This study developed a harmonic-trend model to characterise and project thunderstorm rainfall variability over Makurdi and Jos in the Middle Belt region of Nigeria, using daily and monthly thunderstorm rainfall data obtained from the Nigerian Meteorological Agency (NiMet) for the period 1995–2024. Descriptive statistics, linear trend analysis, and Fourier harmonic modelling were applied to isolate the long-term (secular) trend and the cyclical (seasonal/periodic) components of thunderstorm rainfall, which were then combined into a single hybrid harmonic-trend model. Results showed that thunderstorm rainfall averaged 634 mm annually in Jos and 546 mm in Makurdi, with Jos exhibiting a statistically significant increasing trend (r² = 0.2496) while the increase in Makurdi was not significant (r² = 0.0023). The harmonic component captured strong seasonal periodicity in both locations, reflecting the concentration of thunderstorm activity within the rainy season (March–October). When combined, the harmonic-trend model explained over 97% of the variability in thunderstorm rainfall at both stations, with Nash–Sutcliffe Efficiency (NSE) values ranging from 0.91 to 0.99, indicating excellent model performance and validating its suitability for both historical characterisation and future projection. Extrapolation of the validated model to 2050 projected a continued increase in thunderstorm rainfall intensity at both stations, with a more pronounced upward trajectory in Jos than in Makurdi. These findings highlight the growing dominance and intensification of thunderstorm-driven rainfall in the Middle Belt under changing climatic conditions. The study concludes that harmonic-trend modelling provides a robust and reliable tool for forecasting convective rainfall behaviour in data-sparse tropical environments, and recommends that NiMet incorporate such models into operational forecasting systems, while policymakers integrate the projected rainfall intensification into flood-risk management and climate-adaptive planning for the region.
Keywords: Harmonic-trend model, Makurdi and Jos., Rainfall, Thunderstorm
