EXPLAINER: Why predicted rainfall may not always materialise

The failure of forecast rainfall to occur over parts of Ghana’s coastline between Thursday, July 2 and Sunday, July 5, despite repeated warnings by weather authorities, has sparked public debate over the reliability of weather predictions.

The concerns were amplified after the Ghana Meteorological Agency (GMet) issued several forecasts warning of moderate to heavy rainfall in the coastal belt, including Accra, during the period. An SMS alert circulated on Sunday night also warned residents in Greater Accra of a 40 per cent chance of moderate rainfall between 4:00 a.m. and 7:00 a.m. on Monday, July 6, advising the public to avoid flooded roads and remain alert.

While the alerts prompted many residents to prepare for possible flooding, the anticipated widespread rains in many parts of the capital and other coastal communities did not occur as forecast.

So, why do weather forecasts sometimes fail to materialise?

Forecasts are probabilities, not guarantees

One of the biggest misconceptions about weather forecasts is that they are predictions of certainty.

Meteorologists generate forecasts using sophisticated computer models that analyse atmospheric conditions such as temperature, humidity, wind patterns and air pressure. These models estimate the likelihood of weather events occurring rather than guaranteeing that they will happen.

For instance, the SMS warning indicated a 40 per cent chance of rain, meaning there was a moderate probability that measurable rainfall could occur within the forecast area. It did not mean rain was certain or that every location within Greater Accra would experience it.

Small atmospheric changes can alter outcomes

Weather systems are highly dynamic and can change within hours.

A slight shift in wind direction, temperature or moisture levels can weaken a storm, delay its development or divert it away from an area that was previously expected to receive rainfall.

These seemingly minor atmospheric changes can significantly affect where rain eventually falls.

Rain may fall elsewhere

Forecasts are usually issued for relatively large geographical areas rather than individual neighbourhoods.

This means rain forecast for Greater Accra may occur in only portions of the region while other areas remain dry.

Similarly, rain-bearing clouds may develop offshore or over neighbouring districts instead of directly over the capital.

Computer models have limitations

Although weather forecasting technology has advanced significantly, no computer model can perfectly replicate the atmosphere.

Meteorological agencies rely on data from satellites, radar, weather stations, aircraft and other observing systems. However, there are still gaps in observations, particularly over oceans and remote areas, where fewer measurements are available.

As a result, forecast models must estimate conditions in some locations, introducing a degree of uncertainty.

Local weather is difficult to predict

Short-lived weather events such as thunderstorms are among the most difficult phenomena to forecast accurately.

These storms can form rapidly, weaken unexpectedly or dissipate before reaching populated areas.

This is particularly common during Ghana’s rainy season when weather conditions can change within a short period.

Forecast accuracy decreases over time

Forecasts are generally most accurate within the first one to three days.

As the forecast extends further into the future, uncertainty increases because atmospheric conditions continue to evolve.

Even so, weather agencies regularly update forecasts as new information becomes available.

Why warnings are still issued

Experts say weather agencies often prefer to issue warnings when there is a credible risk of hazardous weather, even if the event does not ultimately occur everywhere.

This precautionary approach is intended to help protect lives and property, particularly in flood-prone communities where delayed warnings could have severe consequences.

In many cases, it is considered safer to alert the public to a potential threat than to underestimate a weather event that later proves destructive.

Are weather forecasts becoming more reliable?

Despite occasional misses, meteorologists say weather forecasting has become considerably more accurate over the past two decades due to improvements in satellite technology, Doppler radar, computer modelling and data collection.

GMet has previously cited logistical constraints as a challenge affecting the accuracy and effectiveness of some of its weather forecasting and monitoring operations.

Notwithstanding the above, because the atmosphere is an inherently complex and constantly changing system, forecasts will always carry some level of uncertainty.

For this reason, weather experts encourage the public to treat forecasts as guidance based on the best available scientific evidence at the time they are issued, while paying attention to regular updates as conditions evolve.