Area Forecast Discussion
Issued by NWS Eureka, CA

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000
FXUS66 KEKA 211247
AFDEKA

AREA FORECAST DISCUSSION
National Weather Service Eureka CA
447 AM PST Wed Feb 21 2018

.SYNOPSIS...Gusty north winds, mountain snow, and possibly coastal
hail, will develop across the region during Thursday. This will
be followed by cold Friday morning temperatures. Additional
showers and mountain snow will be possible this weekend into early
next week.

&&

.DISCUSSION...Light precipitation is gradually spreading south
early Wednesday morning across the coastal waters, and portions
of Humboldt, Trinity, and Mendocino Counties. This activity will
generally dissipate during the remainder of Wednesday morning as
an associated upper disturbance exits to the southeast. However, a
second disturbance will move south across NRN CA Wednesday night
into Thursday. Cold midlevel temperatures (at or below -30C at 500
mb) accompanying the upper wave will contribute toward a
marginally unstable environment favorable for widespread shower
development.

Model forecast soundings show sub-zero saturated conditions in
the low to midlevels over interior Humboldt and Del Norte Counties
during Thursday morning and afternoon, and given the convective
nature of the expected precipitation, will likely yield
accumulating snowfall above 1000 feet. At this time, a broad area
of 1-2 inches of snow appears probable for the high terrain of
Humboldt and Del Norte County, with 3-4 inches possible from
roughly Kneeland to Berry Summit...which prompted the issuance of
a Winter Weather Advisory.

In addition, model output favors at least a moderate risk of
accumulating small hail over the Humboldt and Del Norte coasts.
SPC has also included portions of the CWA in a general thunder
outlook, but this threat appears to be very marginal based on
model forecast soundings. Thus elected not to include in the
latest forecast package. Finally, deepening surface low pressure
over CNTRL CA into the Great Basin, combined with high pressure
over the NERN PAC, will support north-northwesterly winds gusting
to 35 mph for exposed coastal and ridgetop locations during
Thursday afternoon.

The previously mentioned upper wave will move east away from the
region during Thursday night. Subsidence occurring in the wake of
the wave will aid in clearing cloud cover and also contribute to a
stabilizing boundary layer. These processes combined with a cold
antecedent airmass and strong radiational cooling will result in
Friday morning temperatures in the teens and 20s. As a result, a
Hard Freeze Watch has been issued for portions of the Humboldt and
Mendocino coasts.

After Friday, persistent upper troughing is forecast to occur over
the western half of the nation. This pattern will favor periodic
episodes of showers/mountain snow and the maintenance of a
seasonably cool airmass.

&&

.AVIATION...Low clouds continue to move across the area with
ceiling heights remaining above MVFR levels (3500-5000 feet along
the Redwood Coast and around 6000 feet at KUKI). Patches of very
light rain continue to move S-SE with a better defined, but still
light, band presently extending from near KCEC to about 15 miles
offshore KACv. High resolution models indicate the potential for
very light rain to persist mainly along the Redwood Coast until
about mid- morning. More light rain is possible late in the TAF
cycle (after 09Z) along the Redwood Coast. Winds will be generally
light through the forecast period, but winds above the surface
will begin to increase after 09Z Thursday. /SEC

&&

.MARINE...Light E to SE winds early this morning will back around
to northerly by this afternoon and gradually increase. Have
continued the Small Craft Advisory for the outer waters beginning
at 3 PM today. Have hoisted a Gale Watch beginning at midday on
Thursday. This watch continues until 9 PM Thursday for the N outer
waters and until 3 AM Friday for the S outer waters and is for
wind gusts between 35 and 40 knots. N winds will decrease again
during the day Friday from N to S with lighter NW winds expected
for the upcoming weekend.

A NW swell with a period between 13 and 15 seconds will decay
through Thursday. Another NW swell will enter the waters Thursday
afternoon and decay through the end of the weekend. Larger NW
swell at between a 9 and 11 second period will persist through
much of the period as stronger N winds develop farther offshore
the local marine area. /SEC

&&

.EKA WATCHES/WARNINGS/ADVISORIES...
CA...Hard Freeze Watch from Thursday evening through Friday morning
     for CAZ103-109.

     Winter Weather Advisory from 4 AM to 4 PM PST Thursday for
     CAZ105-106.

NORTHWEST CALIFORNIA COASTAL WATERS...Gale Watch from Thursday
     afternoon through Thursday evening for PZZ470.

     Small Craft Advisory from 3 PM this afternoon to noon PST
     Thursday for PZZ470-475.

     Gale Watch from Thursday afternoon through late Thursday night
     for PZZ475.

&&

$$

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