What Is a Bomb Cyclone? Bombogenesis Explained

By
Harris
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Key Points

  • A bomb cyclone is a midlatitude storm that strengthens very quickly. Meteorologists call the process bombogenesis.
  • The benchmark is a drop in central pressure of at least 24 millibars in 24 hours at 60 degrees latitude, according to NOAA. At the latitude of New York City, the bar is about 17.8 millibars.
  • The term describes how fast a storm deepens, not how much damage it does. Impacts can include damaging wind, blizzards, heavy rain or snow, flooding and storm surge.
  • A bomb cyclone is not a hurricane. It runs on the clash between cold and warm air masses, while a hurricane draws its energy from warm ocean water.
  • A nor’easter can also be a bomb cyclone if it deepens fast enough, but the labels describe different things: one is about speed of intensification, the other about location and wind direction.

A bomb cyclone is a storm that intensifies explosively. In weather terms, it is a midlatitude low-pressure system whose central pressure falls so fast, usually over a single day, that it crosses a scientific threshold known as bombogenesis. The name sounds alarming, but it is a measurement, not a warning level.

This guide explains what the label means, how these storms form, how they differ from hurricanes and nor’easters, and what to do when one is headed your way. It draws on NOAA, the National Weather Service and the National Hurricane Center.

What is a bomb cyclone?

Start with the word cyclone. The National Hurricane Center defines a cyclone as an atmospheric closed circulation that rotates counter-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. On a weather map, it is a low-pressure center with winds spiraling around it.

A midlatitude cyclone forms in the latitudes between the tropics and the polar regions. According to NOAA’s National Ocean Service, bombogenesis happens when one of these midlatitude cyclones rapidly intensifies, or strengthens, over a 24-hour period. A storm that does this is popularly called a bomb cyclone.

The National Weather Service glossary uses the even shorter word “bomb,” describing it as a popular expression for the rapid intensification of a low-pressure system with surface pressure expected to fall by at least 24 millibars in 24 hours.

Why pressure is the yardstick

NOAA explains that the intensification is represented by a drop in millibars, the pressure unit meteorologists use. The field has used the millibar since 1929, according to NOAA’s JetStream weather school, and one millibar equals one hectopascal. For scale, standard air pressure at sea level is 1013.25 millibars, or 29.92 inches of mercury, the unit you are more likely to hear on local forecasts.

What is the threshold for bombogenesis?

The often-quoted figure is 24 millibars in 24 hours. NOAA notes, though, that the required drop varies by latitude. At 60 degrees latitude, it is at least 24 millibars in 24 hours. Farther south, the bar is lower. At the latitude of New York City, NOAA puts the required drop at about 17.8 millibars over 24 hours.

For readers, the practical takeaway is this: when forecasters say a storm is “expected to bomb out,” they mean its pressure is forecast to fall fast enough to cross that latitude-adjusted line within about a day.

Location Pressure drop needed in 24 hours (NOAA)
60 degrees latitude At least 24 millibars (24 hPa)
Latitude of New York City About 17.8 millibars (17.8 hPa)

How does a bomb cyclone form?

The fuel for a bomb cyclone is contrast. NOAA says bombogenesis can happen when a cold air mass collides with a warm air mass, such as air over warm ocean waters. The National Hurricane Center’s glossary describes the same engine in technical terms: an extratropical cyclone is one whose primary energy source is baroclinic, meaning it comes from the temperature contrast between warm and cold air masses.

That is why so many bomb cyclones develop over the ocean in fall and winter. NOAA notes that these storms usually, though not always, accelerate and strengthen over water. Off the U.S. East Coast, the National Weather Service explains, the polar jet stream carries cold Arctic air south across Canada and the United States and then east toward the Atlantic, where warm air from the Gulf of America and the Atlantic is trying to move north. The Gulf Stream keeps coastal waters relatively mild in winter, which warms the air over the water. The sharp temperature difference between that air and the frigid air over land feeds the storm.

Can a bomb cyclone form over land?

Yes, though it is less common. In March 2019, NOAA meteorologists said a winter storm over the Central U.S. had reached bombogenesis, which the agency called rare for the landlocked region. The storm brought blizzard conditions, severe thunderstorms and isolated flooding from the Rockies to the Upper Mississippi Valley, with the potential to break all-time low-pressure records for parts of Colorado and Kansas.

Bomb cyclone vs. hurricane: what is the difference?

Both are cyclones, and both can produce destructive wind and flooding. Their engines are different.

The National Hurricane Center defines a tropical cyclone as a warm-core, non-frontal system that forms over tropical or subtropical waters and is maintained by heat energy drawn from the ocean. A hurricane is a tropical cyclone with maximum sustained winds of 74 mph or more. A bomb cyclone, by contrast, is an extratropical storm powered by the clash of air masses. There is also a hybrid: a subtropical cyclone, which the hurricane center says has characteristics of both tropical and extratropical cyclones.

The labels can also change during a storm’s life. The hurricane center uses the term post-tropical cyclone for a former tropical system that no longer has enough tropical traits, and notes that such storms can keep carrying heavy rain and high winds. If you follow hurricane coverage, our guide to the Saffir-Simpson categories explains how hurricane strength is ranked.

Is every nor’easter a bomb cyclone?

No. The two terms answer different questions. A nor’easter, as the National Weather Service describes it, is a storm along the East Coast of North America named for the northeast winds it typically drives over coastal areas. NOAA’s satellite service puts it simply: a nor’easter is defined by its location and the direction its strongest winds blow from. Bombogenesis, on the other hand, is defined only by how fast the storm’s pressure falls.

A nor’easter that deepens fast enough is both. NOAA’s satellite service pointed to a powerful nor’easter over New England in October 2019 that met the bomb cyclone criteria: its pressure dropped more than 30 millibars in less than 24 hours, and it set October low-pressure records in Boston, Providence and Portland, Maine. The National Weather Service office in Boston reported wind gusts of 90 mph in Provincetown, Massachusetts, and nearly half a million people in Massachusetts and Maine were without power as of 11 a.m. ET.

The Weather Service says nor’easters can occur at any time of year but are most frequent and most violent between September and April. They usually develop between Georgia and New Jersey within about 100 miles of the coast and reach peak strength near New England and Canada’s Maritime Provinces. For an early-season example, see our report on an October 2026 nor’easter off the Carolinas.

What impacts can a bomb cyclone bring?

NOAA stresses that bombogenesis is a meteorological term about pressure, and that it does not by itself describe impacts. In practice, rapidly deepening storms are often high-impact storms. NOAA lists the typical hazards as:

  • High winds strong enough to bring down trees and power lines and cause outages.
  • Heavy precipitation, including blizzard conditions with heavy, blowing snow and whiteouts.
  • Flooding, including river flooding when rain falls on snow, and coastal flooding and storm surge along the shore.

The National Weather Service adds that nor’easters nearly always bring heavy rain or snow along with gale-force winds and rough seas, and that damage from the worst storms can exceed a billion dollars. The Weather Service lists the Blizzard of 1888, the Ash Wednesday storm of March 1962, the New England Blizzard of February 1978 and the March 1993 “Superstorm” among the best-known nor’easters. The heavily populated I-95 corridor between Washington, Philadelphia, New York and Boston is especially exposed.

How forecasters warn you

Rather than reacting to the bomb label, watch the hazard-based alerts from the National Weather Service. According to the agency, when conditions look favorable for a nor’easter in the coming days, forecasters may issue winter storm, blizzard, high wind and coastal flood watches. When the threat becomes imminent, those watches are upgraded to warnings.

Each alert has its own definition. A Blizzard Warning, for example, is issued for frequent gusts of 35 mph or more with falling or blowing snow that often cuts visibility to under a quarter mile for three hours or longer. A High Wind Warning over land covers sustained winds of 40 mph or more for at least an hour, or gusts of 58 mph or more. Local offices set some criteria themselves, so the snowfall that triggers a Winter Storm Warning in the Northern Plains is typically much higher than in the Southeast.

How to prepare for a bomb cyclone

Because the label says nothing about local impacts, let the watches and warnings for your area guide you. The National Weather Service recommends these steps when a nor’easter threatens:

  • Keep at least three days of food, water and other supplies in a disaster kit.
  • Plan for power outages, since strong storms can leave you in the dark.
  • Create an emergency communications plan so your family knows how to reach each other if separated.
  • Stay off the roads if local authorities advise it.
  • Never drive into flood waters.

In a blizzard, the Weather Service advises against travel. If you must go, carry a winter survival kit, and if stranded, stay with your vehicle.

For real-time watches, warnings and storm coverage across the country, follow the LiveNewsWorld U.S. Weather Live center, and check your local forecast at weather.gov.

The bottom line

A bomb cyclone is a midlatitude storm that strengthens unusually fast, its central pressure falling at least 24 millibars in 24 hours at 60 degrees latitude and less farther south. It runs on the contrast between cold and warm air, which is why fast-deepening nor’easters can earn the label. As a 2018 NOAA explainer put it, the term is less scary than it sounds. It tells you how quickly a storm is strengthening, not what it will do to your town. For that, follow the warnings from your local National Weather Service office.

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