Ken Clark

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Heavy Rain to Bring Serious Risk of Flooding and Mudslides

November 27, 2012; 1:49 PM ET

The series of storms I talked about yesterday is still on track. The first storm will arrive tomorrow with one almost every day through Sunday. The brunt of these storms will be for about the northern third of California from Monterey and the Santa Cruz Mountains to about Yosemite on north. The cumulative effect of these storms is what concerns me. By Friday and over the weekend, enough rain will have fallen that streams and rivers will be rising and the possibility of mudslides and debris flows around burn areas, and landslides in other areas, increase. This very well could end up being a very serious situation for some.

The model information is remarkably close in many areas depicting how much rain they are forecasting for various areas. Below is a spreadsheet of total rainfall from tomorrow through Sunday night for various locations across California that the GFS and European models are forecasting.

These models are pretty impressive just taking them as they are. But these models do not handle local differences from orographic lift very well. Places like the Santa Cruz mountains, the mountains in the North Bay region, and all west facing slopes of the Sierra, are likely to see higher amounts that modeling data suggests more times than not.

My expectations are for widespread 2- to 4-inch rain amounts in lower elevations from Monterey to the Bay Area and east to Sacramento and south to the northern most San Joaquin Valley with local amounts of 5 to 6 inches. In the Santa Cruz Mountains, mountains in the North Bay region and in the foothills and mountains east of the Sacramento Valley and northern most San Joaquin Valley I am expecting 8 to 12 inches with local amount of 16 to 18 inches. This amount of rain over the five-day period would certainly cause major hydrological problems along streams and smaller river with flooding, this includes the Delta Region. Snow levels in the Sierra are going to be very high as well, until later Sunday, so even up to the 7,000- to 7,500-foot level, a lot of rain will be falling.

As you can see by the rainfall amounts above, by the time one gets into Southern California rainfall amounts are much, much lighter. Now I believe the GFS amounts may be a bit on the low side, especially around Los Angeles, but perhaps the European is too high, but no matter what I also firmly believe the real stormy weather is going to miss this area.

The views expressed are those of the author and not necessarily those of AccuWeather, Inc. or AccuWeather.com

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Ken Clark
Ken Clark's Western U.S. weather blog tackles daily weather events with commentary from one of the most experienced and trusted Western U.S. weather experts.