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

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HYDROLOGIC OUTLOOK
NATIONAL WEATHER SERVICE SAN FRANCISCO BAY AREA
339 PM PST Mon Jan 30 2017

...Another round of rain expected this week...

Our most recent episode of wet weather in California ended early
last week as trailing convective thunderstorms dropped short
bursts of intense hail and rain in sporadic locations across the
Bay Area and Central Coast. Relatively cool temperatures following
the rains led to slow evaporation over several days, keeping
both street and soil surfaces damp. Recently, temperatures have
crept upwards slightly, further drying the uppermost soil layers
as remnants of infiltrated rainwater percolated to shallow and
deep aquifers. Nevertheless, elevated groundwater combined with
charged springs and continued runoff have kept base flow levels high
in many creeks and rivers over the last week. In fact, water
levels in most drainages are roughly equivalent to those seen
prior to the multi-storm system that last began around Jan. 18th.

We are again at the precipice of additional rain for our area, set
to begin on Wednesday and extend through much of the latter half
of the work week. The forecast rain totals are not high, but as
noted above the rain will fall on watersheds that will react
quickly to additional moisture. The few tenths to half an inch
forecast for Wednesday will largely accomplish the priming needed
to efficiently convert the inch or more of rain expected for
Thursday/Friday directly to runoff. Larger creeks and rivers
throughout the Bay Area/Central Coast will rise but won`t likely
flood, while smaller creeks, particularly those feeding into
wetlands, tidal marshes, and sloughs may rise above flood levels.
If any flooding does occur late this week, lowland creeks in the
North Bay will be the likely culprits. Rain totals in the North
Bay are forecast to be generally higher than other areas. The most
likely effect of the rain this week will be additional slope
stability issues in the hilly and mountainous terrain of the Bay
Area and Central Coast. We have already witnessed the abundance of
slides and debris flows with the saturating storms so far this
season, and we are likely to see more of this activity with
renewed precipitation this week.

Of greater concern is the hint from several large-scale models
that additional moisture is expected later in the weekend and next
week. If the outcome of these simulations is realized, more
widespread flooding, mudslides, and ponding of water will occur
next week. Great uncertainty exists, however, so it is prudent to
wait for refinements in forecast simulations over the next several
days to determine how best to plan for the weekend and next week.

As we move through another wet weather pattern, please stay safe,
and adhere to road closures and other safety notifications coming
from local Emergency Managers and law enforcement. Importantly,
avoid driving through ponded water and runoff...turn around, don`t
drown! Stay tuned for updates to weather forecasts from NWS and
your local media outlets.

$$

MWS



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