SerpApi.Org
The potential of slow-moving thunderstorms in the Sierra Nevada on Tuesday has prompted the National Weather Service to issue a flood watch in and around the Lake Tahoe area.
The watch is in effect from noon to 11 p.m. Tuesday for the greater Tahoe basin, including Truckee, South Lake Tahoe, Incline Village and Tahoe City. It also extends south to Mammoth and east to Reno.
"Excessive runoff may result in flooding of creeks, streams, urban areas, burn scars, and areas near steep terrain," the weather service said. "Area rivers and streams could see rapid rises and increased sediment loads with localized heavy rain, but no river flooding is expected."
Widespread cloud cover over Lake Tahoe on Tuesday morning initially limited thunderstorm severity and distribution of thunderstorms there, but clearer skies over Mammoth increased the potential for midday downpours in Mono County. The weather service increased its flood risk for Mono County to a slight (level 2 of 4) risk. Storms are expected to form near Mammoth and move toward Tahoe in the afternoon.
Not everywhere will see flooding - some locations may not even get wet - due to the hit-or-miss nature of popup thunderstorms. But any storms that do form will be juiced up by an unusually humid air mass. A weather balloon launched from the Reno airport Tuesday morning recorded the most atmospheric moisture in four years.
A half-inch of rain or more could fall in an hour under the heaviest storms and could be accompanied by gusts up to 60 mph.
"Locally higher totals are possible where storms move slowly or repeatedly develop over the same area," said weather service meteorologist Chris Johnson.
Moisture from post-Tropical Storm Elida to the southwest and the North American monsoon to the southeast are pushing into California and Nevada, but flooding is only expected from the Sierra eastward. That's because most of the moisture is at high altitudes, closer to the elevation of the Sierra's soaring mountains. Elsewhere in California, a dry layer of air was expected to evaporate much of the rain before it reaches the ground.
Warm air rising up mountain slopes during summer afternoons also provides an important lifting mechanism to convert water vapor to clouds and precipitation over the Sierra.
Lingering moisture could trigger another flood watch in the same areas Wednesday, though drier air is expected to push in Thursday and end the threat of thunderstorms.
Subscribe
There’s more to San Francisco with the Chronicle. Subscribe today for just 25¢.Read this story on sfchronicle.com
