A Synoptic and Mesoscale Analysis of the

27-28 May 2001 Great Plains Derecho

By Diana Blahyj

  On 27-28 May 2001, an organized line of severe thunderstorms moved through Kansas, Oklahoma, and Texas.  The storm began around 2200 UTC on Sunday, 27 May 2001 in west-central Kansas and ended around 0945 UTC on Monday, 28 May 2001 in east-central Texas.  This organized complex of thunderstorms produced significant, widespread wind damage, several hail events, and a few tornadoes (See Fig. 1 ).   The length of this event (approximately 12 hours), along with the spatial coverage of the storms, suggest that this event was a derecho.  This study will show that this severe weather event was indeed a derecho, meeting all of the temporal and spatial criteria needed to be classified as such.  It will also be shown that there was a feature, better defined on the mesoscale than on the synoptic-scale, which helped to focus and organize the long-lived convection of this derecho.

   The storm reports from Kansas, Oklahoma, and Texas are examined to identify the temporal and spatial characteristics of this event.  These characteristics are then compared to the derecho criteria established by Johns and Hirt (1987) to assess whether or not this severe weather event was indeed a derecho.  The results of this comparison indicate that this event meets all six of the derecho criteria, and is therefore, a derecho.   (See Table 1 for a list of the derecho criteria and the characteristics of this event).

     An ingredients-based methodology is used to examine the environmental conditions needed for severe convection.  Four ingredients are needed for the development of severe convection. These ingredients are moisture, lift, instability, and vertical wind shear (Doswell 1987).  All four of these ingredients must be present in order for severe convection to take place.  If even one of the four ingredients is missing then severe convection will not occur.

Two-hourly surface theta-e maps show that moisture was present at the surface throughout most of Oklahoma and north-central Texas, as seen by the higher values of theta-e in these areas.  Two boundaries are also evident in the surface theta-e maps, an outflow boundary and a dry line (See Fig. 2 for the 0200 UTC surface theta-e map).   Thus, these maps show that two of the four ingredients needed for severe convection (moisture and lift) are indeed present in the southern Great Plains.

 The RAOB soundings from Oklahoma City and Dallas/Ft. Worth indicate that the environment south of the derecho had an ample amount of vertical wind shear and a large amount of instability to support severe weather.  This is evident from the high values of sfc-6 km shear, CAPE, and LI (See Fig.3 for the 0000 UTC Oklahoma City sounding).  Thus, the other two ingredients needed for severe convection (instability and vertical wind shear) are also present in the southern Great Plains.  Therefore, all four of the ingredients needed for severe convection were present in the environment that the derecho moved into.

      Further examination of surface theta-e maps, along with upper air maps at 850 mb, 700 mb, 500 mb, and 250 mb indicate that synoptic-scale conditions are weak.  There are no fronts in the southern Great Plains at the time of this event.  There are also no 500-mb troughs or 250-mb jet streaks to assist in the forcing of this event.  Thus, there is a lack of large-scale forcing on this day due to weak synoptic-scale conditions.  The presence of severe thunderstorms in such a weak synoptic environment, therefore, implies strong mesoscale forcing.

      Synoptic-scale maps and mesoscale maps from 0200 UTC to 0500 UTC were analyzed and then compared to see if the mesoscale analyses were better at resolving the surface features than the synoptic-scale analyses.  Both sets of maps were then compared to radar images at the same time to assess the accuracy and utility of each type of analysis.

Both data sets reveal the presence of an outflow boundary, as is evident by the tight thermal gradient seen in Fig. 4 & Fig. 5 , but the mesoscale analyses define it much better than the synoptic-scale analyses.  One example of this can be seen by comparing Fig. 4 , the 0500 UTC mesoscale analysis to Fig. 5 , the 0500 UTC synoptic-scale analysis.  The mesoscale analysis does a much better job at resolving surface features, such as this outflow boundary, than the synoptic-scale analysis does.   Further comparison of both of these analyses with Fig. 6 , the 0500 UTC radar image, clearly shows that the mesoscale analysis also does a much better job of accurately representing the events that are taking place at the surface.

     In conclusion, the severe weather event of 27-28 May 2001 was indeed a derecho, meeting all of the temporal and spatial criteria to be classified as such.  Examination of the synoptic-scale environment shows that there was a lack of large-scale forcing on this day due to weak synoptic conditions.  Severe thunderstorms in a weak synoptic environment imply strong mesoscale forcing.  The mesoscale forcing mechanism in this case was an outflow boundary from the thunderstorm complex.  This outflow boundary was evident in both synoptic-scale and mesoscale analyses, but it was much better defined in the mesoscale analyses than in the synoptic-scale analyses.  Thus, the feature that helped to focus and organize the long-lived convection of this derecho, the outflow boundary, was better defined on the mesoscale than on the synoptic-scale.
 

 
                                       Fig. 1.  Map of storm reports (wind, hail, and tornado) for 27 May 2001.
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Derecho Criteria Established by Johns and Hirt (1987)

 

 Characteristics of 27-28 May 2001 Severe Weather Event
 

Reports of convectively induced wind damage and/or convective wind gusts greater than 26 m s -1 (50 kt) must be located in a well-defined, concentrated area. This area of wind damage must contain a major axis of at least 400 km (250 nm).

 		  

Reports of convectively induced wind damage and/or convective wind gusts greater than 26 m s -1 (50 kt) are concentrated in an area with a major axis of approximately 900 km.
 

The pattern of wind reports must show a path or multiple paths of damage that occurred in a chronological progression.  The reports cannot be random in nature.


 		  

The pattern of wind reports shows damage occurring in a chronological progression.  The first report of wind damage was in Garden City, KS at 4:00 PM on 27 May 2001, and the last report of wind damage was in Madisonville, TX at 3:45 AM on 28 May 2001.

 
At least three reports of either F1 (33-50 m s-1) damage and/or convective gusts 33 m s-1 (65 kt) or greater must be located in the well-defined area, and these reports must be separated by 64 km (40 nm) or more.
  
 

There are approximately 43 reports of winds 65 kt or greater in this well-defined area.  At least five of these reports are separated by 64 km or more.
 

The time between successive wind reports can be no longer than 3 hours.

 		  

The longest time between successive wind reports was 55 minutes.
 

The convective system that is producing this event must have temporal and spatial continuity.

 		  

The convective system producing this event does have temporal and spatial continuity.
 

Multiple paths of wind damage must be associated with the same mesoscale convective system.

 		  

Multiple paths of wind damage are associated with the same mesoscale convective system.
                                     Table 1. A comparison of the six derecho criteria established by Johns
                                                     and Hirt (1987) to the characteristics of the 27-28 May 2001
                                                     severe weather event.
 
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                                      Fig. 2.  0200 UTC 28 May 2001 surface theta-e (K) and winds (kt).
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                                   Fig. 3.  0000 UTC 28 May 2001 Oklahoma City RAOB sounding.
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                                       Fig. 4.  0500 UTC 28 May 2001 surface mesoscale analysis.  Pressure (black) analyzed every
                                                    2 mb, temperature (red) and dew point (green) analyzed every 2 ° C.
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                                      Fig. 5.  0500 UTC 28 May 2001 surface synoptic-scale analysis.  Pressure (black) analyzed
                                                   every 2 mb, temperature (red) and dew point (green) analyzed every 2 ° C.
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                                       Fig. 6.  0500 UTC 28 May 2001 radar reflectivity composite for Oklahoma and northern Texas.
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