Area Forecast Discussion
Issued by NWS Eureka, CA

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919
FXUS66 KEKA 221125
AFDEKA

AREA FORECAST DISCUSSION
National Weather Service Eureka CA
425 AM PDT Tue Aug 22 2017

.SYNOPSIS...Thunderstorms will be possible over portions of
Trinity and northeastern Mendocino Counties through Wednesday
evening. Warmer and drier conditions will then develop across
interior areas during the second half of the week.

&&

.DISCUSSION...The main weather feature of interest this morning
is the upper level low off the southern CA coast. This is starting
to bring some mid level moisture into the area. A few
thunderstorms have been moving across the central valley this
morning north of Sacramento. These are in association with a weak
shortwave rotating around the low. A few of these could move into
Mendocino county early this morning, but the short term models are
indicating they will dissipate before making it that far.
Widespread marine stratus is in place along the coast this
morning.

This afternoon another shortwave will bring additional showers
and thunderstorms to Trinity county and northeastern Mendocino
county. There is some disagreement on the models on exactly where
this wave will go and produce the most thunderstorms. The 00Z NAM
and the 00Z GFS show the best location to be southern Trinity
county, but the 06Z NAM and GFS have the shortwave moving through
a bit faster and farther north. They have also backed off slightly
on the instability as well. This has lowered confidence a bit in
how much thunderstorm coverage there will be. This afternoon the
marine stratus will likely push back towards the coast, but the
immediate coast will likely remain cloudy again.

Tonight there are some indications in the models that we may see
some convection continue into the overnight hours. The best
chances are in northern Trinity county.

Wednesday an upper level trough moves approaches the west coast
and this pushes the upper low off to the east. This will bring
southwest flow and much drier air in the mid levels. So have
trimmed back thunderstorm chances on Wednesday to just far
northeastern Trinity county. If anything forms it will likely
contain only be a few strikes. Thursday morning the upper level
trough moves onshore bringing cooler air. This may help break up
the marine layer at the coast by weakening the inversion.

Friday into the weekend an upper level ridge builds back into the
area. This will bring warming temperatures, likely over 100 for
the weekend. This will also compress the marine layer and may
bring some dense fog at night. There will likely be some
afternoon clearing over the weekend. Early next week the ridge
starts to break down and this may limit afternoon clearing and
bring cooler inland temperatures. MKK

&&

.AVIATION...Nighttime satellite imagery showed widespread coverage
of marine stratus in the area. Both CEC and ACV are in IFR/LIFR due
to low ceiling from the marine stratus. As of writing of this
discussion, UKI is still under VFR conditions. However, the marine
stratus is making good northward progress from Sonoma County. Do
expect the marine stratus to lower conditions to IFR for UKI. UKI
should break out this morning, while CEC and ACV will not be able to
break out and remain in the marine stratus through the forecast
period.

&&

.MARINE...Light to moderate north winds with low seas in store
through tonight. Early this morning, buoys reported winds less than
10 kt with seas less than 5 ft. Thus, we don`t have any advisories
in effect for NW California coastal waters through tonight. Pressure
gradient off NW California will tighten once again starting on
Wednesday, and this will lead to increasing north winds and
resultant steep seas. Have a small craft advisory in effect from
early Wednesday morning through late Thursday night to account for
the strong north winds and steep seas. Winds and seas will improve
by Sunday.

Changes to the previous forecast package were minor. Utilized GFS
and NAM as the models of choice.

&&

.FIRE WEATHER...A red flag warning is in place for much of Trinity
county for this afternoon for abundant lightning. These storms are
forecast to move fairly slowly resulting in wetting rain in the
heavier cores. Lightning may still strike outside these cores.
Late this week and into the weekend warm and dry conditions are
expected to return. This would allow any holdover fires to
intensify and spread. Offshore flow is expected for the weekend,
but winds will be lighter than the offshore flow last week. MKK/JMG

&&


.EKA WATCHES/WARNINGS/ADVISORIES...
CA...Red Flag Warning from 2 PM this afternoon to midnight PDT
     tonight for CAZ283.

NORTHWEST CALIFORNIA COASTAL WATERS...
     Small Craft Advisory until 5 PM PDT this afternoon for
     PZZ470.

     Small Craft Advisory until 5 AM PDT early this morning for
     PZZ475.

&&

$$

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