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Hydrologic Outlook

Hydrologic Outlook

Hydrologic Outlook
National Weather Service Los Angeles/Oxnard CA
413 PM PDT Sun Mar 18 2018


A strong storm system originating from the Pacific Ocean will tap
into a long fetch of deep subtropical moisture and bring the
potential for a very wet and long duration storm event for
Southwest California Tuesday through Thursday night. This
atmospheric river event will likely bring the highest rainfall
totals to some portions of Southwest California, so far this
season. Preliminary storm total estimates range from 2.00 to 4.00
inches across coastal and valley areas, to between 4.00 and 6.00
inches across south and southwest facing foothills and mountains.
Local amounts up to 8.00 inches cannot be ruled out along some
south and southwest facing slopes.

A prolonged period of moderate to heavy rainfall could occur with
this system, with the highest rainfall intensities expected to
occur sometime between late Tuesday night and early Thursday.
Rainfall rates will likely exceed USGS thresholds and bring a
threat of significant flash flooding and mud and debris flows to
recent burn areas. These burn areas potentially include the
Thomas, Whittier, Creek, and La Tuna burn scars. While there is
some uncertainty in the positioning of heaviest rainfall with this
system, areas under the heaviest rainfall are projected to see
rainfall rates ranging between 0.50 and 0.75 inch per hour.
Isolated rainfall rates as high as 1.00 inch per hour cannot be
ruled, especially as storm system`s cold front approaches the
area between Wednesday night and Thursday morning.

In addition to the flash flooding and mud and debris flow risk in
recent burn areas, there will be other flooding threats in non-
burn areas due to the long duration and intensity of this storm.
Widespread urban roadway flooding is possible as well as
rockslides and mudslides, especially near canyon roadways. As a
result, there could be significant travel delays and road closures
across the region between Tuesday and Thursday night. Creeks and
small streams could also be running and it is always advised to
remain out of arroyos and barrancas.

As we draw closer to this event, Flash Flood Watches will likely
need to be issued, especially for the recent burn areas. Pay
close attention to the latest forecasts and updates on this storm
system. Please follow instructions from your local emergency



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