For the past few days (and perhaps quite a few more), a dome of high pressure has settled over the Southern Plains and lower Mississippi Valley. Beneath that atmospheric dome, air is sinking and heating up, producing afternoon highs near or above 100 degrees F; since sinking air retards cloud formation, intense sunshine magnifies the discomfort.
Along the outer rim of the dome, cooler air is sliding beneath and lifting the hot air, igniting thunderstorms. In concert, clockwise winds at the edge of the dome direct these storms in a broad arc; yesterday, these "riders" were moving from northeastern New Mexico and eastern Colorado across the Central Plains and then southeastward through the Tennessee Valley and down to the Gulf Coast.
While not always as widespread and intense, high pressure domes are common over the Southern Plains in August and play a major role in the development of the Southwestern Monsoon. As Gulf moisture is drawn westward across Mexico, following the southern edge of the dome, it joins a moisture plume from the Sea of Cortez; the combined humid air masses flow northward along the western rim of the high pressure dome, feeding thunderstorms across the Desert Southwest and into Colorado (see also The Dome of August).
