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PATTERN TRANSITION: Breaking Down Late-June

The transition from spring to summer is fully underway as the jet stream lifts northward across North America. Spring 2024 will go down as one of the most active for severe weather across the Central US over the last two decades. Many questions remain about Summer 2024, but it does appear we'll get off to a stormy start.

A building ridge of high pressure has the first round of summertime heat expanding across the Midwest and Northeast this week. Cooler temperatures are in place across the Northwest, with a very active jet stream racing between these two contrasting air masses.

Contrasting cooler + warmer air masses and the jet stream racing between them:

The result over the next 5 days will be corridors of rain and thunderstorms from the Northern High Plains into the Upper Midwest with at least localized severe weather risks day to day.

Storm chasers earned a sunburn beneath a stout cap that kept a lid on thunderstorm development in western Nebraska yesterday, June 17th. Today, a line of severe thunderstorms is expected from northern Minnesota and the Upper Peninsula of Michigan down into the Southern High Plains. The tornado risk is probably highest further north in the poor terrain of the Upper Midwest, but slow-moving storms further south down the cold front from west Texas into central Kansas could still provide some good chasing with at least a few transient supercells and a rogue tornado or two possible. By sunset, I expect a pretty cool squall line to expand 500+ miles from Minnesota into Kansas.

Wednesday through Friday (June 19-21) the risk for severe weather probably becomes more isolated and localized in nature, with scattered storm clusters possible anywhere from the Northern and Central High Plains into the Upper Midwest.

The pattern begins to flip in week 2, as the ridge of high pressure slides into the Southwest U.S. with a series of upper-level disturbances riding over the top of the ridge from the Northern Plains into the Great Lakes. At the same time, southerly flow will keep Gulf of Mexico moisture funneled northward.

The combination of active jet stream flow out of the northwest over the top of a meandering surface frontal boundary with plenty of moisture is a recipe for repeated rounds of thunderstorm activity from the Plains into the Midwest.

This pattern is by nature one of low predictability. We're far removed from being able to pin down what days and what geographic areas may be favored for more significant severe weather episodes, but the background state for an active, stormy run is becoming more apparent. The weekend, June 22-23 might feature an organized severe weather event somewhere in the Midwest as an initial area of low pressure and associated cold front sweep through the region. This is essentially the first domino to fall in the pattern transition, aiding in the southwestward migration of the ridge of high pressure, and depositing a frontal boundary for renewed thunderstorm activity into next week.

So for now, I expect the next 5 days to feature corridors of storminess with local, isolated severe risks (excluding tonight's 500+ mile squall line, of course) with a bit of a ramp-up in coverage and intensity of severe weather events in days 5-10, or June 23-28 across the Plains and Midwest.

Beyond that - Dakotas, Midwest, derechos, supercells, I haven't a clue yet.


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