If warm weather causes violent storms, why so many violent storms in cold winters?

Forecasters expect a wet, cold winter for Southwest Washington

Odds are it's going to be a colder, wetter winter than normal, courtesy of the La Nina effect.
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The 1998-'99 La Nina was one of the century's strongest, producing a stream of violent storms. The winter of 1998-99 also experienced record snowfall in the mountains due to an "arctic express" weather pattern - a stream of cold, moist air from Alaska that sometimes accompanies La Nina.

Another arctic express during a La Nina in 2007-08 produced a frigid March and April. A series of squalls and hailstorms April 19, 2008, caused eight one-car accidents in two hours throughout the county.