A massive atmospheric ridge of high pressure extends across much of the U.S., stretching from Southern California to the Mississippi Valley. Within this dome, air is sinking, causing it to heat up and dry out; as a consequence, cloud formation is minimal, augmenting the intensity of the high June sun.
High temperatures will range from the mid 90s (F) to 120 degrees within the dome; the most extreme heat will develop in the low deserts of the Southwest while triple digit heat indices (combining temperature and dew point) will be widespread across the Great Plains and lower Mississippi Valley. Here in central Missouri, we expect a high of 99 degrees F under sunny skies and anticipate afternoon highs in the mid to upper 90s for the next week.
Thunderstorms will erupt along the northern and eastern edges of the dome, where the hot, humid air interacts with a cooler and drier air mass; this clash zone currently stretches from the Great Lakes to the Ohio River Valley and Mid Atlantic Region. On the back (northwestern) edge of the dome, a less intense band of precipitation curves from Northern California to Montana. As the atmospheric ridge inches eastward, the storms will move in concert and relief from the intense heat won't arrive for a week or more. By July, such high pressure domes begin to settle over the Southern Plains, triggering the Southwest Monsoon.