California - Possibly Worst Floods Since 2005
Posted by JAC on 1/18/2010, 7:29 am
Dr. Greg Forbes, Severe Weather Expert



Here's an update to my
Thursday blog about this week's stormy pattern. We're still expecting a series of at least 3 storm systems to blast into California through Thursday, driven onshore by a strong upper-level jet stream. As is shown below, we're forecasting 4 to 8 inches of rain in the coastal areas, up to 20 inches in the mountains and the west- and south-facing slopes, and 6 to 10 feet of snow on mountain tops. In addition, the heavy rain could be accompanied by thunderstorms -- a few possibly severe -- and strong wind gusts that could reach 70 mph at times. Large waves could bring beach erosion.

A big part of the threat associated with the heavy rains, especially in thunderstorms, will be the possibility of mudslides and debris flows -- particularly in recent burn areas such as the Station Fire from last summer. The chart from the National Weather Service below shows that a rainfall of between 0.75 and 1.5 inches in an hour will be enough to cause flash flooding. But in the recent burn areas outlined in black, a half inch of rain in an hour (and as little as 0.20 inches in 15 minutes) could cause flash flooding and mudslides. That type of rainfall could certainly fall on just about any day through Thursday.





Early on January 17, the
National Weather Service in Los Angeles says the series of storms this week may bring flooding threats on a scale not seen since 2005. Heavy rains fell then in a series of storms from January 7-11 and brought widespread flash flooding and even river flooding. One thing about that event was that it came toward the end of a 15-day-record period of rainfall at Los Angeles that began on December 27. It has been drier to start with this year, so let's hope the results won't be as bad as in 2005.

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Large El-Nino Jet hitting CA - JAC, 1/18/2010, 7:20 am
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