Small Craft Advisory 91, NWS Eureka, CA

AREA FORECAST DISCUSSION
National Weather Service Eureka CA
258 PM PDT Sun Aug 27 2023

.SYNOPSIS...Temperatures will be near to slightly below normal
across Northwest California during much of the next seven days. In
addition, breezy northwest winds are forecast to occur Tuesday and
Wednesday. Late this week cool showery conditions are possible
across the region.

&&

.DISCUSSION...
An upper low spinning off the Pacific Northwest coast has brought a
deep marine layer to coastal areas and once again good inland
movement of the stratus into many of the near coastal valleys. The
stratus has pulled back toward the coast but is still widespread
along the Humboldt and Del Norte coastlines. Another round of
stratus pushing inland is expected again tonight.

Conditions will begin to change tomorrow as the upper low begins to
move east over the region and eventually ejects northeast out of the
area as an open wave. This will allow mixing to increase over the
region and north winds to ramp up Monday into Tuesday. Conditions
will become quite gusty in many areas by Tuesday afternoon with some
ridgelines seeing gusts over 35 mph, specially near the southwestern
Humboldt coast into Mendocino and Lake counties. Wind speeds are a
little more uncertain farther north where the pressure gradient will
be less pinched along the coast. However, the trajectories may be
more north to northeasterly the farther north you go, bringing more
of an offshore component to northern Humboldt and Del Norte counties
Tuesday through Wednesday morning. Some gusts in excess of 30 mph
will be possible on ridgetops Tuesday night as the typical offshore
overnight enhancement occurs in these areas.

The winds will begin to decrease in magnitude late Wednesday into
Thursday as the next in the series of upper lows to impact our
region approaches. This time there is a distinct possibility, 30-
40%, that this next low brings with it an actual cold front with
stratiform rain for parts of the area Thursday into Friday, most
likely in the north. The GFS and ECMWF ensembles have been
favoring this possibility while the Canadian ensembles are on the
drier side. This upper low may then spin over the region for the
next couple of days into the weekend bringing additional chances
for showers or thunderstorms over the higher terrain while also
bringing cooler temperatures to interior areas. /RPA

&&

.AVIATION...
The stratus has a strong presence this afternoon, but is showing
some signs of afternoon weakness. The optical depth of the stratus
is low enough that resulting surface heating is disrupting the
low level inversion over the coast. Partial scattering is evident
on visible satellite. VFR conditions that may develop will be
crushed by the evening stratus onslaught. Smoke from nearby
wildfires continues to impact CEC and ACV, but more severely at
CEC where VIS has struggled to exceed 2 SM. Overnight, ceilings
will trend back down to IFR/LIFR levels. The presence of an upper
low to the NW, and associated weak shortwaves will maintain a
slightly deepened the marine layer. A general trend back to LIFR
is however still expected overnight, though the duration may be
limited as was seen Saturday night. Smoke will continue to impact
CEC, especially through the morning hours when visibility will be
most restricted. A trend towards more robust afternoon northerly
to northwesterly winds will better drive the smoke east over the
next couple of days.

&&

.MARINE...
Seas are small and less than 6 feet today, and this will continue
through most of Monday. A slow, steady increase in northerly
winds will begin off of Cape Mendocino this afternoon. A trough
axis will begin to pass through the region later Monday, and the
northerlies will further increase as the pressure gradient
tightens. Isolated gale strength gusts will develop south of the
cape on Monday afternoon, and by Tuesday afternoon gale condition
will spread outward through the southern waters. A Gale Watch is
up for the southern waters for Tuesday afternoon. Further
refinement on the timing of the watch may be needed based on
recent model trends. Gale coverage and strength will be greatest
on Wednesday, and will include portions of the northern zones.
Seas remain primarily locally generated, tied to the nearshore
northerlies... although a small mix of NW and SW swells move
through the weekend and next week.

&&

.FIRE WEATHER...
Enhanced fire weather concerns will begin Monday night into Tuesday
as north-northwesterly winds increase across the area. Wind gusts in
excess of 35 mph will be possible in some areas of southeastern
Humboldt into Mendocino and Lake counties by Tuesday afternoon.
For most areas the northwesterly flow will bring increased RH and
reducing the fire growth potential from these winds. However on
the east side of Mendocino county and Lake County the conditions
could be drier with lower afternoon RH (15-20 percent) and
overnight recoveries decreasing by Wednesday morning as the flow
becomes north-northeasterly. At this time a headline still seems
appropriate for this setup but a red flag may need to be
considered.

Farther north the winds will also become more north-northeasterly
through the day Tuesday into Tuesday night and Wednesday morning,
but with higher humidity. The main issue will be the change in wind
direction with gusty conditions to near 30 mph for the fires in
Humboldt and Del Norte counties.

The potential for wetting rains is increasing for late in the week
as the next in a series of upper lows approaches the area. The
initial approach of the low could bring a stratiform rain band
without the threat of thunder. /RPA

&&

.EKA WATCHES/WARNINGS/ADVISORIES...
CA...
None.
NORTHWEST CALIFORNIA COASTAL WATERS...
     Small Craft Advisory from 4 PM Monday to 2 PM PDT Tuesday
     for PZZ455-475.

     Gale Watch from Tuesday afternoon through late Tuesday
     night for PZZ455.

     Gale Watch from Tuesday afternoon through Wednesday morning
     for PZZ475.

&&

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