There is a tussle underway between a large high pressure ridge dominating the area from the Desert Southwest to the Midwest and core of the Westerlies aloft from the Pacific Northwest to the Northern Midwest. The ridge is a mound of warm air at the surface and aloft which is in the area south of the 588 line on the forecast 500 millibar map for Wednesday, June 22nd at 7:00 a.m. CDT. Temperatures in the 80s and 90s are common under the ridge with readings topping 100 in the Desert Southwest. The main flow aloft at about 18,000 feet (500 millibars) is being steered around the northern edge of the ridge.
During the next few days we should see a northwest flow of air aloft increase over the Upper Midwest. This pattern is favorable for showers and thunderstorms, especially from late afternoon overnight into the morning. The nighttime component of these storms is a major rain producer for the Midwest. Watch for this pattern to develop and for thunderstorms to become more numerous from the Dakotas southeastward through parts of Minnesota, Nebraska, Iowa, Illinois, Missouri, northeastern Kansas, to the east from Monday into Thursday. Locally heavy rain, large hail, strong winds and tornadoes are possible in this situation. Other isolated thunderstorms may form at random under the warm high pressure area.