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Black Hills Area Thunderstorms
A line of thunderstorms in Butte County as seen at sunrise from the shore of Belle Fourche Reservoir on June 13, 1998. Photo taken by Randee Peterson of Black Hills Online using a Casio digital camera.
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With average precipitation on the plains surrounding the Black Hills only around
sixteen and a half inches per year, the climate is classified as semi-arid. Nonetheless, for a semi-arid
climate the region has quite a few thunderstorms and Rapid City averages around forty two a year. While most
storms pass with periods of heavy rain, some lighting, and perhaps some small hail, there is a tendency for
some of them to become severe.
For a thunderstorm to be classified as a severe storm, it must contain hailstones greater than three
quarters of an inch in diameter and/or winds in excess of 55 miles per hour. The most common component
of thunderstorms in the Black Hills region that would cause them to be classified as severe would be large
hail. Whereas tornadoes are more common farther south and east, the Black Hills lie on the northern edge
of a part of the United States known for having the highest frequency of storms with large hail annually.
As illustrated to the right, this area of high hail frequency has a peak in the southeast part of Wyoming,
near Cheyenne, where over eight storms with large hail can be expected each year at any given point.
The Black Hills can expect four to six such storms a year at any given location. Most of the hail that
falls in the Black Hills is small, but it can fall in such quantity as to whiten the landscape and give
the appearance of a freak summer snowstorm. Even small hail, however, can be devastating to crops and
gardens when it falls in sufficient quantities. Hailstones in western South Dakota can also get quite
large. On June 26, 1998 a hailstorm near Howes in northern Meade County produced hailstones four and a
half inches in diameter.
Randee's pickup truck following a close encounter with tennis ball
sized hail during a severe storm in Belle Fourche, South Dakota.
Another component of severe thunderstorms is strong straight line winds. While
tornadoes get most of the press, the reality is that strong thunderstorm straight line winds cause more
damage across the United States each year than tornadoes. One reason for this is that most tornadoes,
though violent, are small…a quarter mile in diameter or less at ground contact. The gust front, a wall of
strong straight line winds associated with a severe thunderstorm, can be several miles long and travel
great distances over an extended period of time. Powerful gust fronts with winds over 100 mph have been
known to travel over a hundred miles, blowing over trees, powerlines, trailer homes and outbuildings in a
wide area.
Cumulus mammatus clouds, like these shot by Dan Carlson
over
Piedmont, South Dakota, indicate turbulence in the parent cumulonimbus cloud.
Damage from such storms is so incredible that witnesses often insist that a tornado
must have been involved, when, in fact, no tornado was present. While tornadoes tend to form for the most
part on a storm's rear flank, the gust front often rushes out ahead of the storm. It is possible for a
storm to produce a combination of large hail, damaging winds, tornadoes and flash floods. Such extreme
tornadic "supercells" are not common in western South Dakota, but they can happen.
The gust front will often precede the area of heavy rain and hail in a severe storm. The shelf cloud
formation, like the one pictured below, is often associated with a strong gust front. This ugly, tumbling
or rolling low cloud mass can also produce gustnadoes. These are small tornado like vortices of strong
wind caused when sections of a gust front collide and create brief, but intense whirlwinds capable of
causing significant damage to weaker structures unfortunate enough to come in contact with them. These
can occur some distance away from the parent cumulonimbus cloud.
Other small strong whirlwinds called "landspouts" can form where surface winds collide under a developing
cumulus cloud containing strong updrafts. As the storm pictured below was developing, it produced a
landspout near Hot Springs around 5:30pm on Wednesday, June 24, 1998. The landspout lifted a mobile home
from the ground, dismantled it, and tossed a young occupant several yards through the air. He was lucky
to live to tell his story. The same storm moved northeast and produced straight line winds in excess of
65 mph across northeast Custer and eastern Pennington Counties before weakening at about 7:30pm in Meade
County. Compared to other severe storms, that one was rather mild. Severe thunderstorms have, at times,
cut paths of damage so wide and long that they are visible on pictures taken by weather satellites over
20,000 miles above the earth!
Sadly, many people dismiss severe thunderstorm warnings as unimportant and fail to take cover until it is
too late.
A shelf cloud associated with a severe thunderstorm
near Bear
Butte on June 24, 1998. Photo by Randee Peterson.
Still another danger posed by Black Hills area thunderstorms is the flash flood. The mere mention of the
words "flash flood" sends a sobering chill through the hearts of long time Black Hills residents. The night
of June 9-10, 1972, after 10-15 inches of rain fell between Rapid City and Pactola Dam, the worst flash
flood in United States history sent a wall of water racing down Rapid Creek and through the heart of Rapid
City. The devastation in human and economic terms when the day dawned on the 10th was staggering. Over 230
people had lost their lives, hundreds more were injured. Homes, businesses, trees, animals, and all manner
of personal possessions were swept away by the raging waters.
Extensive work has been done to reduce the threat of another flood of that magnitude, but the fact
remains that, even with improved monitoring systems and regulations governing development in areas prone
to flooding, the Black Hills will always be vulnerable to this threat. The Black Hills formation is cut
with countless draws, ravines, canyons and stream beds. When rain falls in the Black Hills, it has only
two places to go…into the soil or downhill. If enough rain falls in a short period of time, the ability
of the soil to absorb the water will be exceeded and the downhill runoff will begin. As large amounts of
water pour into natural drainage areas, streams and creeks will rise out of their banks. If the flood
producing storm happens to remain stationary or follow a course over the same drainage for a long period
of time, that is when the danger is at its greatest.
Unfortunately, many Black Hills communities are located in places that, geographically speaking, would
have to be considered at risk for flash flooding. Among these are Spearfish at the end of Spearfish Canyon,
Sturgis at the end of Boulder Canyon, Rapid City along Rapid Creek, Hill City along Spring Creek and
Deadwood near the bottom of a natural bowl. Many campgrounds and recreation areas in the Black Hills are
also in flood risk areas. For this reason, it is always a good idea to keep an eye on the sky and have a
means to monitor the latest weather bulletins when out and about in the Black Hills.
A severe storm over southwest Lawrence County of South Dakota as seen
from the top of Bear Butte. The storm produced golfball size hail and some flash flooding in southern
Lawrence County, and also in northeast Weston County of Wyoming. Photo by Dan Carlson with a Casio digital
camera.
A mature severe thunderstorm on the plains east of Rapid City. At the
time the photo was taken, the white hail shaft in the lower left of the storm was about 2 miles northwest
of the town of New Underwood. Notice the rainbow immediately following the hail shaft. Photo by Dan Carlson
using a Casio digital camera.
A rainbow over Rapid City as seen from Skyline Drive. The
precipitation behind the rainbow is a mixture of heavy rain and hail. Photo by Dan Carlson using a Casio
digital camera.
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