Areal Flood Warning 2, NWS Eureka, CA

AREA FORECAST DISCUSSION
National Weather Service Eureka CA
647 AM PST Thu Nov 21 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 this afternoon into tonight with impacts
lasting into Friday.

&&

.DISCUSSION...Latest GOES-West IR satellite imagery shows a well-
defined atmospheric river and a stationary front currently stalled
over NWRN California. MRMS QPE and observed rainfall reports
0.25 to 0.50 in/hr rates across much of the forecast area,
especially in Mendocino and Lake counties. Caltrans and CHP
reported ponding water at Bayside Cutoff, rock slides and trees
blocking the roadways. 36HR precipitation amounts range from 2.5
to 6.0 inches, locally up to 9 inches over the southwest facing
terrain.

A second cyclone has been developing and quickly strengthening
this morning 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. While this second
rapid cyclone will no be as satrong as the Tuesday-Wednesday
event, it will help to push back northward the atmospheric river
with an enhanced moisture flux and IVT values. This will bring
again widespread moderate to heavy rainfall. 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 today into Friday, with steady
moderate and embedded heavy rainfall elsewhere.

WPC has remain the rare High Risk through the described
locations, with a wide swath of Moderate risk elsewhere. An
additional 4 to 6 inches with isolated 15 inches is forecast
to continue through Friday, with the bulk falling through
tonight. This risk category is warranted with this magnitude of
rainfall expected to fall over already saturated grounds resulting
in Life- threatening flooding across portions of Northwest
California today into Friday. Meanwhile, rapid rises has been
observed along several river points across Humboldt and Mendocino
counties. Please see the Hydrological Section below for more
details and watch for changes in the river forecasts as models
ingest rainfall verification. Prolonged moderate to heavy rainfall
will promote an additional

In addition, a tight pressure gradient associated to a surface
cyclone combined with 45-75 kt south-southeast at 925- 850 mb
layer will promote gusty winds late this afternoon and evening.
Gusts around 40 to 60 MPH are are expected over the more
prominent coastal headlands and exposed ridges across the area.

Snow levels across NE Trinity County will continue to rise today.
Heavy snow is expected to continue for elevations above 3500 feet
this morning, and then above 5000 feet by late afternoon. 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.

&&

.AVIATION...Moderate to heavy rainfall continues this morning as a
strong atmopsheric river provides a steady stream of moisture to
NorCal. Periods of MVFR/IFR conditions will continue intermittently
as locally heavy rainfall reduces visibility's and lowers ceilings.
With the core of the LLJ having shifted south, elevated southerly
winds (15 to 2 5 knots) will continue at UKI while the coastal
pressure gradient briefly weakens at the northern coastal terminals.
Light southerly flow will rapidly increase this evening and
overnight with the arrival of an additional surface low and strong
frontal boundary. LLWS is possible for a few hours this evening
before winds mix down to surface levels. Gusts exceeding 50 knots
are possible at CEC in the few hours before and after midnight.

&&

.MARINE...Elevated wave heights from a NW swell have finally
diminished to 12 to 13 feet after a brief resurgence Wednesday
evening. Small craft conditions and light southerly flow will
continue through this afternoon in all but the southern inner
waters, where gale gusts 35 to 40 knots persisted through the night
in the core of a LLJ near Point Arena. Gale conditions will rapidly
spread north into the remaining zones this evening with the arrival
of an additional low pressure system and strong surface front. High
end gusts may exceed 50 knots in the outer waters in the hours
before and after midnight. This fast moving systerm will generate
rapid wind wave response, driving wave heights back up to 15 to 17
feet early Friday morning. Winds and seas are expected to quickly
diminish this weekend, potentially falling below advisory levels as
the NW swell continues to decay. Unsettled conditions are likely to
develop late this weekend into early next week with additional upper
shortwaves moving through the PNW waters.

&&

.HYDROLOGY...A prolonged period of persistant moderate to heavy
rainfall will continue to lead in urban and small streams as well
as rock and landslides across the forecast area through Friday. A
Flood Advisory is in effect for the southern Humboldt,
southeastern Trinity and portions of Mendocino through this
morning. While a Flood Watch remain in effect for Del Norte,
Humboldt, Mendocnino, Lake and southeast Trinity counties through
Friday night. Latest MRMS soil moisture modeling is indicating
widespread 50% to 70% saturated soils at the moment. Soil
moisture will continue to saturate through tonight and set the
stage for flooding and enhanced runoff today 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 stem rivers have been responsive with the steadier
precipitation across the region. Latest QPF guidance continue
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
1.40 inches today through at least Friday morning. Concerns
increase for rivers reaching flood stage. 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, Mad River at Arcata and Navarro River at
Navarro have been has been rising quickly, with 1 to 7 feet ahead
of the NCRFC forecast. Also, Garcia River and Scott Creek at
Lakeport has reached 10 feet early this morning. Scotts Creek at
Lakeport was forecast to reach 16 feet today and up to 19 feet
Friday, and this may have significant impacts along the creek.
Stay tuned to the latest forecast. Stay tuned to the latest
forecast.

&&

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

     High Wind Watch from 3 PM PST this afternoon through late
     tonight for CAZ101>106.

     Winter Storm Warning until 10 PM PST this evening for
     CAZ107.

     Wind Advisory until 4 AM PST Friday for CAZ109.

NORTHWEST CALIFORNIA COASTAL WATERS...
     Small Craft Advisory until 6 PM PST this evening for PZZ450-
     470.

     Gale Warning from 6 PM this evening to 4 AM PST Friday for
     PZZ450-470.

     Gale Warning until 4 AM PST Friday for PZZ455.

     Small Craft Advisory until 3 PM PST this afternoon for
     PZZ475.

     Gale Warning from 3 PM this afternoon to 4 AM PST Friday
     for PZZ475.

&&

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

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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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