ST. LOUIS – Why? A big question for just three letters.

No two years are the same, but similar large-scale patterns in the atmosphere and in ocean temperatures around the globe can give us some early indications of an active severe weather season.

One big clue for this year’s active severe weather season has been the slowly fading La Nina in the central Pacific Ocean. The cold water associated with La Nina teamed up with a warmer than normal pocket further north up towards Alaska to enhance the northern branch of the jet stream.

That branch of the jet stream is a key contributor to the wind energy needed for an active severe weather season across the Midwest. That wind energy helps contribute to storm rotation and rotating storms produce our most violent severe storms.

The alignment of that jet stream can also promote more frequent cold fronts diving into the Midwest during the spring season. That is at the same time, seasonal wind changes promote warm, humid air more aggressively flowing north from the gulf coastal regions. That warm, moist air is potential energy, or fuel, that when sparked can erupt into massive spring thunderstorms.

But to ignite the fuel, you still need a trigger in the form of strong low-pressure systems, cold fronts and warm fronts. The active jet stream has supplied a constant flow of triggers, which is why severe weather events have come at somewhat regular intervals since March.

The wild card to this year’s severe weather season has been the influx of drought parched air from the southern plains and desert southwest. This extensive area of dryness from Oklahoma back into Texas and New Mexico has allowed a much further east penetration of a weather feature known as a dryline. Drylines are notorious for spawning extremely severe weather in what we traditionally know as the Tornado Alley.

However, that dryline has regularly been force much further east than normal providing a strong focus for severe storms, the kinds that produce all severe hazards from large hail to extreme winds, and violent tornadoes.

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