Area Forecast Discussion
Issued by NWS Eureka, CA

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904
FXUS66 KEKA 232251
AFDEKA

AREA FORECAST DISCUSSION
National Weather Service Eureka CA
351 PM PDT Thu Mar 23 2017

.SYNOPSIS...Moderate to heavy rain and gusty winds can be expected
tonight and tomorrow, with pleasant conditions Saturday. Another
storm system will bring more rain and gusty wind Sunday and Monday
as well.

&&

.DISCUSSION...After a mild and pleasant first half of the day,
active weather is knocking on the door as a potent spring cold
front approaches the region. Satellite imagery reveals a visually
impressive occluded low pressure system well off the Washington
coast, with a sweeping cold front draped just offshore along the
Pacific Northwest Coast before turning westward well out into the
central Pacific. This cold front is steadily approaching the
region, and a leading stratiform rain shield is currently just
offshore from Northwest CA. This rain shield will steadily march
through the area between this evening and early tomorrow
afternoon, bringing moderate to heavy rain to much of the area.
This rain will likely not result in any main stem river flooding,
but the expected 1 to 3" totals will likely result in some minor
nuisance flooding and will increase the potential for mudslides.
Stratiform rain will cease quickly from NW to SE as the cold front
passes through the area tomorrow morning and early afternoon, and
after a brief break, waves of showers with perhaps a strike of
lightning or two will move through mainly Del Norte and Humboldt
Counties Friday afternoon through Saturday morning.

The bigger story may actually be the expected strong southerly
winds, which will steadily increase through the evening and peak
overnight tonight. The strongest wind gusts are expected to occur
in the exposed high elevations of Humboldt and Del Norte counties,
near Point Saint George and Crescent City, and in SW Humboldt
county near the Lost Coast and King Range...with gusts to 55 mph
possible. Gusty winds will also be possible in most other areas,
particularly along the Mendocino coast headlands and other exposed
high elevations across the interior. Wind speeds will decrease
sharply tomorrow morning as the cold front passes through the
area.

Lastly...snow levels will be generally high with this system and
actually rising in most areas through the night due to warm air
advection aloft. However, one exception will likely occur across
the Trinity Horn due to a combination of evaporative and upslope
cooling, bringing snow levels down to around 3000 to 4000 feet in
the vicinity of Trinity lake and the adjacent Highway 3 corridor.
While there is a considerable amount of uncertainty as to whether
temperatures can cool enough between now and then to bring
accumulating snow all the way down to Highway 3, with the
exception of Scott Mountain Pass, this effect is frequently
observed in this region, and model guidance does carry this
signal. Opted not to hoist any winter weather advisories given
the high uncertainty, small geographic region, and limited
impacts of expected snow. However, motorists should be prepared to
encounter some light slushy snow along highway 3 north of
Weaverville overnight tonight and tomorrow morning.

Conditions will improve markedly Saturday as a transient upper
level ridge builds overhead, but this will be brief. Another
potent storm system will bring a round of moderate to briefly
heavy rain, gusty south winds, and high elevation snow Sunday and
Monday, with similar impacts if slightly lower rain accumulations.
Stay tuned for more details regarding this storm system in the
coming days.

Another transient ridge will result in pleasant conditions again
Tuesday, followed by a weaker storm system and perhaps some
modest rain Wednesday and Thursday. /BRC

&&

.AVIATION...Increasing clouds and winds across all terminals this
afternoon. Do have wind shear mention for late this afternoon
through the overnight hours for all terminals. Low level wind shear
up to 50 kt for Crescent City and Arcata airports. Wind shear to 35
kt for Ukiah airport. Rain will also be moving into the area
starting late this afternoon. The heaviest rain will be during the
overnight hours into tomorrow daytime. With conditions lowering to
MVFR. Winds will diminish by early tomorrow morning. /RCL

&&

.MARINE...Active weather pattern will continue through this weekend.
Southerly gales through tonight, another period of strong
southerlies on Sunday.

This afternoon, a strong weather front is approaching the coast of
NW California. Southerly winds have been increasing ahead of the
front. Do expect sustained south winds to 35 kt with gusts to 45 kt.
Strongest winds will be over the northern outside waters. Thus, gale
warning is in effect for most of the coastal waters except for the
southern inside waters. The gale warning will be through most of
tonight. Winds will subside to below gale force level by late
tonight. Winds will be lighter and from the southwest on Friday.
Winds will remain light and from the west on Saturday. By Saturday
night, winds will shift back to the south and increase as another
weather front approaches the coast. Do expect stronger winds to
return to the area on Sunday.

With the increase in winds, the seas will be elevated through
tonight. Have combined seas to 14 ft for tonight. Waves will
subside to around 9 ft tomorrow as the wind wave subsides while
the westerly swell remains. Seas will remain at 9 to 10 ft through
Saturday, before increasing due to wind waves on Sunday.

Changes to the previous forecast package are fairly minor. Used
ECMWF/GFS in refreshing the forecast package.

&&

.EKA WATCHES/WARNINGS/ADVISORIES...
CA...Wind Advisory until 5 AM PDT Friday for CAZ101-102-104>106.

NORTHWEST CALIFORNIA COASTAL WATERS...Gale Warning until 3 AM PDT Friday for PZZ450-470-475.

     Small Craft Advisory until 3 PM PDT Friday for PZZ455.

&&

$$

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