Heat Burst Affects Southwest Iowa

During the evening of August 23, a rare phenomenon known as a heat burst affected portions of southwest Iowa. A heat burst is characterized by a sudden rise in temperature, a drop in humidity, and strong winds that can approach or exceed severe levels. They are associated with high based decaying thunderstorms with a substantial dry layer beneath the base of the storm. As rain from the thunderstorm falls into the underlying dry air, it cools the air immediately around it, which becomes denser than the surrounding air and begins to sink. As this air sinks, it drys and compresses adiabatically, which results in the hot and dry readings recorded with heat bursts.

The sounding from Omaha at 7:00pm on August 23 clearly depicts this low level dry layer, with cloud bases up around 18,000 feet. This dry layer is circled on the sounding below.


Storm Reports from the Evening of August 23

The evening's heat burst produced scattered wind damage across southwest Iowa. The worst damage appears to have been just south of Brayton in southern Audubon County shortly after 7:00pm. Several reports of downed trees and power lines were received from this area. Additional trees and power lines were reported down in the town of Bridgewater in Adair County at 6:50pm.

0650 PM     NON-TSTM WND DMG BRIDGEWATER             41.25N 94.67W
08/23/2011 ADAIR IA LAW ENFORCEMENT

TREE REPORTED DOWN ON A POWER LINE ON NORTH MAIN. DAMAGE POSSIBLY FROM A HEAT BURST.
0705 PM     NON-TSTM WND DMG 1 S BRAYTON             41.53N 94.93W
08/23/2011 AUDUBON IA LAW ENFORCEMENT

TREE REPORTED DOWN ON A POWER LINE. TRAINED SPOTTER ALSO REPORTED LIMBS DOWN IN HIS YARD.
SPOTTER ESTIMATED WINDS OF AROUND 40 MPH AND OBSERVED A TEMPERATURE INCREASE FROM
76 TO 96 DEGREES IN A FEW MINUTES ALONG WITH A SHARP DROP IN THE HUMIDITY.
THIS EVENT WAS LIKELY A HEAT BURST.
0744 PM     NON-TSTM WND GST FONTANELLE              41.29N 94.56W 
08/23/2011 M60 MPH ADAIR IA MESONET

Observations

A number of weather stations recorded the heat burst as it passed through. The highest wind gust recorded was 60 mph at the Fontanelle schoolnet site, while both the Atlantic AWOS and Fontanelle schoolnet sites recorded a temperature of 102 degrees. To show just how quckly the weather conditions changed as a result of the heat burst, the tabular data from the Atlantic AWOS is shown below.

Atlantic AWOS Data
Time (CDT)    Temp (F)  
Dew Point (F)
   Wind    
5:55pm  84  75  Calm
6:05pm  86  61  16 G 28 mph
6:15pm  93  54  21 G 41 mph
6:25pm  88  61  10 G 39 mph
6:35pm  86  59  12 mph
6:45pm  86  61  14 mph
6:55pm  88  64  16 G 26 mph
7:05pm  90 54  20 G 30 mph
7:15pm  99  18*  8 G 38 mph
7:25pm  102  7*  3 mph
7:35pm  91  52  Calm
7:45pm  88  64  10 G 18 mph
7:55pm  84  72  13 G 21 mph
8:05pm  84  75  8 mph

*AWOS sites typically have problems reporting dew points in low humidity environments, so these two dew point measurements are likely incorrect.

Chart of the Atlantic AWOS temperature, dew point, and wind speed.
Courtesy of the Iowa Environmental Mesonet

Below are some plots from local schoolnet stations, courtesy of the Iowa Environmental Mesonet.

 Anita Schoolnet Site  

 Temperature and Dew Point Plot Wind Speed, Gust, and Direction Plot
The dew point sensor at Anita appears to be out, but one clearly see the sudden rise in temperature at about
7:00pm, rising from 82 F to 96 F in 10 minutes.
 At 7:00pm the winds at the site rose from nearly calm to 39 mph, with a maximum recorded gust of 51 mph. There was little change in the wind direction.

 Audubon Schoolnet Site  

 Temperature and Dew Point Plot Wind Speed, Gust, and Direction Plot
This is the earliest of the heat bursts recorded. Shortly after 5:00pm, the temperature spiked from 81 F to 91 F, and the dew point dropped to 57 F.

 At 5:00pm, the winds jumped from calm to 26 mph,with a maximum recorded wind gust of 37 mph.


 Corning Schoolnet Site  

 Temperature and Dew Point Plot Wind Speed, Gust, and Direction Plot
At 8:00pm, the temperature rose from 84 F to 96 F and the dew point dropped from 76 F to 49 F. There was a second heat burst with a similar drop around 9:00pm.  At 8:00pm, the winds at the site rose from nearly calm to 22 mph, with a maximum recorded gust of 37 mph.

 Fontanelle Schoolnet Site  

 Temperature and Dewpoint Plot Wind Speed, Gust, and Direction Plot
There appears to have been multiple heat bursts from 7:00pm to 8:30pm. The highest temperature recorded in this time was 102 F, and the lowest dew point was 50 F.  There were three main wind speed maximums from 6:30pm to 7:45pm between 27 mph and 39 mph. The highest wind gust recorded was 60 mph at 7:45pm.


Radar Data

The heat burst also appeared on velocty radar imagery from the Des Moines Doppler Radar, but there was little evidence on conventional reflectivity data as to the presence of the heat burst. There were only very weak showers in the vicinity of the wind damage reports.

Velocity data from 5:50pm of a likely heat burst over southern Guthrie County. No damage reports were received and there were no observation sites in the area. Maximum velocity values were around 70 kts.
Reflectivtiy data over the Bridgewater area at the time the damage occurred (6:54pm), showing just a weak shower in the vicinity. There was no evidence of a heat burst on velocity data either.
Reflectivity data from 6:59pm at the time wind damage was reported south of Brayton in southern Audubon County. Note the lack of any showers in the area. Velocity radar data from 6:59pm, showing winds of up to 60 kts along the Audubon and Cass County line.


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