Area Forecast Discussion
Issued by NWS Eureka, CA

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FXUS66 KEKA 202147

National Weather Service Eureka CA
247 PM PDT Sun May 20 2018

.SYNOPSIS...Areas of coastal marine stratus will continue to be
very persistent through the upcoming week, with only localized or
temporary areas of sunshine. Monday will have very limited shower
or thunderstorm activity over the interior mountains, with more
widespread storms expected Tuesday and especially Wednesday.


.DISCUSSION...Very deep marine layer around 4000 feet made it well
inland this morning, and with steady onshore flow continuing, has
struggled to retreat out of most of Humboldt and Del Norte
Counties, as well as western Mendocino. With northerly winds
starting to pick up, the divergent flow downwind of Cape Blanco is
resulting in some clearing near Crescent City. As high pressure
builds over Washington and Oregon, and a thermal trough
strengthens over interior California, the low-level northerly flow
will accelerate into Monday. The flow will also tend offshore over
Del Norte County, as well as between Cape Mendocino and the
northern Mendocino coast, which should promote clearer skies there
even tonight into Monday morning. A building ridge aloft should
result in a lowering inversion and a shallower marine layer
heading into the upcoming week, resulting in less of an inland
marine air intrusion, and warmer temperatures inland. While the
northerly trajectory will keep the flow onshore over much of
Humboldt county, the stronger winds and shallower marine layer may
at least mean there will be a better chance for afternoon or
evening clearing along the coast between Ferndale and Trinidad

Aside from stratus and brisk winds along the coast, we have a
few weak thunderstorms around northeast Trinity county, and will
linger along the northeast periphery of Trinity county through
early evening. The main feature driving our weather for the
upcoming week will be an upper-level low tracking southward across
California through Monday, before lifting back to the north
across Nevada, Idaho, and eastern Oregon Tuesday and Wednesday.
Brief high pressure aloft on Monday will bring drier and more
stable air to limit afternoon shower and thunderstorm activity
over the mountains of eastern Trinity and northeast Mendocino
county. However, as the aforementioned low tracks back northward
Tuesday and Wednesday, the low and mid-level flow will turn more
easterly and southerly, with increasing moisture and instability.
Tuesday afternoon will probably feature storms in the same areas
as today. Wednesday looks like the greatest threat for more
widespread and perhaps stronger storms, but do not expect any
threat back toward the coast.

The late week period will feature at least temporarily more stable
conditions as high pressure aloft returns. Inland temperatures
will be slightly above normal for late May. Coastal stratus will
likely become more persistent, even in those areas that do manage
some clearing early in the week. Model guidance is starting to
latch on to another upper- level low and weak associated surface
low which will approach our coast from the west Friday and
Saturday. This will have the potential to deliver a bit more
widespread shower and thunderstorm activity to our region, but
details will remain sketchy for now. AAD


.AVIATION...Expect a period of VFR conditions for much of the next
24 hours around Crescent City, as the low-level flow turns more
offshore, and clearing skies expand south of Cape Blanco. Much of
Humboldt county and ACV will remain socked in the clouds with MVFR
conditions perhaps settling back to IFR conditions at times for
early Monday morning. The marine layer should be shallower on
Monday, so expect less inland intrusion compared to today. That
should include UKI, with clear skies through Monday there. A few
thunderstorms will be found until early evening over Trinity
county, but these will diminish tonight, with only an isolated
cell possible around there on Monday afternoon.


.MARINE...Moderate northerlies will increase tonight bringing near
gale force gusts across the waters by Monday. Short period seas will
build in response to the winds, first south of Cape Mendocino this
evening and then northward through Monday afternoon. As increasing
winds become more widespread, steep seas will approach 7 to 10 feet.
Beyond that, the pressure gradient will relax and allow winds to
decrease and, subsequently, seas will settle mid to late next week.
The existing small craft advisories remain in effect through Tuesday
morning. Additionally, a long period south swell and mid-period
northwest swell will continue to impact the waters over the next
several days. /KML


     this evening to 5 PM PDT Tuesday for PZZ450.

     Small Craft Advisory until 5 PM PDT Tuesday for PZZ455-470-475.



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