Area Forecast Discussion
Issued by NWS Eureka, CA

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000
FXUS66 KEKA 241157
AFDEKA

AREA FORECAST DISCUSSION
National Weather Service Eureka CA
457 AM PDT Mon Jul 24 2017

.SYNOPSIS...
Isolated thunderstorms will be possible today for locations east of
a Gasquet, to willow Creek, to Peanut line. Thunderstorm potential
will increase somewhat in potential and coverage each day through
Wednesday. Otherwise, look for coastal clouds and patchy fog the
next few nights and into the morning hours, with seasonably warm
temperatures expected. For the remainder of the week, high pressure
will reestablish itself across the region, with temperatures
increasing by this weekend.

&&

.DISCUSSION...
.SHORT TERM...
(Today through Wednesday)

Will thunderstorms form over the next few days remains the forecast
challenge. The answer in short is yes, they will form, with
convective potential increasing each day. In fact, a few showers and
an isolated thunderstorm formed this morning around 1 AM over the
southern outer waters, with an altocumulus field (ACCAS)which looks
to be tied to a small vort max rotating around the parent low, which
is currently spinning about 60 nautical miles west of Point Arena.
Additionally, another piece of energy is approaching the region as
it pinwheels around the low, with another pretty extensive ACCAS
field moving westward from the northern Sacramento Valley. Here,
cloud top cooling has been rapid, with temperatures dropping around
27 deg. C/half hour. Thus far, no elevated cores have been detected
on radar, nor any lightning strikes. However, convection developed
in about the same time frame earlier offshore. Soundings from just
northwest of Redding show moistening and rapid destabilization
through the remainder of the morning, with this spreading westward
towards Weaverville with time into the afternoon. Given the very
large inverted-V signature, dewpoint depression over 30 degrees F,
the mention of dry thunderstorms was added to the forecast. This is
highly conditional, but based on what was observed earlier this
morning, the additional vort maxes rotating through this
morning/afternoon, and signatures seen in the satellite imagery, it
seemed like a prudent move.

Farther west and north, the NAM model has come on board with the
GFS, showing thunderstorms developing near the Somes Bar area,
moving westward. The 00z run had it moving into Del Norte County
after 8 PM, while the new 6z run delays it to a few hours before
sunrise. Conversely, the 00z GFS had convection from northeastern
Humboldt into Del Norte County developing after 5 PM. The current
run maintains that general scenario, while also trying to develop a
separate area from southern Humboldt into northern Mendocino
counties. Given this run is an outlier compared to the other its
previous run and various other models, we didn`t bite on this, as
the "game is a foot`, not too much change was made to the forecast
for today.

One thing the majority of the models do agree on is a much better
chance for storms to develop during the day tomorrow, lingering into
Wednesday. Soundings from the GFS/NAM also continue to indicate good
instability, with the main change being seen in the NAM as it has
increased its various convective indices. As better forcing arrives
as the low edges closer these days, the possibility for one or two
strong storms does exist given the very good parameters seen in the
data. However, storm movement will become increasingly slow each
day, as the low moves closer.

.LONG TERM...
(Wednesday evening through Sunday)

With all the action being in the short term portion of the forecast,
the remainder of the week will be more tame. In general, ridging
aloft will build across northwest California, with increasing
heights aloft leading to a gradual warming area-wide, with dry
conditions expected. /PD

&&

.AVIATION...
Marine stratus is present along the coast this morning, and as a
result, ACV and CEC are experiencing IFR ceilings and visibility.
Persistence suggests that low clouds will clear this afternoon,
and then redevelop overnight. Elsewhere, hi-res model guidance
shows an increase in boundary layer RH at UKI late tonight, and
NAM forecast soundings depict thermodynamic profiles supportive of
a shallow cloud layer near the surface. Finally, an upper low
positioned offshore from Mendocino County will yield an
environment that is supportive of isolated thunderstorm
development in the vicinity of NWRN CA. However, confidence in
storm occurrence at any particular TAF site is too low at this
time to include in the morning forecast issuance.

&&

.MARINE...
An isolated thunderstorm developed over the southern outer
waters early Monday morning. This activity has since dissipated,
with the greatest probability for storm development shifting
inland by Monday afternoon. Otherwise, a thermal trough of low
pressure over interior CA combined with high pressure over the
NERN Pacific will continue to support northerly gale force winds
over the northern outer waters through late tonight. These winds
will also aid in the development of large steep seas over the
southern outer and northern inner waters. The pressure gradient is
forecast to slacken somewhat Tuesday and Wednesday, with winds
subsequently weakening...at least temporarily. Despite the
relatively weaker winds, small craft northerlies will still be
likely over the outer waters through much of the week.

&&

.FIRE WEATHER...
The main concern lies with our thunderstorm potential over the next
few days, especially today where dry thunderstorms are expected,
should a storm form. Otherwise, 100 and 1000 hour fuels are more
conducive south of highway 299, compared to further north. Low-
level fuels such as brush/grass is conducive across the entire area
though, so any thunderstorm will pose a concern with its attendant
lightning threat. The good news is moisture will continue to
increase each day through wednesday, which may help with regard to
these concerns, as the storms will become increasingly wet each day
where they do form. For the remainder of the week, dry conditions
are expected for all of our neck of the woods, with increasing
afternoon highs/lower relative minimum humidities a good bet. /PD

&&

.EKA WATCHES/WARNINGS/ADVISORIES...
CA...None.
NORTHWEST CALIFORNIA COASTAL WATERS...Hazardous Seas Warning until 5 AM PDT Tuesday for PZZ450-475.

     Gale Warning until 5 AM PDT Tuesday for PZZ470.

&&

$$

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