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

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FGUS76 KMTR 172246

246 PM PST Tue Jan 17 2017


Beginning Wednesday of this week, three separate storm systems are
set to impact the Central Coast and Bay Area of California. The
first is forecast to arrive Wednesday morning beginning in North
Bay counties and will move quickly southward through the area,
reaching Monterey County by either afternoon or evening hours.
Rainfall amounts are expected to be around 1-3 inches in the Coast
Ranges and generally less than an inch in valleys east of the
coastal mountains. The second system is forecast to arrive in the
Bay Area Thursday night and continue into Friday, with rainfall
totals slightly less than the first system. The third and final
system is expected to arrive Saturday night and extend through the
weekend with lingering showers tapering off into Monday. For this
storm system, rainfall amounts should be higher than either of the
first two systems, with 4-6 inches in the coastal mountains and
1-2 inches in valley settings to the east. All three storms are
expected to be relatively fast moving and strongly orographic.
Rainfall intensities are not expected to exceed 1.00 inch per hour
although some forecast models suggest intensities could
periodically and briefly approach these values. The strong
orographic depiction will result in much of the rainfall being
wrung out over the coastal mountain ranges with little remaining
for valleys east of the Coast Range province.

Only about 5 days have elapsed since our last rains, and
temperatures have been quite cool both during the day and night.
So although local creeks and rivers have generally fallen back
down to somewhat elevated baseflow, characteristic of mid-winter
conditions, evaporation has been limited and soil moisture is
still high. In wetter mountainous locales, spring-fed creeks and
seeps remain active. The first two forecast storm systems are not
likely to cause too many headaches across the Bay Area because of
the limited storm-total rainfall amounts forecast, but urban areas
are likely to experience traffic difficulties associated with
minor to moderate rainfall. Mountainous areas of the Coast Range
may well see additional slope instability problems, although the
instability is not likely to rear its ugly head until the second
(or third) system.

The third system stands the greatest chance to cause slightly more
widespread problems throughout the greater Bay Area, not just
because it is the last in the trio, but also because the projected
rainfall totals are highest in this third system. As alluded
above, the mountains along the coast will be subject to the
majority of expected rainfall, and the already saturated
conditions will prime watersheds to shed runoff quickly and to
liberate overburden in debris flows, shallow slumps, and road-side
erosion. The suggestion here is not that widespread chaos will
ensue, but the potential for mass movement of steep hillslopes is
greater this weekend. Most creeks and rivers will be able to
handle the successive bouts of runoff, but a few may approach or
exceed flood levels. Some reservoirs in the Bay Area continue to
spill and/or are actively being relieved through rule-curve
releases in expectation of additional rainfall over the coming
days. Refined high-resolution forecasts that will become available
in the coming days will allow more refined predictions on flooding
concern, so stay tuned.

Stay safe, and look for additional storm information as we
approach and move through these three storm systems.


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