The propensity for military-related language in the media has brought a rise in the use, and misuse, of of the term weather bomb in the last decade. The term bomb in combination with weather events appears to have first arisen during the war years of the 1940s when meteorologists began calling intense storms over the northwestern Atlantic Ocean "bombs". These "bombs" were storms with all the ferocity of hurricanes, and yet were not truly hurricanes (like 1991's Perfect Storm) that had the potential for damaging ships as great as enemy bombs.
Over the succeeding years, the term evolved into the description of a specific form of storm development characterized by MIT professor Fred Sanders in the 1980s as being an extratropical low pressure cell in which the central barometric pressure drops at least 24 millibars (2.4 kPa) in 24 hours. Sanders called such cyclones, bomb cyclones because of their explosive development.
Bomb cyclones rated headlines during the1990s when several were called the "Storm of the Century" and the "Perfect Storm" by US eastern reporters. These storms generally resulted in severe blizzards forming out of Nor'easters that struck the northeastern coastal states and Canadian provinces. Many of these storms began as Colorado Lows or Gulf of Mexico lows that underwent explosive cyclogenesis, the rapid formation of an intense winter storm over a short period when they moved off the land and into the warm Gulf Stream waters off Cape Hatteras.
As a result weather bombs, or more correctly bomb cyclones, have been associated with the eastern North America coast and offshore waters, though they also form off the Asia coast in the Western Pacific and over the eastern Atlantic and Pacific Oceans. The Pacific Northwest Region of North America has seen some of its most devastating storms arise from bomb cyclogenesis.
The bomb cyclone know as The Perfect Storm forms off US East Coast Satellite imagery, courtesy NOAA)
As I said, bomb cyclones have characteristics similar to hurricanes in their power and precipitation intensity. They may produce winds of hurricane force (greater than 74 mph / 118 km/h), and a few have developed distinct "eyes" when viewed from satellites. However, there are many major differences between the two storm types.
Bomb cyclones form at high (non-tropical) latitudes during the cold season (late-fall, winter, and early spring), the general opposite of tropical storm formation. Bomb cyclones have cold air and fronts associated with them which hurricanes do not, and indeed, cold air is an essential ingredient for a bomb cyclone, while it kills a hurricane. And finally, bomb cyclones require strong upper-level winds to develop, whereas such winds destroy hurricanes.
Synoptic meteorologists have identified several features of the high-altitude atmospheric circulation that must be present for a cyclone to undergo such rapid intensification. Technically, the forecasters look for surface low pressure cells moving into regions of high vorticity advection with increasing baroclinicity at the 500 mb level. Before I scare you off with these technical terms, I can rephrase and simplify the sentence (with my apologies to synopticians) saying that means that the formation of a bomb cyclone occurs in regions where increasingly strong temperature gradients (baroclinicity) due to a strong surge of very cold air, strengthens the jet stream and the tendency for an increase in the cyclonic--counterclockwise--spin (vorticity) over time of the low. When the combination of a strong jet stream and increasing cyclonic spin occurs west of a surface low pressure cell, it increases the rate of ascending air in the low and drops its central pressure.
We believe bomb cyclones are more common along the US East Coast because the topography serves as a focusing and trigger mechanism for the process. The topographical features of importance are the southern Appalachian Mountains and the relatively warm water off the coast. The mountains aid the process by enhancing the increase in vorticity, and the warm waters increase the baroclinicity. Once the "bomb" process begins, the ocean waters provide the moisture needed for heavy precipitation, and the northeastward-flowing jet stream herds the storm up the coast.
The February 1979 Presidents' Day Storm which dropped 5-10 cm (2-4 inches) of snow per hour from Washington, DC to New York began with bomb cyclone deepening. The storm covered sections of the East Coast with over 40 cm (16 inches) of snow. Another bomb blizzard was the "Lindsay Storm" which dumped more than 50 cm (20 inches) of snow on New York City and led to the downfall of New York City Mayor John Lindsay. The first "Storm of the Century" in the new century was a bomb cyclone that grew to be the Nor'easter of January 2000, the most significant snowstorm in the East since the Blizzard of '96. This storm recked havoc from South Carolina to Maine, unofficially dropping 66 cm (26 inches) in parts of North Carolina breaking records for the biggest snowfall ever. The Perfect Storm or Halloween Storm of late October 1991, is another example of a bomb cyclone, though snow was not a major factor in this storm.
Learn More From These Relevant Books Chosen by The Weather Doctor
Williams, Jack: The Weather Book, 1997, Vintage Books, ISBN 0-679-77665-6.
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