How do weather forecasters determine how much rain will fall in a 24-hour period?

John Maytham, standing in for Lester Kiewit speaks to Doctor Peter Johnston, Climate Scientist at UCT’s Climate System Analysis Group.

Listen below:

How do weather forecasters determine how much rain will fall in a 24-hour period?

According to Johnston, the predicted rainfall amount can vary depending on the region and the weather models used. Forecasters rely on a combination of long-term averages, historical rainfall data, and statistical projections to make these estimates.

Typically, around 25-millimeters of rain in a day is considered a significant amount. However, these forecasts are never guaranteed.

Over the past five years, rainfall patterns appear to be shifting. Instead of steady, persistent rain spread over several days, we're now seeing more sporadic but intense downpours.

This trend toward shorter, heavier rainfall events is concerning, as existing infrastructure may not be designed to handle such sudden volumes of water.

Forecasts are based on probability, not certainty.

Johnston points out that a forecast might suggest a 60% chance of receiving a certain amount of rain—not a full 100% guarantee.

"There's a certain amount of unknown in a forecast... that's the way statistics work."

- Doctor Peter Johnston, Climate Scientist - UCT’s Climate System Analysis Group

Scroll up to the audio player to listen to the full conversation.