Helena, California Weather Forecast Discussion

596
FXUS66 KEKA 080821
AFDEKA
AREA FORECAST DISCUSSION
National Weather Service Eureka CA
121 AM PDT Mon Sep 8 2025
.SYNOPSIS...Periods of rain and thunderstorms are forecast today
through mid week. Drier weather will be possible toward the end of
the week, followed by another chance of rain during the weekend.
&&
.DISCUSSION...
A closed mid level cyclonic circulation and surface frontal
boundary will slowly approach the northern California coast
today. Light pre-frontal precipitation is forecast to start this
morning over the coastal ranges as the surface boundary and
surface low stalls offshore. Mid level cooling with a shortwave
trough will likely result in conditional instability over the
coastal waters and few thunderstorm will be possible. Shear
profiles are not very favorable for long-lived organized storms,
but strong storms will still be possible.
Bulk of rain and convective activity will arrive late this
morning, followed by periods light rain into the afternoon and
evening hours. Southerly and southeasterly winds in advance of
the surface boundary will probably keep rain amounts around a few
hundredths for the greater Eureka area. Forecast area wide,
generally light to locally moderate rainfall is forecast, however
there is a 20% chance of isolated thunderstorms and a 10% chance
for heavier rain rates (>0.25in/hr) due to steep lapse rates
associated with cold air aloft for mainly the coastal ranges
and higher terrain of Trinity. The potential for moderate to
heavy rain is forecast to diminish tonight.
The upper level low is forecast to slowly move over the area
Tuesday through Wednesday. Instability and moisture will be
sufficient for more showers and terrain forced thunderstorms both
days. Brief heavy downpours (>0.25in/hr) will be possible (10-20%
chance) with thunderstorms across the interior Tuesday through
Wednesday. On Thursday, wrap around moisture and precipitation
will be possible for Trinity, eastern Mendocino and northern Lake.
Deterministic model soundings indicate sufficient CAPE for storms
as well. Main threat will be lightning strikes, erratic gusty
winds and perhaps very brief heavy downpours.
Drier and warmer weather conditions are anticipated for Friday as
a 500mb ridge axis builds over the area and the mid level low
departs into the northern Great Basin. The dry weather may not
last long as another splitting trough approaches the Northern
California coast by Saturday. Rain chances will increase Sat into
Sun, however the timing, duration and amounts of rain remain
highly uncertain.
&&
.AVIATION...
(06Z TAFs)...High clouds continue to stream into the region ahead
of the approaching trough. Models are suggesting stratus forming
early Monday morning and moving into the coastal terminals,
bringing MVFR to IFR conditions. Some uncertainty remains as the
approaching low is likely to deepen the marine layer further,
which usually lifts ceilings. Even if ceilings drop to IFR, as the
low approaches early Monday morning, ceilings are likely to lift
as mid-level clouds move in. Rain showers begin late morning or
early afternoon, starting first around ACV and CEC, then moving
toward UKI later.
&&
.MARINE...
Winds continue to slightly increase into Monday as an area of low
pressure approaches the area. The low has tracked south of
previous forecasts, and southerly winds are now forecast to be
elevated south of Cape Mendocino Monday, but still wind gusts peak
at 15-20 kts. Showers and thunderstorms are possible, which,
along with lightning, could bring locally stronger winds up to 30
kts and steep seas. Some models have these continuing overnight
into Tuesday, but confidence is lower.
Northerlies return Tuesday, with elevated winds forecast south of
Cape Mendocino. Gusts are currently forecast to peak at around 15-20
kts Tuesday, with slightly higher winds possible by Wednesday. These
winds also spread north to the northern outer waters by Wednesday.
Steep seas are also possible with these winds, with seas building to
5-6 ft by Wednesday. Nearshore conditions are currently forecast to
remain milder. Another system late week will ease the northerly
winds and steep seas. A long period NW swell of 5-6 ft is forecast
to fill in Thursday into Friday.
&&
.FIRE WEATHER...
Cooler temperatures, higher relative humidity, and a chance for
wetting rain (0.10 inches) are expected Monday through Tuesday.
Below normal temperatures and a chance of rain and a threat for
wet interior thunderstorms will continue on Wednesday and Thursday
as an upper level low slowly moves across the area. Warming and
drying is expected on Friday, however another trough or cut off
low will likely bring cooling and more showers next weekend. Gusty
westerly breezes are expected just about every day this week in
the valleys of southern Mendocino and southern Lake. Southern
Mendo and Lake are on average the driest this time of year and
may not get any rain at all this week.
&&
.EKA WATCHES/WARNINGS/ADVISORIES...
CA...
None.
NORTHWEST CALIFORNIA COASTAL WATERS...
None.
&&
$$
NOTE: The full forecast discussion is now regularly issued between
10 am/pm and 1 pm/am. Additional updates will be issued as needed.
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For forecast zone information
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https://www.weather.gov/images/eka/zonemap.png

NWS EKA Office Area Forecast Discussion