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Area forecast discussion 
National Weather Service Eureka California
506 am PDT Thursday Sep 18 2014

Synopsis...the approaching low pressure system will prolong
shower activity through the day today. Isolated thunderstorms will
develop across Mendocino County today and travel northward.
Shower and thunderstorm activity will quickly dissipate this
evening as high pressure builds behind the diving trough.


Short term (today)...a frontal boundary associated with the low
pressure system located offshore brought the first significant
rainfall of the season to northwestern California yesterday
afternoon. Much of the County Warning Area received less than
0.10 inches while the Redwood coast and interior received up to
0.25-0.50 inches. The highest rainfall was just under 1 inch on
higher terrain north of Cape Mendocino.

The approaching low pressure system will swing additional weak
boundaries over northwestern California prolonging the possibility
for showers and a few isolated thunderstorms today. Models have been
doing a poor job leading up to this event and still contain
differences in the positioning of the surface low before it dives
southward tonight. The positioning of the low will dictate where
thunderstorms ignite. One thing models have been consistent with is
highlighting yolla Bolly range as a region where storms will ignite.
In later runs models have been showing convective precipitation
along the Mendocino coastline however the best cape and instability
look to be farther east. Thunderstorms were extended to most of the
eastern County Warning Area and as well as the Mendocino coastline
to account for uncertainty of where convection will fire. Showers
will quickly weaken tonight as high pressure builds behind the
diving trough. \Kml

Long term (fri through wed)...high pressure will build in over
northwest California on Friday as the upper trough slides off to the
south and east...transitioning into a cut off low that then dives
south...settling over Southern California...and moving off to the
east by Monday. Models are in good agreement with this solution. Dry
conditions along with light offshore flow and above normal
temperatures will return to northwest California and dominate the
weather pattern into early next week as heights build. A southerly
coastal stratus surge looks possible Saturday night into Sunday with
a flow reversal over the waters. The light offshore flow will likely
give coastal locations above normal temperatures with the warmest
day looking like Saturday.

By late Tuesday into Wednesday models forecast another upper level
trough to approach northwest California. Have increased probability of precipitation to
slight chance/chance across the northern half of the County Warning Area and this
matches up well with the neighbors. Rmop tool shows low
predictability in the pattern for this time period. This is still
aways out and there remains quite a bit of uncertainty. Will monitor
the situation. Stp


Aviation...the frontal boundary and the approaching trough is
currently bringing middle level clouds and occasional rain showers this
morning. Conditions as low as IFR are possible in the heavier
showers...although these will generally be brief. The threat for
showers will diminish this afternoon and evening as the trough axis
moves through the area. Tonight winds turn offshore so expect this
will keep conditions relatively good despite the additional moisture
around. Mkk


Marine...the upper level trough and associated surface low
continues to approach the area bringing light to moderate southerly
winds to the waters. These winds and the winds farther offshore have
created a southerly wave that has built to around 6 to 9 feet at 8
seconds. These winds and waves will diminish through the day as the
trough moves overhead. Extended the Small Craft Advisory in 475 through 9am and added
a Small Craft Advisory for seas in zone 470 through 9am as well. Tonight the trough
looks to start moving out of the area and winds will become
northerly but remain light. Friday winds will increase to 20 to 25
knots as high pressure builds into the area. A Small Craft Advisory will likely be
needed for at least the northern outer waters. These seas will
likely propagate into the southern waters creating rough conditions
there as well. These northerly winds are short lived however as the
upper level trough that moved over the area becomes a cutoff low and
drift backs towards the area. This brings light southerly winds once
again. Winds look to remain mainly light and out of the south
through late next week. There is the possibility that there will be
some stronger southerly winds on Wednesday as stronger system moves
through...however confidence is low on that at this point.

There are two fairly distinct long period waves complicating the
forecast. The first is a southerly swell around 16 seconds around 2
to 3 feet. This will slowly diminish by Saturday night. The other wave
is around 3 feet at 12 seconds out of the northwest. This will slowly
diminish by Friday night. Saturday another northwesterly swell
builds into the waters. Initially it looks to be around 7 feet at 12
seconds. This diminishes to around 4 feet at 11 seconds by Monday and
then remains fairly persistent through mid-week. Mkk


Fire approaching low pressure system will prolong the
possibility for showers and isolated thunderstorms over northwestern
California today. See the above discussion for details on the
thunderstorm potential this afternoon. Otherwise ridge tops may
observe gusty winds this afternoon as the low slides over the
region. This in conjunction with dry fuels inland could make for
critical fire weather conditions, however, shower activity will
help stifle critical conditions. No fire weather products were
issued since storms are expected to remain isolated in nature.


Eka watches/warnings/advisories...
Northwest California coastal waters...Small Craft Advisory until 9 am PDT this morning for pzz470-475.



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