What Is a Bomb Cyclone? Why This Winter Storm Doesn’t Qualify

Not-quite-a-bomb cyclone brings messy winter weather

By Stéphanie Pappas edited by Andrea Thompson

A winter storm that likely won’t reach “bomb cyclone” status is nonetheless bringing complicated weather to the eastern United States today, with snowfall totaling a foot expected in parts of New York and southern Maine.

This first significant winter blast in the region is due to a low pressure system that has moved eastward from the Midwest through the Ohio Valley and South Central over the past few days, says Alan Gerard, meteorologist and CEO of Balanced Weather. The disturbance causes the development of a second low pressure area off the mid-Atlantic coast. This other low is expected to intensify as it moves north toward Cape Cod during the day, but will likely stay “just short of bomb cyclone territory,” says Ashton Robinson Cook, a meteorologist with the National Oceanic and Atmospheric Administration’s (NOAA) Weather Prediction Center.

A bomb cyclone occurs when a mid-latitude storm rapidly drops in pressure within 24 hours. The precise pressure drop depends on latitude: for example, at 40 degrees latitude (roughly that of New York), the pressure should drop about 18 millibars in 24 hours, according to NOAA. The cyclone moving north off the East Coast will further intensify impressively, Cook says, and will bring strong winds to parts of Maine to accompany the snow.

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But this low pressure, however, keeps the air warmer near the most populated part of the Northeast up to the middle of the Atlantic, explains Gérard. “That’s why Boston and Washington get rain or a little mixed precipitation,” he says. The heaviest snow, about 8 to 12 inches, is expected in southern New England and New York state.

Most of the precipitation will dissipate by Wednesday morning, Cook says, although drafts of cold air will continue to plunge into the central and eastern United States through next weekend. This cold trend could well continue throughout the month of December, says Gérard.

Although it is not always easy to directly relate polar patterns to North American weather patterns, this winter storm was preceded by an episode of what is known as stratospheric warming over the North Pole. This phenomenon occurs when the stratosphere – the layer of the atmosphere above the troposphere, where most of our weather occurs – suddenly increases several degrees in temperature. This warming has weakened the polar vortex, the circle of strong winds around the Arctic that keeps the coldest air around the pole. When it weakens, cold air can escape southward. A second oscillation of the polar vortex could occur in the coming weeks, says Gérard.

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