Area Forecast Discussion
Issued by NWS Eureka, CA

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FXUS66 KEKA 202242

National Weather Service Eureka CA
342 PM PDT Wed Sep 20 2017

.SYNOPSIS...Cool and occasionally showery conditions will continue
through Friday, with a warming trend expected over the weekend and
early next week. Patchy frost may develop tonight and tomorrow
night across some interior valleys, particularly in Trinity and
Mendocino counties.


.DISCUSSION...Light stratiform rain has given way to convective
showers across NW California as the core of an upper level trough
has begun to filter into the region from the north. These showers
are being driven by cold air aloft moving over relatively warm
waters, resulting in modest instability and resulting convection.
While instability is modest, some showers across the Oregon waters
and immediate coast have been strong enough to produce a few
lightning strikes, and the region of lightning production has
slowly shifted southward through the day. By later this evening,
the area of maximum instability will be located across the waters
of NW CA, and a few additional lightning strikes will be possible.
A few strikes will be possible along the immediate coast, and in
particular the Del Norte coast, but by the time this occurs the
overall steering flow will begin to turn more northerly than
westerly, and instability will be weaker than it was earlier in
the day. As a result, most lightning strikes will remain offshore,
and will likely be more sporadic. Scattered showers will continue
through the evening, and a break in the showers is expected late
tonight through approximately midday tomorrow.

As showers and skies clear overnight tonight in between waves of
showers, temperatures across interior valleys are expected to
fall. Model guidance indicates that temperatures are likely to
fall into the low to mid 30s in the valleys of Trinity and
northern Mendocino counties, and perhaps even northern and western
portions of Humboldt county. If the valleys decouple and winds
manage to go light, areas of frost are likely. While there is a
high amount of uncertainty regarding just which valleys will
decouple enough to see frost formation, thereafter.  forecasted
low temperatures are plenty cold enough to warrant the issuance of
a frost advisory. In fact, a few local cold spots may approach or
dip slightly below freezing, particularly in the vicinity of
Hayfork, Douglas City, and Ruth.

As the upper trough sinks southward and eastward, one last gasp of
showers is expected to move through the region sometime late
tomorrow afternoon and evening, primarily impacting the Humboldt
and Del Norte Coasts. Once these showers move through the area
tomorrow night, dry conditions are expected for the foreseeable

Temperatures will remain cool and below normal through Friday, and
a gradual warming trend can be expected Saturday through the
middle of next week thanks to slowly building high pressure. By
Monday, well above normal temperatures are expected, and may last
for the next several days. /BRC


.AVIATION...A frontal boundary pushed across the region overnight
accompanied by bands of rain/showers. This system ushered in IFR
Cig/Vis overnight...and by mid-morning conditions had improved to
mostly MVFR...even at UKI. Skies/Vis quickly turned VFR by Midday
with lingering scattered showers and occasional MVFR north of Cape
Mendocino. Model guidance, satellite imagery and radar indicate
afternoon improved conditions will give way to more cloudiness and
scattered showers as some post frontal energy moves onshore. By
Thursday, improving conditions expected across the region.
However, low visibilities (main concern) and a few areas of IFR
ceilings will be the biggest aviation concern in early morning at
coastal airports.


.MARINE...The northwesterly swell will subside through the next
several days with seas expected to fall below 10 feet over the
next 12 to 24 hours or so. The inner waters` Small Craft Advisory
were allowed to expire on time this morning, however; the outer
waters have been extended to cover tomorrow morning. This may need
to be adjusted on tonight`s swing shift as the conditions may
fall below criteria this evening. Otherwise, there was no change
to the forecast 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. KAR/KML


.FIRE WEATHER...Cool and humid conditions will persist through at
least Friday as an upper level trough pushes through the region.
While a few showers will continue to impact coastal regions of
Humboldt and Del Norte counties through tomorrow evening,
including perhaps a stray lightning strike or two, most other
areas have seen the last of significant rains for the foreseeable
future. A warming trend is expected to develop Saturday and beyond
as high pressure gradually builds back into the region. Saturday
night and beyond, nocturnal offshore flow will also return, and
steadily drying conditions can be expected. By Monday, well above
normal temperatures will return, and humidity values will once
again be very low. This will allow fuels to begin drying again,
although this process will take longer in areas that have seen
substantial rainfall over the past few days. The area of greatest
concern locally will be Mendocino, Lake, and eastern Trinity
counties, as these areas have not seen substantial rains for the
most part. /BRC


CA...Frost Advisory from 3 AM to 8 AM PDT Thursday for CAZ105-107-108-

NORTHWEST CALIFORNIA COASTAL WATERS...Small Craft Advisory until 3 AM PDT Thursday for PZZ470-475.



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