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

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HYDROLOGIC OUTLOOK
NATIONAL WEATHER SERVICE SAN FRANCISCO CA
1250 PM PDT WED OCT 12 2016

...WIDESPREAD RAIN LATER THIS WEEK THROUGH THE WEEKEND...

The region`s first significant storm system is set to
arrive late this week and continue into the weekend. Widespread
rainfall is most likely to begin over the North Bay late Thursday
and spread through the remainder of the region on Friday. A
second, slightly weaker, system will then bring additional
rainfall to the region Saturday into Sunday. Lingering showers
will be likely in between and after the passage of these systems
along with breezy winds.

Rainfall totals beginning late Thursday through the weekend vary
across the region, with the mountainous areas of the North Bay
receiving perhaps as much as 5 inches and the Santa Cruz Mountains
coming in a close second with around 3-4 inches. The Santa Lucia
Range in Monterey County is only forecast to receive a total of
around 1 inch. Local variance in storm behavior, including
unforeseen orographic effects, could enhance storm-total
rainfall, but we don`t anticipate accumulated precip to rise much
above these levels. (Stay tuned to updated forecast information,
as storm predictions may change as we move closer to the event.)

The effect this rain will have on flooding concerns is expected to
be minimal. Local creeks and rivers are not expected to rise above
flood levels. Much of the rain will come at or immediately
preceding the morning commute on Friday, and because this is the
first rainfall event of the season, roads will be slick and rather
dangerous. Leaves and debris from trees will likely clog storm
drains, exacerbating localized urban flooding from any intense
storm cells that may move through the area. But again, localized
flooding in urban areas is expected to be minimal, with slick
roadways and blockages from wind-blown trees being the major
hazard.

In the burned areas of our service area, notably the Loma Fire and
Soberanes Fire, we do not expect slope stability issues related to
intense rainfall. Should rainfall rates exceed a generic threshold
of approximately 0.75" per hour, debris flows on burned slopes may
be triggered, but the current rainfall forecast does not support
the development of these intensities. Ash, burned debris, and
post-fire "gunk" will likely be eroded from hillslopes, along with
fine sediment. These materials will collect in local drainages,
but will likely not overwhelm local channel capacities such that
flooding would occur. The drainages tapping the Loma Fire funnel
first through the Chesbro and Uvas Reservoirs, so downstream
effects are likely to be minimal in more populated areas of Morgan
Hill, San Martin, and Gilroy. Mountain roads and private fire
roads may have cones of sediment and debris from runoff on burned
slopes, in both the Loma and Soberanes Fire burn perimeters. But
again, major debris flows and landslides are not expected to
result in burned areas of either wildfire.

Please stay tuned to updated forecasts from the NWS Bay Area and
your local media outlets as we move closer to the approaching
storm.  Stay safe!

$$

Strudley



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