Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Eureka California
448 am PDT Sat Aug 29 2015
Synopsis...a cold front is providing light to moderate rain
across areas north of the Humboldt Mendocino County border. This
system will move east of the region later this morning...followed
by isolated shower activity through Sunday.
Discussion...the much anticipated cold front is moving rapidly
across Trinity County early this morning. Thus far widespread
light to moderate rain has been observed across areas north of the
Humboldt Mendocino County border. South of the border northern
mendo has seen very light rainfall amounts. Best rainfall
observations were generally over del Norte County where highest
values were a little over an inch. Highest total so far across
the County Warning Area was observed at cooskie Montana RAWS in the King Range where
1.45 inches was recorded. Around Humboldt Bay Eureka and kacv both
recorded about a half an inch. The front was noticeably weakening
as it approached central Trinity County...thus this area may only
see totals in the 0.15 to 0.25 range at best. As of 430am the Post
front air mass appears to be quite dry both on infrared and visual
observations as high ceilings and patches of clear skies are already
being observed. Areas in the del norte coast will have the best
onshore flow component and thus best potential for isolated shower
or light rain/drizzle later today. A couple of weak shortwaves
embedded in the flow aloft will move across the north coast again
on Sunday morning while a secondary much weaker surface reflection
may promote some additional light rain mainly confined to the near
coast region. Models are in general agreement that a long wave
trough will remain in place over the West Coast for much of the
next week resulting in below normal temperatures. This trough will
also allow shortwaves to swing around the trough possibly bringing
brief shots of shower activity to the region middle to late next week
however quantitative precipitation forecast estimates are very low.
Aviation...after a brief period of rain along the Redwood coast...conditions
are expected to gradually improve through the day today. While
isolated showers will be possible over the next 24
hours...particularly at cec...widespread moderate to heavy rainfall
has ended for this event. Ceilings are expected to improve as a
cold front moves farther inland...although MVFR ceilings may be a
bit more stubborn to lift at cec. Late tonight...another weaker
cold front may result in a brief period of light rain along the
Redwood coast. At uki...MVFR ceilings will lift this
afternoon...resulting in VFR conditions through the day. Low clouds
will be possible overnight.
Marine...as expected southerly winds increased markedly overnight as a
weakening cold front moved through the waters. Ascat passes late in
the evening indicated offshore speeds of around 20 to 30 knots...and
this has been verified with buoy observations and at the cec ASOS.
This closely aligns with the previous forecast numbers...and while
low end gale force gusts are possible across parts of the northern
waters this morning...given that the duration of the strongest winds
is expected to be short and that these would likely only be marginal
gale force gusts...small craft advisories appear to be sufficient.
However...fresh to strong breezes will likely continue north of Cape
Mendocino through the day today...allowing steep seas to build and
remain elevated. Thus...opted to extend the small craft advisories
for the northern waters through the evening to account for the
persistent elevated conditions. Also...added a short duration Small
Craft Advisory for the southern outer waters this morning to account
for stronger winds and higher seas across the western half of the
zone...I.E. 30 to 60 miles offshore. Winds will likely be quicker
to subside in this area during the day today allowing seas to slip
below advisory criteria...although they will likely remain somewhat
Winds will begin to finally subside to light to moderate levels by
early Sunday morning...allowing steep short period seas to gradually
subside. While this will result in improving conditions
offshore...a moderate long period northwesterly swell will begin to
move through the waters...building to around 4 feet at 17 seconds by
middle afternoon. Depending on how quickly short period seas
subside...this could result in a moderate sneaker wave threat along
area beaches. Later that evening...yet another northwesterly swell
will move into the waters...however this iteration will likely build
to larger heights /6 to 7 feet/ and at a shorter period /11 to 12
seconds/. Also in the mix through the week will be a series of
small southwesterly long period swell groups. Winds should be
remain generally light to moderate until at least the end of the
week...with the strongest winds south of Cape Mendocino.
Fire weather...as expected rain amounts have been fairly
impressive over areas north of Humboldt County this morning.
As of 4am Crescent City has reported about an inch...iraws42
has reported 0.83. Farther inland Camp Six also about an inch
and Gasquet 0.64 inches...iraws41 also near an inch. For inland
Humboldt County iraws14 0.29 inches...while Mad River only 0.08
inches. At this time the rain band is just moving across Trinity
County and so far less than 0.10 inches have been observed there.
Not anticipating much more than perhaps 0.20 over the Trinity area
as the front is rapidly weakening. Behind the front stars are
already being observed at the weather forecast office and radar does not show much
else coming...although models are pretty insistent some residual
isolated shower activity will occur later today. Nonetheless a
cool moist day is in store for most of the active fires and
region. This trend will continue through most of the upcoming work
Northwest California coastal waters...
Small Craft Advisory until 9 PM PDT this evening for pzz450.
Small Craft Advisory until 3 am PDT Sunday for pzz470.
Small Craft Advisory until 9 am PDT this morning for pzz475.
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