What is a "bomb" cylone?
As we progress into winter, you will likely hear the term "bombogenesis" or "bomb" cyclone used time and time again. While this name may seem sensationalized, it is in fact a legitimate meteorological term. Bombogenesis refers to a cyclone, or low pressure system, with a rapidly deepening barometric pressure. The criteria is a 24 millibar pressure drop over the course of 24 hours.
Where and when do they occur?
"Bomb" cyclones occur most frequently along coastlines where warm water is available. The warm water creates a strong temperature gradient from its surface to the atmosphere just above it. A temperature gradient simply means the amount a temperature changes over a distance, with stronger temperature gradients leading to strengthening cyclones. In the winter, there is an abundance of cold, Arctic air pouring into the United States creating these sharp gradients, especially over coastal areas. As a result, the majority of "bomb" cylcones occur from October to March.
Nor'Easters are most notorious for going through the bombogenesis process, often providing large swaths of 1 to 2 feet of snow during mid-Atlantic and Northeast winters. However, there have been cases of bombogenesis occurring with storm systems moving from the Rockies into the Midwest. These cases require the collision of two, opposing regimes of air — an Arctic air mass and a tropical air mass.
A "bomb" cyclone does not necessarily have to occur over land or achieve a certain size, the pressure drop is the only requirement for a storm to officially be undergoing "bombogenesis". The characteristics of these storms can be compared to that of a strong tropical storm or even a hurricane.
Bombogenesis in the Pacific Ocean
A low pressure centered off of the British Columbia coast is currently undergoing bombogenesis and will likely achieve a central pressure similar to that of a hurricane by Thursday morning. The storm system will not bring any hurricane-like impacts to the Pacific Northwest but it does open the door to a procession of storms bringing beneficial rainfall and mountain snow. With such a sudden switch to such great amounts of precipitation, the risk for mudslides and flash floods is extremely high. This is particularly true for burn scar areas as the soil is especially unstable.
As for the storms' impacts on Montana, they look to be rather minimal. Western Montana has the greatest chance of seeing increased precipitation throughout the next week.