There has been various instances of extreme weather across portions of the United States the last few days. Northern California has seen increased rainfall in short periods of time leading to flash flooding instances trapping people in their vehicles. The same system that is causing flooding in California is causing up to 10" of snowfall in parts of western Nevada.
The reasoning behind this dramatic flooding and snowfall is because of an atmospheric river. This occurrence is pretty much moisture being drawn in from the tropics in the lower Jet-stream in a narrow band back towards California. This moisture then meets a front forming precipitation. The image above displays the atmospheric river depicted in relative humidity in green.
Thankfully the atmospheric river is beginning to lose strength allowing for the system to fall apart. However, things are still ramping up in southeastern portions of the United States as the same cold front that brought Mid-Missouri showers Monday night is firing off strong storms. There has been various tornado warned storms and possible touchdowns in this area stem from Arkansas all the way to the east towards Georgia.
The main driving force behind the southeastern storms is wind shear. Wind shear occurs when winds are moving at different speeds/ directions at different levels in the environment. This causes turbulence and can lead to rotation which has fired up dangerous storms.