During the morning of June 10, an area of wind and thunderstorms moved from South Dakota and Nebraska into Minnesota and Iowa.  These thunderstorms brought heavy rainfall to portions of the area.  In addition, as the rain came to an end, several locations received 30 to 60 minutes of very strong winds.  The strongest recorded wind gust was 63 mph in Hendricks, Minnesota at 10:40 am.  Both Madison, South Dakota and Brookings, South Dakota had wind gusts up to 58 mph with some tree limbs down in the city of Brookings. 

Maximum wind gusts observed on June 10



Peak wind Gust

Chamberlain, SD

7:53 am

38 mph

 Gregory, SD

 7:55 am

29 mph

 Geddes SD

8:00 am

34 mph

 Wessington, SD

8:00 am

41 mph

 Huron, SD

 8:12 am

 40 mph

 White Lake, SD

8:35 am

45 mph

 Mitchell, SD

9:10 am

54 mph

 Parkston, SD

9:35 am

43 mph

 Menno, SD

10:02 am

36 mph

 Madison, SD

10:05 am

58 mph

Brookings, SD 

10:15 am

58 mph

Sioux Falls, SD 

10:31 am

40 mph

Hendricks, MN 

10:40 am

63 mph

Beresford, SD 

10:40 am

43 mph

Le Mars, IA

10:55 am

28 mph

 Luverne, MN

10:58 am

45 mph

 Pipestone, MN

11:13 am 

45 mph

Marshall, MN 

11:35 am 

44 mph

 Worthington, MN

11:35 am 

 45 mph

 Sheldon, IA

11:35 am 

 44 mph

Sioux City, IA

11:47 am

32 mph

 Tracy, MN

11:54 am 

50 mph

 Spencer, IA

11:54 am 

36 mph

 Slayton, MN

 11:58 am

 45 mph

 Jackson, MN

12:12 pm 

39 mph

 Storm Lake, IA

12:15 pm 

43 mph 

What is a wake low?

 A wake low is an area of low pressure that develops behind a line of thunderstorms or squall line.  Not all squall lines have a wake low associated with them.  In order for a wake low to develop, one typically needs a large area of rain behind the line of thunderstorms.  Because this area of rain can take a few hours to develop, wake lows are generally seen with squall lines that last for several hours. 

The diagram shows the development of the wake low in the pressure field behind a squall line.  The closer the lines of pressure, the stronger the winds will be.  This schematic is from Fujita (1963).

Evolution of a wake low

As the squall line forms, an area of high pressure, known as a mesohigh (H in the diagram), will form due to the rain-cooled air (W1).  To the east of the high pressure, an outflow boundary will form.  As the squall line continues to mature, the mesohigh will increase in strength with a tightening gradient just behind the outflow boundary.  This will be one area of strong winds (W2).  In the mature stage, a large area of rain will form behind the thunderstorms which extend behind the mesohigh.  With the right conditions, an area of low pressure will form behind the mesohigh.  This is the beginning of the wake low (W3).  Note that as the wake low matures, the pressure gradient increases which will result in an increase in the southeast winds between the mesohigh and wake low.  As the squall line continues to move east, the wake low can continue to develop which can result in an extremely tight pressure gradient and very strong winds.  During the dissipation stage of a squall line, the winds between the wake low and mesohigh may be stronger than that seen with the outflow boundary.

The wake low of June 10, 2010

The squall line that went through the area today was an asymetric squall line.  Below is a diagram of a asymetric squall line and a radar image from the NWS Sioux Falls radar at 8:00 am CDT, June 10.


Diagram of assymmetric squall line Radar picture from 8:00 am CDT, 10 June 2010

The radar picture (right) does not show the entire squall line which extended farther southwest into Nebraska.  However, the similarity to the diagram of an asymmetric squall line (left) can be seen as the large area of stratiform rain across most of southeastern South Dakota, northwestern Iowa, and southwestern Minnesota.  To the south of this area of heavy rain, a line of strong thunderstorms can be seen extending from northeast Nebraska toward Interstate 80.  With this type of squall line, the wake low is going to be located on the western side of the stratiform area which is located between the James River and Missouri River.

To get a sense of how quickly the pressure changes with a wake low, five minute observations were collected from three locations in South Dakota - Winner, Mitchell, and Sioux Falls.  Twenty minute observations were also collected from Madison, South Dakota, Brookings South Dakota and Worthington, Minnesota.

5 minutes wind and pressure observations from Winner, SD
Winner South Dakota pressure observations Wind speed trace from Winner, SD

In Winner, the pressure began to slowly fall after 5 am CDT as the thunderstorm complex moved over the area.  Winds also gradually increased out of the southeast peaking at around 30 mph between 6 and 7 am CDT.  The wake low approached the area after 7:15 am and moved over Winner between 7:30 and 7:50 am CDT.  Between 7:15 and 7:30 am CDT, the pressure fell at the rate of 0.03 inches per 5 minutes (~1 mb/5 min) reaching a minimum of 29.56 inches at 7:45 am CDT.  The strongest winds occurred with the lowest pressure - reaching 39 mph around 7:40 am.  Winds over 30 mph were observed for approximately 40 minutes in Winner.

Pressure and wind observations from Mitchell, South Dakota
5 minutes pressure observations for Mitchell South Dakota Wind speed and gust observations from Mitchell, SD

 As the wake low moved east, it continued to strengthen.  Similar to Winner, as the thunderstorms and rain moved in after 5 am CDT, the pressure in Mitchell was steady before starting a slow fall after 7 am CDT.  The pressure fall accelerated after 8 am CDT at the rate of 0.03 inches (~1 mb) per five minutes from 8:45 am to 9:10 am CDT.  Between 8 am and 9:10 am CDT , the pressure fell 0.24 inches (~8 mb).  The impact of the wake low was also more dramatic with winds gusting over 35 mph for almost 2 hours and over 45 mph for 45 minutes.  As in Winner, the maximum wind speed of 54 mph was seen with the minimum pressure at 9:10 am CDT.  However, unlike Winner, the pressure quickly rose behind the wake low which may account for the stronger winds observed in Mitchell.

Pressure and wind observations from Sioux Falls, South Dakota
Pressure observations from Sioux Falls, South Dakota Wind observations from Sioux Falls, South Dakota

The pressure trace in Sioux Falls was similar to Winner and Mitchell.  The pressure fell rapidly between 10:10 am and 10:45 am CDT.  Over that time, the pressure fell 0.15 inches (~ 5 mb) reaching a minimum of 29.67 inches.  Over half of this pressure fall occurred between 10:15 am and 10:20 am CDT where the pressure fell 0.08 inches (~2.7 mb).  Similar to Winner, the pressure remained low for almost 30 minutes.  The observed wind gust was also lower than Mitchell - around 40 mph.

Observations from Madison and Brookings, South Dakota
Pressure observations from Madison, South DakotaWind speed observations from Madison, South Dakota  
Pressure observations from Brookings, South DakotaWind observations from Brookings South Dakota  

Two of the stronger wind gusts were observed in Madison and Brookings, South Dakota.  Although observations were only every 20 minutes, the pressure trace was very similar to Mitchell, South Dakota.  In Madision, the pressure 0.27 inches (~9 mb) between 8:45 am and 10:05 am CDT.  In one 20 minute period, the pressure fell 0.16 inches (~5.3 mb).  During the time of the rapid pressure fall, the winds gusted over 35 mph reaching a peak gust of 58 mph at 10:05 am - when the pressure reached a minimum of 29.59 inches.  Also note that wind continued to gust over 35 mph through noon.

In Brookings, the pressure fall was not quite as extreme (0.24 inches or ~8 mb in 1 h and 20 minutes).  However, a similar rapid fall of 0.12 inches (~4.0 mb) occurred in 20 minutes.  The maximum wind of 58 mph was observed at the same time as the minimum pressure (29.63) at 10:15 am CDT.

Pressure and temperature trace from Worthington, Minnesota
Pressure trace from Worthington, MinnesotaWind observations from Worthington, Minnesota  

Finally, the pressure and wind observations from Worthington, Minnesota show a similar observation to Sioux Falls.  The pressure fell rapidly, 0.20 inches (~6.7 mb) in 40 minutes, and then remained at the minimum value for at least 20 minutes.  Afterward, the pressure rose rapidly, 0.1 inches (~3.3 mb) in 20 minutes - only Winner had a similar rapid rise in pressure.  Winds remained strong for only a short period of time with gusts over 40 mph observed for only 20 to 30 minutes.

To better illustrate the relationship between where the wind gusts were with respect to the area of rain, there are a series radar images taken every 30 minutes below.  Each image depicts the maximum wind gust observed with the wake low within 15 minutes of the image.  As can be seen, the maximum wind gusts (and minimum pressure) occurred at the western edge of the rain shield.


Radar imagery and wind observations from 8:00 am to Noon CDT, June 10, 2010
Radar image and wind gusts around 8:00 am CDT
Radar image and wind gusts around 8:30 am CDT.
Radar image and wind gusts around 9 am CDT.
Radar image and wind gusts from around 9:30 am CDT.
 Radar imagery and wind gusts from around 10 am CDT.
 Radar imagery and wind gusts from around 10:30 am CDT.
 Radar imagery and wind gusts from around 11 am CDT.
 Radar imagery and wind gusts around 11:30 am CDT.
 Radar imagery and wind gusts from around Noon CDT.



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