With the summer months approaching, we will start to see the increased probability of supercell thunderstorms developing. Supercells are strong thunderstorms that give us the majority of our tornadic severe weather. They form in environments with rising air and moderate directional and speed wind shear. The interaction of these factors and the relative strength of the two can affect which type of supercell is formed. The National Weather Service, NWS, defines wind shear as how the wind changes speed and/or direction with height. These two elements are essential for supercell development. The traditional and most common supercell development is referred to as a classic supercell. Supercells differ from traditional thunderstorms because they have a mesocyclone. A mesocyclone is a rotating updraft within the storm. Also, classic supercells can show a clear “hook echo” on radar which can be evidence of a tornado developing. A hook echo is when the mid-level mesocyclone wraps rain around the updraft. On radar, you can identify this by looking for a strong hook shape within the supercell. The presence of a hook echo does not necessarily indicate a tornado is present but shows mechanisms that are important for tornado genesis are present. Classic supercells can also come in two variations: Low Precipitation (LP) and High Precipitation (HP).
Low Precipitation supercells are supercells that produce relatively low amounts of rain. With (LP) supercells, there are high upper-level winds that push the rain far from the base of the supercell. This shows in the storm structure visibly as the storm can be tilted in the horizontal more than the traditional supercell. They are traditionally low moisture environments that aid in there not exhibiting large amounts of rain. However, hail is not uncommon with this type of supercell. The available water can suspend high in the cloud and rain out as hail. Tornadoes are unlikely with this type of supercell because the base of the storm is typically too high up in the atmosphere for a tornado to extend to the surface. They have this elevated base because rising air must go higher up in the atmosphere before clouds can form due to its dry environment. However, if one does produce, they are highly detectable because there are no strong amounts of rain to obscure it. They are common in the Texas and Oklahoma areas because of the lower amounts of moisture.
On the opposite side of the supercell spectrum, high precipitation supercells produce large amounts of rain. They have lower upper-level winds, approximately 35 knots or less. This heavy rain-fall can obscure different features like tornadoes and wall clouds. Obscure tornadoes can be particularly dangerous because they can’t be spotted by the human eye, and observers would not know when the tornado was approaching. They occur in high moisture environments and high CAPE environments, which means there are high areas of rising air. If there is high wind shear and high CAPE, you can have dangerous flooding, tornadoes ,and hail. Tornadoes formed in (HP) supercells can be extremely wide in diameter as the base of the storm is lower than normal. The El Reno tornado in 2013 had a width of 2.6 miles as the entire mesocyclone touched the surface. High precipitation supercells can have high impacts such as flooding, hail, lightning, and tornadoes.
Photo Creds: Supercell Diagram- Weather Underground
Low and High Precipitation Supercells - University of Illinois at Urbana
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©2020 Weather Forecaster Dakari Anderson
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