Features Generated by a Strong Thunderstorm
- Gust front
- Boundary between warm, sticky air and the cold downdraft air
- Source of downdraft air: Evaporation of rain into dry air. Sources:
- Dry air under the cloud as rain shaft falls through
- Entrainment of dry air surrounding cloud into thunderstorm.
- Trough of low pressure
- Wind shifts dramatically
- Can have straight line damaging winds just behind wind shift
- Occasional gustnado
- Thunderstorm or outflow high
- Produced by cold air sinking in downdraft
- Downdraft at rear of high
- Air rushes through high towards gust front (not geostrophic)
- Cold air did not exist before thunderstorm development
- Region of heavy rain and hail
- Wake depression or wake low
- Least frequently observed feature
- To rear of thunderstorm high
The general purpose of the EAS 486 class project was to analyze the mesoscale features associated with the bow echo that developed in eastern South Dakota and moved through southern Minnesota on July 28, 2002. Click here for an animated radar loop (1.67 Mb)
In addition, new thunderstorms developed along several weak surface boundaries as the main thunderstorm complex moved towards other boundaries.
The spacing of stations in the Dakotas is quite sparse. Even though the surface station network in Minnesota is more dense, the thunderstorm-generated features are small enough and change rapidly enough to make analysis difficult
2100 UTC 28-July-2002 Radar Reflectivity Composite
(click on image for a radar loop)
Fujita (1962) pioneered the techniques needed to analyze such small-scale features. In order to use the surface observations, biases of the surface data caused by topographic and instrument differences must be eliminated. The altimeter setting must be used since it is reported by all stations.
Stages:
- Compute 24-hour averages of every surface station used in the analysis on a non-convective day.
- Construct a smoothed pressure analysis every 1 mb.
- Determine an adjustment for any station which deviates from the smoothed analysis by at least 0.5 mb
Click on image for large view of Brookings time series
Click on image for large view of Pipestone time series
The hourly surface mesoanalysis shows the interaction of the thunderstorm between Huron and Brookings, SD and its trip eastward across southwest Minnesota. During the trip across southern Minnesota,
- The pressure difference between the gust front and the high pressure system reached as large as 5 mb, which occurred in less than 30 minutes.
- The synoptic scale low was lost as the gust front dominated the signal.
- The gust front/outflow boundary interacted with the synoptic scale warm front and a surface trough across southern Minnesota. Some of the most severe weather occurred near the point where the gust front intersected the other boundaries, including a brief tornado.
- A small low pressure system moved along the southern Minnesota trough. When the gust front from the bow echo caught up, a brief tornado was spawned.
- By the time of the final analysis, the outflow boundaries have overwhelmed the surface analysis
Surface map for 16Z 28-July-2002
Surface map for 17Z 28-July-2002
Surface map for 18Z 28-July-2002
Surface map for 19Z 28-July-2002
Surface map for 20Z 28-July-2002
Surface map for 21Z 28-July-2002
Surface map for 22Z 28-July-2002
