Area Forecast Discussion
Issued by NWS Eureka, CA

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FXUS66 KEKA 092352

National Weather Service Eureka CA
352 PM PST Fri Dec 9 2016

.SYNOPSIS...A wet and cloudy pattern will persist over the next
several days, with periods of widespread rainfall. The heaviest
rains are expected tonight as an atmospheric river impacts the


.DISCUSSION...After a slight break in widespread rainfall today,
another batch of widespread rains is presently moving inland along
the Redwood Coast. This is response to an atmospheric river that
has set up from Hawaii to SW OR and NW CA. This deep feed of
moisture is very apparent on the latest H2O vapor imagery with
cold cloud tops in most of the band. Several vorticity maxima
pivoting around a 524 dam upper low W of Vancouver are helping to
increase jet dynamics and upper divergence over the N 1/2 of our
area. In fact, cloud tops are cooling further and the areal extent
of cold clouds is increasing. 1 to 3 inches of additional
rainfall are likely area wide tonight and Saturday morning with
locally higher amounts over the S-W facing terrain (especially
over the northern interior and the King Range of SW Humboldt.
Snow levels will start the night around 6000 ft but will fall to
near 4500 ft over Del Norte and extreme N Trinity County by
morning. As a result of the increased precipitation forecast, have
bumped snow totals up over the N Trinity mountains. However,
significant snows are not expected below 4500 feet.

More rains, more showery in nature, are expected late Saturday
through Sunday night, with the most coverage and highest QPF
values over Del Norte and N Humboldt counties. Another disturbance
is expected to affect the area late Monday into Tuesday. There
remains a high amount of uncertainty regarding this feature. The
ECMWF develops a 1000 mb surface low off off the NW CA coast by
late Tuesday with a period of significant QPF over the forecast
area. The GFS is far more conservative, with no significant
surface low development and a much lighter and showery
precipitation field. Have capped PoPs below 70 percent over the S
portion of the area during this period until better model
agreement arises.

The main forecast adjustment was to reduce PoPs during the middle
of the coming week as the models are indicating a bit more ridging
over the area ahead of the another disturbance late in the week.


.AVIATION...MVFR ceilings this afternoon will give way to MVFR to
IFR conditions this evening as a band of sub-tropical moisture from
Hawaii(an Atmospheric River) moves into northern California around
0300Z this evening. This band of precipitation will slowly move
southeastward and the precipitation will become showery in nature
around 0900Z over Del Norte and Humboldt and Trinty Counties and
around 1200Z in Mendocino county. The surface winds become westerly
to northwesterly as the precipitation turns to showers.


.MARINE...The light winds will persist across the marine zones through
Monday,and then are expected to increase. A west to northwest swell
will propagate across the waters Saturday increasing the seas. A
band of sub-tropical moisture from Hawaii (an Atmospheric
River) will move across the coastal waters overnight bringing
periods of light to moderate rain. The Small Craft Advisory across
the northern outer waters was cancelled this afternoon as the
short period seas ended. A Small Craft Advisory for Hazardous Seas
was issued for the northern outer waters valid Saturday night
through late Sunday afternoon due to the anticipated building
northwest swell.


     Small Craft Advisory for hazardous seas from 6 PM Saturday to
      6 PM Sunday for PZZ470.


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