The storm that hit large swaths of the United States and Canada is what weather forecasters call “tornado bomb.” While this type of storm is not uncommon, it is very powerful, with high winds bringing snow or heavy rain to many areas.
Storms can form when a low-pressure air mass meets a high-pressure mass. Air moves from an area of high pressure to an area of low pressure, creating wind. What defines a bomb storm is how quickly the pressure drops in the low pressure mass – at least 24 millibars in 24 hours. This rapidly increases the pressure difference, or slope, between the two air masses, making the wind stronger. This rapid enhancement has a name: bomb-making.
When the wind blows, the rotation of the earth creates a tornado effect. The direction is counter-clockwise in the Northern Hemisphere (when viewed from above).
John Moore, a meteorologist and spokesman for the National Weather Service, said the conditions for a bomb storm were met over the Great Lakes, where arctic cold air from the polar vortex bends. The section meets very warm air in the east.
Air pressure drops to at least 962 millibars, while in other places as high as 1047 millibars. “It’s a really sharp slope,” he said.
Mr. Moore said as the area where the two air masses meet, known as the Arctic front, move north and east, the conditions for bomb generation will also continue to move.
But as arctic air spreads over most of the country, it will eventually warm, reducing the pressure differential. The storm will melt. And forecasts call for above-average temperatures across most of the country next week, he said.
