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
Wearing of the Green- Are Green Skies usually associated with Severe Weather? (Photo Credit: Jennifer Shoemake)
Shelf cloud taken from Albuquerque, New Mexico's west side on July 26, 2018.
Across the Great Plains, folklore says that if the sky turns green it’s time to head inside. But, is there truth to this saying? While it is not uncommon for green skies to accompany severe weather, there appears to be no direct correlation between the two.
During the day, the sky appears blue because the shorter-wavelength, bluer end of the light spectrum bounces off of air molecules better than the long wavelength red end of the spectrum. As the sun shifts lower in the sky (and after the peak heating of the day), the spectrum of direct sunlight is shifted from blue wavelengths of the sky to the red, orange, and yellow of the sunset due to the longer trip of the sun’s rays through the atmosphere. A thick cumulonimbus is composed of many water droplets and ice particles, resulting in air molecules scattering and attenuating. Since water reflects blue and green light better than red, there appears to be a green-ish tint to the sky. The same phenomenon is present when you place a glass of water with blue food coloring in front of a glass with blue that will produce the same green tint that the light in the sky transmits.
A study from Pennsylvania State University concluded that the relative contribution of hail to the green color was actually quite small. It isn’t the presence of hail needed to produce a green sky, but the size of the droplets/particles in the cloud may dictate the shade of green (for example, smaller drops may lead to more of a blue-green sky than larger drops which may produce more of a yellow-green color).
These intense thunderstorms that produce green-tinted skies also have the potential to generate large hail, damaging winds, frequent lightning, flash flooding, and even tornadoes, which may make the green color appear threatening. But, this is not always the case. Either way, it is best to head indoors when thunder roars.
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©2020 Meteorologist Sharon Sullivan