Severe Weather Outbreak Jon: Powerful And Long-Lasting; Tornadoes Likely


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The jet stream flow this morning shows a sharp trough associated with Severe Weather Outbreak Jon entering the central plains. Graphic courtesy of NOAA

A low-level flow of warm, humid air from the Gulf of Mexico will combine with dry air above and a kink in the jet stream to produce a severe weather outbreak worthy of a name in the central and eastern US the next three days.

Decoded Science is breaking with tradition. We will no longer name weather events after members of specific categories; nor will we keep an alphabetical list. Each weather event will receive a designation on its merits.

The merit of this severe weather event, which began yesterday and threatens to last until the weekend, is that it commenced on my birthday. Therefore I have named it after myself. A little narcissistic, perhaps, but why not give a meteorologist a little notoriety?

What Conditions Create The Most Violent Weather?

For the most part, the atmosphere is a placid place. Occasionally a large system such as a tropical or extra-tropical cyclone interrupts the calm. These systems are necessary to keep the temperature in balance; more radiation is coming in than going out at the equator, and the reverse is true at the poles.

Lest we all either freeze or cook, Mother Nature is kind enough to provide an equalizing phenomenon — wind, which transports warm air poleward and cold air equatorward. Thanks, Mother Nature.

Even in the most vigorous cyclone, movement of air is mostly systematic and predictable. In a tropical cyclone, air spirals into the center and flows out at the top. In an extratropical cyclone, a cold front wedges beneath warm air and a warm front rides over cold. The lifting of the air occurs in a regular way over a large area.

Occasionally the atmosphere becomes wild and chaotic: Air currents overturn like water in a pot heated to the boiling point.

Instability In The Atmosphere

In a tropical cyclone, air spiraling into the center has nowhere else to go but up. Similarly, air is forced upwards at a cold front or a warm front. A lifted parcel of air will find itself cooler than surroundings and want to return to where it came from, but it is continually forced upward.

Instability occurs when the actual decrease of temperature with height (called the lapse rate) is larger than the change in temperature that a displaced parcel of air undergoes if it is displaced upward. Then the parcel finds itself warmer than the surroundings and wants to keep on going.

Due to the decrease in pressure with height, a lifted parcel of dry air cools (the ideal gas law says pressure is directly proportional to temperature) at about 5.5 degrees per thousand feet (the dry adiabatic rate). If the air is saturated, moisture will begin to condense as the air rises (cooler air cannot hold as much water vapor as warmer air), and energy, which has been stored in the water vapor molecules, is released as heat. This added heat somewhat offsets the cooling from lower pressure. Thus rising saturated air cools at the wet adiabatic rate — only 3.5 degrees per thousand feet.

Normally the atmosphere is stable: The actual lapse rate is less than either the dry or wet adiabatic rates, and any displaced parcel of air feels a force that tries to return it to its original position. However, there is a situation called conditional instability that results in very unstable conditions and violent up- and downdrafts over a fairly large area.

Conditional Instability

If there is saturated air below dry air and the whole column is lifted, for example by a cold front, the lower layer cools at the wet rate and the upper layer cools at the dry rate. The actual lapse rate becomes steeper (temperature falls more with height), and if this continues long enough the actual lapse rate becomes greater than the change of temperature of a lifted parcel.

At this point, any displacement of air will then cause a parcel to continue moving in the same direction. Violent columns of rising and falling air then produce thunderstorms and tornadoes.

Where Does Conditional Instability Form?

The central United States is a perfect place to find conditional instability. Humid air flows north from the Gulf of Mexico at lower levels, and if a westerly flow in the jet stream which has lost its moisture crossing the Rockies lies above, any lifting mechanism such as a front or daytime heating can turn the conditional instability into actual instability and set off severe weather.

Severe Weather Outbreak Jon

Warm air from the Gulf of Mexico has covered the southern plains and southeastern states for a week. One outbreak of tornadoes has already occurred in Oklahoma, and scattered tornadoes have spun up from Kansas to Alabama. Everything is in place for the next event, and a sharp dip in the jet stream is just entering the central plains. Though unstable conditions could occur over a large area, the jet stream dip will provide an extra lift.

Today’s tornadoes will be centered in a rhombus with Oklahoma City, Little Rock, Saint Louis and Kansas City at its vertices.

Thursday’s worst weather will occur in a wide swath from Texas to Michigan, including Dallas and Chicago.

Though these places are at the epicenter of the severe weather because of the passing of the jet stream wave, the area of potential instability covers most of the eastern two-thirds of the nation, and thunderstorms with dangerous lightning, hail, and flooding rains — even a tornado — could occur outside the area of maximum conditional instability.

Vertical wind shear in the form of change of speed with height can lead to tornado formation. Graphic courtesy of NOAA.

Vertical wind shear in the form of change of speed with height can lead to tornado formation. Graphic courtesy of NOAA.

Mesoscale Regions Of Instability

Tornado outbreaks tend to cover areas intermediate between the sizes of a single tornado and an extra-tropical cyclone. Such a mesoscale feature could develop with Jon, especially in the area of the passing jet stream dip.

What Kind Of Severe Weather Can We Expect?

Once instability starts on a mesoscale, the weather becomes violent very rapidly.

Directional vertical wind shear can lead to tornado formation. Graphic courtesy of NOAA.

Directional vertical wind shear can lead to tornado formation. Graphic courtesy of NOAA.

As the lapse rate crosses the threshold of instability, thunderstorms develop rapidly, and if there is rotation to the individual storms, tornadoes form. Tornado formation depends critically on the vertical wind shear — the turning and change of wind speed with height.

Modern forecasting methods do not allow predicting a specific tornado occurrence beyond being able to provide a few minutes’ warning, so anyone in a tornado watch area should be prepared to take cover on short notice.

The Most Dangerous Time

As a cold front lifts a column of air, the greatest instability occurs during the hours of maximum daytime heating — mid to late afternoon.

The cloud pattern has a lot of influence on the exact location of tornado formation, and this feedback loop can sometimes stifle the instability.

Mesoscale areas of instability can persist once they form, so tornadoes are common after sundown. When visual clues to tornado formation and motion are lost, the danger increases.

The jet stream flow for a week from today looks very much like today's. More strong thunderstorms and tornadoes are likely. Forecast courtesy of NOAA.

The jet stream flow for a week from today looks very much like today’s. More strong thunderstorms and tornadoes are likely. Forecast courtesy of NOAA.

A New Weather Pattern

After a winter of large undulations in the jet stream flow, the upper air pattern has changed to a more zonal (west-to-east) circulation, with sharp dips moving within the general flow.

This pattern, possibly a response to El Niño Eggplant, is forecast to persist for at least a couple of weeks, so more outbreaks of severe weather are likely in the US heartland as the calendar advances towards the annual tornado maximum in late May.

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