Area Forecast Discussion
Issued by NWS Eureka, CA

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FXUS66 KEKA 151223

National Weather Service Eureka CA
523 AM PDT Sun Sep 15 2019

.SYNOPSIS...As a cold front approaches, increasing clouds and
rain will begin to spread across northwest California this
afternoon and evening. Scattered showers will linger into Monday,
with noticeably cooler temperatures inland. Another storm system
will bring a second dose of rain later Tuesday into Wednesday.
Drier and warmer weather will return for the end of the week.


.DISCUSSION...As the first widespread rain event of the season
approaches, the initial forecast challenge before the rain arrives
will be sky cover. The GOES-17 night fog product is depicting a
fairly deep layer of marine clouds extending well inland up the
river valleys, however this layer has scattered along portions of
the coast, with partially clear conditions in some areas. This is
likely due to some offshore flow induced as an arm of Pacific high
pressure is shoved inland in advance of the cold front approaching
offshore. Meanwhile, some patches of lower clouds and even some
fog have redeveloped near the immediate coast. The best way to
sum it up would probably be `mixed sun and clouds` for most of the
coastal plain to start the day, while the river valleys will
probably be tougher to scour out until the late morning hours.

In any case, the more impactful weather will roll in later today
and tonight with the cold front, in the form of a period of
beneficial rain. Showers should reach the Del Norte coast before
the morning is out, while the rest of the coast down to Eureka
will likely have to wait until the afternoon hours. Low-level
southerly flow ahead of the front may keep the Humboldt Bay
vicinity from seeing more than a few showers until more
substantial rain pushes in this evening. The main frontal rain
band may only last a few hours at any one location as it crosses
northwest California from northwest to southeast this evening.
Scattered showers will follow for Monday, mainly north of
Mendocino county, however there will likely be a relative lull
with even partial clearing later tonight into Monday morning. The
main upper trough axis will then cross our area, but will begin to
fill as it does so. Conditions will be unstable enough on Monday
for downpours with the showers, and even a rumble or two of
thunder, but am not too excited about the strength of any
convection given the filling trough and marginal CAPE. The trough
axis will shift east of our area in the afternoon as well, which
will tend to result in diminishing shower activity especially
toward the coast, and likely some partial clearing there by day`s
end. Total rainfall from the latest high-resolution guidance are
looking a bit more meager over eastern Trinity and Mendocino, but
still around a tenth of an in inch in most valleys, with higher
amounts in the mountains. A quarter to three-quarters of an inch
is expected for most of Humboldt and Del Norte, locally higher in
the mountains of Del Norte county. Winds will not be an issue with
this front, with even the more wind prone spots struggling to
gust to 20 mph (exposed mountain ridges aside).

Tuesday will start off drier as high pressure temporarily builds
in, with plenty of valley fog but also some sunshine. Clouds will
increase over Humboldt and Del Norte ahead of the next front.
Model guidance has come into good agreement with this next system,
with a cold front pushing southward across our area Tuesday
afternoon and Tuesday night, followed by an area of low pressure
tracking southward along the Pacific Northwest coast and
enhancing shower activity Wednesday. The latter may result in some
heavier rainfall amounts of one to two inches in the higher
terrain of Humboldt and Del Norte, otherwise expect a similar shot
of rain compared to the first system. There will be some brief rises
on area creeks and streams with these rains, but very limited
response is forecast on the main stem rivers by the California-
Nevada River Forecast Center. Overall the rain will be very
beneficial, and will help quell our wildfire season. The main
impacts will be to travel, with possible slick roads from this
summer`s oil patches, as well as some localized rock fall near
steep slopes.

At this point, the last couple of days of the upcoming week into
next weekend are looking drier and sunnier, with temperatures
warming back closer to seasonal averages. /AAD


.AVIATION...Broken patches of stratus lingering along the North
Coast overnight have made for a difficult forecast this morning. ACV
has been mainly VFR as the stratus bank remains just offshore,
however CEC has fluctuated between categories from LIFR to VFR as
clouds and patchy fog drift in and out of the terminal
intermittently. Thinking conditions will settle in around MVFR with
some variations during the morning as a more extensive area of
stratus approaches from offshore. There should then be enough
sunshine in advance of the trough to scatter out this initial cloud
layer and produce a few hours of VFR conditions before rain moves in
later in the afternoon with the arrival of the front. This should
bring IFR conditions to the coastal terminals between 21-00z
continuing into the evening, with conditions becoming more showery
in nature during the overnight hours. Conditions will also
deteriorate across the interior from this evening into the overnight
as the front pushes inland. /CB


.MARINE...Small Craft Advisories have come to an end this morning as
northerly winds and short period seas continue to trend downward
south of Cape Mendocino with the approach of the upper level trough.
A brief period of moderate to fresh southerlies will sweep across
the northern waters in the vicinity of Point Saint George later this
afternoon as the surface front reaches the coast. The trough will
also bring a showery regime which could result in localized gusty
winds in the vicinity of the most vigorous showers. Otherwise,
expect generally light winds over the next couple days as seas are
dominated by a building NW swell on the order of 7-8 feet at 12
seconds. A smaller, longer period SW swell will also remain present
through much of the coming week. A stronger push of southerly winds
looks to accompany the next trough on Tuesday, with the possibility
for some eventual headlines north of Cape Mendocino in that time
frame. A northerly regime may then return late in the week behind
the last in the coming series of troughs. /CB




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