Area Forecast Discussion
Issued by NWS Eureka, CA

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FXUS66 KEKA 241133
AFDEKA

AREA FORECAST DISCUSSION
National Weather Service Eureka CA
433 AM PDT Tue Apr 24 2018

.SYNOPSIS...High pressure will maintain mainly sunny skies and
above normal temperatures through Thursday across interior
Northwest California, while onshore flow brings cooler weather and
increasing marine stratus to coastal areas. An upper-level low
will likely bring cooler weather and showers Friday into the
weekend.

&&

.DISCUSSION...
As a thermal trough shifts offshore and an upper-level ridge
axis shifts inland, the surface and low-level flow will
transition from weakly offshore to southerly and onshore today
into Wednesday. This transition has already occurred along the
Mendocino coast. While satellite imagery is not especially
revealing due to high clouds, surface observations there indicate
the likely presence of marine stratus along the coast. Model
guidance supports this `southerly surge` of stratus expanding
northward to offshore the Humboldt coast this afternoon, likely
shifting inland by later this evening. The exact timing of this
progression is uncertain, so temperatures this afternoon along the
Redwood Coast may end up anywhere from the upper 50s to the upper
60s depending on the timing of the arrival of stratus. For now
expect clouds to increase and temperatures to cool late this
afternoon or early this evening between Eureka and Crescent City.

The marine layer will deepen as onshore flow increases Wednesday,
with fairly persistent clouds and limited afternoon sunshine
through Thursday along much of the Northwest California coast.
Meanwhile, as an upper-level ridge gradually gives way to an
approaching offshore upper-level low, inland temperatures will
remain above normal through Thursday; warmest today, then
gradually dropping each day this week. The upper-level low will
begin to push onshore on Friday, with inland high temperatures
dropping into the 50s and low 60s Friday through Sunday, and a
good chance of showers for the bulk of our region. There will
continue to be a bit of model disagreement on the track and depth
of the upper-low; for now favor a blend of the ECMWF and GFS
Ensemble Mean. The operational GFS and maybe half of the GFS
ensemble members have a weaker and farther north system which
would be a drier solution, especially south of Cape Mendocino. In
either case, do not expect significant rainfall amounts, but
something in the tenth to half-inch range Friday through Sunday.
The potential for some thunderstorms over interior areas would be
the more hazardous threat, but instability may be lacking, so
will only keep a slight chance of thunderstorms in the forecast.

The general consensus of model solutions heading into next week
advertise an upper-level ridge building in after the upper-level
low passes by, which would result in warming temperatures and
drier weather. However, the exact timing of that remains
uncertain. A secondary trough or two dropping southeast out of the
Gulf of Alaska may delay the arrival of that warmer and drier
weather early next week. For now, made little to no change to the
forecast Sunday through next Tuesday. AAD

&&

.AVIATION...A southerly stratus surge is moving north along the
coast. High clouds are making it difficult to see how far north it
has progressed on the night satellite. It appears they have made
it most of the way to Cape Mendocino and are currently impacting
the Mendocino coast. The southerly winds are expected to continue
to move them north through the day today. They are expected to
push onto the Humboldt and Del Norte coasts in the afternoon or
early evening and impact KCEC and KACV bringing IFR or LIFR
conditions. Confidence is low on the timing, but they are expected
to be here before 06Z. Winds will be southerly at the coast, but
fairly light. Ukiah and most in areas are expected to continue to
see VFR conditions and light winds through the TAF period. MKK

&&

.MARINE...Winds have turned southerly a bit more quickly than
expected. Light southerly winds are expected to persist through
Saturday as an upper level low approaches the area. Occasional
periods of 10 to 20 kt are possible but generally the winds will
be fairly light. Confidence in the details diminish for the
weekend as model solutions start to diverge, however most
solutions keep the winds fairly light through Sunday. Sunday or
Monday high pressure starts to build into the area and bring a
return to northerly winds. They may be strong at times.

A west to northwest swell has started to build into the waters
this morning. Currently it is building in around 3 feet at 18
seconds. There is also a mid period swell around 12 seconds, but
the longer period swell is quickly becoming the dominant swell.
MKK

&&

.SURF ZONE...The long period swell entering the waters this
morning will increase the potential for sneakers along the coast.
Waves heights build to around 8 feet at 16 seconds by early
afternoon. This will create enhanced set behavior meaning there
will be longer lulls between the bigger waves. The main threat is
today as the periods diminish tomorrow. Never turn your back on
the ocean. MKK

&&

.EKA WATCHES/WARNINGS/ADVISORIES...
CA...None.
NORTHWEST CALIFORNIA COASTAL WATERS...None.
&&

$$

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