Area Forecast Discussion
Issued by NWS Eureka, CA

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
000
FXUS66 KEKA 241401 CCA
AFDEKA

AREA FORECAST DISCUSSION...CORRECTED
National Weather Service Eureka CA
601 AM PST Fri Nov 24 2017

.SYNOPSIS...Dry conditions can be expected Friday and most of
Saturday. Windy and wet conditions can be expected the remainder
of the weekend, likely with some travel inconveniences. Holiday
travelers should monitor weather conditions closely over the
weekend.

&&

.DISCUSSION...Light rain is quickly coming to an end throughout
the region early this morning as a very weak cold front slips to
the south and high pressure temporarily reclaims the north coast.
Weather conditions will be generally pleasant today as a result,
with cool temperatures but no rainfall. Any lingering low clouds
and fog along the immediate coast should dissipate quickly during
the morning hours today, and will give way to mostly passing high
clouds through the day.

Attention then turns to a series of storm systems that will bring
unsettled conditions to the North Coast over the weekend. The
first of these storm systems is decidedly sub-tropical in nature,
and currently located well out over the open ocean and south of
the 30th parallel. Satellite imagery reveals a building plume of
tropical moisture within it`s warm sector, and this moisture
stream is slowly heading in the direction of northern California.
During the day Saturday, this wave will quickly move northward
through the California waters, pulling a warm front through the
state as it does. It is at this point that weather conditions will
take a turn for the worse across the area, presenting several
potential hazards for holiday travelers. The remainder of the
discussion below will be separated into sections based on those
hazards.

.WIND...

Strong winds will be the most immediate weather hazard with this
storm, as southeast winds will quickly increase throughout the
day Saturday as the warm front passes through the area. These
winds will likely peak during the evening and persist through the
overnight hours, with the strongest winds occurring in exposed
high elevations. Wind gust speeds may reach or exceed 50 mph in
these wind-prone areas, and this may include some of the higher
passes of the region like South Fork Summit on Highway 36 or Berry
Summit on Highway 299. Winds in the lower valleys will generally
be protected from the stronger winds, and winds along the coast
will certainly be gusty but not quite to the same extent as the
higher ridges. One possible exception will be in the vicinity of
the Eel River Delta...places like Fortuna and Ferndale...as
southeast winds can often lead to strong gusts in those areas.
Meanwhile, these winds will likely result in warm conditions along
the coast Saturday, thanks to the same downsloping process that
brought the warm temperatures earlier in the week. Winds will
quickly veer to the southwest and decrease during the day Sunday
as the first cold front passes.

Beyond Sunday, winds become highly uncertain. Sometime late
Sunday a compact low pressure system will move onto the
California coast, bringing with it strong westerly winds on its
southern flank. The big question mark here is...where will this
system move onto the coast? Models have struggled mightily with
this, and as of now are beginning to bring this system onshore
much farther south, perhaps even south of Cape Mendocino, whereas
earlier solutions carried it into Oregon. As a result, wind speeds
may not be quite as strong as originally anticipated. Still,
there is a high degree of uncertainty here, so stay tuned.

Winds will decrease quickly Monday as this system moves inland.

.PRECIPITATION...

While a few light showers will be possible at
high elevations as the initial warm front moves through the area
during the day Saturday, this activity will be very light and
primarily virga showers. More substantial precipitation will not
begin until the core of a rich plume of subtropical moisture -
i.e. an atmospheric river - moves onshore and interacts with local
terrain. The timing of this is somewhat uncertain, but will likely
occur sometime between Saturday evening and early Sunday morning.
Models continue to struggle with timing and placement, but it
appears that the heaviest rain will fall from Cape Mendocino
southwards into Mendocino County during this first event. There is
some concern that models are under-forecasting the total
accumulations given the nature of the moisture plume, but with so
much uncertainty surrounding the timing and location of heaviest
rain, it is difficult to pin down any particular area. This will
be watched closely. The first wave of rain will cease sometime
Sunday afternoon as the first cold front moves through the area.

Another storm system will move onto the coast late Sunday and
early Monday, bringing with it another round of precipitation.
Much like the wind, uncertainties abound regarding precipitation
timing and location as models struggle to depict this system.
While uncertainty is high, the trend is for a more southerly
track, which would push the heavier accumulations into areas from
Cape Mendocino southward. We do know that this storm will be
colder than the one before it, and as a result precipitation will
likely be more showery and convective. Thunderstorms will be
possible along with small hail, and snow levels will eventually
lower to around 3000 to 4000 feet overnight Sunday and Monday
morning. Whether or not the passes see any snow depends on where
and when this low pressure system moves onshore, and how quickly
snow levels fall. In any case, light snow accumulations remain
possible along the high passes of Highways 36 and 3, and to a
lesser extent Highway 299, overnight Sunday.

Overall, the primary threat to travel that this precipitation will
bring is the increased potential for rock slides across mountain
roads, standing water on roads, and to a lesser extent light snow
on high elevation passes.

Showers will cease during the day Monday, and a dry pattern will
reclaim the California Coast for the foreseeable future, aside
from perhaps some very light showers toward mid-week. /BRC

&&

.AVIATION...A front is slowly moving east through Northwest
California with Del Norte County and coastal Humboldt now west of
the mid/low level cloud cover and rain showers. As the front
continues to move east and weaken, low and mid level cloud cover
will gradually dissipate. However, at the same time, high level
clouds will spread back over the region ahead of the next storm
system. Any valley fog development this morning will dissipate by
mid day. Winds will remain generally light today.

Winds tonight will gradually shift to out of the south tonight
and increase into Saturday, particularly at the immediate coast
and mountain ridges of Humboldt and Del Norte counties. CEC could
see gusts over 40 mph Saturday evening/night. Rain will also
redevelop and spread over the region Saturday night with ceilings
gradually falling near or into MVFR. /RPA

&&

.MARINE...Light winds and relatively low seas will persist into
tonight as high pressure moves over the waters. A low pressure
system will begin to approach tonight turning winds out of the
south. These winds will rapidly increase with gales likely in the
outer waters by late Saturday afternoon or early evening and
continuing overnight. Very steep seas will build in response to
the winds, peaking between 10 and 15 feet early Sunday morning.

Winds will decrease on Sunday after the first system moves inland
followed by a return to strong southwest winds once again Sunday
evening. Model guidance is trending toward this second system
being a little weaker in the Northwest California waters than
originally thought, but small craft force winds still look likely.
In addition to the winds, a large northwest swell will build into
the waters on Sunday. Seas in excess of 10 feet at 14-16 seconds
are expected, potentially exceeding 15 feet in the southern waters
Sunday night into Monday. Additional large northwest swells will
build into the waters later this coming week.

North winds are anticipated to return Monday through mid week.
Small craft to near gale force northerlies will be possible with
the strongest winds expected on Wednesday. /RPA

&&

.EKA WATCHES/WARNINGS/ADVISORIES...
CA...None.
NORTHWEST CALIFORNIA COASTAL WATERS...Gale Watch from Saturday afternoon through late Saturday night
     for PZZ470.

     Gale Watch Saturday evening for PZZ475.

&&

$$

Visit us at http://www.weather.gov/eureka

Follow us on Facebook and Twitter at:
http://www.facebook.com/NWSEureka
http://www.twitter.com/nwseureka

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



USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.