Wednesday, February 15, 2017

Special Statement from the National Weather Service - Altadena

One product issued by NWS for: Altadena CA
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Hydrologic Outlook

Hydrologic Outlook

Hydrologic Outlook
National Weather Service Los Angeles/Oxnard CA
540 AM PST Tue FEB 15 2017

...A major storm system will affect Southwest California Thursday
night through Saturday resulting in flooding concerns...

Confidence is building for a significant 24-36 hour rainfall event
for Southwest California beginning Thursday night through
Saturday, bringing the potential for numerous hydrological impacts.

The first in a series of storms is expected to bring light to
moderate rainfall initially to San Luis Obispo and Santa Barbara
Counties Thursday morning, before overspreading Ventura and Los
Angeles Counties Thursday evening through the overnight hours. A
second more powerful storm system will begin to intensify as it
approaches Southern California as early as Friday morning. A long
narrow band of subtropical moisture, otherwise known as an
Atmospheric River will take aim at Southwestern California. Santa
Barbara, Ventura and Los Angeles Counties should take the brunt of
this powerful storm Friday morning through Saturday morning. Between
2 to 4 inches of rainfall is expected across coast and valleys, with
between 4 to 8 inches across the Santa Ynez Range, as well as the
Ventura and Los Angeles County Mountains. Most of San Luis Obispo
County should receive 1 to 3 inches, with 2 to 5 inches in the
coastal foothills.

Rainfall rates are expected to reach one inch per hour. With this
storm system expected to be slow in moving through the forecast
area, there will be several potential hydrological impacts. The
potential for widespread flooding and flash flooding is a definite
threat. Not just for the recent burn areas, but anywhere there is
the potential for flooding.

The expected high rainfall amounts and intensities could bring
numerous hydrological impacts. Many rivers and streams could see
significant flows on Friday into Saturday...but main-stem river
flooding is not expected at this time. There is a potential for
swift water rescues due to the heavy rain and resulting increased
flow rates. There will be the threat of flash flooding with mud and
debris flows in and around the recent burn areas, especially with
Friday`s storm. Also, widespread and significant urban and small
stream flooding can be expected at times through Monday. Lastly rock
and mudslides will be a significant threat during this period,
especially across canyon roads.

Models are indicating a few more weaker storm systems to move
through the region into early next week. Any additional rainfall
could continue to bring hydrological impacts.

Watch for updates and possible watches and warnings at as we get closer to the event.



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