Northern California is expected to experience thunderstorms and heavy rain as an atmospheric river moves through the region, according to the National Weather Service.
“A weather system brings active weather back to (Northern California) mid- to late week, with widespread rain, isolated thunderstorms, gusty winds and mountain snow,” the National Weather Service wrote on X on Sunday, Nov. 9.
The event will primarily affect Northern California and the Central Valley. The agency added: “While forecast specifics will become more apparent in the coming day, users are encouraged to stay tuned as isolated flooding and heavy elevation snowfall are possible with this upcoming system.”
Rainfall is forecast to begin Wednesday, Nov. 12, in areas including Sacramento, Modesto, Merced and Fresno. Mountain regions such as the Sierra Nevada are expected to receive snow above 5,000 feet. South Lake Tahoe could see up to six inches of snowfall.
The National Oceanic and Atmospheric Administration describes an atmospheric river as a region in the atmosphere that transports water vapor outside of tropical areas. According to its website, “The average atmospheric river carries an amount of water vapor roughly equivalent to the average flow of water at the mouth of the Mississippi River.” When these systems reach land they often release moisture as rain or snow. The administration notes that those carrying significant amounts of water vapor can lead to extreme rainfall and flooding.
There is a high risk for heavy precipitation during this period which could result in localized flooding and increased snowfall in higher elevations. Flash floods are most commonly caused by intense rainfall from thunderstorms but can also depend on factors like topography and soil conditions.
Starting Wednesday there will be risks for flash floods across parts of the Bay Area, Northern California and Central Valley — including Merced and Fresno — though NOAA estimates that “the probability of rainfall exceeding flash flood guidance” remains below five percent.
In Modesto specifically, cloudy skies are expected on Tuesday with temperatures reaching a high near 75 degrees before dropping to around 56 degrees overnight. On Wednesday highs will be about 73 with lows near 59; there is an 80% chance of rain after 4 p.m., increasing Thursday with a 90% chance after 10 a.m., when temperatures drop further. Showers may continue into Friday before clear skies return by Saturday.
The National Weather Service expects Modesto will receive approximately 1.5 inches of rain from this system. Since July 1st Modesto has recorded about 1.2 inches so far this season; the measurement period continues through June next year.
Drivers are advised to use caution as wet conditions can make roads slippery especially when combined with high speeds which may cause vehicles to hydroplane. The Department of Motor Vehicles recommends slowing down by up to ten miles per hour when it rains: “Slow down at the first sign of rain, especially after a dry spell,” according to its website. If hydroplaning occurs drivers should slow down gradually without using brakes.
