The uncertainty comes as Nairobi and several highland areas continue experiencing unusually cold conditions, with Kenyans waiting for the expected onset of the short rains.
Weather expert Maithya wa Vilivū said medium-range forecasting models were currently giving conflicting indications about when the rains would start.
According to Maithya, the uncertainty has been influenced by the interaction of three major weather patterns: a strong El Niño, a developing positive Indian Ocean Dipole (IOD) and an unclear Madden-Julian Oscillation (MJO).
The IOD measures differences in sea surface temperatures between the western and eastern parts of the Indian Ocean, while the MJO is a tropical weather system that moves around the globe and can influence rainfall and thunderstorms.
The interaction between these weather systems can make it difficult for forecasting models to accurately determine where and when significant rainfall will occur.
Maithya noted that recent runs of both the European and Global Forecast System (GFS) models had produced major differences in their projections regarding rainfall amounts, distribution and consistency.
The uncertainty is particularly noticeable in Nairobi, Central, Western, Rift Valley and Nyanza regions, where experts currently estimate forecast predictability at between 50 and 70 per cent.
Despite the uncertainty over the exact start date, the broader outlook is increasingly pointing towards a wetter-than-normal season.
Forecast models are gaining confidence that a positive IOD could develop alongside the strong El Niño, potentially increasing rainfall across parts of East Africa.
The Australian Bureau of Meteorology's latest projections indicate that the positive IOD could strengthen between September and December, reaching its highest levels of between +0.8 and +1 degree Celsius around October and November.
Some American weather models are projecting an even stronger positive IOD, with the index potentially reaching between +1.05 and +1.1 degrees Celsius during the same period.
Experts have warned that the combination of a strong El Niño and a firmly positive IOD could result in prolonged and intense rainfall across parts of Eastern Africa between October 2026 and January 2027.
Maithya said the GFS model was consistently showing a plume of moisture moving from the Congo Basin through Uganda into Western Kenya, the Rift Valley and parts of Central Kenya and Nyanza from the third week of September.
He also warned that some areas could experience heavy afternoon rainfall and hailstorms, particularly Nandi, Nakuru, Kericho, Bomet and northern Narok.
The coastal region, meanwhile, is expected to receive significant rainfall over the next 10 days.
The outlook comes as the Kenya Meteorological Department forecasts generally dry conditions across most parts of the country between September 8 and 14.
However, several counties could still experience showers and thunderstorms during the period.
Temperatures are also expected to fall below 10°C in some highland areas, including Nairobi, Nakuru, Nyeri, Nyandarua and Laikipia, as Kenyans await clearer indications of how the upcoming short-rains season will unfold.
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