In Minneapolis, there is no immediate cause for concern. The storm at
Worthington, although heavy, is far from life threatening for those who stay
indoors, away from windows (which can be broken by high winds or wind-
driven hail) and electrical appliances, bathtubs, shower stalls, swimming pools,
and antenna installations (which are hazardous because of lightning).
At 5:00
P.M., we hear the announcement that Minneapolis and its suburbs are
under a severe thunderstorm warning. “A line of heavy thunderstorms has been
sighted. Movement is toward the east–northeast at 26 kt (30 mi/h).” A group of
thunderstorms, called a multicell storm, is approaching. The warning is effective
until 6:30
P.M. for Hennepin County. A similar warning for Ramsey County will
soon follow. Severe thunderstorm warnings are typically issued for much
smaller areas than that of the associated watch. It is possible that severe thun-
derstorm warnings may have to be issued for localities not within the original
watch rectangle.
SEVERE OR NOT?
Thunderstorms frequently develop as isolated cells, in which case they are called
single-cell storms. Groups of two or three thunderstorms compose multicell
storms; this is the type of system now bearing down on us. The heaviest thun-
derstorms develop internal vortices. Deep within the main cloud, a clockwise, or
anticyclonic, vortex can form in the lefthand portion of the storm, and a coun-
terclockwise, or cyclonic, vortex can form in the righthand half (as the cell is
viewed from the rear). In the center of the thunderstorm, updrafts occur. These
updrafts can reach speeds of 150 kt (170 mi/h). The double-vortex structure
is illustrated in Fig. 4-5. This type of thunderstorm is called a supercell. The
cyclonic vortex is called a mesocyclone.
The internal circulation of a severe thunderstorm is revealed by means of
dual-Doppler radar. This instrument shows the locations of rain masses within
showers and thunderstorms, and also indicates the velocities (speeds and direc-
tions) of water droplets, thereby showing the internal wind circulation. With the
aid of computer graphics and animation programs, severe thunderstorms, moni-
tored by dual-Doppler radar, can be vividly portrayed in three dimensions. When
a thunderstorm has developed as a supercell structure, severe weather is likely,
and tornadoes are a particular danger.
Sometimes a supercell will split into two separate thunderstorms after it has
traveled some distance. The clockwise spinning vortex tends to turn toward the
left as the thunderstorm moves forward, and the counterclockwise vortex veers
toward the right. After the two vortices have separated sufficiently, the supercell
divides. If the process is repeated several times over a period of hours, a single
CHAPTER 4 Rain and Lightning
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