Severe Weather Outbreak Elephant: A Warm Cap Puts A Lid On Tornado Formation


Home / Severe Weather Outbreak Elephant: A Warm Cap Puts A Lid On Tornado Formation
The forecast for Monday, June 30. The greatest chance of tornadoes is in the yellow area. Forecast courtesy of NOAA

The forecast for Monday, June 30. The greatest chance of tornadoes is in the yellow area. Forecast courtesy of NOAA

Severe Weather Outbreak Elephant has already left a heavy footprint in parts of the plains, midwest, and south.

And he isn’t finished with his rampage of floods, thunderstorms, and tornadoes.

But what could have been a serious outbreak of tornadoes Sunday never materialized due to a ‘warm cap’ on the system.

What’s A Warm Cap?

The dip in the jet stream that is primarily responsible for Elephant flattened out somewhat on Sunday, with a tongue of warm air just below jet stream level — a warm cap — putting a lid on the instability.

When saturated air rises, it cools at a rate of 3.3 degrees per thousand feet. If the lapse rate (change of temperature with height) is greater than that, a rising parcel of air finds itself warmer than the surroundings and continues to rise — the air is unstable. With the warm cap, the air had to rise higher to reach an unstable level.

Typically the air cools with height in the atmosphere at a rate of around four degrees per thousand feet. The warm layer around five thousand feet above the ground produced a zone of stability in which the lapse rate was less than the critical value of 3.3 degrees per thousand feet. A rising parcel of air was cooler than its surroundings and sank back where it came from — the air was stable.

Finally, late in the day, after the sun’s heating forced enough lifting for the air to reach the unstable level, tornadoes and thunderstorms formed; but they were not as powerful as some forecasters had feared they would be.

In addition, some low clouds interfered with the sun’s heating. Once the low clouds burned off by mid-afternoon, heating was intense and instability began to affect Iowa, Minnesota, Nebraska, and Missouri. Some tornadoes were reported and there were large hail and damaging straight-line winds. But big tornadoes never formed.

Elephant Floods Memphis

Like Severe Weather Outbreaks Aardvark and Beaver before him, Severe Weather Outbreak Elephant is connected with a breakdown in the jet stream, which has taken a more southerly track than average for most of the past six months. When a wave in the jet stream spins into a closed low pressure swirl, it becomes cut off from the normal west to east movement of weather systems in the mid-latitudes and might not move at all for a few days. That is the case with Elephant, which is bringing unsettled weather to the same places day after day.

In addition, specific places can receive copious amounts of rain, as individual thunderstorm cells move slowly, and a series of storms can affect a single location. Memphis experienced such a ‘train’ of storms on Sunday, with widespread flooding.

Flooding Continues In The Upper Midwest

Many cities in Iowa and Minnesota have already experienced their wettest Junes on record — some their wettest all-time months. As Elephant has stalled, the hard-hit areas got more rain yesterday and will receive additional rain today.

Elephant Will Move East — Finally

The threat of severe weather continues today, as the warm cap weakens and instability continues in the same areas, generally from Texas to Michigan. With the humid air still in place, and the dry flow above cooling somewhat, large ‘supercells’ could develop. The southerly flow near the surface coupled with the westerly flow at jet stream levels will provide the spin that could turn thunderstorms into strong tornadoes.

Tomorrow, Elephant will move to the midwest, with the severe weather threat extending as far east as New York state. The rest of the week, Elephant may linger again, this time over the east coast, while the soggy midwest dries out.

The Next Severe Weather Outbreak

The next severe weather outbreak this summer, if there is one, will be named Fox. Right now extended forecasts call for a weakening of the jet stream and a return to a more normal polar vortex far to the north. But we have seen this forecast repeatedly and it has often been wrong. Fox is not now in sight, but she may be lurking.

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