To forecast thunderstorms it’s most important that you can differ between the thunderstorm cell types. Lear how-to.
To be able to forecast thunderstorms, it is necessary to classify the thunderstorm cells into certain classes. In this blog, we explain what types of thunderstorms there are and what cloud structures you can photograph in these thunderstorms.
Every thunderstorm cell fulfils only one purpose in the weather system: the unstable condition of warm air at ground level and cold air aloft must be balanced. The warm air rises vertically in the cumulonimbus cloud, this is the updraft of the thunderstorm cell. To compensate, cold air including precipitation falls back to the ground, this is the downwind of the thunderstorm cell. One of the most complex jobs of a meteorologist is to be able to forecast thunderstorms. To do this, countless parameters from the weather models are compared.
In VIEWFINDR we use the simulated radar reflectivity in Central Europe and North America. This parameter shows the expected position of thunderstorm cells and systems. To forecast thunderstorms, this parameter can easily be called up in the VIEWFINDR app. By setting different times, the displacement of thunderstorm cells can be detected. This method gives an overview of the situation in a few seconds.
Forecasting thunderstorms involves a lot of uncertainty. As a layman, it is therefore not necessary to consider the total number of different weather parameters in the forecast. The information about an approximate location and time is sufficient. It is much more important to look at the precipitation radar in the short term. By observing the radar signals, thunderstorms can be predicted by interpolating the current radar image for the next 1-2 hours. Only on the live data of the precipitation radar can the individual types of thunderstorms be predicted.
The simplest form of a thunderstorm is the single cell. This type of thunderstorm cell has only a short life span of 20min – 60min before the single cell dies because the updraft does not find any more warm air in the vicinity. Single cells typically occur when there is little wind shear and jet stream. The absence of wind ensures that the single cell cannot reach further warm air. Single cells therefore remain local and move little from the spot.
Single cells are the best thunderstorms for photographing lightning. The fact that they are local and small-scale means that there is little rain in the area that could prevent the lightning from being seen. If single cells are visible in the thunderstorm forecasts and on the precipitation radar, it is worthwhile to photograph lightning at dusk and at night. During the day, single cells are not interesting to observe. They rarely have interesting cloud structures. It is only worthwhile to photograph single cells when lighting conditions are added in the evening or morning.
On precipitation radar, the single cell can be recognised by a rapid intensification and enlargement of the radar signature, soon followed by a weakening. Predicting the lifespan of single-cell thunderstorms is difficult, so photographing single cells always requires a little luck.
A multicell is the union of several single cells at different stages of the life cycle into one larger cell. The multicell can exist for several hours and cover long distances. Visually, the single cell can be recognised by the fact that small cumulus clouds, up to cumulonimbus clouds, appear staggered next to each other. The individual updrafts have joined together to form a common updraft base. Through this, the warm air rises in the updraft towers.
The downwind area of several individual cells in the decay stage has also merged. In the multicell there is a greater spatial separation of upwind and downwind. This means that new warm air can always rise in the upwind area of the multicell, and it is difficult for the cold air to prevent access to new warm air.
Multicells exist in environments with a moderate to strong jet stream combined with moderate wind shear. The wind shear contributes to the spatial separation of the upwind and downwind regions. The jet stream allows the multicell to move over the land, so that it can always reach new warm air masses. Forecast thunderstorms is much easier on precipitation radar. Multicells can be recognised by the fact that there are several so-called nuclei in the immediate vicinity on the radar. Each core corresponds to a single cell in a different stage of maturity.
The strongest form of thunderstorm cell is the supercell. Thunderstorm forecasts warning of supercells often speak of large hail and, of course, the risk of tornadoes. A supercell is a single cell, as there is only one updraft. However, there is a big difference between this and a single cell. A supercell occurs when strong wind shear prevails in the atmosphere. If the wind on the ground comes from the south, for example, but the jet stream comes from the west, the wind turns with altitude. Forecast thunderstorms in areas with high wind shear to predict possible supercells.
This ensures that the supercell always receives a laminar inflow of warm air into its updraft, while this is carried on by the jet stream. In addition, the warm wind from the south pushes the cold air in the downwind area of the supercell out of the way. This prevents the cold air from the downwind area from reaching below the upwind base, and the supercell continues to receive an influx of warm air.
Predicting the path of the supercell as a thunderstorm is quite easy on precipitation radar. A supercell can be recognised by its enormous size, and it usually has a V-shaped structure on the precipitation radar, with a long precipitation plume extending to the north. Some supercells form a so-called hook echo. This radar signature is the indication of extremely strong rotation of the thunderstorm cell, which can become a tornado.
Supercells are photogenic throughout the day, with interesting cloud structures forming on them. During the night, a lot of lightning can be photographed. As a photographer, being able to predict supercell thunderstorms is very helpful if you want to photograph great cloud structures.
The LP supercell, from the American “low precepitation”, is a form of supercell with very low precipitation. This supercell can be predicted as a thunderstorm on precipitation radar, looking for a roundish signature with a long precipitation plume. LP supercells have a very low risk of a tornado, but very large hail can occur in the precipitation area of the supercell. In the LP supercell, the upwind area is clearly more dominant than the downwind area. Therefore, a good view of the entire upwind area is always to be expected in this thunderstorm.
In this supercell, there is an ideal balance between the upwind and the downwind. This ensures that the greatest risk of tornadoes is found at the classic supercell. The classic supercell is characterised by the typical hook echo on the precipitation radar. Being able to predict the track of these thunderstorms is enormously important, because it is the only way meteorologists can estimate which path a tornado will take. To forecast thunderstorms which can produce tornados is one of the most important tasks in thunderstorm prediciton.
Probably the most monstrous cloud structures are produced by HP supercells. The “high precipitation” supercells have a more dominant downwind area, there is particularly much precipitation in the vicinity of this thunderstorm cell. Large hail and gale-force winds occur, and tornadoes are particularly dangerous because they are enveloped in rain and are difficult for storm chasers to detect.