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Just a week after the U.S. East Coast was ravaged by two powerful winterstorms, another strong storm brought a blizzard to portions of the Plains. Like the blizzard on the East Coast, heavy snow, strong winds, and cold temperatures combine to create dangerous winter weather across the Northern and Central Plains. However, one may wonder why this blizzard tracked across the middle of the U.S., rather than up the East Coast.
To understand how blizzards and winterstorms develop and where they move, it's important to understand a little about how the atmosphere works. What happens on the ground (or surface of the earth), is intricately connected to what happens high in the atmosphere. Upper level winds at about 18,000 feet (500mb) help steer storms at the surface, so it is important to look at both the surface and 500mb if you want to forecast where winterstorms will develop and move.
Meteorologists generally look for troughs, or elongated areas of low pressure, at 500 mb to identify the area where strong storms at the surface may develop. The 500 mb map forecasted for January 12, 1996 shows a deep trough over the Eastern U.S. The arrows indicating wind direction show the air diving south over the Central U.S. and then turning north again up the Eastern Seaboard. The axis, or middle, of the trough stretches from Ohio to Georgia. Storms at the surface usually intensify as they move into troughs. Simply put, the counterclockwise winds associated with the trough help strengthen the low pressure system at the surface. The surface map forecasted for the same time as the 500mb map, shows a strong low pressure system over the Carolinas. Note the position of the surface low in relation to the upper level (500mb) trough. The most intense storms, including blizzards, usually occur just to the east of upper level trough axes where the atmosphere is most rapidly being lifted. A legendshows how to read the stations plotted on the maps.
To understand why the blizzard of January 17-18, 1996 tracked across the Central U.S., rather than up the East Coast, look at the 500mb map from the morning of January 18, 1996. Note that the upper level trough is in a different location than on January 12th. The upper level trough on the 18th is located over the central U.S. Winds dive southeast over the Rockies and then turn north again over the Mississippi River Valley. The axis of this trough stretches from North Dakota to Oklahoma. The position of the upper level trough is the key to understanding why the surface low pressure system (that brought the blizzard to the Plains on January 17-18) developed and tracked where it did.
The surface map from 00Z on January 18, 1996 shows the center of the surface low in northern Texas. Twelve hours later, the surface map from 12Z on January 18 shows the low over western Wisconsin. Notice that the storm track generally followed the 500mb winds. Also note from the surface maps that blizzard conditions occurred just north and west of the surface low track (and just east of the 500mb trough axis). Surface winds blowing counterclockwise and inward toward the low pressure center provide a continuous supply of very cold arctic air to the region just north and west of the storm track. The surface map from 00Z January 18 shows temperatures well below 0F over Montana, Wyoming and the Dakotas. East of the storm center, winds are blowing from the south or southeast, bringing warm, moisture-laden air northward from the Gulf of Mexico. Over eastern Texas temperatures were in the low 70's. Even Chicago is a balmy 55 F!
In summary, by using 500mb maps and surface maps it is possible to forecast where strong storms will probably develop, and subsequently where they will generally move. Blizzards usually occur just east of the upper-trough axis where the surface low becomes strongest AND to the northwest of storm track, where northerly winds supply a flow of cold, arctic air.
