AREA FORECAST DISCUSSION
National Weather Service Eureka CA
240 PM PST Thu Jan 25 2018

.SYNOPSIS...Showers through tonight, then diminishing on Friday.
Next rain system will move into area Friday night and continuing
through Saturday daytime. Dry weather after that.

.DISCUSSION...The active weather we have been seeing will continue
for the next few days. Through this evening, showers will continue
to impact NW California. The atmosphere is unstable enough for these
showers to thrive. The 850 mb temperature is around -2 C. According
to SPort Satellite SST analysis, SST is around 54 F or 12 C. This
gives us a lapse rate of around 14 C from surface to 850 mb.
Compared to the 13 C rule used for lake effect snow, the 14 C
temperature difference suggest a very unstable atmosphere. We have
seen some pretty strong showers since the overnight hours through
this afternoon. Early this morning, a strong convection went over
Eureka area producing occasional lightning strikes and small hail.
The small hail was significant enough to cover Highway 101 south of
Eureka leading to hazardous driving conditions. A layer of black ice
formed on the roadway after cars driving through small hail
compressed the hail into an icy layer on the roadway. Through today,
some of these showers had small hail, lightning strikes, and weak
rotation in them.

These showers moved from west to east, made landfall then moved into
interior locations. Snow was observed as low as 1200 feet. Snow
level was generally around 2000 feet. During periods of heavier snow
showers, snow level was lowered and visibilities were significantly
reduced. We had snow accumulation reports ranging from around 4 to
14 inches of snow across the region. Highest snowfall was in higher
elevation locations in Trinity County. Do expect these snow showers
to continue through tonight, with additional snow accumulation to 4
inches.

Showers activity will be diminishing overnight tonight, as a
transitory area of high pressure quickly moves into the area. We
will have a brief dry day on Friday. However, the next weather front
is coming in right behind. Precipitation will move into the area
Friday night, and continue into Saturday daytime.

For the Friday night, early Saturday rain event, the Atmospheric
River Forecasting Tool shows that this will be a non-AR event.
Ensemble models are showing Integrated Vapor Transport of around 150
kg/m/s, which is below the 250 kg/m/s to threshold to be considered
as a typical AR event. SREF ensemble shows the same idea, suggesting
Eureka to get around 0.5 to 1 inch of precipitation, which is not
particularly heavy for this time of year. The highest rainfall will
occur north of our area, in the state of Oregon. Southern Mendocino
County will stay mostly dry. Precipitation will end on Saturday
night as the front lifts northeastward.

The ridge of high pressure will then build over the West Coast of
North America, and this will steer the AR towards Vancouver Island.
Sunday and Monday looks like the warmest days. We have 850 mb
temperatures going up to around 10 to 15 C for those days. If there
is enough downslope winds happening, coastal Plains can see
temperatures in the low to mid 60s. There may be a break in the dry
weather on Tuesday. After that, the mild and dry weather will
return to the area.

.MARINE...The large swell has built into the waters as expected
this afternoon. Currently it is around 13 to 15 feet at 14
seconds. This is expected to slowly diminish tonight, but remain
over 10 feet through Monday. West winds will persist tonight
around 10 to 20 Kt.

Friday winds will become southerly again and increase ahead of the
next front. These winds will mainly increase in the northern outer
waters. The 12Z NAM was showing fairly widespread gale force
winds, but the GFS had only a very small area of gale force
winds. Held off on issuing a gale watch due to low confidence.
Steep waves associated with these winds are expected to peak
around 10 feet.

Fairly light southerly winds are expected Sunday through Monday
night as the next system moves through. Tuesday through the middle
of the week northerly winds are expected to return as high
pressure builds back into the region.

.AVIATION...The clouds and showers associated with an unstable
airmass in the wake of the cold front yesterday will continue to
move onshore this afternoon and evening. This continues to bring a
chance of a thunderstorm to the coastal sites this afternoon. The
axis of the upper level disturbance that has been enhancing these
showers today has passed the coast this morning and as it moves
inland the atmosphere will begin to stabilize. Expect clouds and
some showers to continue through the night, lessening toward
morning. Once these heavier showers this afternoon weaken we expect
coastal sites will stay vfr through the night as there will still be
enough instability for low clouds and fog to form. In KUKI expect
IFR condition to develop by early morning as low clouds or fog
develop as the lower atmosphere settles down.

/MKN

.EKA WATCHES/WARNINGS/ADVISORIES...
CA...Winter Weather Advisory until 4 AM PST Friday for CAZ102-105>108-
     111.

NORTHWEST CALIFORNIA COASTAL WATERS...Small Craft Advisory until 9 PM PST Saturday for PZZ455-475.

     Small Craft Advisory until 3 AM PST Sunday for PZZ450-470.

$$