Special Weather Statement, NWS Eureka, CA

AREA FORECAST DISCUSSION
National Weather Service Eureka CA
508 AM PDT Fri Mar 23 2018

.SYNOPSIS...A cold upper level trough will generate occasional
showers and mountain snow through the weekend. High pressure will
build next week resulting in dry weather conditions.

&&

.DISCUSSION...A cold and unstable air mass offshore the Pacific
Northwest has been slowly sagging southward toward NW California
through the night. A band of colder cloud tops has been developing
near the ORCA border where a few lightning strikes have been
detected. BUFKIT soundings and convective parameters were not
looking all too impressive for thunderstorms today. Considering
current satellite and lightning data trends, we added them to the
forecast for today, mainly over the waters and near the coast
north of Cape Mendo.

The rain and show has been slow to develop through the night,
but will start to ramp up in coverage and intensity by late this
morning into this afternoon as a shortwave trough pivots
southeastward toward the north coast. Moderate to locally heavy
precip should last into the evening hours before diminishing
overnight. Significant snow accumulations are expected today
through this evening for the mountains of Del Norte, Humboldt and
Trinity counties. The main forecast challenge has been figuring
out the snow levels. For this afternoon and evening when the bulk
of the heavy precip arrive, models indicate snow levels rising to
3000 feet or higher. This is just above most of the major passes,
except South Fork Mtn Summit and Buckhorn Summit. We could still
see pockets of snow down to 2000 feet in northeastern Trinity
county, however it is looking like mainly a high elevation snow
event. Thus we will maintain the Winter Storm Warning through this
evening for several inches of new snow.

The rain and snow showers should decrease in coverage and
intensity late tonight or early Saturday morning before the main
pool of cold air aloft swings toward the coast. High resolution
mesoscale models indicate bands of showers developing offshore
and moving through the area through the day on Saturday. With
500mb temperatures falling to -35C and steepening lapse rates,
chances for thunderstorms and small hail should increase. Snow
levels may also fall to around 1000 feet Saturday morning, though
the precip will most likely be light. Also, daytime ground
heating may once again pump levels back up to 2000 to 3500 feet.
Nonetheless, another winter weather advisory or heavy snow warning
will be necessary for Saturday through Saturday evening.

The cold upper trough will slowly transition across the area Sat
night and shower/convective activity will likely continue. We
will be on the backside of the trough on Sunday, but there will
still be a potential for showers. The air aloft will still be
cold and the surface flow onshore.

High pressure will build toward the west coast early next week
and hold through the week. This ridge will keep major storm
systems well to our north. No rain is expected. The main concern
will be the potential for early morning frost for coastal areas
and for the interior valleys of Mendocino county Mon and Tue.
Frost advisories or freeze warnings will be necessary. The sky
forecast will also be tricky for the interior valleys, especially
for Trinity county where low clouds and fog tend to form at night
after several days of rain or snow. The days are getting longer
and the nights shorter, so the chances for nocturnal river valley
stratus should decrease as we head into mid week. What may become
an issue is coastal stratus. Persistent northerly flow offshore
coupled with warming aloft will probably result in diurnal coastal
stratus along the north coast.

&&

.AVIATION...Another frontal system will pass over northwest
California today bringing gusty winds, rain, and perhaps even some
thunderstorms to coastal terminals. Conditions are expected to
flux between VFR and MVFR with IFR conditions being possible in
heavier bands of rain or thunderstorms.

&&

.MARINE...Another front will bring strong southerly winds to the
waters this morning and then briefly shift to the west tonight.
This will be shortly lived though as strong southerlies will
return to the waters on Saturday as the final front in this storm
system passes over head. High pressure will be building behind
this front which will cause increasing north winds for the middle
of next week. The wind-driven wave environment will be steep and
chaotic over the next few days with the constantly changing wind
field. Nevertheless, expect steep building waves with each frontal
passage today and tomorrow. Otherwise a large northwesterly swell
system will keep seas elevated through the weekend. /KML/KR

&&

.EKA WATCHES/WARNINGS/ADVISORIES...
CA...Winter Storm Warning from 10 AM this morning to 11 PM PDT this
     evening for CAZ107-108-111.

     Winter Storm Warning until 11 PM PDT this evening for CAZ102-105-
     106.

NORTHWEST CALIFORNIA COASTAL WATERS...
Small Craft Advisory until 9 AM PDT Sunday for PZZ450-455-470-475.

&&

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For forecast zone information
see the forecast zone map online:
http://www.weather.gov/eureka/zonemap.png

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