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

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FGUS76 KMTR 041830

1130 AM PDT Wed Apr 4 2018

...Potential flooding later this week in the North Bay and Santa
Cruz Mountains due to Atmospheric River Event...

A late season storm system with unseasonably high moisture
content will bring periods of rain, possibly heavy at times, to
the San Francisco Bay Area and Monterey Bay Area later this week.
Light rain is forecast to develop in the North Bay by Thursday
afternoon. Rainfall is then expected to spread south across the
rest of the San Francisco Bay Area and into the Monterey Bay Area
late Thursday and Thursday night. Rainfall intensity through
Thursday night is expected to be light to moderate.

Rainfall intensity is forecast to increase on Friday and rain
rates could be heavy at times by Friday afternoon. Periods of
heavy rain are then possible through Friday night and into early
Saturday morning. Rainfall is expected to taper off by midday
Saturday. Rainfall totals from late Thursday through midday
Saturday are forecast to range from 2 to 4 inches across the North
Bay, except 4 to 6 inches in the North Bay Mountains. Rainfall
totals forecast for the Santa Cruz Mountains are also in the 4 to
6 inch range. Local amounts of more than 6 inches are possible in
the wettest locations of the North Bay Mountains and Santa Cruz

Based on current rainfall projections, mainstem rivers such as the
Russian River and Napa River are forecast to see significant
rises late this week and into the weekend, but remain well within
their banks. However, smaller rivers and streams could flood,
especially if heavy rain rates persist across a watershed longer
than anticipated. Flooding would be most likely in low lying and
flood prone areas. In addition, heavy rain rates across the North
Bay burn scars could trigger flash flooding or mudslide/debris
flows across and downstream from the burn scars. Outside of the
burn scars, widespread landslides are not expected from this
event. This is because there has been a sufficiently long period
of time since the last rain event for drying to occur. However,
localized landslides or rockslides may still be possible outside
of burn scar areas, especially in areas of steep terrain.

Persons living near rivers and streams or other flood prone areas
should be alert for the possibility of flooding during this
Atmospheric River event late in the week. Flood watches may be
issued once forecast confidence concerning rainfall amounts
increases, especially if subsequent forecast models indicate
higher rainfall totals or more intense rain rates. Please stay
tuned to bulletins from NWS, NWS social media (NWSBayArea), or
from other sources of local weather information.


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