Area Forecast Discussion
Issued by NWS Eureka, CA

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FXUS66 KEKA 232227

National Weather Service Eureka CA
327 PM PDT Wed Aug 23 2017

.SYNOPSIS...An upper trough will produce interior cooling on
Thursday. Warmer weather is expected for the weekend as high
pressure builds over the area. Coastal locations will continue to
see the usual dose of low clouds and fog through the week before
the marine layer decreases this weekend.


.DISCUSSION...An upper trough to our NW will slowly sag southeast
toward the northern California coast tonight through Thursday.
Drier southwesterly flow aloft in advance of this trough has been
developing across the region today. The thunderstorm threat has
shifted well to our NE with only cloud buildups over the Trinity
mountains expected through early this evening.

The upper level trough will deepen the marine layer tonight
and some light coastal drizzle is expected. Shallow cooling
from a weak surface front will result in lower high temperatures
across the interior on Thursday. Increasing northerly winds
offshore behind the front should result in better chances for
clearing along the Mendo coast as downslope winds develop in the
lee of the King Range. For the north coast, full clearing seems
less probable.

The cool down for the interior will be shortlived. High pressure
aloft and stronger large scale sinking will build back over the
area on Friday. Temperatures will ramp up even more for the
weekend as upper level ridging and stronger adiabatic compression
develops. Interior valleys will once again be flirting with 100F
degree heat. Coastal stratus is expected to compress and become
shallower this weekend. This should result in faster clearing.
The offshore flow is not forecast to become very robust, so only
Del Norte county may be spared from the stratus this weekend.
However, smoke from the wild fires in western Siskiyou county may
result in more smoke filled skies for Del Norte county. Also, the
latest high resolution models indicate a flow reversal taking
shape south of the Cape this weekend. This will advect stratus
northward and may keep low clouds and fog locked onto the
Mendocino coast through the weekend. Confidence in sunshine for
coastal areas this weekend is declining.

Going into early to mid next week, the ridge aloft will hold
firm then start to shift eastward mid to late next week. This may
be enough increase the depth of the marine layer and allow the
stratus to be a bit more persistent. Inland temperatures will
remain above normal while coastal areas remain near normal.


.AVIATION...The marine layer continued today with IFR-LIFR
cigs/vis impacting local airports/airfields...especially ACV &
CEC. The layer remained fairly deep through morning. A received
PIREP just before noonday reported cig tops 2200 feet down to 200
feet as a airplane descended into ACV. Although the stratus
remained quite entrenched into afternoon, VIS Satellite indicated
some thin spots vicinity of Humboldt Bay and a little bit of cloud
erosion just east of ACV. "Any" mixing out of clouds will once
again be very brief...allowing the stratus to intensify and again
overwhelm coastal sites by late afternoon/early evening. With no
dynamics to change conditions...stratus should persist. Inland
area airfields will remain VFR through the period.


.MARINE...Winds increased steadily today. The ASCAT pass showed
15 kt across the northern outer waters and 20 kt to the south.
North winds will continue to increase through Thursday bringing
more substantial gusts across the outer waters Thursday afternoon
through Friday. A gale warning was hoisted for the southern outer
waters with 35 to 40 kt gusts expected Thursday afternoon.
Additional gale force gusts will pepper the northern waters but
are not expected to be enough to warrant a warning. Stronger gusts
will be possible nearshore primarily around Point Saint George
and Cape Mendocino. Strong north winds will cause steep waves to
hike up Thursday and Friday. A mid period northwesterly wave
system will also propagate into the waters the next few days.
Conditions will begin to settle Saturday with winds and seas
falling below warning and advisory levels late in the weekend into
next week. The existing small craft advisories may need to be
extended into Saturday if winds or seas are slow to settle. /KML


.FIRE WEATHER...Primary concern going into this weekend will be
gusty northeasterly and easterly winds and poorer RH recoveries
for the mid and upper elevations of mainly zones 203 and 204. A
thermal trough will develop along the coast this weekend as the
air aloft warms in response to stronger adiabatic compression.
Expect another round of triple digit heat and very low daytime
RH`s across the interior valleys. The warm air aloft will result
in stronger and lower nocturnal inversions. This coupled with
light NE/drainage winds will allow smoke from the fires in
western Siskiyou county to snake downstream into the
Klamath/Trinity river valleys. The trapped smoke may result in
degraded air quality this weekend. Current indications are for
light to moderate mixing with decent dispersion in the afternoon
hours as the inversion breaks.


Small Craft Advisory 5AM Thu to 10PM Fri PZZ450.
Small Craft Advisory until 4AM Sat PZZ470.
Small Craft Advisory 9PM this evening to 10PM Fri PZZ455.
Small Craft Advisory til 3PM Thu PZZ475.
Gale Warning 3PM Thu to 3AM Fri PZZ475.



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