Atmospheric rivers, bomb cyclones and red tags: A guide to storm lingo

The closed and damaged pier at Seacliff State Beach in Aptos on Tuesday.
(Kevin Painchaud / Lookout Santa Cruz)

As storms continue to rage through Santa Cruz County, there’s a hurricane of technical terms and official statements whirling across the region. In this quick guide, Lookout defines some essential words and phrases about the recent extreme weather.

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As the storm continues to rage in Santa Cruz County, there’s a hurricane of technical terms and official statements whirling across the region. In this quick guide, Lookout defines some essential words and phrases about the recent extreme weather.

Evacuation orders and warnings

Highway 9 was closed just past the Tannery heading north on Monday, Jan. 9, 2023.
Highway 9 was closed just past the Tannery heading north Monday due to flooding from the San Lorenzo River.
(Kevin Painchaud / Lookout Santa Cruz)

An evacuation order represents an immediate threat to life. California’s Office of Emergency Services states that it is a lawful order to leave once it is issued and that the area is lawfully closed to public access. It is a resident’s choice whether or not to leave, but the Santa Cruz County Sheriff’s Office warns that emergency services could be delayed or cut off from people in need within evacuation zones.

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“We don’t take evacuation orders lightly,” said Ashley Keehn, a spokesperson for the sheriff’s office. “We know it takes a lot for people to get everything that’s most valuable to them and leave their homes for any period of time.”

An evacuation warning, meanwhile, represents a potential threat to life and/or property. The Office of Emergency Services says that those who require additional time to evacuate, and those with pets and livestock, should leave once a warning is issued.

When an evacuation warning is issued, the sheriff’s office recommends preparing to leave at a moment’s notice when an evacuation order is announced. A go-bag of medications, important documents, a first-aid kit and a cellphone charger are among the items residents should consider packing once they receive a warning.

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Property damage

A yellow tag notes on a business on the Capitola Esplanade
A yellow tag notes that a business on the Capitola Esplanade suffered a limited amount of damage and some access is allowed.
(Kevin Painchaud / Lookout Santa Cruz)

After a building has been damaged by a storm, county building inspectors assess the severity of the damage and label the structure with a color-coded tag.

A red tag posted on The Sand Bar in Capitola, signifying it is unsafe for people to enter.
(Kevin Painchaud / Lookout Santa Cruz)

According to the Santa Cruz County Sheriff’s Office and the Capitola Police Department, a red tag denotes a seriously damaged structure that is too dangerous to enter. If your home or business is red-tagged, stay out. Only once the threat has subsided and the damage has been repaired and certified by county officials can residents return.

A yellow tag is for moderately damaged buildings that are safe to enter but require clearance by an engineer before they are reopened completely.

A green tag means that a building is not damaged or only slightly damaged and is safe to inhabit.

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Extreme weather

Satellite view of bomb cyclone and atmospheric river.
An unusually deep low-pressure system and an atmospheric river as seen from space.
(National Oceanic and Atmospheric Administration & Los Angeles Times)

Bomb cyclone

The term “bomb cyclone” comes from the equally explosive meteorological term bombogenesis, which describes a rapid and dramatic drop in pressure in an already low-pressure weather system. Low-pressure systems such as cyclones suck in air from around them to try to balance out the pressure difference, and in turn they can cause high winds, serious rain and foul weather.

For a bomb cyclone, when that bombogenesis pressure drop happens, precipitation intensifies quickly and winds pick up fast, sounding the alarm for meteorologists and those in its path.

Atmospheric river

An explainer on atmospheric rivers from the National Oceanic and Atmospheric Administration.
(Via National Oceanic and Atmospheric Administration)

The National Oceanic and Atmospheric Administration describes an atmospheric river as a “river in the sky” that transports water vapor along long, narrow channels in the atmosphere. When an atmospheric river runs over land, it typically releases its moisture as rain or snow.

“These columns of vapor move with the weather, carrying an amount of water vapor roughly equivalent to the average flow of water at the mouth of the Mississippi River,” NOAA writes.

Landslides, mudslides, debris flows and flash floods

An aerial phot of a landslide that closed Highway 9 in Ben Lomond on Monday.
(Kevin Painchaud / Lookout Santa Cruz)

Landslides, mudslides and debris flows all involve the movement of soil, rock and debris, like trees and vegetation.

Landslides are mainly gravity-driven and can occur at varying depths within the soil. They are not always triggered by rainfall but are in some cases.

Mudslides and debris flows are essentially the same thing. Mudslides are almost always driven by precipitation, and they occur at depths much shallower and closer to the surface than landslides can, said Laura Sullivan-Green, professor and department chair of civil and environmental engineering at San Jose State University.

“Mudslides are usually quite fast, and they are the types of events that we typically see with the kind of precipitation that we are experiencing right now,” said Sullivan-Green.

Because these debris flows are caused by intense rainfall, mudslides are typically much wetter than landslides. Mudslides flow like a liquid, whereas landslides often tumble like a solid.

The National Weather Service defines a flash flood as major flooding that occurs shortly after or during intense rainfall. Flash floods strike quickly, often stranding people and damaging property. Fast-moving water or flooded roadways can also harm people when traveling during a flash flood.

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