Northern California is in for a rare weather event in the middle of October this weekend. This significant storm is expected to bring up to 5 feet of snow to the mountains and potentially a foot of rain at the epicenter of its atmospheric river. The storm, which is forecast to hit Sunday and last through Monday, has been classified as a bombogenisis or “bomb cyclone,” meaning we’re in for a wild ride.
We hear about “atmospheric rivers” in the Pacific Ocean all the time, but rarely a “bomb cyclone” or “bombogenesis.” So what the heck is a bomb cyclone? Here is a definition from the National Oceanic and Atmospheric Administration:
Bombogenesis, a popular term used by meteorologists, occurs when a midlatitude cyclone rapidly intensifies, dropping at least 24 millibars over 24 hours. A millibar measures atmospheric pressure. This can happen when a cold air mass collides with a warm air mass, such as air over warm ocean waters. The formation of this rapidly strengthening weather system is a process called bombogenesis, which creates what is known as a bomb cyclone.
In layman’s terms, a bombogenesis is a rapidly intensifying weather system of low pressure that occurs when cold air collides with warm air over the ocean, which strengthens the system. While the weather phenomenon isn’t rare, it’s also something that isn’t seen in this area regularly.
According to the NOAA, fourteen of 20 hurricane-force wind events underwent bombogenesis in the North Atlantic during the first two months of 2014. To be classified as a weather bomb, or having undergone bombogenesis or “bombing out,” the central pressure of a low-pressure system must drop at least 24 millibars within 24 hours.
Northern California had a run-in with a bomb cyclone in November 2019, which brought a few feet of snow to the mountains and heavy rain and wind to the valley. In fact, the storm was so powerful that it broke the record for lowest pressure ever measured in California. The reading occurred in Eureka:
Pressure has bottomed out at around 28.69" at Crescent City, which is (unofficially) the lowest for California since records have been kept.— NWS Eureka (@NWSEureka) November 27, 2019
See for yourself: https://t.co/FcbR5VqPxs https://t.co/ZZ45IPlwJ0 https://t.co/YlpTyKIWQy
The result of a bombogenesis? Typically a storm is intensified when it reaches “bombing out” and includes increased rain, snow, extreme wind and even lightning. The fact that the NWS used the term means that the storm has intensified significantly over the course of its development.
Current forecasts show heavy rain and wind in the valley, with the potential for blizzard-like conditions in the mountains:
So it looks like the first storm of winter is going to more intense than initially expected. Here we go, Northern California – winter is here.