Winter Storm Watch 5, NWS Eureka, CA

AREA FORECAST DISCUSSION
National Weather Service Eureka CA
508 AM PDT Mon Mar 10 2025

.SYNOPSIS...Mostly dry weather today although conditions are
changing synoptically as the ridge breaks down and the cutoff low
continues south and offshore of SoCal. Mid to high level clouds
are expected as moisture bands roll into the PACNW. Gusty south
winds expected Tuesday night into early Wednesday. Heavy rain and
heavy mountain snow is forecast late Tuesday night through
Wednesday. Additional bouts of rain and mountain snow expected to
follow late week into the weekend. Snow levels are forecast to
fall below 2000 feet Wednesday night and Thursday as rain showers
persist. Small hail possible Wednesday night and Thursday.

&&

.DISCUSSION...

KEY MESSAGES:

-Wet weather will return late Tuesday with moderate rain through
 the end of the week.

-Possible damaging wind for high elevations late Tuesday into
 early Wednesday. Greatest risk for populated areas along shore
 and in Lake County.

-Snow levels will drop through the week, dropping below 2000 feet
 by Thursday with periods of heavy snow over highways 299, 36, 3,
 and 199.

-Small hail and hazardous road conditions possible Wednesday
 night through Thursday

WEATHER PATTERN OVERVIEW:

The high pressure pattern, currently over the CWA, is
transitioning eastward and will give way to a large scale synoptic
troughing pattern Tuesday. Probabilities of precipitation are
low, with the exception of coastal drizzle. Conditions for today
remain warm and dry, with interior highs in the low to mid 60s
backing off just a couple degrees from the past couple of days.

A series of deep troughs will bring a return to wet and unsettled
weather by late Tuesday through the end of the week. Southerly wind
will begin to increase ahead of the troughs by late Tuesday
afternoon, peaking early Wednesday morning. A moderate jet of 35
to 50 kts at 800 mb is forecast to sweep the area early Wednesday
morning. This jet supports a 20 to 40% chance of gusts up to 55
mph on high ridges. That said, a generally stable environment will
limit the extend of winds mixing to populated lower elevation
locations. Models consistently show the lower elevation areas
with great wind potential are Crescent City and Clear Lake with
both have about a 20 to 40% chance of 45 mph gusts. Such speeds
are consistent with a minor risk of tree damage and isolated power
outages.

A consistent supply of moisture along the trough will help generate
widespread rain across the area, mostly during the day Wednesday.
Moisture transport is generally consistent with a weak to moderate
atmospheric river event, though the duration of the AR appears
limited. Most likely rain totals are about 2 to 3 inches at
higher elevations and 1 to 2 inches at lower elevations. Down
sloping effects will generally greatly decrease low elevation rain
amounts, especially around Humboldt Bay. Such amounts will
generate only minor to moderate urban and small stream flooding
concerns with less than a 10% chance of minor flooding along any
mainstem rivers. Convective potential increases late Wednesday
and persists into Thursday with more showery precipitation and a
slight risk for small hail as 500mb temperatures plunge to -30C.
Another pulse of organized rain with a post frontal trough is
probable on Friday.

The deep nature of the troughs will advect cold air across the area,
peaking Wednesday night into Thursday. Though snow levels will
starts as high as 4000 feet Tuesday night, there is a consistent
signal than snow levels will drop below 2000 feet by early
Thursday with upslope and evaporative cooling helping to drop snow
levels as low as 1000 feet, especially in Trinity County by Thursday
evening. Though precipitation will be inconsistent by this time,
impactful snowfall is expected on high elevation highway passes of
299, 36, 199, and 3 by early Thursday. 4 to 8 inches of snow is
generally most likely. Snow levels will most likely reach their
minimum early Friday morning, with a 25 to 50% chance of 1 inch of
snow as low as high elevations of highway 101 in Mendocino County
and around Cobb.

FORECAST CONFIDENCE AND DEVIATIONS:

Timing of the incoming system has remained remarkably consistent over
the past several model runs, with rain and wind set to begin to
Tuesday evening and peaking early during the day Wednesday. Rain
amounts, however, have proved more variables. While the chances of
high rain amounts have generally diminished, there remains a 20%
chance of localized amounts over 4 inches in terrain favored areas.
Such amounts would bring more moderate flood potential.

Similarly, the overall wind risk has trended downwards, but there
remains significant uncertainty, mostly dependent on the exact
strength and track of surface low pressure which remains very
uncertain. Generally speaking, however, only 10% of ensemble members
show a 800 mb jet any stronger than 50 mph. This generally caps the
overall maximum wind speed. In line with this, ensembles now show
essentially no chance of wind gusts over 70 mph along high terrain
with chances for damaging wind gusts over 55 mph dropping to near 30
mph even in the most exposed areas. Of note, however, the
possibility for stronger wind next weekend has actually increased
with 20% of models showing a low level jet up to 70 mph. As the
midweek event has generally trended more moderate, some attention
will have to paid to the weekend event.

On the snow side, confidence has remained high that snow levels will
drop strongly by Wednesday night. There is an 80% chance of
accumulating snow at locations such as Berry Summit and Collier
Tunnel by late Wednesday evening. These locations serve as a good
indicator of when the strongest impacts are most likely along area
highways. Snow is more uncertain further south and later in the
week. That said, the low snow level signature remains strong, most
of the uncertainty has to do with waning QPF potential during the
coldest periods. The possibility for impactful snow at Cobb and
along highway 101 bears close watching, but the potential for 1
inch or more of snow in these locations remains relatively low
around 30%. /DB/EYS

&&

.AVIATION...VFR conditions are present at the terminal spaces
currently. Offshore flow is expected overnight, keeping terminals
from being heavily impacted by low cloud ceilings. Winds will remain
light along the coast due to weaker pressure gradients than the last
few days. Come mid-morning today, lower cloud layers are forecasted
to fizzle out leaving only a BKN ceiling of high clouds. Tonight
into tomorrow, model guidance is suggesting possible NNW breezes
along exposed coastal terminals followed by possible fog/low clouds
forming briefly overnight. Later this week, Low level turbulence is
possible Tuesday afternoon/evening as a Low Level Jet stream is
forecasted to pass over coastal terminals.

&&

.MARINE...High pressure builds in again to start the week causing
northerly winds return by Monday afternoon. This weather pattern is
expected to be short-lived as another incoming low pressure system
will force strong southerly winds on Tuesday. There is potential for
a Gale Warning to be issued as the first of the extended Hi-Res
model guidance (FV3) is showing +34kt gusts from Tuesday late
evening through Wednesday morning as a frontal boundary sweeps
across the coastal seas. On Wednesday afternoon, forecasting
uncertainty increases significantly due to a potentially strong area
of low pressure forming in the NE Pacific, forcing areas of gale
force winds could form. As this system gets into the range of the
higher resolution models in the next couple days (48-56hrs out) this
uncertainty should be resolved better. Thursday winds look to become
north or northwest, yet again this will depend on the exact position
of the to-be formed low pressure center. Friday and into the weekend
it looks like another strong system will again create high winds.

The current westerly swell is generally expected to continue around
7-8 ft @ 14-15s through today. Also on Monday, a southwest swell
around 4ft @ 8s moves into the waters and persists into Tuesday.
Also on Tuesday afternoon, another long period swell is forecast to
start building to around 8-10ft @ 15s on Wednesday. Later in the
week, a larger swell is possible and is currently forecast to peak
around 10-12 ft @ 15s, but confidence is lower given the uncertainty
of the forecast pattern with more updates/forecast confidence coming
within the next few days. MKK/DS

&&

.EKA WATCHES/WARNINGS/ADVISORIES...
CA...
     Winter Storm Watch from Tuesday evening through Wednesday
     afternoon for CAZ107-108.

     Frost Advisory until 8 AM PDT this morning for CAZ110-111-
     114.

NORTHWEST CALIFORNIA COASTAL WATERS...
     Small Craft Advisory from 3 AM to 3 PM PDT Wednesday for
     PZZ450-455.

     Small Craft Advisory from 9 PM Tuesday to 3 PM PDT
     Wednesday for PZZ470-475.

&&

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