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Area forecast discussion 
National Weather Service Eureka California
246 PM PDT Monday Apr 20 2015

Synopsis...afternoon and early evening showers across the
interior mountains will diminsh late this evening. This will allow
for a generally fair night while fog and low clouds again plague
the immediate coast. Tuesday will be similar to today but a touch
cooler across the interior.


Short term (tonight through thu)...developing north flow will
send stratus back onto the north coast and into the coastal river
valleys this evening. With shallow moist air coming in from the
ocean and a slightly deeper cloud layer this evening we may see
some light misty drizzle. Expect the marine layer to deepen more
on Tuesday with fog becoming less problematic, near mean sea level,
through the day as northerly winds create better mixing in the
boundary layer. Not sure if it will clear out on Tuesday as the nam12
and high resolution models show high boundary layer humidity being
imported onshore through the day.

High temperatures will continue trending downward through middle week
as a splitting upper trough approaches the north coast. This
trough along with an upper low meandering offshore the central
California coast will bring a threat of thunderstorms today and
again on Tuesday across the northwest cal interior. Convective parameters
from the nam12 and GFS were once again quite impressive...MUCAPES
in excess of 2000j/kg with lifted indices near -5c during peak
heating this afternoon. Moisture is marginal with precipitable waters around
0.70in and 700- 500mb layer humidity around 50%, but should be
sufficient to trigger some deep convection. The storms probably
will not produce much if any precipitation. Model based soundings are not
text book inverted-v profiles...however there may be some strong
outflows from a few of the storms.

Convective parameters are also impressive on Tuesday. The southwest
flow aloft in advance of the approaching trough will likely force
the convective activity farther to our east. There is still a good
chance of storms in Trinity County and along the crest of Mendocino
National Forest.

The trough aloft will split on Wednesday...with the northern branch
moving eastward across the Pacific northwest and the southern branch heading
southward down the California coast. All the models were in good
agreement on the evolution of the large scale trough...but
differed on the convective precipitation generation as the tail end of
the trough slides over northwest cal. The GFS and nam12 remain not as
bold with convection on Wednesday...but continue to generate some precipitation
over the interior...mainly Trinity County. Thus have not made many
changes here and maintained a slight chance of showers and
thunderstorms on Wednesday. Thursday will be a transition day with a few
showers possible as a trough clips the northern portions of northwest cal
and a ridge transits the southern portions bringing fair weather.

Long term (fri through mon)...another trough will be dropping into
the Pacific northwest by Friday with a couple of short-waves moving over the
area in northwest flow. Models keep the bulk of the precipitation north
and east of the area...but kept slight chance probability of precipitation in on Friday to
account for some possible showers. Ridging will build back in over
northwest California late Friday into Saturday...before another trough
drops in from the northwest later in the weekend. Both the GFS and European model (ecmwf)
show precipitation riding over the ridge and approaching the area
Saturday night into Sunday...with a frontal system possibly
bringing more rain to the region Monday night into Tuesday. The
GFS remains the faster of the models. Due to the continued
uncertainty...have kept probability of precipitation mostly unchanged from the previous
forecast. Stp


Aviation...a persistent marine layer has kept the stratus locked
in along the coast today with IFR to LIFR ceilings and visibilities observed.
As of 20z cumulus was building over the interior mountains and showers
and thunderstorms are expected this afternoon/evening. MVFR
conditions...gusty winds...small hail...and some mountain
obscurations are possible near any shower/T-storm activity.
Convection will diminish during the evening with the loss of
diurnal heating. Going into the overnight...expect IFR to LIFR
ceilings and visibilities once again for coastal locations with some -dz
possible. Think that low clouds will once again make it up into
the Russian River valley affecting kuki overnight with IFR
conditions possible. Stp


Marine...light to moderate winds will prevail through the early
evening hours before marine conditions begin to deteriorate
drastically starting tonight. The culprit is a strong Pacific
storm off the coast of British Columbia that pinches the ridge of
high pressure just offshore. This will tighten the pressure
gradient across the northwest California coastal waters bringing strong
gales Tuesday afternoon through early Thursday morning. The
persistent strong winds will cause large, steep seas to develop
near 15 to 20 feet around 10 to 11 seconds late Tuesday into
Wednesday. Due to increasing confidence, the watches have been
upgraded to gale warnings for the outer waters.

Small craft advisories have been issued for the other remaining
zones...first for the southern outer waters this afternoon which
precedes the Gale Warning. Second is for the inner waters starting
tonight (north of cape mendocino) and Tuesday morning (south of
cape menocino). Although both of the nearshore zones will be
initially marginal I decided to go ahead and issue the advisories
a bit early as steep seas are anticipated to develop. One thing
that we will need to keep a special eye on is whether a hazardous
seas warning will be needed at some point for the inner waters. I
anticipate large, steep seas to propagate from the outer waters
but will wait on issuing the warning at this time. Either way you
look at it conditions will be pretty crummy due to both the winds
and seas.

Winds and seas across the coastal waters will begin to slowly
diminish on Thursday with advisories looking like a good possibility
after the gales expire. Otherwise the trend for improvement should
continue into the weekend. Kml


Eka watches/warnings/advisories...
Northwest California coastal waters...
Small Craft Advisory from 9 PM this evening to 9 PM PDT Thursday
for pzz450.

Small Craft Advisory until 3 PM PDT Tuesday for pzz470.

Gale Warning from 3 PM Tuesday to 9 PM PDT Thursday for

Small Craft Advisory from 9 am Tuesday to 9 am PDT Friday for

Small Craft Advisory until 3 PM PDT Tuesday for pzz475.



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