not let your guard down or attempt to drive
across flooded roads. Additional period of moderate to locally
heavy rain is expected this evening through Saturday afternoon.

Soils are nearly completely saturated and streamflow are all
running higher. Thus, flash flooding, rock and mudslides, down
trees, and rivers reaching flood stage were observed. Additional
rockslides and landslides are expected to increase as soils become
completely saturated. As of 4AM PST, the Eel River has reached
moderate flood stage (22 ft) and is expected to crest to 25 feet
this afternoon promoting significant flooding continuing through
Saturday morning. The Russian River at Hopland has reached minor
flood stage (15 feet) early this morning. Other main stem rivers
such as the Van Duzen at Bridgeville, Mad River at Arcata, and
Navarro River at Navarro are also forecast to reach minor flood
stage today. Many other smaller streams and creeks will likely
reach bankfull or come out their banks. Stay tuned to our latest
warnings and advisories. DB/ZVS


&&

.EKA WATCHES/WARNINGS/ADVISORIES...
CA...
     Flood Watch through this evening for CAZ101>106-108>115.

     High Wind Warning until 8 AM PST this morning for CAZ102-
     104>106.

     Winter Weather Advisory from 4 PM this afternoon to 4 PM
     PST Saturday for CAZ107.

NORTHWEST CALIFORNIA COASTAL WATERS...
     Gale Warning until 9 AM PST this morning for PZZ450-455-470-
     475.

     Small Craft Advisory from 9 AM this morning to 9 AM PST
     Saturday for PZZ450-455-470-475.

&&

$$

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For forecast zone information
see the forecast zone map online:
https://www.weather.gov/images/eka/zonemap.png