Area Forecast Discussion
Issued by NWS Eureka, CA

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FXUS66 KEKA 201136
AFDEKA

AREA FORECAST DISCUSSION
National Weather Service Eureka CA
436 AM PDT Sun Aug 20 2017

.SYNOPSIS...Northeasterly winds will continue to transport smoke
across the Redwood Coast through early next week. Thunderstorms
will be possible over portions of Trinity and northeastern
Mendocino Counties during the afternoon and evening hours of
Monday, Tuesday, and Wednesday.

&&

.DISCUSSION...High pressure off the coast and low pressure over
southern CA continues to bring a strong pressure gradient over the
northern portion of the area. North to northeast winds of 10 to 20
mph with gusts to 35 mph continue this morning. This offshore flow
is keeping most of the area clear of clouds and fog. There is
still fairly widespread smoke, especially in the northern portion
of the area. As the low starts to move north towards the area the
pressure gradient is weakening along the Mendocino coast. This is
allowing a southerly surge of winds and stratus to start pushing
northwards.

Tonight offshore flow weakens considerably. Models are showing
that this will allow the stratus to spread northward. There is
still some weak offshore flow at 925 mb limit the stratus slightly
in Humboldt and Del Norte counties. This is expected to start
burning off by mid morning, but will likely persist at the
immediate coast through late morning. In Mendocino county the
southerly winds at the surface will likely push the stratus up
into the coastal areas from Point Arena to Cape Mendocino. This
may be slow to burn off as well.

Monday afternoon there is the potential for some thunderstorms
over the interior. Current models show the best potential to be
over the Yolla Bolla`s. A storm or two could form in this area,
but it is not expected to be very widespread. Monday night stratus
will likely be more widespread at the coast and the marine layer
will deepen.

Tuesday afternoon looks to have the best chance for
thunderstorms. The upper level low continues to push north in this
pattern putting us in southeast flow. There is decent instability
and mid level lapse rates are around 7C/KM. Storm motion will be
weak, around 5 to 10 kt. This should help limit the potential for
lightning fires, but they may hold over until the warm weather
later in the week. Tuesday night the marine layer is expected to
be deep once again. Wednesday there is another chance for
thunderstorms. It certainly does not look as favorable as on
Tuesday. Another upper level trough starts to approach the area
bringing us southwest flow. This is much drier and mid level lapse
rates are poor as well. Left thunderstorms in the forecast, but if
models continue to show conditions this stable they may need to be
removed.

Thursday the upper level trough moves past the area. This will
likely be the coolest day with temperatures in the 80s to low 90s
in the inland areas. Friday into the weekend high pressure builds
back in the area bringing above normal temperatures again. This
also brings some more offshore flow which should limit the coastal
clouds. Additional smoke is possible, but this is difficult to
predict due to uncertainty on the size of the fires. MKK

&&

.AVIATION...Due to fire in SW Oregon near Brookings and low level
northeasterly winds, there were smoke in the area including Crescent
City and Arcata. With the steep seas, there were also sea sprays in
the area. Do expect a combination of sea sprays and smoke will
reduce visibilities in Crescent City and Arcata airports today.
Think that smoke will be more dominant, so put that as the weather
group. Do expect conditions to lower to MVFR at times, otherwise it
should remain mostly VFR for KCEC and KACV. Do expect the gusty
north winds in Crescent City to continue.

Meanwhile, marine stratus is trying to sneak into Ukiah from the
south early this morning. If it does so, do expect the conditions to
lower to MVFR or IFR. If there is any marine stratus, it should
quickly burn off this morning, giving way to VFR conditions for the
rest of the day for Ukiah.

&&

.MARINE...Steep seas and gale conditions will continue through
tonight. Overnight last night, buoys in NW California reported gale
force wind gusts with steep seas up to 16 to 18 feet. The active
marine weather is caused by a tight pressure gradient off NW
California coast, as a strong high pressure pinches against the West
Coast while thermal low persists over Central California. Do expect
improving conditions by Monday. Before then, we have gale warning
for the outside waters, small craft advisory for the inside waters,
and hazardous seas warning for the northern inside waters through
tonight.

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

&&

.FIRE WEATHER...Northeasterly winds gusting from 20-30 mph are
occurring this morning for elevations above 1500 feet in Del
Norte County. These winds combined with poor overnight RH recovery
of 30 to 45 percent and sufficiently dry fuels will yield a
locally critical fire weather threat. A red flag warning remains
in effect for fire zone 203 (Upper Smith) in interior Del Norte
County from through 8am Sunday for elevations above 1500 feet.
Surrounding the red flag warning, slightly weaker winds will limit
the severity of the

There is a chance of thunderstorms during the afternoon and
evening hours of Monday, Tuesday, and Wednesday across Trinity and
northeastern Mendocino Counties. Current models show the best
chance for storms on Tuesday. These storms are expected to move
very slowly, which will favor wetting rainfall collocated with
lightning. Late in the week warm and dry conditions are expected
to return which will allow any holdover fires to continue to burn
and strengthen. MKK

&&

.EKA WATCHES/WARNINGS/ADVISORIES...
CA...Red Flag Warning until 8 AM PDT this morning for CAZ203.

NORTHWEST CALIFORNIA COASTAL WATERS...
     Small Craft Advisory until 3 AM PDT Monday for PZZ450-455.

     Hazardous Seas Warning until 9 AM PDT this morning for PZZ450.

     Gale Warning until 3 AM PDT Monday for PZZ470-475.

&&

$$

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