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The intrusion of upper-level low pressure systems into the continental United States from Alberta is always pesky business. Dubbed “clippers” in the winter, they’re notorious for transcontinental swaths of light to moderate snow. But their most vicious impacts generally arise in warm weather, when they can prompt the development of violent thunderstorms. Early Wednesday morning, an upper-level low crossed the border into Montana. Though it will have no special nomenclature as it does in the winter, this system (like many others this spring and summer) will leave a trail of scattered transcontinental thunderstorm damage as it races from the Central Plains Wednesday to the Northeast Friday. Nebraska and South Dakota will be the first to face the storms. As the upper-level shortwave crosses the Rocky Mountains Wednesday, stretching of the air beneath it will intensify the system both aloft and at the surface. A surface cyclone will drag the boundaries…