Indicators for Predicting Severe Weather
Recently we have added CAPE and Gust Maps to help you predict severe weather, and avoid these conditions.
CAPE stands for Convective Available Potential Energy and is the amount of fuel available to a developing thunderstorm. More specifically, it describes the instability of the atmosphere and provides an approximation of updraft strength within a thunderstorm. So CAPE is used by weather officials to understand what the potential might be for thunderstorms and how powerful those storms might become if they do materialize.
The standard measurement of energy is represented as Joules Per Kilogram. A high CAPE value might also be expressed by weather experts by using the term “high instability”. When we talk about a highly unstable atmosphere, CAPE values are usually in excess of 2500 J/kg’s which would supply ample energy for strong updrafts and violent thunderstorms, should they develop.
Severe thunderstorms require high CAPE values and the higher the CAPE value, the more energy available to promote thunderstorm growth. However, the CAPE variable is not a “magic number” that will predict the certainty of a thunderstorm. But if a thunderstorm does develop, and the CAPE value is high, then there is a good chance for an “explosive storm”. Below is a typical CAPE Map in the Caribbean. The Yellow/Orange areas show high CAPE values.
The table below is a good guide for Thunderstorm/Lightning.
Gusts are sudden, brief increases in the speed of the wind. According to U.S. weather observing practice, gusts are reported when the peak wind speed reaches at least 16 knots and the variation in wind speed between the peaks and lulls is at least 9 knots. The duration of a gust is usually less than 20 seconds.
All sailors know that it is the Gust that can do major damage and make boat control difficult, and often more dangerous than a high average wind strength. It is prudent practise to view the Gust Map, to check the maximum wind strengths you may experience. A large difference between the average wind strength, and the gusts can be very dangerous, especially if you are not prepared. So we advise that you animate your weather routing, over the Gust Maps to check for these situations.
See a typical Gust Map from the North Atlantic. Dark red areas show Wind gusts of over 35 knots. Grey areas over 50 knots.
Rain squalls can also affect the wind strength in a dramatic way. These clouds ‘push’ air down to the surface, and so if the cloud was directly upwind, you can expect the wind speed to increase before it passes overhead, and decrease after.