What is the most difficult kind of weather system to forecast a path for?

Dear Tom,
What is the most difficult kind of weather system to forecast a path for?
—Alicia MacGreger

Dear Alicia,

Every type of weather presents forecast challenges. Strong low pressure systems are associated with major upper-air features that allow computer models to produce accurate surface tracks. But as these systems move from the Plains into the Midwest, a tiny deviation in the predicted path can result in significant forecast errors of precipitation type or amount at any given spot. Weak low pressure systems present difficulties because their paths can be erratic. Other forecast problems involve changes in system intensity, with powerful systems weakening or weak systems strengthening. Finally, thunderstorms are fickle. Individual thunderstorm cells, because they change from hour to hour, cannot be predicted more than a few hours in advance.