For the past two months, a dome of high pressure has been sitting over the southeastern U.S., aggravating a prolonged regional drought. As Pacific fronts move toward this dome, they are deflected to the north and east, depriving the southeast of much needed moisture. Clockwise winds move along the outer edge of the dome, pulling humid Gulf air into the Heartland and bringing heavy rain and severe thunderstorms to the southern Plains and Midwest.
Within the dome, sinking air prevents the development of "pop up" thunderstorms which are usually common in the southeast. Furthermore, the prolonged drought has produced a dry landscape that feeds little moisture to the atmosphere; any showers that do manage to form drop their rain into a low humidity air mass and most of the precipitation evaporates before reaching the ground. Such conditions lead to wildfires, a lowering water table and widespread water shortages. A potent, slow-moving tropical storm may prove to be the only hope for breaking this cycle.
Many of Earth's deserts have developed under similar conditions. They occur at latitudes where high pressure dominates and where the air is sinking. Air warms up and dries out as it sinks and, as noted above, high pressure deflects fronts and prevents cloud formation. Where oceanic currents and jet stream patterns keep such domes in place, deserts form.