Jet Stream is weaker so storms move slower and are easier to catch. Jet stream may disappear completely meaning nothing but uninteresting summer thunderstorms. Often time most severe thunderstorms occur within a few hour drive from Oklahoma City. Many storm chasers take their annual vacations in May, so extreme traffic congestion occurs around storms. Can have several days of great storms in the high plains meaning little travel.
Less chasers are out meaning no traffic congestions.
Roads are sparse in the high plains. There are usually several areas which look equally favorable making for difficult forecasts. Some years, the dryline never makes it out of New Mexico until June. Active dryline years results in a record number of chaser's jockeying for position. Some of the most prolific Texas Panhandle tornado events have occurred in June.