Area Forecast Discussion
Issued by NWS Eureka, CA

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000
FXUS66 KEKA 201210
AFDEKA

AREA FORECAST DISCUSSION
National Weather Service Eureka CA
510 AM PDT Wed Sep 20 2017

.SYNOPSIS...Cool weather will persist through the week as a cold
air mass drops south from the Gulf of Alaska. Periods of showers
will persist through Thursday and then wind down on Friday. High
pressure will rebuild this weekend leading to a warming and drying
trend that will last into the early portion of next week.

&&

.DISCUSSION...A weakening front will move southeast across the
area this morning. As of 4AM, the front was just south of Pt St
George where southerly winds have been decreasing. Rain thus far
has been light to moderate with most reliable rain gages
reporting a quarter to half an inch for Del Norte and northern
Humboldt counties. Light rain will continue to spread south and
east across the remainder of the forecast area this morning. Most
locations should see measurable rain.

A break in the light rain is expected by mid or late morning as a
post frontal ridge develops. Cold air aloft coupled with moist NW
flow will likely produce on and off showers through the day,
primarily for the north coast. The potential for low topped
convection will increase this evening over the coastal waters as
500mb temperatures plummet to -25C or less. An isolated low top
storm will certainly be possible offshore over the coastal waters.

A colder air mass will start to settle over the area tonight.
Skies should become partly cloudy to clear across eastern Trinity
and northern Mendocino counties tonight. The longwave cooling
will allow temperatures to fall into the mid 30s in these
locations. Therefore, a frost advisory has been hoisted. The
threat for frost will continue Thursday night into early Friday
morning. If skies clear out completely, a few valley sites below
3000 feet in central/eastern Trinity county may dip to 32F or
less. Early morning minimums should moderate on Saturday, however
a few valley sites below 3000 feet may still dip into the mid 30s.

The chance for showers will continue on Thu. The models were
still not in agreement. Another trough will drop down from the
north on Thu. The ECMWF and GFS are now much drier and have this
wave dampening out considerably as it slides southward just
offshore over the coastal waters. The 00z NAM12 was quite wet,
bringing another round of showers into the area north of the
Cape. The 06z NAM dried out considerably and is now similar to
the two spectral models. We definitely could see more showers on
Thu due to the cold air aloft. A few terrain driven showers may
sprout up with the daytime heating.

The cold weather will moderate this weekend as high pressure
builds back over our region. An offshore flow will result in a
warming and drying trend this weekend that will last into the
early portion of next week. Dry weather and above normal
temperatures are expected next week as a longwave ridge dominates.

&&

.AVIATION...LIFR conditions will improve as the band of rain
continues to track southeastward through the morning hours. Expect
predominantly VFR conditions across most of northwest California
this afternoon with some cumulus and a possible thunderstorm
primarily along the coast. MVFR to IFR conditions should then return
to the Humboldt and Del Norte county coastlines tonight. Moisture
from the rains should help generate some interior valley clouds
tonight as well. /KML

&&

.MARINE...The northwesterly will subside through the next several
days with seas expected to fall below 10 feet today. As a result,
the small craft advisories were adjusted to expire today. First the
inner waters will expire later this morning then the outer waters
later this afternoon. Moderate winds will persist through the rest
of the work week before building high pressure brings increasing
north winds this weekend. Although, some gusty south winds will be
possible in the northern inner waters with the passage of a weak
boundary. /KML

BEACH/SURF CONDITIONS...A beach hazard statement remains in
effect until 9AM PDT this morning. Seas have been trending
downward with nearly squared periods.

&&

.EKA WATCHES/WARNINGS/ADVISORIES...
CA...Beach Hazards Statement until 9 AM PDT this morning for CAZ101-
     103-104-109.

     Frost Advisory from 3 AM to 8 AM PDT Thursday for CAZ107-108-110-
     111.

NORTHWEST CALIFORNIA COASTAL WATERS...
Small Craft Advisory until 3 PM PDT this afternoon for PZZ470-
475. Small Craft Advisory until 9 AM PDT this morning for
PZZ450-455.

&&

$$

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