WEATHERAmerica SUMMARY, Saturday, April 13, 2024

[Larry Cosgrove]

Springtime thunderstorms are holding off the heat ridges in the lower 48 states.

The recent closeness of the subtropical highs to the U.S. is a reminder that before long (about a month or so) you will be seeing "hot and humid" turn up in the forecast with regularity. At issue here is the vigorous storm sequence, easily seen on satellite and water valor images across the Pacific Basin. There is a clearly defined injection of moisture and energy from the Madden-Julian Oscillation. Typically, during the spring, this equates to enhanced chances for severe weather and flooding rains along and below 40 N Latitude in the USA.

You will see an example of the prevailing pattern in the near term. The first storm in the series will progress from California to Colorado, and then into the Upper Midwest/Great Lakes on Tuesday. A rapid surge of deep tropical moisture will destabilize the atmosphere from Texas and Louisiana into Iowa and Wisconsin. A dry tongue and cold air mass surge will meet the unstable regime across the middle and upper Mississippi Valley. This familiar surface convergence pattern against a 500MB cold pool and splintered vorticity maximum offers almost a textbook case for a severe weather outbreak featuring large hail and tornadoes, To worsen matters, an upper impulse riding the subtropical jet stream may kick off an MCS in SE TX into LA by Wednesday morning.

A possible complication will be the introduction of a new shortwave from the Pacific Northwest. Another round of intense convection seems probable next weekend, perhaps skewed more toward the south central states into the Ohio and Tennessee Valleys. This may seem like a familiar scenario. Indeed I can see another such event April 28 - 30. The "lift out" of the jet stream will start getting underway in the second week of May. When the worst of the convection moves above the Interstate 70 cities (Denver CO to Baltimore MD), complaints about summer heat will replace the worries about tornado outbreaks.