Thunderstorms & Tornadoes

Thunderstorms & Tornadoes

Ordinary Thunderstorms • Three stages have been identified in ordinary thunderstorms: • an unstable atmosphere and vertical updrafts keep precipitation suspended • entrainment of dry air that causes cooler air from evaporation, triggering downdrafts and falling precipitation and gust fronts • weakening updrafts and loss of the fuel source after 15 to 30 minutes.

Mature Stage Thunderstorm During the mature stage, updrafts may stop at the troposphere where the cloud ice crystals are pushed horizontally by winds and form an anvil top, or they may overshoot further into the tropopause.

Dissipating Stage of Thunderstorm Once downdrafts dominate updrafts, the storm ends as precipitation leaves the cloud faster than it is replenished by rising, condensing air. Often, lower level cloud particles will evaporate leaving an isolate cirrus anvil top section.

Multicell Storms Cool downdrafts leaving a mature and dissipating storm may offer relief from summer heat, but they may also force surrounding, low-level moist air upward. Hence, dying storms often trigger new storms, and the successive stages may be viewed in the sky.

Severe Thunderstorms • Storms producing a minimum of • 3/4 inch hail and/or • wind gusts of 50 knots and/or • tornado winds, classify as severe. • In ordinary storms, the downdraft and falling precipitation cut off the updraft. • In severe storms, winds aloft push the rain ahead and the updraft is not weakened and the storm can continue maturing. • The single supercell storm shown here maintained its structure for hours.

Gust Front & Microburst Turbulent air forms along the leading edge of the gust front, which can generate tumbling dust clouds. Such gust fronts and associated cold dense air often feel like a passing cold front, and may cause a 1 to 3 mb local rise in pressure, called a mesohigh.

Gust Front Shelf Cloud When unstable air is prevalent near the base of the thunderstorm, the warm rising air along the forward edge of the gust front is likely to generate a shelf, or arcus, cloud.

Microbursts from Dense Air Dry air entrained into the thunderstorm will evaporate and cool the falling mix of precipitation and air, which may create dry, and in humid areas wet, microbursts of strong winds.

Pre-Frontal Squall Line Storms Pre-frontal squall lines identify major storms triggered by a cold front that may contain several severe thunderstorms, some possibly supercells, extending for more than 1000 kilometers. This 1989 storm spawned 25 tornadoes, the worst killing 25 people.

Trailing Stratified Clouds An extensive region of stratified clouds may follow behind a squall line. This figure shows a loop of rising and falling air that supplies the moisture to the stratiform clouds and associated light precipitation.

Mesoscale Convective Complex An organized mass, or collection, of thunderstorms that extends across a large region is called a mesoscale convective complex (MCC). With weak upper level winds, such MCC's can regenerate new storms and last for upwards of 12 hours and may bring hail, tornadoes, and flash floods. They often form beneath a ridge of high pressure.

Dryline Thunderstorms Abrupt geographic changes from moist to dry dew-point temperature, called drylines, form in western TX, OK, and KS in the spring and summer. The diagram illustrates how cool cP air pushes hot and dry cT air, at the height of the central plains, over the warm moist mT air. Such mixing causes large scale instabilities and the birth of many supercell storms.

Thunderstorm Movement Figure 15.17 Middle troposphere winds often direct individual cells of a thunderstorm movement, but due to dying storm downdrafts spawning new storms, the storm system tends to be right-moving relative to the upper level winds. In this figure, upper level winds move storms to the northeast, but downdrafts generate new cells to the south, which eventually cuts off moisture to the old cell.

Lightning & Thunder Charge differences between the thunderstorm and ground can cause lightning strokes of 30,000°C, and this rapid heating of air will creates an explosive shock wave called thunder, which requires approximately 3 seconds to travel 1 kilometer.

Lightning Stroke Development Charge layers in the cloud are formed by the transfer of positive ions from warmer hailstones to colder ice crystals. When the negative charge near the bottom of the cloud is large enough to overcome the air's resistance, a stepped leader forms. A region of positive ions move from the ground toward this charge, which then forms a return stroke into the cloud.

Types of Lightning Nearly 90% of lightning is the negative cloud-to-ground type, but positive cloud-to-ground lightning can generate more current and more damage. Several names, such as forked, bead, ball, and sheet lightning describe forms of the flash. Distant, unseen lightning is often called heat lightning.

Lightning Rods & Fulgurite Metal rods that are grounded by wires provide a low resistance path for lightning into the earth, which is a poor conductor. The fusion of sand particles into root like tubes, called fulgurite, may result.

Tornado A rapidly rotating column of air often evolve through a series of stages, from dust-whirl, to organizing and mature stages, and ending with the shrinking and decay stages. Winds in this southern Illinois twister exceeded 150 knots.

Tornado Occurrence Tornadoes from all 50 states of the U.S. add up to more than 1000 tornadoes annually, but the highest frequency is observed in tornado alley of the Central Plains. Nearly 75% of tornadoes form from March to July, and are more likely when warm humid air is overlain by cooler dryer air to cause strong vertical lift.

Tornado Wind Speed As the tornado moves along a path, the circular tornado winds blowing opposite the path of movement will have less speed. For example, if the storm rotational speed is 100 knots, and its path is 50 knots, it will have a maximum wind of 150 knots on its forward rotation side.

Suction Vortices & Damage A system of tornadoes with smaller whirls, or suction vortices, contained within the tornado is called a multi-vortex tornado. Damage from tornadoes may include its low pressure centers causing buildings to explode out and the lifting of structures. Human protection may be greatest in internal and basement rooms of a house.

Fujita Tornado Scale Tornado watches are issued when tornadoes are likely, while a warning is issued when a tornado has been spotted. Once the storm is observed, or has passed, the Fujita scale of F0 – F5 is used to classify tornadoes according to their rotational speed based on damage done by the storm.

Atmospheric Conditions A specific pattern of events often coincide during the formation of tornadoes and severe thunderstorms. This may include when an open-wave mid-latitude cyclone mixes together cold dry air with warm moist air at the surface, and 850 mb warm moist and 700 mb cold dry air aloft flow north and north east, as shown in this figure. Further, at the 500 mb level a trough of low pressure pressure forms to the west of the surface low, and the 300 mb polar jet swings over the region. Figure 15.34

Thunderstorm Sounding Temperature and dew point have typical vertical profile in the warm sector before a tornado occurs, including the shallow inversion at 800 mb that acts like a cap on the moist air below. The cold dry air above warm humid air produces convective instability and lifting.

Vorticity from Horizontal to Vertical Spinning horizontal vortex tubes created by surface wind shear may be tilted and forced in a vertical path by updrafts. This rising, spinning, and often stretching rotating air may then turn into a tornado.

Tornado Breeding Supercell Storm Supercell thunderstorms may have many of the features illustrated here, including a mesocyclone of rotating winds formed when horizontal vorticity was tilted upwards.

Rear Flank Downdraft Supercell thunderstorm development may create an area where the updraft and counterclockwise swirl of upper winds converge into a rear flank downdraft. This downdraft can then interact with lower level inflow winds and spawn a tornado.

Rotating Clouds The first sign that a supercell may form a tornado is the sight of rotating clouds at the base of the storm, which may lower and form a wall cloud, shown in this picture.

NonSupercell Tornadoes If a pre-existing wall cloud was not present, than any tornado formed is not from a supercell storm, and is often called a funnel cloud, or may be a gustnado if the form along a gust front.

Landspouts, which form over land but look like waterspouts. They form when surface winds converge along a boundary where opposite blowing wind creates a horizontal rotational spin. If a storm passes above, its updraft may lift and stretch the horizontal spinning air, causing it to narrow and increase in rotational speed. Landspouts

Doppler Radar Analysis A single Doppler radar unit can uncover many features of thunderstorm rotation and movement, but cannot detect winds parallel to the antenna. As such, data from two or more units might be combined to provide a complete view of the storm. Dopplar lidar (light beam rather than microwave beam) provides more details on the storm features, and will help measure wind speeds in smaller tornadoes.

NEXRAD Wind Analysis NEXt Generation Weather RADar (NEXRAD) is operated by the National Weather Service and uses Doppler measurement to detect winds moving toward (green) and away (blue) from the antenna, which indicates areas of rotation and strong shear.

Waterspout Funnel Warm, shallow coastal water is often home to waterspouts, which are much smaller than an average tornado, but similar in shape and appearance. The waterspout does not draw water into its core, but is a condensed cloud of vapor. A waterspout may, however, lift swirling spray from the water as it touches the water surface.

Thunderstorms & Tornadoes