Way more ice than usual this winter, say Penn State weather experts

University Park, Pa. — If it seems like this winter has brought more ice storms than normal, there's a good reason, according to Penn State weather experts. It really has.

According to Paul Knight, Pennsylvania state climatologist, the Keystone State feels the effect of 12 to 18 winter storms during a typical winter. Of those storms, only two or three would normally produce widespread icy conditions. "Many parts of Pennsylvania have already exceeded their usual number of icing events for a year, and the winter is not even half over," he says. "There is no doubt that the frequency of freezing rain during December and so far in January is unusual, particularly for the central part of the state."
 
So far this season, a different weather pattern has caused each ice storm, but all of the storms have one element in common, explains Knight, a meteorology instructor. An elevated wedge of mild air several thousand feet above the ground melts the falling snowflakes. As the precipitation continues to fall it reaches a shallow layer of colder air near the ground, which causes the rain to freeze on contact. 
 
"The wedge of above-freezing air is a result of the storm path that has brought storms directly into Pennsylvania or westward into the Great Lakes," he says. "Winds aloft from the south have brought in this mild air, which is key for an ice storm."
 
Don't look for winter to get milder anytime soon, warns Bryan Swistock, a water resources extension associate in Penn State's College of Agricultural Sciences. Less icy, perhaps, but still nasty. "This winter, the weather pattern has been sending a lot of storms up into the Great Lakes region and even up into northern New England, which has been getting pummeled with snow," he says. "This storm path is one reason why Pennsylvania has been seeing a lot of mixed precipitation.
 
"As the jet stream cuts up to our west, we end up on the warmer side of these storms, and we get either rain or some mixture of rain, snow and ice," Swistock explains. "Pennsylvania has been right on the line where precipitation mixes, especially in the northcentral and northern areas of the state. For southern Pennsylvania, more rain has been a side effect of these storm paths." 
 
A faraway phenomenon is related to these unusual weather patterns for Pennsylvania, Swistock points out. "Most meteorologists are attributing the shifted storm track to the ocean current, La Niña," he says. "Not to be confused with El Niño, which is more commonly seen and is famous for causing certain weather patterns. La Niña is known for producing the kind of milder, icy-weather pattern we've been experiencing thus far this winter."
 
La Niña occurs when equatorial water in the Pacific is cooler than normal during the autumn and winter months. Knight agrees that there is a subtle connection between the early winter storm track across the United States and water temperature anomalies observed in the tropical Pacific.
 
Weather patterns that result from currents such as La Niña are cyclical, but predicting them years in advance is not an option, the Penn State weather experts note. Monitoring ocean temperatures annually and looking for the associated changes can help meteorologists to predict weather patterns for the upcoming season. "A lot of predictions coming into this season were projecting what's happening now," says Swistock. 
As this winter progresses, storm tracks should dip farther south as cold air spreads deeper into the eastern part of the continent, Knight says. As a result, the weather patterns that have been producing the warmer, icy conditions should break down and we should see colder weather, says Swistock. Both Swistock and Knight suggested that winter is probably moving into a period of more snow and colder temperatures, which means that the risk of ice storms will likely decrease in late January and into February.
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Last Updated November 18, 2010