The US Northeast is bracing for a snowstorm that could dump over 30cm of snow with 80km/h winds in certain cities. But it might not be just any snowstorm. It could be a weather bomb.
"Bombogenesis" and "weather bomb" are fairly arbitrary terms -- they describe a threshold to represent intense changes in weather bringing a strong storm. What that means for the US East Coast is that a snowstorm with the energy of a hurricane could be coming their way tomorrow.
"Bomb is a fancy way of saying a really quickly intensifying storm," Bob Henson, Meteorologist at Weather Underground, explained to Gizmodo -- one in which atmospheric pressure drops one millibar per hour for a whole day. "There's nothing magic about 24 millibars in 24 hours. It's like saying a high fever is over 101 [Fahrenheit]. You still have a fever if it's 100.9 and 101.1, but you need a level to say what's extreme."
Average atmospheric pressure at sea level is around 76cm or 1013 millibars. "Low pressure" might be 75cm or 1000 millibars. Some models predict the pressure in tomorrow's storm will drop from an already low 1002 millibars to 978 millibars, the pressure at the centre of a category 1 hurricane. If that happened, it would translate to a 24 millibar change in 24 hours, said Henson.
That low pressure system will develop off of the United States' southeastern coast, where cold, dry air from the north will meet and creep on top of warm, moist air in the south. Warm air rises and rushes upwards, bringing the declining pressure with it. The temperature gradient between polar and tropical air masses is especially strong around March, so the drops in pressure can be more profound -- resulting in weather bombs.
The low pressure void causes air to rush in from all directions, spiralling into a cyclone. The air reaching the centre gets trapped and has no where to go but back up, bringing moisture from the ocean with it. The rising moisture condenses and turns into rain, sleet or snow, depending on the temperature differences and the shape of the boundary between the two air masses as you increase in altitude. As of now, Washington DC may see rain or snow, while Boston and New York are definitely expected to get some white stuff -- up to 45 and 50cm of accumulation respectively, according to the National Weather Service.
Eventually, the storm will weaken when the temperatures and pressures equalise across the cold air-warm air gradient. Until then, it will remain a powerful storm. "They're very much like hurricanes," Jeff Masters, another Weather Underground meteorologist, told Gizmodo. "It won't have winds as strong at the centre because hurricanes have eyewalls which concentrate the energy in. [This storm] will have energy spread over a much longer area."