Bomb cyclone, atmospheric river to give 1-2 punch to West Coast this weekend

(KXAN) — The most significant of storms in a parade of active weather for the West Coast is now underway.

Over the past week, parts of Northern California up to Washington have seen several inches of valley rain and mountain snow, as storm after storm has pummeled the region. By Sunday, the strongest of the series will be moving in creating a phenomenon called an “atmospheric river” in combination with a “bomb cyclone.”

A bomb cyclone is a low pressure system that drops 24 millibars in 24 hours or less — generally speaking, the lower the pressure, the stronger the storm system is.

The storm system is expected to reach its maximum intensity offshore, but its effects will still be very impactful from Seattle down to the California coast.

An atmospheric river is a relatively narrow band of concentrated atmospheric moisture that can release a tremendous amount of rain and snow when it makes contact with land (and is exacerbated when it comes in contact with a mountain range).

When a low pressure system (a.k.a. mid-latitude cyclone) drops 24 millibars of pressure in 24 hours or less, it undergoes “bombogenesis.”

While not officially deemed a hurricane, the effects from a bomb cyclone can “be equivalent to a hurricane in the Gulf of Mexico or Atlantic,” Carly Kovacik, a lead meteorologist for the National Weather Service office in Seattle told CNN. “A lot of these storms produce very strong winds and high seas, but since they are mid-latitude storms, we just don’t call them hurricanes even though the impacts can be very similar.”

In addition to the effects from a bomb cyclone, the Pacific Northwest will also endure the heavy precipitation from another phenomenon known as an “atmospheric river.”

Think of these rivers as exactly that: a river. But instead of liquid water moving along the surface of the earth, they transfer water vapor in narrow channels in the sky. Water vapor is liquid water after it evaporates and is in it’s gaseous phase.

The combination of a bomb cyclone and atmospheric river will bring significant rain and snow to the West Coast in the coming days. While a lot of this precipitation will be welcome for the drought-stricken region, burn scars from a relentless wildfire season will mean a lot of the precipitation will have a hard to absorbing into the soils.

Widespread 3-6 inch rainfall totals are possible in northern California, with isolated areas potentially receiving anywhere from 8-10 inches. Flash flood watches are in effect, as well as having the possibility of ash and debris flows in the mountainous areas.

Atmospheric rivers are key players in the western United States’ water cycle, as these events provide significant rains and snows to the Sierra and Cascade Mountains — where a majority of their water supply comes from.