Determining the type of supercells that may occur within a given severe weather environment is important for a multitude of reasons. A supercell is a thunderstorm that rotates. There are three types of supercells: low-precipitation (LP), classic, and high-precipitation (HP). The configuration of the wind shear profiles within a severe weather environment is critical in diagnosing what types of supercells can be expected on a given day.
While there are many contributing factors that revolve around determining the storm mode of a supercell, one of the most important factors is analyzing the storm-relative wind, particularly in the upper levels (or anvil level) of the supercell. A corollary for the storm-relative wind can be thought of as viewing an object in motion while in a vehicle, versus watching a vehicle pass you by as you stand on the ground. Detailed information about the storm-relative wind can be found in this scientific journal.
Storm-relative winds are important to analyze in a severe weather environment since they can allow a storm to ingest energy and moisture by taking advantage of the air within the inflow layer regions of the storm (the layer where air flows into the supercell’s updraft).
Strong winds moving into a storm can generate an increase in storm-relative helicity as well, which can enhance the potential for tornadoes. The stronger the storm-relative winds, the greater the likelihood that vertical wind shear will increase as well.
Low Precipitation Supercells
Low-precipitation supercells can occur if the upper-level storm-relative winds are ≥60 knots. This is due to the strong winds in the upper levels of the storm that can evacuate the mass flux of precipitation far away from the updraft. Thus, very little precipitation is left to fall out of the base of the storm.
It is clearly evident in the photo above that the upper-level storm-relative winds are quite strong. This is visualized through the excessive tilting of the updraft, with a barber pole like structure to the supercell.
Notice that the base of the supercell is not obscured by any precipitation and that the majority of the precipitation is being “flung” to the northeast, or to the right of the image. Below was the observed sounding profile from that evening from a weather balloon that was launched from the National Weather Service in Norman, Oklahoma.
In the sounding profile above, you will notice that the thermal instability is roughly 1,300 J/kg. While the Convective Available Potential Energy, or CAPE, is sufficient for thunderstorms, the wind shear was important to analyze as the bulk wind from the ground to the mid-levels of the storm were averaged at about 40 knots — which is more than conducive for a supercell despite the rather weak CAPE in place.
However, you will also see that the upper-level storm-relative winds were roughly 60 knots on the evening of March 7th, which meant LP supercells were likely. Typically, LP supercells have elevated bases for a multitude of reasons including a high shear and low CAPE environment, or a rather dry environment with weak wind shear and high CAPE. They often produce large hail given strong mechanical forcing or high values of CAPE, and can sometimes produce tornadoes if the environment is not too dry.
Classic supercells often occur in an environment where the upper-level storm-relative winds are between 40-60 knots. These types of supercells can be coined as the “Goldie Locks” of supercells such that there is not too much precipitation that falls out of the downdraft to obscure the mesoscale features involved with the storm.
Unlike LP supercells, the removal of precipitation mass from the updraft is not as significant. As a result, there is precipitation seen falling within the rear-flank and forward-flank downdrafts. While it may be slightly difficult to discern the rear-flank and forward-flank downdrafts from the photograph above, the Doppler radar image below gives a clear visual of the distribution of precipitation.
In the radar image above, the rear-flank downdraft (RFD) is where the core of heavy rain and hail is occurring (pink pixels), and the forward-flank downdraft (FFD) is to the east, or right, of the RFD. This supercell drifted eastward very slowly, and was nearly stationary at times.
Below is the observed sounding profile from Dodge City, Kansas, on the evening of May 21st. Strong CAPE was in place, with nearly 3,000 J/kg that overspread across western Kansas. In addition to strong instability, strong low-level wind shear was established across the area coupled with modest upper-level storm-relative winds near 50 knots. Usually, classic supercells form in environments where CAPE is ≥1,000 J/kg, to as high as ≥3,000 J/kg, near the dryline.
Classic supercells can sometimes transition into high-precipitation (HP) supercells with time, depending on the mesoscale and storm-scale dynamics that it takes on. However, classic supercells have a clear separation of the updraft and downdraft. This is evident in the radar image above as the “notch” in the radar image between the rear-flank and forward-flank downdraft is the location of the supercell’s updraft.
These types of supercells are sometimes the most photogenic, as the precipitation produced by the storm does not completely obscure mesoscale phenomena such as wall clouds, inflow tails, and tornadoes.
Classic supercells have some variability in their cloud base height, or the lifting condensation level (LCL). Notice in the photos above that there is a clear separation of the updraft and downdraft regions, as well as variability in their LCLs. The tornadoes produced by the storms are visible as well as other mesoscale features, despite a variation in their LCL.
Depending on the environment the supercell occurs in often determines what characteristics the storm will take on in the later part of its life cycle. Sometimes, classic supercells can merge with other nearby storms and form into a squall line. Usually, all storm hazards including damaging winds, large hail, and tornadoes, are associated with classic supercells.
High Precipitation Supercells
High-precipitation supercells can occur in environments where the upper-level storm-relative winds are ≤35 knots, and often form along the intersections of an outflow boundary, dryline, and a warm or stationary front (this is known as the triple point).
HP supercells are prolific rainfall produces, as there is little separation between the updraft and downdraft regions. As a result, the distribution of precipitation is highly concentrated within the rear-flank and forward-flank downdraft regions. Important features such as wall clouds and tornadoes can be invisible to the naked eye due to heavy rainfall.
Due to weak storm-relative winds, the evacuation of the precipitation mass within the updraft is minimal, and extremely heavy rainfall can occur within HP supercells. They can occur in environments with extreme CAPE, sometimes with values exceeding 4,000 J/kg. In environments with extreme instability and strong wind shear, HP supercells can produce enormous hail in excess of 4.00 inches in diameter, violent tornadoes, and flash flooding.
In the sounding profile above from May 31st, 2013, it revealed an environment highly favorable for HP supercells across central Oklahoma given extreme instability and weak storm-relative winds in the upper levels of the atmosphere.
HP supercells often have low cloud base heights, usually ≤1.0km. In rare cases, the entire mesocyclone of HP supercells can touch the ground. The mesocyclone associated with the El Reno supercell nearly touched the ground as per mobile Doppler radar observations, making it nearly impossible to discern any storm related structures.
Even in a case like this where there was strong wind shear, the excessive water loading and the depth of moisture would result in extreme rainfall that would obscure what ended up as the widest tornado ever recorded at 2.6 miles wide.
The region underneath HP supercells where heavy precipitation can obscure views of tornadoes is known as the “bears cage”, where precipitation wraps around the entire mesocyclone. The “bears cage” is a metaphor that describes the danger of a tornado that cannot be seen by the naked eye as it is completely shrouded in rain.
HP supercells often move to the right of the mean wind, which can increase the tornado potential tremendously if there is high storm-relative helicity present. They can also merge with nearby storms, turning into massive supercells that can produce widespread flash flooding in addition to destructive straight-line winds and tornadoes. Sometimes, clusters of HP supercells can evolve upscale into a forward-propagating mesoscale convective system depending on the orientation of the wind shear, instability, and other variables.
Below is a radar animation from the NOAA Hazardous Weather Testbed’s Phased Array radar that sampled the cluster of supercells that merged together over El Reno, Oklahoma, on May 31st, 2013. Unlike the previous radar image above in the classic supercell section, you can clearly see how there is widespread heavy precipitation across all regions of the storm as the updraft and downdraft regions are not well separated.
When observing HP supercells, it is best to exercise extreme caution and maintain a safe distance. This is due to the fact that the rainfall associated with HP supercells can be dangerous, as visibilities can drop to less than a quarter mile and it can thus obscure views from all angles. Rainfall rates within HP supercells can near 2.0-3.0″ per hour, and in extreme cases, sometimes >3.0″ per hour.