Severe Weather

Severe thunderstorms

Severe thunderstorms

Model of the major features and circulation patterns associated with a severe thunderstorm.

are defined as convective storms with frequent lighting, accompanied by local wind gusts of 97 kilometers per hour, or hail that is 2 centimeters in diameter or larger. Severe thunderstorms can also have tornadoes!

The figure on the right illustrates the features associated with a severe thunderstorm.

The green arrows represent the updrafts which are created as warm moist air is forced into the front of the storm.

At the back end of the cloud, the updrafts swing around and become downdrafts (blue arrows).

The leading edge of the downdrafts produces a gust front near the surface. As the gust front passes, the wind on the surface shifts and becomes strong with gusts exceeding 100 kilometers per hour, temperatures become cold, and the surface pressure rises.

Warm moist air that rises over the gust front may form a roll cloud. Bellow the anvil, mammatus clouds might be visible.

Roll cloud	Mammatus cloud

Updrafts and Downdrafts

Just as the ordinary thunderstorm, they form as moist air is forced to rise into a conditionally unstable atmosphere.

All thunderstorms require instability (potential) and lift. The lift is the mechanism that releases the instability. Lift is produced by such things as fronts and low pressure troughs, or by air rising upslope.

We say that the atmosphere is unstable when air rising in a cloud is warmer than its environment, like a hot-air balloon. It is the heat released by condensation within a cloud that permits the rising air to stay warmer than its surroundings, and thus to be buoyant through great depths.

In the same way, air that is cooler than its environment tends to sink as long as it can stay cooler than its surroundings.

The upward moving air in a thunderstorm is known as the updraft, while downward moving air is the downdraft. The atmosphere can be unstable for updrafts but stable for downdrafts, stable for updrafts but unstable for downdrafts, stable for both, or unstable for both. The degree of atmospheric instability is one of the two major factors in determining the strengths of thunderstorm updrafts and downdrafts.

When the low-level air is unstable but relatively dry and adequate mid-level moisture is present, a storm may develop with a weak updraft but a strong downdraft with the latter the result of strong negative buoyancy and cooling through evaporation of precipitation into the dry air. This high-based storm resembles high terrain, western U.S. storms which occasionally produce dry microbursts. Significant hail and rain are unlikely.

A storm which contains a strong updraft and weak downdraft; will not produce wind damage, but can foster heavy rains and/or damaging hail. Single and multicell storms comprise this category. They include storms that dump heavy rain, but little or no hail because of warm conditions aloft, and multicell storms that are capable of producing hail because of lower environmental freezing levels. Strong updraft, weak downdraft storms often form in very moist atmospheres where there is little, if any, dry air and evaporational cooling to drive downdrafts.

Relatively weak updrafts and downdrafts are found with non-severe showers and thunderstorms. The last possible combination is a storm with strong updrafts and downdrafts. These storms frequently produce destructive downbursts, hail, heavy rain, and tornadoes. As one would expect, the most severe storms, including supercells, have strong vertical drafts and occur in the most unstable atmospheres.

Wind Shear

But, severe thunderstorms also form in areas with strong vertical wind shear.

Vertical wind shear is the second critical factor in the determination of thunderstorm type and potential storm severity. Vertical shear, or the change of winds with height, interacts dynamically with thunderstorms to either enhance or diminish vertical draft strengths.

Thunderstorms which occur in weak vertical wind shear usually have an erect appearance. These storms don't last as long as strong storms in a sheared environment since the rainy downdraft quickly undercuts and chokes off the updraft. If any severe weather occurs with these weak-shear storms, it will be brief, occurring just prior to dissipation.

We refer to storms in sheared environments as organized convection. Organized storms are longer-lived, usually have preferred areas of new updraft development, and often allow for some predictability of periodic severe weather events.

Dangers associated with thunderstorms:

• lightning - kills an average of 93 people and injures about 300 every year in the US.
• downbursts - exceptionally strong downdraft that exits the base of the thunderstorm cloud and spreads horizontally along the ground.
• flash floods - sudden overflow of the water in a river channel or other drainage way. Takes an average of 140 lives each year.

  A particularly severe event occurred during the summer of 1976 and is comonly know as the The Big Thompson Canyon Flash Flood

  NOAA: Big Thompson Canyon Flash Flood

  FEMA: Flood Hazards

Spreading microburst in the desert.