Winter Storm Watch 1, NWS Eureka, CA

AREA FORECAST DISCUSSION
National Weather Service Eureka CA
230 PM PST Fri Dec 31 2021

.SYNOPSIS...Showers will dissipate in the vicinity of Cape
Mendocino early this evening. Clearing skies will then favor cold
Saturday morning temperatures, with freeze conditions expected to
occur along portions of the coast. Otherwise, a winter storm will
produce periods of strong south winds, locally heavy rainfall, and
high elevation snow Sunday night through Tuesday.

&&

.DISCUSSION...An upper trough was exiting east away from the West
Coast this afternoon. However, coastal convergence occurring in
the vicinity of Cape Mendocino combined with a marginally unstable
boundary layer airmass was aiding in weak shower development over
southwestern Humboldt County. The boundary layer is forecast to
dry and stabilize tonight as upper heights build and large-scale
subsidence overspreads the region. Skies will trend clear for part
of the night...though model guidance shows high-level cirrus
increasing toward sunrise. Thus, near certain freezing
temperatures occurring across the interior will be less certain
along the coast. Nevertheless, ensemble probabilities remain
solidly within warning criteria, and therefore the inherited
freeze headline has been maintained.

Saturday will feature dry and mainly pleasant weather as a
shortwave ridge moves east across the region. Thereafter, upper
heights quickly begin to fall over the NERN PAC as a trough digs
toward the West Coast. A frontal boundary associated with a deep-
layer cyclone centered over the southeastern Gulf of Alaska will
advance toward the coast of NWRN CA Sunday afternoon and night. A
southerly coastal barrier jet, characterized by 925 mb winds
ranging from 40 to 70 kt, will develop in response and aid in a
high probability of advisory level gusts across exposed coastal
areas and ridges Sunday evening through Monday morning. In
addition, both frontal and orographic ascent will aid in
precipitation development Sunday night into Monday morning.
Rainfall totals with this initial round of precipitation will
range from 1-2 inches in Del Norte County to a few tenths in Lake
and southern Mendocino County. In addition, southerly upslope flow
over the Trinity Alps will dynamically lower snow levels,
initially to around 2000 feet Monday morning that then rise to
3000-4000 feet by Monday evening. Mountain snowfall will occur
over Trinity County as a result, though highway impacts will
mainly be confined to Scott Mountain Pass on Highway 3, where
storm total snow is forecast to be around 12 inches during the
Monday through Tuesday time period.

Otherwise, model guidance shows mean deep-layer flow trending
toward zonal on Tuesday, which will yield moisture transport
vectors perpendicular to north-south oriented coastal terrain.
Those terrain features will focus the highest rainfall totals
ranging from 1 to 3 inches on Tuesday...highest amounts in
Humboldt and Del Norte Counties...lowest over Lake and southern
Mendocino Counties. Beyond Tuesday, ensemble forecast guidance
shows upper heights building Wednesday over the NERN PAC, followed
by a ridge becoming centered over the West Coast on Thursday. That
evolution continues to yield low confidence in additional rainfall
occurring during the day 5 through 7 forecast time period.

Garner

&&

.AVIATION...Pesky scattered showers, mainly around Humboldt
County, have slowly begun to diminish in coverage heading into
this afternoon. Brief MVFR CIG and even VSBY with rain have been
observed there, but the majority of the time has been VFR for the
bulk of northwest California. All guidance along with radar and
satellite trends suggest aviation conditions will only improve
through this evening, with skies continuing to clear as daytime
instability diminishes. High pressure should then provide VFR
conditions for much of the weekend. However, there will likely be
some patchy inland valley fog the next couple of mornings,
especially in the more sheltered locations. UKI may have seen
enough mixing and drying of the boundary layer to limit fog
formation Saturday morning, but think there still is a possibility
for at least some brief patchy MVFR conditions near the terminal.
Some northwesterly wind gusts over 20 kt will be locally possible
over coastal headlands and ridgetops into this evening, followed
by very light winds on Saturday.

&&

.MARINE...Had to expand the Small Craft Advisory to the inner
waters through this evening, mainly due to a mid-period northwest
swell that has been a little higher than earlier advertised. That
has put it in the 8 to 10-foot range at many of our buoys, with
combined seas occasionally touching 11 feet. Additionally,
northerly winds are a bit more stout than earlier forecast, with
at least gusts into the 20 to 25 kt range over a significant
portion of the coastal waters. These conditions will linger into
this evening, then both winds and that swell should relax
substantially through Saturday. That window for nice sea
conditions may last into Sunday morning, especially south of Cape
Mendocino.

Winds will begin turning southerly Saturday night as a cold front
approaches from the north. By Sunday evening, Gale conditions are
looking rather likely for our northern waters, and some gale
force gusts may extend a ways south of Cape Mendocino when winds
peak around Monday morning. There is also a good chance of storm
force gusts adjacent Pt St George during the peak winds. Later
shifts may consider a Storm Watch or Warning for portions of the
northern waters, but current forecast coverage/duration/confidence
of storm force gusts do not support it just yet.

As if that weren't enough, a large west-northwest swell is still
on track to build into our coastal waters just after the winds
ease up; by Monday afternoon or certainly Monday night. That will
result in combined seas building to 15 to 20 feet, probably
lingering into through Tuesday. Winds and seas should greatly
diminish Wednesday and Thursday. /AAD

&&

.HYDROLOGY...A multi-day, multi-inch rainfall event is forecast to
occur across northwest California Monday and Tuesday. Heavy rain
occurring over mountain snowpack will favor rises on creeks,
streams, and rivers. Currently, the probability of area rivers
reaching monitor stage is around 25 to 40%, and the probability
of flood stage being achieved ranges from 5 to 25%, with rivers
generally cresting on Wednesday.

Garner

&&

.BEACH HAZARDS...Astronomical high tides will reach 8.81
feet at the north spit tide gauge on Sunday January 2nd, and nearly
that high on the 3rd. Coastal flooding around Humboldt Bay typically
starts around 8.8 feet. Thus, confidence is high that significant
coastal flooding will occur across low-lying areas around the bay,
particularly as strong south winds develop Sunday through Tuesday.

Along the open coast confidence is lower, but strong south winds
could exacerbate wave run-up onto the shore. This could yield waves
moving across Highway 101 near the south side of Crescent City on
Monday. A NW swell will start to build on Monday, however the peak
in the swell looks to be out of sync with high tide on Monday. The
large swell will likely start to subside on Tuesday, but surf may
still remain sufficiently elevated to pose a threat for increased
wave run up on the beaches with high tide forecast around noon or
shortly thereafter.

Otherwise, surf heights along west-northwest facing beaches will
increase to around 18-21 feet Monday evening through Tuesday
morning. Beach-goers should exercise caution during that time
period and remember to never turn your back on the ocean.

&&

.EKA WATCHES/WARNINGS/ADVISORIES...
CA...
     Freeze Warning from midnight tonight to 9 AM PST Saturday
     for CAZ101-103-109.

     Coastal Flood Advisory from 8 AM to noon PST Saturday for
     CAZ103.

NORTHWEST CALIFORNIA COASTAL WATERS...
     Gale Watch from Sunday evening through Monday afternoon for
     PZZ450.

     Small Craft Advisory until 9 PM PST this evening for PZZ410-
     415-450-455-470.

     Gale Watch from Sunday afternoon through Monday afternoon
     for PZZ470.

     Small Craft Advisory until 3 AM PST Saturday for PZZ475.

&&

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For forecast zone information
see the forecast zone map online:
https://www.weather.gov/images/eka/zonemap.png

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