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Area forecast discussion 
National Weather Service Eureka California
358 PM PDT Tuesday Jul 7 2015

Synopsis...a disturbance off the California coast will bring a
chance of showers and thunderstorms to portions of interior northwest
California through the end of the week. There will be a slight
chance that storms may impact coastal areas this evening through
Thursday.

&&

Discussion...

Short term...(through late thursday)...the primary item of
interest in the short term will be the track and evolution of a
large closed low currently off the coast of California...as this
feature will result in chances for thunderstorms throughout much
of the region over the next several days. This afternoon the
center of the low remains well offshore...placing the northern
half of California within a regime of deep southeasterly flow. As
a result...ample moisture exists throughout the region...with
surface dewpoints well into the 50s and precipitable water values near and
slightly above one inch. Likewise...instability has gradually
built to around 1000 to 1500 j/kg of cape throughout the interior
mountains and farther east into much of the Great Basin...and
vigorous convection has developed in earnest across northeast
California and the northern sierras where minimal inhibition
exists. However...convection has been much slower to mature across
our area...likely due to the presence of a more substantial cap in
the low to middle levels. Eventually though...isolated to scattered
thunderstorms are expected to emerge from the developing cumulus
field...particularly across the Trinity Alps and other sub ranges
of the northern Klamath Mountains...including possibly a storm or
two in the vicinity of the yolla Bolly mountains. While there
appears to be moderate flow in the upper third of the storm
depth...steering flow in the middle and low levels remains relatively
weak. This is reflected in the nature of already existing storms
nearby...and any storms that do develop are expected to move
somewhat slowly to the northwest and will be capable of producing
heavy downpours...gusty winds...and small hail. A few storms may
drift onto the Redwood coast through the evening and overnight
hours...particularly in northern Humboldt and del norte counties.
Storms should gradually diminish by the early morning hours
tomorrow...although lingering convective debris may result in
light showers persisting through the morning in places.

By middle day tomorrow...model guidance suggests that the center of
the upper low will likely move onshore somewhere near and slightly
south of the Bay area. However...as mentioned
previously...forecast models traditionally struggle with lows such
as these given a lack of in situ data and also a relative lack of
dynamics. Still...there does appear to be relative agreement
amongst solutions both in regards to the upper low and the
redevelopment of convection early tomorrow afternoon across the
interior. While models are unanimously more aggressive in
developing widespread convection...there remains some concern
regarding the degree of instability that will be achieved in the
wake of the previous days storms...and specifically whether or not
cloud cover will sufficiently relinquish its hold on the area
prior to peak heating. Still...increasing moisture...a potentially
weaker cap...and very good model agreement do support a
conditional threat for scattered to numerous thunderstorms. Much
like today...the strongest wind flow will likely be restricted to
the upper levels of the storm column...and with increasing
moisture expected...slow moving but periodically strong pulse-type
storms with heavy rain rates are most likely. Given the persisting
easterly wind component...storms may reach the coast yet again
during the late afternoon and evening...and a few may persist
through the night.

By Thursday...uncertainties become compounded after two
consecutive days of potential convection. However...the upper low
will likely have moved into western Nevada and weakened by the
afternoon...resulting in generally weaker and more northerly flow
across the region. Also...cooler temperatures and poor lapse rates
will likely result in relatively meager instability although ample
and deep moisture will remain in place. Still...its hard to say
with certainty how Thursday will unfold at this stage aside from
continuing chances for showers and storms.

Long term...(thursday night through tuesday)the upper level low
continues to push inland over central California. This moves the low out of
a favorable area for thunderstorms with much less instability.
Also...the low becomes more of an open trough. Models continue to
show some showers and due to the uncertainty on the track of the low
trimmed back thunderstorms to slight chance...but left the chance
for showers in the forecast. Saturday left the slight chance for
thunderstorms...but the atmosphere is pretty capped so it is
starting to look less likely. Saturday night into Sunday morning the
GFS brings an upper level trough to the area which looks to bring a
few showers overnight. Temperatures will remain fairly cool and
there will be quite a few clouds around. The marine clouds will be
fairly deep...but with the cooler air aloft the inversion may be
less sharp. So there will likely be some clearing of the low clouds
each afternoon...although the high clouds are expected to remain
fairly persistent.

Early next week the models start to struggle with the pattern. The
Canadian brings another cut off low down the coast...while the GFS
and European model (ecmwf) keep the flow more zonal with fairly high heights.
Currently favor the GFS/ec solution which keeps things drier.
Temperatures will slowly start to warm again...although the really
hot weather is not expected to return in the near future. Mkk

&&

Aviation...mainly VFR conditions across the area this afternoon.
Later this evening some showers are expected to move over del
Norte County...there is the potential for a thunderstorm or two.
Have added some vicinity thunderstorms to the cec taf...but this
will may need to be monitored. Tonight some stratus may return at
the coast...although with the increased mixing due to the
convection the ceilings may not be very consistent. Kept acv and cec
in MVFR to account for this...although periods of lower conditions
are possible. Mkk

&&

Marine...the upper level low off the California coast will slowly move
off to the east over the next several days keep the pressure
gradient fairly week. Although the pressure gradient is
weak...northerly winds will increase slightly...peaking around 15
knots over the next several days. Late in the week winds will become
southerly as an upper level trough starts to slowly approach the
area. These southerly winds will persist over the area for the
weekend. Early next week the European model (ecmwf) and GFS show very weak high
pressure building into the area and bringing a return of weak to
moderate northerly winds.

In addition a couple southerly swells are moving through the
waters...one around 13 seconds and another one around 20 seconds.
The 20 second swell is expected to increase over the waters over
the next 24 to 36 hours. Models shows this peaking around 3 to 4
feet...but have kept it trimmed at 2 feet as the models are currently
overdoing it and they typically overdo these southerly swells.

&&

Fire weather...isolated to scattered thunderstorms will be
possible across portions of the interior late this afternoon
through the overnight hours...and a few storms may reach the
Redwood coast by this evening. While lightning will be possible
with any storms that develop...storms will likely be slow moving
and relatively wet.

Fine fuels will gradually moisten over the next few days as a
combination of scattered showers, cloud cover, cooler temperatures
and high relative humidity values blanket the region. Although a fair amount of
lightning is possible over the next few days...due to the
expectation of wetter fuels...fire weather watches or red flag
warnings will not be necessary. Jt/bfg/brc

&&

Eka watches/warnings/advisories...
California...none.
Northwest California coastal waters...none.
&&

$$

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