February 14, 2019

Flash Flooding Is Likely In Central And Southern California On Thursday



Flash flooding is likely across parts of California and Arizona on Thursday as an approaching storm system threatens to bring several inches of rain in a short period of time. Flood watches stretch from northern California to southeastern Arizona in anticipation of the heavy rain. It doesn't take much heavy rain in this part of the country to cause significant flooding problems.

A potent upper-level trough moving toward California this hour is responsible for the low-pressure system that will cause all of the headaches on Thursday. The latest forecast from the Weather Prediction Center calls for several inches of rain across most of the state, save for some of the rain shadows and the southern half of the Central Valley.



Accordingly, flash flood watches (that light green Mr. Yuk color) and flood watches (dark green) are in effect for much of California and parts of Arizona. Urban flooding, overflowing waterways, and flash flooding in rough terrain is likely during the heaviest rain. Mudslides are also possible on burn scars.

There's a ton of moisture in the atmosphere for the developing showers and thunderstorms to work with. Southwesterly flow around the southern end of the low is dragging deep tropical moisture north toward California. This ribbon of moisture, known as an "atmospheric river," is usually what's responsible for the intense rain events we see on the West Coast.

Source: CIMSS


You can see the atmospheric river approaching southern California in the image above, which shows precipitable water (PWAT) across the eastern Pacific. Precipitable water is the amount of rain that would fall if you condensed all the water vapor in a column of the atmosphere. If the PWAT over your house is 1.15", it means that you'd get about 1.15" of rain if you condensed all of the water vapor in the atmosphere above your town. PWAT isn't the whole story, of course, but higher PWAT values indicate a better opportunity for heavy rain—it's a deeper reservoir of moisture for showers and thunderstorms to tap into.

It's not just the heavy rain you have to look out for. Some of those thunderstorms could be on the stronger side. The Storm Prediction Center says there's a marginal risk of severe thunderstorms in the Central Valley between Sacramento and Fresno. The agency's late-night forecast on Wednesday said that any thunderstorm that can get strong enough could produce gusty winds, small hall, or even a brief tornado. Tornadoes aren't all that uncommon in California, and this area is exactly where you'd expect to see them develop.

Source: NOHRSC


Also: snow. Lots of it. Not only is this good for ski resorts, but this kind of snowpack will help replenish bodies of water downstream once it all starts to melt during the warmer months. The above analysis from NOHRSC shows seasonal snowfall accumulations between 10 and 20 feet near the mountain peaks, with some areas seeing seasonal totals closer to 30 feet. The latest snowfall forecasts easily add 3+ feet to those totals through this weekend.


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I graduated from the University of South Alabama in 2014 with a degree in political science and a minor in meteorology. I ran Gawker's The Vane for two years and I've contributed to Mental Floss, Forbes, Popular Science, and the Washington Post's Capital Weather Gang. I also teamed up with Outdoor Life to write a book called The Extreme Weather Survival Manual, which came out in October 2015.

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