899
FXUS66 KEKA 212343
AFDEKA

AREA FORECAST DISCUSSION
National Weather Service Eureka CA
343 PM PST Thu Nov 21 2024

.SYNOPSIS...A strong atmospheric river storm system will continue
to bring widespread moderate to heavy rainfall tonight through
Friday, resulting in life threatening flooding. Strong and potentially
damaging southerly winds are expected tonight with impacts lasting
into Friday. Moderate to locally heavy rain, mountain snow and
isolated thunderstorms will follow Friday night into Saturday.

&&

.DISCUSSION...Strong atmospheric river storm will continue to
impact Northwest California tonight with moderate to heavy
rainfall and strong damaging wind gusts. Key message are:

* Life-threatening and dangerous flooding tonight and Friday.
* Strong and damaging wind gusts tonight for Del Norte and Humboldt.

Satellite imagery showed a cyclone spinning up due west of Eureka
and just outside 130W this afternoon. Wavy frontal boundary has
been shifting slightly southeast this afternoon, though moderate
and heavy rain rates continued across Mendo, Lake, southern
Humboldt and Trinity this afternoon. The wavy boundary and heavy
rain rates will begin to shift northward again this evening and
overnight as the offshore cyclone tracks NE and deepens. This
surface low is not forecast to intensify or deepen as rapidly as
the last one we had on Tuesday, but it will be closer. Surface
gradients will tighten this evening and southerly winds will
increase over the North Coast and over the waters. A 925mb coastal
jet will develop along and over the coast, reaching 70-80kt around
midnight, give or take an hour. This is when we expect the strongest
winds. A high wind warning is in effect for all of Del Norte and
Humboldt Counties for gusts to 45-65 mph along the coast and
interior mountains. A few of the more wind prone remote mountain
areas will probably have gusts to 80 mph or more. One remote station
(Mattole Rd) has already been gusting to 56 mph as of 2 pm. Even
though the rate of pressure falls will not be as fast as the last
storm on Tue (explosive cyclogenesis), this one could have greater
impacts now that soils are saturated, making it easier for trees
to uproot and topple over.

Moderate to heavy rainfall rates are forecast to continue through
at least Friday morning. The widespread heavy rain rates will
wind down during the afternoon on Friday, but flood waters will
be slow to drain. Additional rain and shower activity is still
expected during the afternoon on Friday and some showers may be
locally heavy as cooler air aloft arrives with an upper trough.

Potential for low-topped convection or thunderstorms will increase
mostly over the coastal waters Friday afternoon and evening. Looking
at the HREF, NBM, legacy SREF and deterministic model soundings,
there does appear to be a chance of thunderstorms as 500mb
temperatures fall to -25C and eventually to -30C Sat and Sun.
There is certainly a chance for small hail as 850mb temps fall
to -1C late Friday night into Saturday morning and freezing
levels fall to around 3000-3500 ft. There may also be a concern
for winter weather products for the mountains over the weekend.
Based on 6 hourly snow amounts and snow levels, the impacts appear
to be low at the moment.

A brief break will be possible over the weekend, before another
series of fronts impacts the area Sunday through Tuesday. These
fronts will be progressive and have less moisture. 6-hourly
rainfall and snowfall rates and storm total rain and snow amounts
are forecast to be much less than our current storm with less
potential for major impacts.

Upper ridging is forecast to restore more peaceful and dry weather
mid to late next week, though after multiple days of rain, valleys
will no doubt fog up during the night and morning hours. DB

&&

.AVIATION...An atmosphere river has continued to provide a
steady influx of moisture to the region today. The result has
been substantial impactful weather conditions (cig/vis/rain/wind0
to the the TAF sites. However, a somewhat loose coastal pressure
gradient has kept winds mostly light, while inland surface winds
remained marginally gusty.

Tonight-Friday: another strong frontal system will rotate over NW
California...and bring additional adverse weather to the region.
Conditions includes: SELY-SLY winds rapidly increasing this evening
and overnight...especially at CEC, MVFR-IFR Cig/Vis, moderate-heavy
precipitation that will add significant rain measurements to the air
terminals...and probable LLWS in the evening/early morning. /TA

&&

.MARINE...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
system 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 Fri-Sat with additional
upper shortwaves moving through the PNW waters. Locally strong
winds associated with showers and isolated thunderstorms are
possible. Another low pressure system will approach over by
Sunday and provide another ramp up over southerly winds and
elevated steep seas. Wind and waves should start to settle down
early next week. DB

&&

.HYDROLOGY...Life-threatening flooding tonight through Friday.
Main stem rivers, smaller streams and creeks continued to rise
sharply today in response to widespread moderate to heavy
rainfall last night through today. Moderate to heavy rainfall
rates will continue tonight and early Friday morning as a long
duration and high magnitude atmospheric river intersects the
coastal terrain. Even after the widespread heavy rain subsides
through the day on Friday, flood waters will be slow to recede. Do
not let your guard down or attempt to drive across flooded roads.
Significant flooding is expected for the Eel River at Fernbridge
tonight through Friday night and could reach 25 feet which will
result in major flooding. Other main stem rivers such as the Van
Duzen at Bridgeville, Mad River at Arcata, Russian River at
Hopland and Navarro River at Navarro are also forecast to reach
minor flood stage. Many other smaller streams and creeks will
likely reach bankfull or come out their banks. Stay tuned to our
latest warnings and advisories. DB

&&

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

     High Wind Warning from 9 PM this evening to 4 AM PST Friday
     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 until 4 AM PST Friday for PZZ475.

&&

$$

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