What is a bomb cyclone? What the term means, where to see it and what to do

What the term means

The National Weather Service glossary entry for "Bomb" reads: "Popular expression of a rapid intensification of a cyclone (low pressure) with surface pressure expected to fall by at least 24 millibars in 24 hour." (That is the glossary's wording.) So a bomb cyclone is a storm whose central pressure falls quickly.

NOAA's explainer on bombogenesis, published January 3, 2018, puts it the same way: it is a mid-latitude cyclone that rapidly intensifies, with a central pressure that drops at least 24 millibars over 24 hours. NOAA says the term is a meteorological one and does not describe the impacts of the weather system.

That last point is the useful one. NOAA says the impacts of such systems often include high winds, heavy precipitation and even storm surge. The nickname tells you the storm strengthened quickly. It does not tell you what will happen on your street.

A real example: Colorado, March 13, 2019

The National Weather Service office in Denver/Boulder describes the March 2019 storm. It says an extremely powerful low pressure system developed over southern Colorado and set a record for the lowest pressure ever recorded over Colorado, at Lamar, of 970.4 mb. It officially met the criteria of a "Bomb Cyclone", in which barometric pressure readings dropped in excess of 24 mb (0.71 in Hg) over a 24-hour period.

The office lists widespread blizzard conditions, wind gusts from 60 to 80 mph, nearly 1,400 flight cancellations at Denver International Airport, closed highways and interstates, and at one point 445,000 customers without power, some into Friday. It also records one known fatality, a Colorado State Trooper. That is one storm in one place; other bomb cyclones have had different effects.

Where to see it: NWS and WPC maps

For your own forecast, enter your city or ZIP code at weather.gov. Your local National Weather Service office issues winter weather warnings, watches and advisories, and the National Weather Service says each office does so using local criteria. Its example: the amount of snow that triggers a "Winter Storm Warning" in the Northern Plains is typically much higher than the amount needed to trigger one in the Southeast.

For the big picture, NOAA's Weather Prediction Center (WPC) publishes surface analyses with North America, United States and regional views. See the WPC surface analysis. WPC's menu also lists winter weather products and storm summaries. A "bomb cyclone" label on a news story is not an official warning level; the warnings on weather.gov are.

Our alerts page shows active National Weather Service alerts and National Hurricane Center storms, and our state pages show alerts for each state.

Know the warning words

The National Weather Service describes these warnings, which are the ones to act on:

• Blizzard Warning: issued for frequent gusts greater than or equal to 35 mph accompanied by falling and/or blowing snow, frequently reducing visibility to less than 1/4 mile for three hours or more. The NWS says: do not travel, and if you must travel, have a winter survival kit with you.
• Winter Storm Warning: a significant winter weather event including snow, ice, sleet or blowing snow, or a combination. The NWS says to delay travel plans until conditions improve.
• Ice Storm Warning: usually issued for ice accumulation of around 1/4 inch or more, which the NWS says will make travel dangerous or impossible and likely lead to snapped power lines and falling tree branches.
• Winter Storm Watch: conditions are favorable for a significant winter storm. The NWS labels watches "Be Prepared" and warnings "Take Action!"

What to do before and during the storm

Ready.gov says winter storms create a higher risk of car accidents, hypothermia, frostbite, carbon monoxide poisoning, and heart attacks from overexertion, and that a winter storm can last a few hours or several days and cut off heat, power and communication services. Its headline advice: if you are under a winter storm warning, find shelter right away.

Before: Ready.gov says to prepare your home with insulation, caulking and weather stripping, to install and test smoke alarms and carbon monoxide detectors with battery backups, to gather supplies in case you need to stay home for several days without power (including medication and pet needs), and to have extra batteries for radios and flashlights. Our winter storm preparedness guide and power outage checklist turn that into a list.

During: Ready.gov says to stay off roads if at all possible and, if trapped in your car, to stay inside. It says to limit your time outside, wear layers of warm clothing and watch for signs of frostbite and hypothermia, and to reduce the risk of a heart attack by avoiding overexertion when shoveling snow. Ready.gov says hypothermia is an unusually low body temperature, and a temperature below 95 degrees is an emergency.

If the power goes out, our stay warm without power guide and frozen pipes guide apply, and the car kit list is in our winter car emergency kit guide.

Heating and generators: carbon monoxide

Ready.gov says to avoid carbon monoxide poisoning: only use generators and grills outdoors and away from windows, and never heat your home with a gas stovetop or oven. It says using an oven or stovetop can also increase the risk of fires and burns and could damage the appliance.

On generators, Ready.gov says generators and fuel should always be used outdoors and at least 20 feet away from windows, doors and attached garages, and to install working carbon monoxide detectors on every level of your home. Read our generator safety guide and carbon monoxide poisoning guide.

Do you need to buy anything?

Probably not because of the nickname. If you already have a few days of food and water, warm clothes and blankets, working flashlights and a way to get alerts, you have the core of what Ready.gov's winter page asks for. If something is missing, Ready.gov names NOAA Weather Radio as a source of emergency alerts and says to install carbon monoxide detectors with battery backups, so a weather radio and carbon monoxide alarms are the items that match that advice. Skip anything sold on the word "bomb" in a headline.