Flood Potential Outlook
Issued by NWS San Francisco Bay Area, CA

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HYDROLOGIC OUTLOOK
NATIONAL WEATHER SERVICE SAN FRANCISCO CA
431 PM PST Tue Jan 3 2017

...POTENTIALLY PROLIFIC RAIN EXPECTED THIS WEEKEND...

At the current time (Tuesday afternoon), we are watching the
beginnings of a moderate Atmospheric River event make landfall
along the coast of California. Several inches of rain are expected
through the overnight hours across the Bay Area and Central Coast,
with 24-hour totals potentially reaching 6 inches or more in the
Santa Cruz Mountains and 6-10 inches in the Santa Lucia Mountains
of Monterey County. While we are focusing our immediate attention
at the current system, another and potentially more potent
atmospheric river (AR) is expected over the coming weekend.

Forecast models suggest that the next AR system may arrive
Saturday and extend through the weekend and possibly into early
next week. While the current system (Tuesday) is focused more
discretely at the Central Coast, this weekend`s AR system is
expected to deliver heavy rain to a more widely distributed
portion of both the Bay Area and Central Coast. Weather Prediction
Center long-time-scale models suggest that rain totals including
both today`s AR event plus this weekend`s AR event could reach 15
inches or more in the coastal mountains of our area, suggesting
this weekend`s AR system is much more potent. Using these
numbers, coastal mountains of the North Bay, Santa Cruz County,
and Monterey County could see 10 or more inches from this
weekend`s forecast AR event alone. As we move closer to the event
this weekend, higher resolution simulations will become available
that will shed additional light on expected rainfall amounts. The
general consensus and punchline at this time is that next
weekend`s system will be stronger than the system currently
affecting our service area.

If the forecast above is realized, the hydrologic impacts of such
a weather pattern are expected to be numerous and widespread
through many sectors of the service area. With mostly light rain
and brief reprieves expected between tonight`s AR system and the
weekend, soils will likely remain well-saturated in all but the
sandiest of soils. Base flows in creeks and rivers will remain
high, reflecting continual drainage of saturated soils, subsurface
storm flow, and activation and/or rejuvenation of locally spring-
fed systems. Some smaller reservoirs may continue to safely spill
through the week. In combination, this means that the runoff
response to this weekend`s AR event will be quick and most of the
rain will efficiently convert to runoff and streamflow.

Main-stem rivers: River Forecast Center forecasts of stream flow
and stage that extend through this weekend`s AR system have not
yet been simulated at this point in time, but in the coming days
the forecast window will include this weekend`s AR system and so
we will have a more complete picture of expected water levels in
larger rivers of our area. If rainfall predictions remain
consistent to that described above, it is likely some of our
larger river systems will flood. More information on this topic
will be refined and released in the coming days...please stay
tuned.

Smaller creeks and streams: It is very likely that many flood-
prone creeks and rivers of the Bay Area and Central Coast will
rise above their banks. Lowland valley settings of the North Bay,
which frequently flood, will likely do so with this weekend`s
storm system. The drainages of the Coast Range will also be very
susceptible to flooding. Notably, the watersheds within the Santa
Cruz Mountains and Santa Lucia Mountains will be prone to flooding
because rainfall rates and accumulation are again expected to be
high this weekend. Flash Flood Watches and Warnings will highlight
problem spots and creeks expected to flood, so stay tuned for NWS
issuance of such products in the coming days.

Urban and agricultural areas: With heavy and intense rains
expected, storm drains will likely become overwhelmed in many
urban locations. Storm drains could also be easily backwatered
where receiving water bodies are elevated due to storm runoff
(creeks and rivers). Agricultural areas and rangeland in flatter
valley settings may easily be inundated, especially where soils
have been compacted and are clayey. With a strong focus on the
Central Coast, the lower Pajaro and Salinas Valley agricultural
areas could experience problematic drainage and flooding from this
weekend`s storm system.

Wildfire burn scars: We have already issued a Flash Flood Watch
for the Soberanes and Chimney Burn Scars in Monterey County to
reflect our concern with tonight`s storm and the potential for
heavy rain to engender debris flows in steeper terrain. Our
concern for debris flows within the Soberanes and Chimney Burn
Scars will continue this weekend, but we will also be concerned
with the Loma Burn Scar and Santa Cruz/Santa Clara County and the
Sawmill Burn Scar in Sonoma County. Rainfall rates that exceed
roughly 3/4" to 1" per hour or more will likely cause slope
failure and debris flows within steeper and more intensely burned
terrain. These rates are not outside the realm of possibility for
this weekend`s AR system.

Debris flows and landsliding: Outside of burn scars, elevated
rainfall rates and saturated soils could cause debris flows and
shallow landsliding in the coastal mountainous areas of the Bay
Area and Central Coast.

Please stay tuned for additional information from NWS SF/Monterey
Bay Area, including important Watches, Warnings and Advisories.
Most importantly, be prepared, and stay safe through these storms.

$$



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