The Northwest is bracing for a powerful storm system to move this week, bringing gusty winds, heavy rain and mountain snow. According to the National Weather Service, this is the first major storm of the season. A atmospheric river And bomb cyclone are what makes this storm so powerful – but what do these terms mean?

What is an atmospheric river?

Atmospheric rivers are no strangers to the western US, especially during the fall and winter months. Atmospheric rivers, or ‘ARs’, are elongated, narrow areas of moisture that move outside the tropical regions.

They are responsible for producing heavy rain and snow, especially when pushed against mountain ranges such as the Cascades and Sierra Nevada. A common type of atmospheric river is called a “Pineapple Express” because it flows from the Hawaiian Islands.

Strong ARs transport water vapor approximately equal to 7.5 to 15 times the average flow of the Mississippi River. Annually, 30% to 50% of precipitation occurs on the West Coast with only a few atmospheric river events.

What is a bomb cyclone?

Bomb cyclones are low-pressure systems that undergo what meteorologists call “bombbogenesis.” Bombogenesis occurs when a midlatitude cyclone (“midlatitude” meaning the area between the tropics and the polar regions) rapidly intensifies over a 24-hour period.

If atmospheric pressure drops by at least 24 millibars within 24 hours in most regions, it is considered a bomb cyclone.

Bomb cyclones can form when a cold air mass collides with a warm air mass, which is why some significant winter storms can form.

When an atmospheric river and a bomb cyclone occur at the same time, a major weather event is expected. Atmospheric rivers provide the moisture, and the bomb cyclone provides the intensity and increased winds.

This week’s big event will see a total of more than 4 to 8 inches of rain in some places. Total mountain snowpack is expected to exceed a foot in most areas; higher heights can reach more than 2 to 3 feet.