Module 5: Thunderstorms, Tornadoes, and Hurricanes

Severe Weather Statement (SVS)

A National Weather Service product which provides follow up information on severe weather conditions (severe thunderstorm or tornadoes) which have occurred or are currently occurring.

Tropical Cyclone Plan of the Day

A coordinated mission plan that tasks operational weather reconnaissance requirements, describes reconnaissance flights committed to satisfy both operational and research requirements, and identifies possible reconnaissance requirements for the succeeding 24-hour period. (NWS)

Tropical Disturbance

A discrete tropical weather system of apparently organized convection--generally 100 to 300 nautical miles in diameter---originating in the tropics or subtropics, having a nonfrontal migratory character, and maintaining its identity for 24 hours or more. It may or may not be associated with a detectable perturbation of the wind field. In successive stages of intensification, it may be subsequently classified as a tropical wave, tropical depression, tropical storm, or hurricane. (NWS)

Cold Core Funnel

A funnel cloud or (rarely) a small, relatively weak tornado that can develop from a small shower or thunderstorm when the air aloft is unusually cold (hence the name). They are much less violent than other types of tornadoes.

Lightning

A sudden visible flash of energy and light caused by an electrical discharge from thunderstorms.

Extratropical

A term used in advisories and tropical summaries to indicate that a tropical cyclone has lost its "tropical" characteristics. The term implies both poleward displacement of the cyclone and the conversion of the cyclone's primary energy source from the release of latent heat of condensation to baroclinic (the temperature contrast between warm and cold air masses) processes. It is important to note that cyclones can become extratropical and still retain winds of hurricane or tropical storm force. (NWS)

Lightning Stroke

Any of a series of repeated electrical discharges comprising a single lightning discharge (strike). Specifically, in the case of a cloud-to-ground discharge, a leader plus its subsequent return streamer.

Tropics

Areas of the Earth within 20°North/South of the Equator. (NWS)

Tropical Depression

Cyclones that have maximum sustained winds of surface wind speed (using the U.S. 1-minute average) is 33 kt (38 mph / 62 km/h) or less. They are either located in the tropics or subtropics. They characteristically have one or more closed isobars. They usually intensify slowly and may dissipate before reaching Tropical Storm intensity. (NWS)

Tropical Cyclone

It is a warm-core low pressure system which is non-frontal. It originates over tropical and subtropical waters and a has a organized cyclonic (counter-clockwise) surface wind circulation. (NWS)

Tropical Storm

It is a warm-core tropical cyclone that has maximum sustained surface wind speed (using the U.S. 1-minute average) ranges from 34 kt (39 mph / 63 km/h) to 63 kt (73 mph / 118 km/h). (NWS)

Eye Wall

It is an organized band of cumuliform clouds that immediately surrounds the center (eye) of a hurricane. The fiercest winds and most intense rainfall typically occur near the eye wall. VIP levels 3 or greater are typical. Eye wall and wall cloud are used synonymously, but it should not be confused with a wall cloud of thunderstorm. (NWS)

Cloud-Ground Lightning (CG)

Lightning occurring between cloud and ground.

Storm Relative

Measured relative to a moving thunderstorm, usually referring to winds, wind shear, or helicity.

Gustnado

Slang for a gust front tornado. A small tornado, usually weak and short-lived, that occurs along the gust front of a thunderstorm. Often it is visible only as a debris cloud or dust whirl near the ground. Gustnadoes are not associated with storm-scale rotation (i.e. mesocyclones); they are more likely to be associated visually with a shelf cloud than with a wall cloud.

Cloud-Cloud Lightning (CC)

Streaks of lightning reaching from one cloud to another.

Cloud-Air Lightning (CA)

Streaks of lightning which pass from a cloud to the air, but do not strike the ground.

Eastern North Pacific Basin

The region north of the Equator east of 140°W. The National Hurricane Center in Miami, FL is responsible for tracking tropical cyclones in this region. (NWS)

Lightning Discharge

The series of electrical processes by which charge is transferred along a channel of high ion density between electrical charge centers of opposite sign. This can be between a cloud and the Earth's surface of a cloud-to-ground discharge.

Thunder

The sound emitted by the rapidly expanding gases along the channel of a lightning discharge. Thunder is seldom heard farther than about 15 miles (24 kilometers) from the lightning discharge, with 25 miles (40 kilometers) an approximate upper limit and 10 miles (16 kilometers) a typical value.

Stepped Leader

A leader which initiates the very first stroke and establishes the channel for all subsequent streamers of a lightning discharge.

Thunderstorm (TS)

A local storm produced by cumulonimbus clouds. It is always accompanied by lightning and thunder. It is estimated that nearly 2,000 thunderstorms occur simultaneously around the Earth at any given instant. There are 3 types of thunderstorms. They are the following: Single Cell Thunderstorms, Multicell Thunderstorms, and Severe Supercell Thunderstorm.

Storm Prediction Center (SPC)

A national forecast center in Norman, Oklahoma, which is part of NCEP. The SPC is responsible for providing short-term forecast guidance for severe convection, excessive rainfall (flash flooding), and severe winter weather over the contiguous United States.

Hook or Hook Echo

A pendant or hook on the right rear of a radar echo that often identifies mesocyclones on the radar display. The hook is caused by precipitation drawn into a cyclonic spiral by the winds, and the associated notch in the echo is caused by precipitation-free, warm, moist air flowing into the storm. A hook often is associated with a mesocyclone, and indicates favorable conditions for tornado development.

Hurricane Local Statement (HLS)

A public release prepared by local National Weather Service offices in or near a threatened area giving specific details for its county/parish warning area on (1) weather conditions, (2) evacuation decisions made by local officials, and (3) other precautions necessary to protect life and property. (NWS)

Ball Lightning

A relatively rare form of lightning consisting of a luminous ball, often reddish in color, which moves rapidly along solid objects or remains floating in mid-air. Also known as globe lightning.

Tropical Cyclone Report

A report summarizing the life history and effects of an Atlantic or eastern Pacific tropical cyclone. It contains a summary of the cyclone life cycle and pertinent meteorological data, including the post-analysis best track (six-hourly positions and intensities) and other meteorological statistics. It also contains a description of damage and casualties the system produced, as well as information on forecasts and warnings associated with the cyclone. NHC writes a report on every tropical cyclone in its area of responsibility. (NWS)

Storm Surge

A rise above the normal water level along a shore caused by strong onshore winds and/or reduced atmospheric pressure. The surge height is the difference of the observed water level minus the predicted tide. Most hurricane deaths are caused by the storm surge. It can be 50 or more miles (80 kilometers or more) wide and sweeps across the coastline around where the hurricane makes landfall.

Tornado Family

A series of tornadoes produced by a single supercell, resulting in damage path segments along the same general line.

Severe Storm

A storm with a tornado, hail 1" (25 mm) or greater, or wind gusts 50 knots (58 mph / 93 km/h) or greater, or all three.

Severe Thunderstorm

A thunderstorm that produces either of the following: winds of 50 kt (58 mph / 93 km/h) or greater (these speeds can result in structural or tree damage), hail 1" (25 mm) in diameter or larger, or a tornado. Lightning frequency is not a warning criterion for issuing a severe thunderstorm warning. Severe thunderstorms can result in the loss of life and property. They can also produce a tornado with little or no advanced warning. A table of hail sizes can be found in this glossary, under the definition of hail. See approaching (severe).

Pulse Storm

A thunderstorm within which a brief period (pulse) of strong updraft occurs, during and immediately after which the storm produces a short episode of severe weather. These storms generally are not tornado producers, but often produce large hail and/or damaging winds. See overshooting top and cyclic storm.

Multiple-vortex (or Multi-vortex) Tornado

A tornado in which two or more condensation funnels or debris clouds are present at the same time, often rotating about a common center or about each other. Multiple-vortex tornadoes can be especially damaging. See suction vortex.

Tropical Wave

A trough or cyclonic curvature maximum in the trade wind easterlies and it is not classified as a tropical cyclone. The wave may reach maximum amplitude in the lower middle troposphere. (NWS)

Tornado (+FC)

A violently rotating column of air in contact with the ground and extending from the base of a thunderstorm. A condensation funnel does not need to reach to the ground for a tornado to be present; a debris cloud beneath a thunderstorm is all that is needed to confirm the presence of a tornado, even in the total absence of a condensation funnel. It nearly always starts as a funnel cloud and may be accompanied by a loud roaring noise. Tornadoes are classified by the amount of damage that they cause. See Fujita Scale.

Hurricane

A warm-core tropical cyclone in which the maximum sustained surface wind (using the U.S. 1-minute average) is 64 kt (74 mph or 119 km/h) or more. The term hurricane is used for Northern Hemisphere cyclones east of the International Dateline to the Greenwich Meridian. It has a diameter of 250 to 500 miles (400 to 800 kilometers) and a cyclonic circulation typically extending to near 50,000 feet (15,000 meters). It is called a Typhoon in the western Pacific north of the Equator and west of the International Dateline and Indian Ocean. See Saffir-Simpson Hurricane Wind Scale. (NWS)

Easterly Wave

A wavelike disturbance in the tropical easterly winds that usually moves from east to west. Such waves can grow into tropical depressions. (NWS)

Hurricane Warning

An announcement that sustained winds of 64 knots (74 mph or 119 km/hr) or higher are expected somewhere within the specified area in association with a tropical, subtropical, or post-tropical cyclone. Because hurricane preparedness activities become difficult once winds reach tropical storm force, the warning is issued 36 hours in advance of the anticipated onset of tropical-storm-force winds. The warning can remain in effect when dangerously high water or a combination of dangerously high water and waves continue, even though winds may be less than hurricane force. (NWS)

Hurricane Watch

An announcement that sustained winds of 64 knots (74 mph or 119 km/hr) or higher are possible within the specified area in association with a tropical, subtropical, or post-tropical cyclone. Because hurricane preparedness activities become difficult once winds reach tropical storm force, the hurricane watch is issued 48 hours in advance of the anticipated onset of tropical storm force winds. (NWS)

Tropical Storm Warning

An announcement that tropical storm conditions (sustained winds of 39-73 mph/63-118 km/h) are expected somewhere within the specified coastal area within 36 hours. (NWS)

Tropical Storm Watch

An announcement that tropical storm conditions (sustained winds of 39-73 mph/63-118 km/h) are possible within the specified coastal area within 48 hours. (NWS)

Isolated Storm

An individual cell or a group of cells that are identifiable and separate from other cells in a geographic area.

Storm

Any disturbed state of the atmosphere, especially affecting the Earth's surface, and strongly implying destructive and otherwise unpleasant weather. Storms range in scale from tornadoes and thunderstorms through tropical cyclones to widespread extratropical cyclones.

Severe Supercell Thunderstorm

It is potentially the most dangerous of the convective storm types. Storms possessing this structure have been observed to generate the vast majority of long-lived strong and violent (EF2-EF5) tornadoes, as well as downburst damage and large hail. It is defined as a thunderstorm consisting of one quasi-steady to rotating updraft which may exist for several hours. Supercells usually move to the right of the mean wind. These are called "right movers" and they are favored with veering winds. Occasionally, these thunderstorms will move to the left of the mean wind. These thunderstorms are called "left movers". These supercells typically don't last as long as their "right mover" cousins and they usually only produce large hail (1" in diameter or greater) and severe wind gusts in the excess of 50 knots (58 mph / 93 km/h). Left Movers are favored when you have backing winds. Radar will observe essentially one long-lived cell, but small perturbations to the cell structure may be evident. The stronger the updraft, the better the chance that the supercell will produce severe (hail greater than 1" in diameter, wind gusts greater than 58 mph (93 km/h), and possibly a tornado) weather. Severe supercell development is most likely in an environment possessing great buoyancy (CAPE) and large vertical wind shear. A Bulk Richardson Number of between 15 and 35 favor supercell development. Typically, the hodograph will look like a horse shoe. This is due to the wind speed increasing rapidly with height and the wind direction either veering or backing rapidly with height.

Heat Lightning

Lightning that occurs at a distance such that thunder is no longer audible.

In-Cloud Lightning (IC)

Lightning that takes place within the cloud.

Tropical Advisories

Official information issued by the Tropical Prediction Centers describing all tropical cyclone watches and warnings in effect along with details concerning tropical cyclone locations, intensity and movement, and precautions that should be taken. Advisories are issued to describe tropical cyclones prior to issuance of watches and warnings, and subtropical cyclones. (NWS)

Tropical Analysis and Forecast Branch (TAFB)

One of three branches of the Tropical Prediction Center (TPC). TAFB generates analyses and forecasts over the tropical and subtropical eastern North and South Pacific and the North Atlantic basins year-round. TAFB products include but are not limited to: High Seas, Offshore Waters, Marine Weather Discussion, Tropical Weather Discussion, and graphical surface and ocean wave analyses and forecasts. This branch also produces satellite-based rainfall estimates for the international community. (NWS)

Tropical Weather Summary (TWS)

The National Hurricane Center issues a monthly summary of tropical weather is included at the end of the month or as soon as feasible thereafter, to describe briefly the past activity or lack thereof and the reasons why. (NWS)

Tropical Cyclone Position Estimate (TCE)

The National Hurricane Center issues a position estimate between scheduled advisories whenever the storm center is within 200 nautical miles of U.S. land-based weather radar and if sufficient and regular radar reports are available to the hurricane center. As far as is possible, the position estimate is issued hourly near the beginning of the hour. The location of the eye or storm center is given in map coordinates and distance and direction from a well-known point. (NWS)

Saffir-Simpson Hurricane Wind Scale

The Saffir-Simpson Hurricane Wind Scale is a 1 to 5 categorization based on the hurricane's intensity at the indicated time. The scale provides examples of the type of damage and impacts in the United States associated with winds of the indicated intensity. The scale does not address the potential for other hurricane-related impacts, such as storm surge, rainfall-induced floods, and tornadoes. It should also be noted that these wind-caused damage general descriptions are to some degree dependent upon the local building codes in effect and how well and how long they have been enforced. (NWS)

Tornado Alley

The area of the United States in which tornadoes are most frequent. It encompasses the great lowland areas of the Mississippi, the Ohio, and lower Missouri River Valleys. Although no state is entirely free of tornadoes, they are most frequent in the Plains area between the Rocky Mountains and Appalachians.

Lilapsophobia

The fear of tornadoes and hurricanes.

Lightning Channel

The irregular path through the air along which a lightning discharge occurs. A typical discharge of flash between the ground and the cloud is actually a composite flash which is composed of several sequential lightning strokes, each of which is initiated by a leader and terminated by a return streamer.

Lightning Ground Flash Density

The number of cloud-to-ground flashes per unit time per unit area.

Flash Multiplicity

The number of return strokes in a lightning flash.

F scale

The old Fujita tornado intensity scale. It was replaced by the Enhanced Fujita Scale (EF).

Fujita Scale (F-scale)

The old scale used to classify the strength of a tornado. It was devised by Dr. Theodore Fujita from the University of Chicago. The F-scale gave tornadoes a numerical rating from F0 to F5. It was replaced by the Enhanced Fujita Scale (EF).

Hurricane Season

The portion of the year having a relatively high incidence of hurricanes. The hurricane season in the Atlantic, Caribbean, and Gulf of Mexico runs from June 1 to November 30. The hurricane season in the Eastern Pacific basin runs from May 15 to November 30. The hurricane season in the Central Pacific basin runs from June 1 to November 30. (NWS)

Eye

The relatively calm center in a hurricane that is more than one half surrounded by wall cloud. The winds are light, the skies are partly cloudy or even clear (the skies are usually free of rain) and radar depicts it as an echo-free area within the eye wall. The hurricane eye typically forms when the maximum sustained tangential wind speeds exceeds about 78 miles an hour. The eye diameter, as depicted by radar, ranges typically from as small as 5 to 10 miles (8 to 16 kilometers) upwards to about 100 miles (160 kilometers). The average hurricane eye diameter is a little over 20 miles (32 kilometers). When the eye is shrinking in size, the hurricane is intensifying. (NWS)

Storm Motion

The speed and direction at which a thunderstorm travels.

Leader

The streamer which initiates the first phase of each stroke of a lightning discharge. The first stroke is led by a steeped leader, which may be preceded by a pilot streamer. All subsequent strokes begin with a dart leader.

Lightning Flash

The total luminous phenomenon accompanying a lightning discharge. It may be composed of one to a few tens of strokes that use essentially the same channel to ground.

Pulse Severe Thunderstorms

These are single cell thunderstorms which produce brief periods of severe weather (1" hail, wind gusts in excess of 58 miles an hour, or a tornado).

Tropical Weather Discussion (TWD)

These messages are issued four times daily by the Tropical Analysis and Forecast Branch (TAFB) to describe significant synoptic weather features in the tropics. One message will cover the Gulf of Mexico, the Caribbean, and the Atlantic between the equator and 32 degrees North and east of 140 degrees West. Plain language is used in these discussions. (NWS)

Storm Data (SD)

This National Climatic Data Center (NCDC) monthly publication documents a chronological listing, by states, of occurrences of storms and unusual weather phenomena. Reports contain information on storm paths, deaths, injuries, and property damage. The December issue includes annual tornado, lightning, flash flood, and tropical cyclone summaries.

Tropical Cyclone Update (TCU)

This brief statement is issued by the National Hurricane Center in lieu of or preceding special advisories to inform of significant changes in a tropical cyclone or the posting or cancellation of watches and warnings. (NWS)

National Hurricane Center (NHC)

This center maintains a continuous watch on tropical cyclones over the Atlantic, Caribbean, Gulf of Mexico, and the Eastern Pacific from 15 May through November 30. The Center prepares and distributes hurricane watches and warnings for the general public and also prepares and distributes marine and military advisories for other users. During the "off-season" NHC provides training for U.S. emergency managers and representatives from many other countries that are affected by tropical cyclones. NHC also conducts applied research to evaluate and improve hurricane forecasting techniques and is involved in public awareness programs. (NWS)

Severe Thunderstorm Watch (WWA)

This is issued by the National Weather Service when conditions are favorable for the development of severe thunderstorms in and close to the watch area. A severe thunderstorm by definition is a thunderstorm that produces 1" (25 mm) hail or larger in diameter and/or winds equal or exceed 50 kts (58 mph / 93 km/h). The size of the watch can vary depending on the weather situation. They are usually issued for a duration of four to eight hours. They are normally issued well in advance of the actual occurrence of severe weather. During the watch, people should review severe thunderstorm safety rules and be prepared to move a place of safety if threatening weather approaches. A Severe Thunderstorm Watch is issued by the Storm Prediction Center in Norman, Oklahoma. Prior to the issuance of a Severe Thunderstorm Watch, SPC will usually contact the affected local National Weather Service Forecast Office and they will discuss what their current thinking is on the weather situation. Afterwards, SPC will issue a preliminary Severe Thunderstorm Watch and then the affected NWFO will then adjust the watch (adding or eliminating counties/parishes) and then issue it to the public by way of a Watch Redefining Statement. During the watch, the local forecast office will keep the you informed on what is happening in the watch area and also let the you know when the watch has expired or been canceled.

Tornado Watch (SEL)

This is issued by the National Weather Service when conditions are favorable for the development of tornadoes in and close to the watch area. The watch is in a form of a rectangle whose size and orientation varies depending on the weather situation. They are usually issued for a duration of four to eight hours. They normally are issued well in advance of the actual occurrence of severe weather. During the watch, people should review tornado safety rules and be prepared to move a place of safety if threatening weather approaches.

Tornado Warning (TOR)

This is issued when a tornado is indicated by the WSR-88D radar or sighted by spotters; therefore, people in the affected area should seek safe shelter immediately. They can be issued without a Tornado Watch being already in effect. They are usually issued for a duration of around 30 minutes. A Tornado Warning is issued by your local National Weather Service office. It will include where the tornado was located and what towns will be in its path. If the tornado will affect the nearshore or coastal waters, it will be issued as the combined product--Tornado Warning and Special Marine Warning. If the thunderstorm which is causing the tornado is also producing torrential rains, this warning may also be combined with a Flash Flood Warning. After it has been issued, the affected NWFO will followed it up periodically with Severe Weather Statements. These statements will contain updated information on the tornado and they will also let the public know when warning is no longer in effect.

Severe Thunderstorm Warning (SVR)

This is issued when either a severe thunderstorm is indicated by the WSR-88D radar or a spotter reports a thunderstorm producing hail 1" (25 mm) or larger in diameter and/or winds equal or exceed 50 kt (58 mph / 93 km/h); therefore, people in the affected area should seek safe shelter immediately. Severe thunderstorms can produce tornadoes with little or no advance warning. Lightning frequency is not a criterion for issuing a severe thunderstorm warning. They are usually issued for a duration of one hour. They can be issued without a Severe Thunderstorm Watch being already in effect. Like a Tornado Warning, the Severe Thunderstorm Warning is issued by your National Weather Service Forecast Office (NWFO). Severe Thunderstorm Warnings will include where the storm was located, what towns will be affected by the severe thunderstorm, and the primary threat associated with the severe thunderstorm warning. If the severe thunderstorm will affect the nearshore or coastal waters, it will be issued as the combined product--Severe Thunderstorm Warning and Special Marine Warning. If the severe thunderstorm is also causing torrential rains, this warning may also be combined with a Flash Flood Warning. After it has been issued, the issuing weather forecast office will followed up periodically with Severe Weather Statements.

Tropical Weather Outlook (TWO)

This outlook normally covers the tropical and subtropical waters, discussing the weather conditions, emphasizing any disturbed and suspicious areas which may become favorable for tropical cyclone development within the next day to two. (NWS)

Severe Weather Potential Statement (SPS or HWO)

This statement is issued designed to alert the public and state/local agencies to the potential for severe weather up to 24 hours in advance. It is issued by the local National Weather Service office.

Single Cell Thunderstorm

This type of thunderstorm develops in weak vertical wind shear environments. On a hodograph, this would appear as a closely grouped set of random dots around the center of the graph. They are characterized by a single updraft core and a single downdraft that descends into the same area as the updraft. The downdraft and its outflow boundary then cut off the thunderstorm inflow. This causes the updraft and the thunderstorm to dissipate. Single cell thunderstorms are short-lived. They only last about 1/2 hour to an hour. These thunderstorms will occasionally become severe, but only briefly. In this case, they are called Pulse Severe Thunderstorms.