How Winter Storms Form

How Winter Storms Form

In North America, winter storms, like other storms at other times of the year, require just the right dynamics for a specific weather type to form.

Some of the ingredients include the proper positioning of the jet stream in the middle latitude regions of the United States. The proximity of a relatively warm air mass accompanied by plenty of moisture flowing up from the south is important.

A sufficient amount of cold polar air flowing down from the north is also needed, cold enough to drop temperatures so that frozen or freezing precipitation will fall.

The intensity of a storm depends upon several items, such as the strength and positioning of the jet stream and associated upper air disturbances, the related strength of the horizontal temperature gradients, and the availability of moisture.

The major moisture sources for winter storms in the United States are the North Atlantic Ocean, the Pacific Ocean, and the Gulf of Mexico.

If cold temperatures are in place and a significant amount of moisture is pumped into a storm system from any of these sources, the result could be a major winter storm.

Winter Storm Development
Winter storms have various components, including low pressure centers, warm fronts, and cold fronts. In the continental United States, winter storms are common from November through April, and sometimes as early as October or as late as May.

The winter dip in the jet stream allows polar air to surge south. This cold, dry air brings with it temperatures cold enough for snow, sleet, or freezing rain to develop.

Warm tropical air filled with moisture from the Gulf of Mexico often continues to flow up from the south during the winter months. When this warm, moist air mass from the south meets the cold, dry air mass from the north, winter storms can result.

Low Pressure Systems 

The term low pressure is used to indicate an area of air pressure that is lower than other areas of pressure around it. Air always moves from areas of high pressure to areas of low pressure.

Winds would blow in a straight line if not for the coriolis force, in which the earth's rotation causes winds to turn as they move away from high pressure areas into areas of low pressure. In the Northern Hemisphere, these winds circulate counterclockwise around areas of low pressure, or cyclones.

As the low develops, the warmer air from the south begins to flow northward on the eastern side of the low. At the same time, colder air from the north flows southward around the low's west side.

Areas of low pressure that produce winter storms often form along a developing or pre-existing frontal boundary. When conditions throughout the troposphere (the lowest layer of the atmosphere) are right, an intense winter storm can form.

Wintertime Warm Fronts
A warm front does not have to wedge and push its way into a colder air mass. Warm air is both lighter and less dense than cold air. Because it is lighter, warm air merely lifts over the cold air it encounters.

The leading edge of a warm air mass encountering a retreating cold air mass is a warm front. As a warm front approaches, the clouds become thicker and lower in the sky. These clouds produce a variety of precipitation types.

Wintertime Cold Fronts
The leading edge of an advancing cold air mass that displaces warm air in its path is called a cold front. Because cold air is heavier and more dense than warm air, an advancing cold front must wedge its way under a warm air mass, then lift and push it out of the way.

Gusty winds and a sharp drop in temperature often accompany and follow cold fronts in winter.