The Christmas Day Storm

A series of warm atmospheric rivers continues to bring moderate to heavy rain and wind to the Central Valley, the foothills, and the mountains this Christmas morning.

The National Weather Service has issued a Flood Advisory for Mariposa County until noon today. Additionally, the Flood Advisory issued for Tuolumne and Calaveras Counties will continue until 9 PM tonight. The overall Flood Watch remains in effect for the Mother Lode, the San Joaquin Valley, the Sierra Nevada, Yosemite National Park, and the Mariposa County foothills until Friday afternoon.

Doppler radar indicated heavy rain this morning. As of this early Christmas morning posting, between 0.2 and 0.5 inches of rain has fallen. Additional moderate to heavy rainfall amounts of 0.5 to 0.7 inches are expected over the area.

Flooding caused by excessive rainfall is either occurring now or is expected. There is some minor flooding in low-lying and poor drainage areas as well as water over the roadways and overflowing poor drainage areas. Some low-water crossings may become impassable.

Excessive runoff is resulting in rises along area rivers, creeks, streams. Small streams and creeks may rise out of their banks. Flooding may occur in urban areas. Storm drains and ditches may become clogged with debris. Extensive street flooding and flooding of creeks and rivers are possible. Area creeks and streams are running high and could flood with more heavy rain. Mudslides and rockslides may occur in the mountain and foothill areas. Debris flows are not expected over recent burn scars in northern California, but do anticipate enhanced runoff in and below the scars.

You should monitor later forecasts and be alert for possible Flood Warnings. Those living in areas prone to flooding should be prepared to take action should flooding develop. Turn around, don’t drown when encountering flooded roads. Most flood deaths occur in vehicles.

Separately, a High Wind Warning has been issued for the Mother Lode and the northern San Joaquin Valley, until 4 PM today. A Wind Advisory is also in effect for Mariposa County, the Yosemite National Park Valley floor and the central San Joaquin Valley, until 4 PM Thursday.

South to southeast winds of fifteen to thirty-five mph are forecast, with gusts ranging from forty to sixty mph.

Strong winds and saturated soils could blow down trees and power lines resulting in widespread power outages. Gusty winds will cause impacts to loose and unsecured items, including temporary structures and holiday decorations. Watch for falling debris and tree limbs.

Winds this strong can make driving difficult, especially for high profile vehicles. Use caution if you must drive. Prepare for potential power outages.

Finally, the Winter Storm Warning issued for the western slope of the northern Sierra Nevada and Yosemite National Park, above the Valley floor, will remain in effect until 10 PM Friday.

Heavy snow is likely. Snowfall rates of one to two inches per hour are possible at times.

The snow levels now range between 4,500 to 5,500 feet.

Total snow accumulations of up to a foot is likely between 4,500 to 5,500 feet. Four to eight feet of snow is likely above the 5,500 foot elevation. The higher you go, the more the snow.

Upper elevation winds could gust as high as sixty mph, which could cause tree damage. Visibilities may drop below a quarter of a mile due to falling and blowing snow. There will also be a heavy snow load on area roofs.

Whiteout conditions are expected and will make travel treacherous and potentially life-threatening. Travel may be very difficult to dangerous to impossible with chain controls, road closures, localized tree damage, power outages, and low visibility due to the combination of heavy wet snow and strong winds. If you must travel, keep an extra flashlight, food, and water in your vehicle in case of an emergency.