Winter storm Gemini affected nearly the entire continental United States. Ice storms raged in Utah, Oklahoma, and Maine; more than half a foot of snow fell in a dozen states. Violent thunderstorms caused hurricane force winds in much of the southeast, and flooding occurred from Missouri to Georgia. How can a storm cause such a variety of weather in so many places?
Hurricanes Versus Non-Tropical Cyclones
The naming of winter storms invites a comparison with hurricanes: hurricanes have a defined low pressure center around which the wind and rain occur. A winter storm, however, though it often has a deep low pressure center, can also be spread out along a frontal boundary with multiple minor low pressure centers. Gemini was in the latter category.
This Winter’s Weather Pattern
The late fall and early winter of 2013 have been characterized by a powerful jet stream sweeping across the United States, with a dip in the middle of the country. The vigorous jet stream is associated with a strong temperature gradient (difference in temperature). This temperature gradient forms when cold air moves southward from Canada and meets warm air from the Gulf of Mexico and the Pacific Ocean. The result has been a frontal zone stretching from the west coast through the plains and sweeping up the east coast. The word ‘front’ is the term used to identify the boundary between competing armies, and it applies to the weather, as warm and cold air collide and battle for terrain.
Along the front, warm air rides over cold, trying to displace it all the way to the ground; in other places, cold air plows under warm. Various types of precipitation occur, depending on the success of the invading armies.
Wind, Rain, Sleet, Snow, and Ice
When clouds build high enough to cause serious precipitation, they are cold enough for the condensation to take the form of snow. If the air is cold all the way to the ground, the precipitation will fall as snow; if there is enough warm air near the surface, the snowflakes will melt as they descend and fall as rain. The most damaging weather develops when the competing armies have come to a standstill and a layer of warm air overlies a shallow layer of freezing air at the surface: the precipitation will be freezing rain, as the raindrops, though they fall through air below freezing, don’t have time to freeze before they hit a solid surface. Sleet falls when the cold army wedges a thick enough layer under the warm to cause the raindrops to freeze in the air.
No Major Low Pressure Center But a Lot of Weather
The warm and cold air masses can lie next to each other for some time without any skirmishes. Then the shooting can start all along the front. That is what happened in Gemini. Where the cold air was entrenched but the warm air overrode it, there were serious ice storms in Utah, Oklahoma, and northern New England. Heavy snow fell just north of the front from Oklahoma to Michigan. In the south, where the warm air got overrun by a dry flow from the west, severe thunderstorms were accompanied by strong winds. The warm air was also moisture-laden, so some flooding occurred along the path of the thunderstorms.
The pattern of a dip in a vigorous jet stream over the United States shows no sign of changing. Temperatures are well below normal in the upper midwest and northern plains, while high temperature records are being broken in the southeast. The warm and cold armies are retrenching and reloading for the next storm. The shooting could start again as early as the end of this week.