Area Forecast Discussion
Issued by NWS Eureka, CA
FXUS66 KEKA 301046 AAA
AREA FORECAST DISCUSSION...UPDATED
National Weather Service Eureka CA
346 AM PDT FRI SEP 30 2016
.SYNOPSIS...An upper level low approaching the west coast will
bring a couple rounds of rain to the area this weekend. First some
light showers are expected in the north on Saturday. Saturday
night into Sunday morning coverage will increase and amounts over
a half inch are possible in the mountains in the north. Additional
light rain is possible through early next week.
DISCUSSION...An upper level low approaching the west coast will
be the main influence on the weather in northwest CA over the next
several days. Currently it is bringing onshore flow and a deep
marine layer to the area. The profiler at the Arcata Airport and
the surface obs show the marine layer to be nearly 3000 feet deep.
This may be deep enough that it will start to break up this
afternoon and bring some clearing to the immediate coast, while
areas just inland may continue to see some clouds. Confidence is
low on this.
Late tonight the first shortwave moving around the low is
expected to move through bringing some light showers to northern
and coastal Humboldt county as well as Del Norte county. Rainfall
amounts in these areas with this first wave are expected to be
less than a quarter of an inch. Farther south and east Trinity,
southeastern Humboldt and Mendocino counties are expected to only
see trace to a few hundredths of an inch of rain with this first
wave. Snow levels will drop to around 6000 feet. Also
temperatures in the interior will cool significantly with highs
only in the 60s to low 70s in the interior. Low temperatures will
be in the mid to upper 30s in the inland valleys. This rain will
be falling on roads that have not experienced rain in several
months, which may make roads more slick than normal due to the
build of oil.
Saturday afternoon and evening a few showers may linger, but
generally there will be a break before the next wave moves
through. Models are in decent agreement on the rain spreading
across the area late Saturday night into Sunday morning. The ECMWF
is still significantly wetter than the GFS. For now am sticking
closer to the GFS forecast as the climatology would suggest large
amounts of rain are not as likely. Still rainfall amounts expected
to be around 1.5 inches in the hills of northern Humboldt and Del
Norte counties. This is not expected to cause flooding, but rapid
rises on small streams and creeks are possible. Farther south and
east rainfall amounts will taper off quickly. Much of Trinity and
Mendocino counties will likely only see a quarter to a half of an
inch of rain. Snow levels will be around 6000 feet with most of
precip with this system. Confidence is growing in this system and
the rainfall amounts, however it still a cut off low and the
models tend to struggle with these so confidence is lower than
average with this system.
Sunday afternoon once the main frontal band moves through there
will be quite a few showers around. Snow levels are expected to
drop to 5000 feet Sunday night with the showers over the area.
Temperatures do no look quite cold enough aloft for small hail to
be a significant threat. Thunder is possible, but for now have
held off on adding. The model profiles don`t look particularly
favorable despite 200 to 400 j/kg of cape. This will need to be
monitored as it gets closer. Temperatures inland will be cool on
Sunday with highs only in the 50s in some areas.
Monday is where the models start to disagree. The GFS keeps Monday
dry while the ECMWF brings another weak system through in the
afternoon. For now kept a slight chance of rain, but am leaning
toward the GFS solution. If the drier solution does pan out, there
may be some frost in the interior valleys Monday morning. Tuesday
there is a better agreement on a weak warm front bringing some
light rain to the area. Wednesday and Thursday generally look dry,
although the GEM tries to bring some rain to the area. Friday the
ECMWF brings a fairly strong system and quite a bit of rain to the
area, while the GFS has a very week system with only a few
sprinkles. Added some pops to the forecast, but this obviously
remains in flux. MKK
AVIATION...MVFR to IFR conditions and light winds to calm
conditions will continue at KCEC and KACV through this morning.
The lower conditions are due to reduced ceilings and/or
visibilities. VFR to MVFR conditions and light winds will prevail
at KCEC and KACV late this morning through this evening. VFR
conditions and light winds to calm conditions will continue at
MARINE...An offshore low pressure trough will maintain a weak
pressure gradient across the coastal waters the next few days.
Light southerly winds and low seas will continue across the waters
through tonight. The seas will increase slightly Saturday as the
low pressure trough approaches the coast and a low pressure system
across the Northeast Pacific Ocean strengthens.
NORTHWEST CALIFORNIA COASTAL WATERS...None.
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