Three elements are necessary for thunderstorm development; humidity, atmospheric instability, and a lifting force.
Humidity
Humidity is important for thunderstorm formation because moisture in the air provides energy for storm development in the form of latent heat. Latent heat is released from the air when water vapor condenses into liquid water, and this takes place as the air cools. When air at the surface rises, it also cools. This is known as adiabatic cooling. As the water vapor in humid air condenses during cooling, clouds are formed and latent heat is released. The release of latent heat means that humid air cools more slowly than the surrounding air, making it more buoyant and increasing its rate of ascent. At sea and near the coast, humidity is almost always high.
Atmospheric Instability
Atmospheric instability is a measure of the likelihood that surface air will rise. Generally, instability manifests as a large temperature decrease with height and the atmosphere is considered to be unstable if temperatures decrease with height by more than 6.5 °C. Under these conditions, rising humid air is more likely to be warmer than its surroundings, increasing its buoyancy and rate of ascent. Once humid air commences its journey aloft in an unstable atmosphere, its progress can be rapid and unrelenting, forming cumulonimbus clouds and eventually thunderstorms.
Lifting Force
Before humid air at the surface can create towering cumulonimbus clouds, force needs to be applied to the air to generate the initial lift. The force has to be sufficient to lift the air to the height at which the humid air will begin to condense, known as the lifted condensation level. Near the coast, the lifting force can be supplied by terrain that causes the air to rise, known as orographic lift. The lift can also be supplied by the sun heating the air at the surface, or through atmospheric instability near the surface.
Forecasting Thunderstorms
Thunderstorms can be isolated events or a part of a larger weather system such as a tropical wave, or a squall line associated with a cold front. Generally, if a squall is part of a larger weather system, it will be well forecast in both weather models and interpreted forecasts. However, isolated thunderstorms are more difficult to forecast. This is because thunderstorms are relatively small (with an average diameter of about 20 kilometers) and short lived, lasting about 90 minutes. Because of this, the effects of a thunderstorm will be felt differently over a small area.
The ECMWF model provides a thunderstorm forecast that is available through windy.com. This is a forecast of the expected number of lightning flashes in a region. However, this forecast is likely to have errors in time and space, and you should not rely on this forecast to identify isolated squalls. For example, https://www.ecmwf.int/en/about/media-centre/news/2018/how-predict-lightning states that,
“(forecast) skill crucially depends on the size of the area and the duration for which a prediction is made. When averaging over the next 24 hours and the whole of Europe, for example, impressive agreement with observations can be achieved.”
In other words, while ECMWF can forecast the number of lightning strikes over Europe on any given day, it is not going to be able to tell you whether those strikes will be in your bay or the one next door.
Forecasts of convective available potential energy (CAPE) are available from resources such as windy.com and Predict Wind. The CAPE index is measured in joules per kilogram and provides an indication of atmospheric instability. CAPE values below 1000 mean that thunderstorms are unlikely, while CAPE values over 3000 mean that the risk of thunderstorms is extreme. When considered with other factors, such as hot and still conditions or high sea surface temperatures, CAPE can provide a good guide as to your thunderstorm risk.
Weather radar and real time lightning trackers can also provide you with useful information on the current location of thunderstorms. The windy.com weather radar overlay provides information on the location of heavy rain and real time lightning strike data. You can also download phone apps which can provide alerts if lightning strikes are detected nearby. In the Mediterranean, the European Storm Forecast Experiment is a valuable resource for determining the likelihood of thunderstorm activity.
It can be difficult to predict the expected wind strength from a squall or thunderstorm. Some storms will have wind gusts over 40 kts while the wind from other systems will barely impact the wind speed. This has a lot to do with the lifecycle of the thunderstorm and the prevailing wind conditions. For example, the winds are usually the strongest and tend to radiate outwards from the base of the cloud when the system is mature and producing rain.
Further reading:
Comments