Coastal Flood Warning 1, NWS Eureka, CA

AREA FORECAST DISCUSSION
National Weather Service Eureka CA
419 PM PST Mon Jan 9 2017

.SYNOPSIS...Scattered thunderstorms are expected along the Del
Norte and Humboldt county coastline tonight, with numerous mostly
harmless showers elsewhere. Heavy rain is expected Tuesday morning
through evening, with more showers and lowering snow levels
Tuesday night. Strong winds are also expected in many areas
Tuesday, along with flooding of main stem rivers, small streams,
and coastal areas near the tidal zone.

&&

.DISCUSSION...Weather conditions continue to closely match
forecasted conditions for the most part, as most of the region is
reaching the tail end of a relative calm before the storm. Light
showers have continued across primarily the interior hills today
with mostly dry but cloudy conditions right along the immediate
coast. Accumulating snow levels have generally ranged from around
3300 feet near the coast to around 2500 to 3000 feet across Trinity
county, although a few protected valleys have seen light snow to
around 2000 feet. Meanwhile, river levels continue to recede after
several reached above flood stage in the past 24 hours, although the
Eel River at Fernbridge remains slightly above flood stage as of
this writing (330 PM). Several instances of flooding have been
observed throughout the region, particularly along the Eel,
Vanduzen, Navarro, and Russian rivers. Additional flooding of these
rivers and others is likely tomorrow, with more details written
below. Winds have been generally breezy but not damaging today, but
much stronger winds can be expected tomorrow. The item of most
pressing concern in the near term is the approach of a broken line
of thunderstorms off the Humboldt/Del Norte coasts, which will move
onshore around 4 PM this afternoon bringing lightning and localized
strong wind gusts.

.HEAVY RAIN/HYDROLOGY...Another round of heavy rain is expected to
move through the region over the next 24 to 36 hours as a series of
potent shortwaves moves onshore. By the time the heavy rain ends
Wednesday morning, most of the region will see an additional 3 to 7
inches of rain, with a slight preference towards Humboldt and Del
Norte counties with the heaviest totals. As usual, the Humboldt Bay
region and Trinity/Mendocino interior valleys will measure the
lowest totals, with the highest amounts in the mountains. Rain rates
will begin to increase this afternoon as a line of thunderstorms
moves onshore, and this increased shower activity will continue
through the night. However, the heaviest period of rain will likely
occur tomorrow morning through late afternoon as the stronger of two
storm systems moves onshore. Flooding is expected to become an issue
in many areas tomorrow due to excess rain and saturated soils,
including many main stem rivers, small streams, low lying rural
and urban areas, and coastal communities near the tidal zone.
Rivers forecast to reach flood stage as of this writing include
the Eel River at Fernbridge, Van Duzen River at Bridgeville,
Navarro River at Navarro, Russian River at Hopland, and Mad River
at Arcata. This will almost certainly result in flooding along
Highway 128 near Navarro, 175 in Hopland, throughout many parts of
the Eel River Delta, and portions of highway 36. Flooding issues
may crop up along parts of highway 1 as well, particularly near
the Garcia River. Several other rivers are forecast to reach
monitor stage as well, and some very near flood stage...including
the Klamath and Smith rivers, among others. Peak flows are
expected to occur Tuesday evening through Wednesday morning, with
the larger rivers like the Eel and Klamath rivers being a bit
slower to recede given their larger basins.

.WIND...Strong and potentially damaging winds are expected tomorrow
morning through much of the day for many areas, with an extra
emphasis on high elevation ridges. Some damaging wind gusts will be
possible along the immediate coast, and in particular on exposed
coastal headlands like Patrick's Point and Point Saint George,
including Crescent City. Gusts to 50 or 60 mph are almost certain
along many coastal ridges, with gusts even higher than that possible
as well. Gusts to 35 to 45 will be more common along the coast, with
occasionally stronger gusts there as well. Recently saturated soils
will likely make trees more vulnerable to toppling as well, and
this was already seen during the strong winds that occurred earlier
in the week. Winds will diminish following the cold frontal
passage Tuesday night. Very localized strong gusts will be
possible with any thunderstorms that move onshore this evening as
well.

.SNOW...Snow levels are expected to slowly rise to around 4000 feet
overnight across mainly areas west of Trinity county tonight, which
will keep winter weather impacts to a minimum across Humboldt, Del
Norte and Mendocino counties. However, snow levels will be slow to
rise across Trinity county, and as precipitation rates increase
additional light snow accumulations will be possible this evening
and overnight. This will primarily impact the high passes of Highway
36 and 3, the former of which has become an important east/west
travel corridor due to the closure of highway 299. Expect another 1
to 4 inches of wet, heavy snow in these areas. Snow levels will rise
to around 4500 feet ahead of tomorrows storm, which should keep most
snow above pass levels during the day tomorrow. Higher elevation
areas will likely see snow measured in feet, but this will likely be
restricted to wilderness areas like the Trinity Alps.

Snow levels are expected to drop quickly behind the cold front
Tuesday night, and convective showers will move onshore through
early Wednesday and maybe slightly longer. Light snow will be
possible at pass levels during this period, although accumulation
amounts are uncertain. A winter Weather Advisory may need to be
considered in future forecasts as this becomes more clear.

.LIGHTNING...A line of broken thunderstorms will move onto the
northern Humboldt coast this afternoon, which may bring some strong
gusts and occasional lightning strikes to localized areas. Waves
of these thunderstorms will be possible through the evening and
early overnight hours before instability and dynamic forcing
wanes.

.LONG TERM...A building ridge will result in a nice break from the
wet and windy weather Thursday and Friday, allowing water levels to
recede. Furthermore, the pattern beyond Friday looks to be a bit
less active than previously anticipated, although additional rounds
of what appears to be light to moderate rain will be possible over
the weekend and early next week. /BRC

&&

.AVIATION...Unsettled and wet weather conditions will continue
through the forecast period. Gusty southerly winds and rain will
ramp up tonight into Tue as a vigorous front approaches the coast.
Conditions will deteriorate at all terminals tonight with IFR
conditions possible in moderate to heavy rain on Tue. Low level wind
shear will be possible at all terminals on Tue as a surface front
moves across the airports.

&&

.MARINE...Thunderstorms have been firing up in the northern
waters this afternoon. A marine weather statement has been hoisted
to warn of the potential for gusts to 34 kt or more through the rest
of today and perhaps into the early evening hours.

Primary forecast concern will be the potential for storm force winds
on Tuesday. Latest guidance from the GFS, ECMWF and NAM continue to
indicate a surface low winding up offshore tonight. This low will
rapidly approach the northern waters on Tue, generating robust
southerly winds. Gales are highly probably with short period
southerly seas ramping up to 15 to 20 ft by afternoon. The GFS and
NAM continue to indicate boundary layer winds around 45 kt and 925
to 850mb winds in excess of 50kt. There is a potential for gusts to
around 50 kt with the atmosphere becoming unstable in the afternoon
on Tue. For now will beef up the wind forecast, but hold off on
hoisting a storm warning due to the convective nature of the gusts.

The models continue to indicate a secondary low following Tue night.
The latest GFS was in better agreement with the NAM/ECMWF and no
longer has blasting westerly and northwesterly winds around 40kt
developing Tue night. We probably will see blustery NW conditions
behind the low, though the NAM and ECMWF were not as strong as the
GFS. Confidence is therefore not high.

Conditions should ease up Thu and Fri, though seas may still be
rough on Thu due to short period northerly wave generation. Another
front will approach over the weekend and light to moderate southerly
winds should return offshore.

&&

.EKA WATCHES/WARNINGS/ADVISORIES...
CA...Flood Watch from 4 AM PST Tuesday through Tuesday evening for
     CAZ101>113.

     Wind Advisory from 10 AM to 7 PM PST Tuesday for CAZ103-109-110-
     112-113.

     Coastal Flood Advisory from 8 AM to 11 AM PST Tuesday for CAZ101-
     104-109.

     Winter Weather Advisory until 10 PM PST this evening for CAZ107-
     108.

     High Wind Warning from 10 AM to 7 PM PST Tuesday for CAZ101-102-
     104>106.

     Coastal Flood Watch Tuesday morning for CAZ103.

NORTHWEST CALIFORNIA COASTAL WATERS...Small Craft Advisory until 10 AM PST Tuesday for PZZ450-455-470-
     475.

     Gale Warning from 10 AM to 10 PM PST Tuesday for PZZ450-455-470-
     475.

&&

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For forecast zone information
see the forecast zone map online:
http://www.weather.gov/eureka/zonemap.png

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