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Microburst Winds Strike Basin
Strong wind affected portions of
Basin on Saturday
evening, July 31, 2004 causing damage to at least one residential roof, a
garage, and a passenger van. Much of the damage was the result of numerous large tree limbs
that were downed
across the town.
The phenomena responsible for the damaging wind is
called a
microburst. Microbursts typically affect areas less than 2.5
miles in length with a relatively narrow swath of damage. These
"bursts" of strong wind generally last less than
10 minutes. Indeed,
most of the damage in Basin (click
here for a damage map) occurred in the southwest-half of town between
5:19 and 5:25 p.m. MDT.
A
weather station located in Basin near the northwest fringe of the
damage only reported a wind gust of 20 mph as the storm began around
5:19 p.m. Phone calls to the Big Horn County Sheriff's Office began
about the same time with a report of a tree blown over into the southbound
lanes of US Highway 16/20 near Wyoming Avenue. Northwest of town,
three power poles were toppled by the strong wind (see pictures below).
Soon after, a residence was reported to have
lost the top layer of roofing
material. By 5:26 p.m. calls were coming in from around town of
numerous trees and limbs down. The greatest concentration of damage
appeared to have occurred between 6th and 9th streets and South Street and
Montana Avenue. A garage was
struck by several large tree limbs, causing a gouge in the roof and
tearing the gutters from the roof. Another report indicated that a van travelling south
on US Highway 16/20 through Basin was struck in the driver's-side
windshield by a flying tree limb. Thankfully, no injuries or deaths were
reported during the event.
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Toppled power lines near the Basin cemetery northwest of
Basin and south of Highway 30.
(Click image for larger view) |
A double
power pole snapped from its base northwest of Basin.
(Click image for larger view) |
Weak rain showers were present in the vicinity of
Basin, but little, if any, rain was reaching the ground. This was
the result of very dry air that was present between the cloud base and the
ground. As rain falls into the dry air it evaporates and cools.
Typically, the drier the air, the greater the amount of evaporation and,
therefore, cooling of the air. Since cloud bases in the western
United States can routinely be 12,000 feet or more above the ground, this
cool air (which naturally likes to sink toward the ground) has a long time
to accelerate before reaching the ground. When the "burst" of cool
air reaches the ground it spreads out creating the strong wind. For
example, imagine emptying a large bucket of water onto the ground.
First, empty the bucket from a height of one foot, and then from a height
of six feet. As you would guess, when emptied from six feet the
water accelerates and spreads out in many directions; whereas, when
emptied from one foot the speed and distance of spread is much less.
The same thing occurs when the cool air reaches the ground. It was estimated that wind speeds in
Basin likely ranged from 50 to 70 mph. However, microburst winds
have been known to reach speeds of over 100 mph.
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WSR-88D radar image from Riverton at 5:17 p.m. MDT. The rain
shower indicated by the arrow was likely responsible for the
microburst.
(Click image for larger view) |
Upper-air profile of the atmosphere above Riverton at 6:00 p.m. MDT.
Note the large area of very dry air below the cloud base. This
dry air aided evaporation and cooling which helped to produce strong
outflow wind.
(Click image for larger view) |
The National Weather Service in Riverton would like
to thank the Big Horn County Sheriff's Office and the Basin Police
Department for their help in surveying the damage and in reconstructing
the events of Saturday evening.
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