Blocking Highs

Domes of high pressure may sit above an ocean or land mass for an extended period of time, shunting storm systems around their periphery; in such cases, these atmospheric ridges are called blocking highs. Within the dome, air is sinking and heating up, suppressing cloud formation and precipitation; indeed, most of Earth's deserts lie along latitudes where persistent domes of high pressure tend to form.  In the Northern Hemisphere, winds move clockwise around the dome while, in the Southern Hemisphere, winds move counterclockwise along their periphery.

Stationary high pressure domes are often responsible for droughts, shunting rain or snow producing storms away from the area over which they lie; they may also induce air stagnation, preventing the inflow of fresh air and aggravating pollution levels.  On the other hand, blocking highs may be protective, impairing the formation of hurricanes and tropical storms or steering them away from the coast; then again, they may direct those storms toward other coastal areas (e.g. a blocking high over the Southeastern U.S. may shunt a hurricane toward Mexico or Texas).

The severe conditions that gripped the central and eastern U.S. last winter was due, primarily, to a persistent ridge of high pressure over the Western U.S.  Beneath that dome, those in the West enjoyed mild, sunny, dry weather (exacerbating California's drought) while Pacific storms were directed into southern Alaska and western Canada; this mild flow severely diminished the snow pack in those regions but dislodged polar air, causing it to plummet through the central and eastern U.S.  This past week, a high pressure ridge has covered the southeastern quadrant of North America, bringing summer conditions well ahead of schedule; fortunately, the dome is sliding eastward and a Pacific cold front is expected to arrive today, igniting thunderstorms and sweeping in cooler, more seasonable weather.