Dial-an-Expert Tip Sheet: Severe Thunderstorms

BOULDER -- The 1997 storm season has gotten off to an early start, with deadly tornadoes striking Florida before the end of February and twisters raking Minnesota in March and the South in early April. Here are some tips to help you cover severe local storms this spring and summer.

Thunderstorm terminology

As forecasters sharpen their skills in predicting severe weather, they've come up with a lengthening list of names for storms. Here's a roundup of some important storm types described in the latest forecast training tools from UCAR's Cooperative Program for Operational Meteorology, Education and Training (COMET).

Mesoscale convective system (MCS): The all-encompassing term for groups of thunderstorms that act as a system. An MCS can range across an entire state and might appear as a solid line, a broken line, or a cluster of cells. It can include any of the storm types below.

Multicell: A typical, garden-variety thunderstorm in which new updrafts form along the leading edge of rain-cooled air (the gust front). Individual cells usually last 30 to 60 minutes, while the system as a whole may last for many hours. Multicell storms may produce hail, strong winds, brief tornadoes, and/or flooding.

Supercell: An especially long-lived thunderstorm with a rotating updraft that continually regenerates on the storm's flank (often on the south or right-hand side). Supercells form in regions of strong vertical wind shear. They typically produce large hail and high winds and may also produce violent, long-lived tornadoes, such as those that recently struck in Alabama and Florida. Supercells often feature a hook-like appearance on radar precipitation displays as rain wraps around a tornadic circulation.

Squall line: A linear arrangement of storms, often accompanied by squalls of high wind and heavy rain (see graphic on next page). Squall lines tend to pass quickly and are less prone to produce tornadoes than are supercells. They can be hundreds of miles long but are typically only 10 or 20 miles wide.

Bow echo: The radar signature of a squall line that "bows out" as winds descend behind the line and circulations develop on either end (see graphic on previous page). A strongly bowed echo may indicate high winds in the middle of the line, where the storms are progressing most quickly. Brief tornadoes may occur on the leading edge of a bow echo. Often the north side of a bow echo becomes dominant over time, gradually evolving into a comma-shaped storm complex.

Derecho: A violent, widespread windstorm caused by a long-lived MCS with a series of bow echoes. Derecho (Deh-RAY-cho) is a Spanish word meaning "straight ahead." Derechoes produce winds of 60 to 100 miles per hour, downing trees and power lines over paths tens of miles wide and hundreds of miles long.

Mesoscale convective complex (MCC): A particular type of MCS, an MCC is a large, circular, long-lived cluster of showers and thunderstorms identified by satellite. It often emerges out of other storm types during the late-night and early-morning hours. MCCs can cover an entire state; they played a role in the 1993 Midwest flooding.

Severe-storm experts

Morris Weisman
303-497-8901
weisman@ucar.edu
NCAR/Mesoscale and Microscale Meteorology Division
Specialty: Thunderstorm modeling and observations; mesoscale convective systems (MCSs).

Wendy Abshire
303-497-8477
abshire@ucar.edu
UCAR/Cooperative Program for Operational Meteorology, Education and Training (COMET)
Specialty: Thunderstorms, radar meteorology, convection-related training.

Greg Thompson
303-497-2805
gthompsn@ucar.edu
NCAR/Research Applications Program (RAP)
Specialty: Thunderstorm observations, stormscale modeling, storm photography. Thompson also operates the NCAR/RAP Real-Time Weather Data Web page (see below).

Pertinent sites on the World Wide Web

National Severe Storms Laboratory
Part of the National Oceanic and Atmospheric Administration (NOAA), this center in Norman, Oklahoma conducts a wide range of research on tornadoes and severe storms. Its home page includes reports on recent studies, including VORTEX (Verification of the Origins of Rotation in Tornadoes Experiment), a major program aimed at discerning the factors behind tornado formation.

Storm Prediction Center (SPC)
Also based in Norman, the SPC is the official NOAA agency responsible for issuing severe thunderstorm and tornado watches and other forecast guidance on storm development. Its Web site includes current watches and daily, monthly, and yearly statistics.

NCAR/RAP Real-Time Weather Data
This Web site includes output from a wide range of short-term computer models, including a high-resolution version of the NCAR/Pennsylvania State University mesoscale model (MM5) used for research on storm prediction.

Tornado Project Online
This small company, located in Vermont, compiles historical and climatological data on tornadoes. Its Web site includes a vast array of information on how, when, and where tornadoes develop from severe storms.

filename: bowecho.tif

Bow echoes are one of the many severe-storm phenomena named and first described by Theodore Fujita (University of Chicago). This image shows the evolution of a bow echo, including the development of a rear-inflow jet at the center and "bookend" vortices on either end. The northern vortex tends to dominate later in the life cycle, creating a comma-shaped system. (Image copyright 1997 the COMET(r) Program.)

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This computer simulation shows a line of supercells transitioning into a bow-echo pattern. Several of the cells have maintained their own surface wind-flow patterns, as shown by the bright arrows flowing into and around the storms. The shaded contours denote radar reflectivity, a sign of rainfall rate. (Image copyright 1997 the COMET(r) Program.)

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This cross section through a mature squall line shows the formation of new cells on the line's forward flank. There, air is being forced up by a rain-cooled gust front (the short dark line extending up from the lower-right corner). (Image copyright 1997 the COMET(r) Program.)