Flash Flooding and Debris Flow in California, October 24 to 25, 2021

Area Forecast Discussion
National Weather Service Hanford CA
152 PM PDT Sun Oct 24 2021

.SYNOPSIS...
A very strong and wet storm system will approach the region by
this evening. Breezy to gusty conditions are possible along the
coastal range through the morning on Monday, as well as over the
Kern County desert during mainly Monday afternoon and evening.
Heavy rain below 8,000 feet and heavy snow above 8,000 feet will
begin to impact the region by this evening and continue through
Monday afternoon. Residual, upsloping showers may persist along
the Sierra Nevada and Tehachapi mountains during Monday evening
into Tuesday morning. Expect a drier pattern with a warming trend
beginning Wednesday.

&&

.DISCUSSION...Current radar images show light to moderate rain
showers over the Sierra, mainly towards Yosemite. The band of
heavier precipitation remains north of our forecast area and is
not expected to arrive into Yosemite & Mariposa/Merced Counties
until this evening. Otherwise, expect cloudy skies and occasional
light to moderate showers over the Sierra Nevada through the
remainder of this afternoon. Winds will continue a bit gusty over
the Sierra Nevada ridge tops and over the crest through Monday,
with the southerly fetch ahead of and along the main precipitation
band until late tonight. The wind shift towards westerly will
occur beginning early Monday morning due to the passage of the
cold front. In short, expect an active weather pattern until
Monday evening, as abundant rain, high elevation snow, and gusty
winds are in store.

During the overnight hours tonight, heavy precipitation will work
its way southward over our portions of the Sierra Nevada and into
the San Joaquin Valley. The heaviest amounts will likely occur
over Yosemite to the Creek Fire burn scar, or where we anticipate
the greatest impacts. Heavy rainfall rates over much of the Sierra
Nevada are likely during late tonight into early Monday morning,
and there will be a potential for at least debris flows as rates
could reach one inch per hour or greater. Expect around 3 to 6
inches of rain in quite a few locations from Yosemite to Fresno
County. Rain will continue to move southward towards Sequoia
National Park, where the KNP Complex Fire is still burning, as
well as the Windy Fire to the south, and rainfall rates may be
more brief but just as intense. Rainfall amounts in the mountains shape
up to be around 1.5 to 3 inches for Tulare County and southward
towards the French Fire burn scar. Amounts in much of Kern County
south of Lake Isabella will still be pretty decent, or up to
around an inch. However, the arrival of heavy rain will occur
later, or beginning early Monday morning, at these locations,
while the system weakens a bit. Above elevations of 8,000 feet,
expect mainly heavy snow. Snow amounts in the highest elevations
of the Sierra will likely reach as much as 3 to 4 feet, especially
from Yosemite to Fresno County. Snow levels will lower to around
6,000 feet by Monday afternoon, although most of the precipitation
is expected to have ended by that point. There is a slight chance
for afternoon thunderstorms in the San Joaquin Valley and lower
Sierra Nevada foothills on Monday, due to a colder, more unstable
airmass.

Precipitation amounts in the San Joaquin Valley will most likely
set record highs for October 25th. In addition, the probability
of an inch or greater precipitation remains around 80 percent at
quite a few valley locations for late tonight into Monday morning.
This amount of rain typically causes nuisance roadway flooding
and is expected to impact morning commuting on Monday, not to
mention the greater amount of rain that will fall in much of the
Sierra Nevada. Once the cooler air and wind shift arrives, gusty
winds are likely in the prone areas in eastern Kern County by
Monday afternoon and evening.

On Monday night into Tuesday morning, residual light showers or
drizzle will likely remain along with lingering low clouds in
southern and eastern portions of the Central Valley, lower Sierra
foothills, as well as towards the Grapevine and Tehachapi
Mountains. Cool temperatures will persist until at least Wednesday
before a more noticeable warmup occurs during late in the week
when daytime highs return to more seasonal values.

&&

.AVIATION...Mountain obscuration over the Sierra Nevada north of
Kings Canyon, extending south over the Tehachapi mountains by 12Z
Monday. IFR visibility and ceilings in heavy rain and snow in the
Sierra Nevada from 03Z Monday until 18Z Monday, with precipitation
tapering off after 18Z Monday. Rain developing with areas of MVFR
conditions in the central SJV after 03Z Monday. Gusty southerly
winds across the west side hills and central SJ Valley with gusts
25-35 kts mainly through 12Z Monday. Gusty west/southwest winds
with gusts of 40-45 kts at times in eastern Kern County from 17Z
Monday through 06Z Tuesday. Otherwise, VFR conditions will prevail
across the central CA interior for the next 24 hours.

&&

.AIR QUALITY ISSUES...
None.

&&

.CERTAINTY...

The level of certainty for days 1 and 2 is high.
The level of certainty for days 3 through 7 is medium.

Certainty levels include low...medium...and high. Please visit
www.weather.gov/hnx/certainty.html for additional information
an/or to provide feedback.

&&

.HNX WATCHES/WARNINGS/ADVISORIES...
Wind Advisory from 10 AM to 11 PM PDT Monday for CAZ338.

Flash Flood Watch from 5 PM PDT this afternoon through Monday
evening for CAZ318-320-325>327.

Winter Storm Warning from 5 PM this afternoon to 11 PM PDT
Monday for CAZ323-326>331.

Flash Flood Watch from 11 PM PDT this evening through Monday
evening for CAZ322-329>331.

Wind Advisory until 11 AM PDT Monday for CAZ300-304-308.

Wind Advisory from 10 AM to 8 PM PDT Monday for CAZ337-339.

&&

Public/Aviation/Fire Wx...BSO
PIO/IDSS..................BS

weather.gov/hanford