Area Forecast Discussion
Issued by NWS Eureka, CA

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000
FXUS66 KEKA 201302
AFDEKA

AREA FORECAST DISCUSSION
National Weather Service Eureka CA
502 AM PST Sat Jan 20 2018

.SYNOPSIS...After a mostly dry Saturday, wet and windy weather can
be expected most of the day Sunday, with significant snow possible
across primarily Trinity County. Monday will be mostly dry, but
wet weather is expected to return towards the middle of the week,
particularly late Tuesday through at least Thursday.

&&

.DISCUSSION...

.SATURDAY...While shower coverage continues to gradually diminish
along the coast as of 3 AM, waves of moderate showers continue to
impact southern Humboldt county from roughly Patrick`s Point to
the Lost Coast. These showers are not particularly strong, but may
be capable of producing bursts of very small hail. Meanwhile,
snow levels are hovering between 2000 and 2500 feet, so some of
the taller coastal ridges may continue to see some light snow as
well. Shower activity will continue to wane through the day and
become very sporadic to almost nonexistent by this evening.
Meanwhile, temperatures are expected to be cool this afternoon,
ranging from the low 50s in the coastal lowlands to the mid 40s
across interior valleys.

.SUNDAY STORM SYSTEM...

.RAIN...Attention then turns to an approaching cold front which
will bring another round of substantial rainfall, strong high
elevation winds, and substantial accumulating snows for some areas
on Sunday. Precipitation is expected to begin very early Sunday
morning in parts of Del Norte county and gradually move southward
through Humboldt, Trinity and Mendocino counties during the day
and evening. The last of the precipitation is expected to end
across southern Mendocino county by very early Monday morning.
Rainfall rates will likely peak during the afternoon hours, and
are expected to be quite substantial across the coast ranges of
southern Humboldt and Mendocino County, in particular. By the time
all is said and done, storm total accumulations will likely range
from 2 to 3 inches in these areas, perhaps approaching 4 inches
in the King Range of southwestern Humboldt county. Rain rates of
up to .5 inches per hour will be possible at times in this area.
Elsewhere, total accumulations of 1 to 2 inches can be expected
below the snow line. While rain rates will be heavy at times,
significant hydrologic impacts and flooding are not expected aside
from nuisance ponding on roadways and swollen creeks. In
addition, typical falling rocks and rockslides will also be
possible along mountain roads.

.SNOW...As mentioned previously, some areas are in line to see
significant snow accumulations. The timing of the onset of
precipitation will play a major role here, as it is currently
expected to begin during the morning when temperatures are at
their lowest. Meanwhile, this precipitation will initially fall
through a layer of dry air in the low levels of the atmosphere and
partially evaporate, which will further cool the atmosphere.
Combine this with persistent easterly and southeasterly winds
through the day, and you have a recipe to keep cool temperatures
locked into place and snow levels low throughout much of the event
on Sunday. This phenomenon will primarily affect Trinity County
in areas east of South Fork Ridge, and especially in the vicinity
of Trinity Lake. Accumulations will vary widely throughout the
county at at differing elevations, but in general, snow
accumulations are expected to be heaviest along the Highway 3
corridor, particularly north of Highway 299 where temperatures
will remain coldest through the day. Accumulations of 4 to 7
inches between 2500 and 3500 feet will be possible here, with 14
inches or more possible at higher elevations...including Scott
Mountain Pass where snow is not typically removed on weekend days.
Accumulations will likely be lower along the Highway 36 corridor
in Southern Trinity County, but still significant enough to cause
travel impacts. Snow will likely accumulate near Buckhorn Summit
on Highway 299 as well. Other areas closer to the coast are not
expected to maintain this cold air and low snow levels through the
day, and as such rain will be the primary precipitation type
aside from the taller peaks.

.WIND...Strong southerly and southeasterly winds are also
expected to impact the region, particularly from Sunday morning
through Sunday afternoon. The strongest winds are expected to
occur at high elevations and along coastal headlands of Humboldt
and Del Norte counties, where gusts to 45 mph and possibly higher
can be expected. Gusty winds and also be expected in the Eel River
Delta and around Humboldt Bay, with gusts to 40 mph possible here
as well. Wind speeds will quickly diminish behind the cold front
as it passes through the area early in the evening.

.MONDAY THROUGH FRIDAY...Aside from perhaps some lingering light
showers at high elevations, Monday will be generally dry with cool
afternoon temperatures. This break in the activity will be short-
lived, though, as another storm system will bring rain, wind and
high elevation snow during the middle of the week. Uncertainty is
still relatively high at this stage regarding when precipitation
will start, particularly with regards to potential warm-frontal
precipitation on Tuesday. Regardless, it appears that the heaviest
precipitation and strongest winds can be expected sometime
between Tuesday night and Wednesday when the cold front moves on
shore. Another round of showers and lowering snow levels is
expected Wednesday night through potentially Friday, although
uncertainty in potential snow amounts remains high.

A building ridge of high pressure over the weekend is expected to
push the storm track farther north into the Pacific Northwest
over the weekend, resulting in a drying trend. /BRC

&&

.AVIATION...Sporadic shower development will impact aviation
interests during Saturday morning, followed by shower dissipation
during Saturday afternoon. A strong storm system will approach the
region late tonight, with frontal rains spreading across the
northern coast by 12Z Sunday. Beyond the immediate TAF period,
gusty south-southeast winds appear probable during Sunday
afternoon, and will be accompanied by heavy rain and wind shear at
KACV and KCEC. Farther inland, mountain snow is expected over
Trinity county during Sunday afternoon, and moderate rains can be
expected at KUKI and surrounding areas.

&&

.MARINE...Model guidance indicates wave heights associated with
an ongoing long period swell will gradually subside during
Saturday afternoon. However, southerly winds will be on the
increase Saturday night into Sunday as a frontal system approaches
the region. At this time, gale force winds appear probable over
the northern outer waters...thus marine zone 470 has been upgraded
to a warning. A gale watch remains in place for the remainder of
the coastal waters. Wind waves from 10 to 12 feet will combine
with another 14-16 foot westerly swell by Sunday afternoon,
resulting in a chaotic sea state. Seas and winds are forecast to
relax during early next week. This will be followed by potential
small craft southerlies on Tuesday. Another westerly swell near 16
feet at 14 seconds will spread across the waters by Thursday.

&&

.EKA WATCHES/WARNINGS/ADVISORIES...
CA...Winter Storm Watch from Sunday morning through Sunday evening
     for CAZ107-108.

     Wind Advisory from 10 AM to 4 PM PST Sunday for CAZ101-102-
     104>106.

NORTHWEST CALIFORNIA COASTAL WATERS...Small Craft Advisory until 3 AM PST Sunday for PZZ450-470-475.

     Gale Warning from 3 AM to 5 PM PST Sunday for PZZ470.

     Gale Watch from Sunday morning through Sunday afternoon for
     PZZ455.

     Small Craft Advisory until 10 AM PST Sunday for PZZ455.

     Gale Watch from late tonight through Sunday afternoon for PZZ450-
     475.

&&

$$

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