Nicolaus, California 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

771
FXUS66 KEKA 202332
AFDEKA

AREA FORECAST DISCUSSION
National Weather Service Eureka CA
332 PM PST Wed Nov 20 2024

.SYNOPSIS...A strong atmospheric river storm system will continue
to bring prolonged moderate to heavy rainfall tonight through
Friday. This could lead to life threatening flooding late tonight
and Thursday. Strong and damaging southerly winds are forecast to
ramp up again late Thursday into Thursday night with impacts
lasting into Friday.

&&

.DISCUSSION...Strong and damaging winds have diminished for most
all the forecast area today. Gusty winds up to 40 mph continued
for Mendocino and Lake Counties, however. South winds are forecast
to increase tonight through Thursday night as another frontal
wave develops. Strongest winds for the North Coast will most
likely occur late Thursday afternoon into Thursday night as a
surface cyclone brushes the area. This next surface low is not
forecast to undergo explosive deepening and winds are forecast to
slightly less. High resolution ensemble and individual members
indicate another 925mb speed max of 70kt+ developing Thursday
evening and strong wind gusts to 50 to 70 mph will likely develop
across the North Coast. Even though the magnitude of winds with
this next surface cyclone appears to be less, impacts may be
higher due to saturated soils and greater risk for trees uprooting.
A high wind watch has been hoisted for Humboldt and Del Norte
Counties. Advisory level winds appear probable for the Mendocino
Coast late this evening through Thursday as the frontal boundary
lifts northward. Winds may drop off some in the morning, before
increasing again late Thursday afternoon into Thursday night.
Thus, a long duration advisory has been issued for the Mendocino
Coast zone. HREF ensemble mean gusts may be too high and the
strongest gusts around 50 mph may be confined to the higher
elevations of the Mendocino coast.

Latest radar imagery and rain gage network indicated widespread
moderate to heavy rainfall across the area this afternoon. 24 hour
amounts ranged from 3 to 5 inches over the higher elevations with
1 to 3 inches for valley and coastal areas.

Guidance continues to hold steady on the general pattern, with with
nearly unanimous agreement depicting an atmospheric river tracking
southward today then stalling over Mendocino this afternoon. A
second cyclone is forecast to take shape and quickly strengthen this
afternoon far offshore as it is drawn in around the parent deep
trough, previously stalled off the Pacific Northwest. The two
surface lows will likely interact with each other complexly, winding
around each other in a Fujiwhara Effect. This places some question
as to how close this strengthening cyclone is to our coast as it
passes northward Thursday, which would have implications to the wind
strength. What is more certain is a reinvigorated atmospheric river
with even higher moisture flux and IVT values. A stream of
persistent, high rainfall rates is forecast to occur in South
Humboldt along Cape Mendocino and through portions of coastal
Mendocino County with this storm Thursday into Friday, with steady
moderate and embedded heavy rainfall elsewhere.

WPC has included our area in a rare High Risk on Thursday through
the described locations, with a wide swath of Moderate risk
elsewhere. An additional 4 to 6 inches with isolated 8 inches is
forecast Wednesday night through Friday, with the bulk falling
Thursday through Thursday Night. This risk category is warranted
with this magnitude of rainfall expected to fall over already
saturated grounds. Meanwhile, mainstem rivers will have continued
upward, with the Eel River at Fernbridge to touch into Moderate
Flood Stage on Friday. Please see the Hydrological Section below for
more details and watch for changes in the river forecasts as models
ingest rainfall verification.

Snow levels across NE Trinity County will continue to rise tonight
through Thursday. Heavy snow is expected to continue for
elevations above 3500 feet this evening, and then above 4000 feet
by early Thursday. The Winter Storm Warning has been extended
through late tonight for the highest elevations of NE Trinity
where snow levels may take longer to rise.

After the passage of the secondary cyclone Thursday, most ensemble
guidance shifts the parent trough westward on Friday, which would
put NWRN CA in the cold sector and shut off our subtropical moisture
tap, marking a change from persistent moderate rainfall to showery
rainfall. Rain will likely continue during this period through the
weekend, however be generally less, and with more sparse coverage. DB

&&

.AVIATION...Conditions associated with an atmospheric river
continued to impact flying conditions and TAF sites today. Steady
advection of light, moderate and heavy rainfall pummeled the
region. At the Coast: the pressure gradient sufficiently relaxed
which resulted in subsidiary winds; Cig/Vis decreased in the
afternoon resulting in MVFR-IFR conditions at ACV and CEC.
Marginal LLWS remained possible along the coast into mid-
afternoon.

As the frontal band moved southeastward today, interior Mendocino
and Lake counties experienced more adverse conditions: southerly
gusty winds and MVFR Cigs and MVFR-IFR Vis.

Expect major impactful weather at the TAF sites on Thursday as the
low pressure system, and atmosphere river continues to affect the
region. /TA

&&

.MARINE...Southerly winds have generally weakened this afternoon
over the waters. Seas from a post frontal WNW swell have pulsed
higher, up to 22 ft this afternoon, and have been covered by an
ongoing Hazardous Seas Warning. That swell will then fade over the
next couple of days. A small jet of strong, southerly winds has
developed in the far southern waters, mainly in the inners along a
stalled occluded frontal boundary. Only a couple of high-res models
are picking up on the quite strong winds in that small area. Buoy 14
has been recording gusts over 45 kts, with a high of 51 kts earlier
this afternoon. The front will begin to lift back north as a
strengthening surface low begins pushing by the waters Thursday. The
focused, gale strength southerlies will lift north and expand in
southern zones response, starting tonight. A very sharp increase in
southerly winds to Gale strength is expected Wednesday afternoon as
the developing cyclone nears. The timing of this may shift a little
so watch the forecast closely. A Gale Warning is now out in the
Southern Inner zone 455 beginning tonight. Gale Watches are for all
other zones in anticipation of strong southerly winds that will
reach higher end gale criteria in zone of the highest gusts likely
exceeding 45 kts. Southerly wind driven seas are forecast to reach
at least 16 feet Thursday night into Friday. JJW

&&

.HYDROLOGY...A strong atmospheric river is forecast to impact
Northwest California with prolonged moderate to heavy rain through
the end of this week. NAEFS mean IVT is showing a prolonged
period of anomalously high values (500 to 800 kg/m/s) from the SW,
perpendicular to the local terrain. This will likely lead to a
prolonged period of persistent rainfall tonight through Friday.
Latest QPF guidance is showing 6 hourly rates of 1 to 3 inches
over SW facing aspects of topographic enhancement while 6 hourly
rate vary from 0.40 to 1.25 inches tonight through at least Friday
morning. Widespread heavy precip rates will likely start to wind
down Friday afternoon, though locally heavy rates of 0.50-1.00 in
6 hours are expected Friday night

Latest MRMS soil moisture modeling is indicating 50% to 70% saturated
soils at the moment in Humboldt and Del Norte, with 20% to 30% in
Mendocino and Lake Counties. Soil moisture will continue to
saturate today through tonight and set the stage for flooding and
enhanced runoff Thursday into Friday as a second pulse of heavy
rain arrive. The concern for major life threatening flooding will
increase during this time frame as small rivers, streams and
creeks reach or exceed bankfull. Main step rivers are forecast
to rise sharply and could exceed flood stages. In particular, the
Eel River at Fernbridge is forecast to exceed 20 feet by early
Friday morning. Also, the Russian river at Hopland may also exceed
flood stage of 15 feet Friday. Other main stem rivers such as the
Van Duzen at Bridgeville and Mad River at Arcata may reach
Action/Monitor stages. Stay tuned to the latest forecast. Scotts
Creek at Lakeport is forecast to reach 16 feet Thursday and up to
19 feet Friday, and this may have significant impacts along the
creek. Stay tuned to the latest forecast. DB/JJW/TDJ

&&

.EKA WATCHES/WARNINGS/ADVISORIES...
CA...
     Flood Watch through Friday evening for CAZ101>106-108>115.

     High Wind Watch from Thursday afternoon through late
     Thursday night for CAZ101>106.

     Winter Storm Warning until 3 AM PST Thursday for CAZ107.

     Wind Advisory from 10 PM this evening to 4 AM PST Friday
     for CAZ109.

     Wind Advisory until 10 PM PST this evening for CAZ114-115.

NORTHWEST CALIFORNIA COASTAL WATERS...
     Hazardous Seas Warning until 9 PM PST this evening for
     PZZ450-455-470-475.

     Gale Watch from Thursday afternoon through late Thursday
     night for PZZ450-470-475.

     Gale Warning from 9 PM this evening to 4 AM PST Friday for
     PZZ455.

&&

$$

Visit us at https://www.weather.gov/eureka

Follow us on Facebook and Twitter at:
https://www.facebook.com/NWSEureka
https://www.twitter.com/NWSEureka

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