Severe Weather Outbreak Elephant Will Last Several Days

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The National Weather Service Forecast for Sunday. The area in yellow is most at risk for tornadoes. Courtesy of NOAA

The National Weather Service forecast for Sunday. The area in yellow is most at risk for tornadoes. Courtesy of NOAA

Decoded Science has named severe weather outbreak Elephant.

Like severe outbreaks Aardvark and Beaver before it, Elephant is associated with a cutoff low pressure system in the jet stream, this one farther north than the previous two.

The weather pattern now has similarities to the one that brought such awful weather to the eastern US last winter, with a lobe of the polar vortex over central Canada dipping southward from time to time.

Warm air is streaming northward from the Gulf of Mexico to set the stage for the severe weather.

Severe Weather Explained

The atmosphere is a complicated system, but certain simple relationships provide an explanation for much foul weather.

  • The atmosphere gets colder as you go up.
  • The pressure decreases as you go up.
  • Rising air cools due to the decrease in pressure with height.
  • Air can hold more moisture (water vapor, the gaseous form of water) the warmer it is.
  • Rain is a result of air rising to the point where, due to its decreasing temperature, it cannot support the amount of water vapor it holds; the water vapor condenses and falls as precipitation.

All weather systems, large and small, follow the rules: Rising air leads to precipitation; since the atmosphere does not fly off into space, rising air and subsiding (falling) air must balance.

In large-scale systems such as hurricanes and nor’easters, the air rises over a wide area. Somewhere else there is a similarly large area of subsidence — a high pressure system with fair weather.

On a smaller scale, the air can rise and fall over tens of miles or less. Severe weather outbreaks consist of many small-scale updrafts. The whole system may cover hundreds of miles, but the local rising and subsiding air columns are narrow — and intense.

Conditional Instability

Severe weather outbreaks are characterized by what meteorologists call conditional instability. Several methods are available to the atmosphere to cause air to rise: Cold fronts, warm fronts, tropical waves, and daytime heating by the sun.

If a ‘parcel’ of air rises and cools, then finds itself cooler than the surrounding air, it will sink back to where it came from: The air is stable. Rising dry air cools at a known rate of about 5.5 degrees per thousand feet. If the actual rate of cooling with height in the atmosphere (the lapse rate) in the vicinity of a rising parcel is less than that, the air will be stable.

If the rising parcel cools to the point at which it is saturated, water vapor will condense. The condensation process releases heat; so a rising, saturated parcel does not cool as fast as a dry one. The rate of cooling of a saturated parcel of air, taking into account the added heat of condensation, is also known: 3.3 degrees per thousand feet. A rising saturated parcel may, therefore, find itself warmer that its surroundings and continue to rise.

This is conditional instability. The air is stable as long as it is dry, but when the air is lifted enough so that condensation occurs, it becomes unstable. When the air is unstable, further lifting creates powerful rising currents, which can produce thunderstorms and even tornadoes.

Where Will Elephant Roam?

As the low pressure in the jet stream moves slowly eastward, Elephant will affect the plains from Texas to the Dakotas today, the eastern plains and upper midwest on Saturday. Then it appears Elephant may linger in the same area on Sunday. After that, Elephant is likely to become a little gentler, but unsettled and possibly severe weather is possible from the midwest to the east coast through Wednesday, when Elephant will finally amble off the coast and disappear into the Atlantic Ocean.

What Kind Of Severe Weather Will Elephant Cause?

With the strong low-level flow of humid air from the Gulf of Mexico, powerful thunderstorms with copious rain will be the focus on Saturday and Sunday, though tornadoes are possible. Rain from Elephant will exacerbate the flooding in areas already hard-hit as a result of the persistent weather pattern. Many cities in Minnesota and Iowa have set June rainfall records.

On Sunday, since the jet stream winds will veer more to the west, the wind shear (change of direction with height) will increase the chance of tornadoes, possibly some very powerful ones. The large area at risk is centered on Iowa, Nebraska, Wisconsin, and Minnesota. The dangerous conditions may propagate eastward next week, though the jet stream is forecast to lose a little of its punch.

Will This Be The End Of This Year’s Severe Weather Outbreaks?

The peak of tornado season is well past; the atmosphere in general becomes more stable as we move through summer and the sun’s heating becomes less intense. Nevertheless, the unusual positioning of the jet stream and the tendency for cutoff lows to form suggest that more outbreaks are possible. If there is another severe weather outbreak this summer worthy of a name, it will be called Fox.

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