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Storm Spotter's Guide

An illustrated guide to identifying severe storms

Introduction

The following is an illustrated guide to what types of features are often seen before and during severe weather events. Since supercell thunderstorms produce the majority of severe weather and are common in this part of the country, this page focuses on this phenomenon.

Caution: Storm spotting is an activity performed only by trained individuals! This page is not meant to be a substitute for official storm spotter training. Contact your local NWS office for details on spotter training in your area. The NWS does not sponsor spotters or storm chasers, but works with them to obtain and provide up to the minute information on severe weather.

Supercells

Photo courtesy of NOAA (photo by Gene D. Rhoden)

Thunderstorms are common in this part of the world, and most area residents are quite familiar with them and the severe weather they can bring. Most individual thunderstorms only last several minutes, however some individual thunderstorms can last several hours. These long-lived thunderstorms are called supercell thunderstorms. Supercell thunderstorms are solely responsible for producing the majority of severe weather, such as large hail and tornadoes, however, strong squall lines can also produce widespread severe weather, primarily very strong winds and/or microbursts.

Charles Doswell and others at the National Severe Storms Laboratory (NSSL), have published many references on thunderstorms, in which the concept of the supercell thunderstorm is described in more detail. A supercell thunderstorm, in brief, is a thunderstorm with a persistent rotating updraft. This rotation maintains the energy release of the thunderstorm over a much longer time than typical, pulse-type thunderstorms which occur in the summer months in the U.S (Lilly, 1986). A 3-D visualization of a simulated supercell thunderstorm is shown below. Notice the rotating streamlines around the orange, carrot-shaped object which indicates the location of the rotating updraft.

(Click on the image for a clearer view)

A diagram of a "classic" supercell is shown below.

Notice the wall cloud indicated above, which is often associated with supercell thunderstorms, and can possibly lead to the formation of a tornado.

The following section lists terms and definitions associated with supercell thunderstorms.

The terms and definitions given are from NOAA Technical Memorandum NWS SR-145, A Comprehensive Glossary of Weather Terms for Storm Spotters.

Photo courtesy of Alan Moller

Rain-free base - A dark, horizontal cloud base with no visible precipitation beneath it. It typically marks the location of the thunderstorm updraft. Tornadoes may develop from wall clouds attached to the rain-free base, or from the rain-free base itself - especially when the rain-free base is on the south or southwest side of the main precipitation area. Note that the rain-free base may not actually be rain free; hail or large rain drops may be falling. For this reason, updraft base is more accurate.

Photos courtesy of Steve Tegtmeier

Wall cloud - A wall cloud, according to the storm spotters glossary, is a local, often abrupt lowering from a rain-free base. Wall clouds can range from a fraction of a mile up to nearly five miles in diameter, and normally are found on the south or southwest (inflow) side of the thunderstorm. When seen from within several miles, many wall clouds exhibit rapid upward motion and cyclonic rotation. Rotating wall clouds usually develop before strong or violent tornadoes, by anywhere from a few minutes up to nearly an hour. Wall clouds should be monitored visually for signs of persistent, sustained rotation.

Tornado - A violently rotating column of air in contact with the ground. 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. A description of tornado intensity classification is shown below.

LP storm (or LP supercell) - Low-Precipitation storm (or Low-Precipitation supercell). A supercell thunderstorm characterized by a relative lack of visible precipitation. Visually similar to a classic supercell, except without the heavy precipitation core. LP storms often exhibit a striking visual appearance; the main tower often is bell-shaped, with a corkscrew appearance suggesting rotation. They are capable of producing tornadoes and very large hail. Radar identification often is difficult, so visual reports are very important. LP storms almost always occur on or near the dry line, and therefore are sometimes referred to as dry line storms.

Photo courtesy of NOAA (photographer unknown)

HP storm or HP supercell - High-Precipitation storm (or High-Precipitation supercell). A supercell thunderstorm in which heavy precipitation (often including hail) falls on the trailing side of the mesocyclone. Precipitation often totally envelops the region of rotation, making visual identification of any embedded tornadoes very difficult and very dangerous. Unlike most classic supercells, the region of rotation in many HP storms develops in the front-flank region of the storm (i.e. usually in the eastern portion). HP storms often produce extreme and prolonged downburst events, serious flash flooding, and very large damaging hail events. Mobile storm spotters are strongly advised to maintain a safe distance from any storm that has been identified as an HP storm; close observations (e.g., core punching) can be extremely dangerous.

Tornado Intensity

Tornadoes vary in size and destructive potential. The winds of a tornado have only recently been estimated with specialized radars (FM-CW), however up until then, the Fujita Scale or F-Scale has been used to quantify tornadic winds according to the extent of damage to buildings and structures:

Photos courtesy of Tim Marshall, George Kuikendall, and NSSL, respectively

            weak                    strong                     violent 

F-Scale         Winds        Character of Damage  Relative Freq.
----------------------------------------------------------------------
F0 (weak)      40-72  mph    light damage   29%
F1 (weak)      73-112 mph    moderate damage    40%
F2 (strong)   113-157 mph    considerable damage  24%
F3 (strong)   158-206 mph    severe damage    6%
F4 (violent)  207-260 mph    devastating damage   2%
F5 (violent)  261-318 mph    (rare) incredible damage < 1%

Photo courtesy of the Institute of Disaster Research

Notice the relative frequency of each category tornado, less than 3% of all tornadoes become violent, with winds that can level brick homes. The image below shows what a violent tornado can do to a car, dramatically illustrating why so many who remain in their vehicles when a tornado approaches ... lose their lives.

The following section lists other hazards and their definitions associated with thunderstorms

Photo courtesy of NOAA (photographer unknown)
Downburst - A strong downdraft resulting in an outward burst of damaging winds on or near the ground. Downburst winds can produce damage similar to a strong tornado. Although usually associated with thunderstorms, downbursts can occur with showers too weak to produce thunder.

Photo courtesy of NOAA (photographer unknown)
Flash Floods - Heavy thunderstorms which move very slowly or thunderstorms which move quickly but redevelop over the same area can produce flash flooding. If you are storm spotting and encounter water running over a roadway, report it immediately. DO NOT TRY AND DRIVE THROUGH THE WATER! Roadways underneath may not always be intact as the image below shows.