Area Forecast Discussion
Issued by NWS Eureka, CA

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FXUS66 KEKA 242138

National Weather Service Eureka CA
238 PM 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


.DISCUSSION...The marine layer has expanded while fog and a well
defined stratus deck move north along the mendo coast this morning
reaching cape mendocino and gradually making the turn toward
humboldt bay. Expect the low clouds and fog to invade the north
coast tonight bringing an end to the warm afternoons and clear
nights along the near coast. The marine layer will continue to
deepen as onshore flow increases Wednesday, with fairly persistent
clouds and limited afternoon sunshine through Thursday along much
of the Northwest California coast. Meanwhile, the persistent but
weak upper level ridge is gradually giving 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 numbers are not super impressive and
plenty of cloud cover may inhibit cell growth...thus will keep
a minimum threat 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/JT


.AVIATION...A ridge of high pressure continued to allow for mostly
VFR weather across the region...and even at the coast north of
Cape Mendocino through mid-morning. However, the Mendocino coast
was blanketed by marine clouds early on as a stratus surge moved
northward. Shortly before 12 noon, stratus clouds had blown into
ACV`s airport and nearly an hour later into CEC`s airport. Cigs
and Vis rapidly dropped into IFR. IFR conditions will probably be
TEMPO along some areas through afternoon before totally becoming
settled over the coastal terminals this evening. Clouds may spread
into local coastal valleys and over nearby hills. Interior
mountain areas will generally remain VFR through Wednesday even
though model guidance is suggesting some lower clouds spreading
into UKI in the morning. Right now, uncertainty! /TA


.MARINE...A southerly surge has resulted in gusty winds from 20 to
25 kt today, primarily around Cape Mendo. These gusty southerlies
should die down quickly this evening. Wind fields overall will
remain light (15kt or less) to borderline moderate (20kt), Wed
through Sat as a weak surface low offshore maintains diffuse
pressure gradients over NW California waters. It will take til late
Sunday or more likely Monday before northerly winds ramp up to
formidable levels (25kt or more) after the passage of an upper level

Seas should remain dominated by mid period westerly swell and long
period s-sw swell through much of the forecast period. Any short
period wave generation will remain small (around 2ft or less) until
Sun and Mon when northerly wind waves build to significant levels.
Short period seas should reach 10 ft or greater on Mon.




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