Rapid developing storms over western Kenya

By Kizito Amua, Sarah Kimani, Scholastic Maloba and Patricia Nyinguro (IMTR/WMO-RTC)

Western Kenya provides favourable conditions for the development of thunderstorms. First there is large scale convergence between the trade winds from both hemispheres since it is over the equator. Second, the Convective Available Potential Energy (CAPE) is big enough to support convective activities, due to high temperatures over the region. Lastly, there is the availability of the moisture within the Inter-Tropical Convergence Zone (ITCZ) region of convergence.

The short rain season in Kenya occurs over the October-November-December (OND) period, as the ITCZ tracks southwards. Over those months, Kenya received depressed rains, and the OND seasonal forecast indicated below normal rains over several areas.

Convective storms take a short time to develop and reach maturity, usually only about an hour, resulting in thunderstorms and showery precipitation, that can cause flash floods. If a series of cumulonimbus clouds develop in same region, then thunder and showery precipitation can occur for a longer period.

This case shows a convective storm that rapidly developed on 3 October in Kitale in western Kenya. Heavy rainfall from the storms led to flooding in Kitale during the afternoon of 3 October.

Rapid Developing Thunderstorms (RDT) is one of the Nowcasting SAF (NWC SAF) nowcasting products that highlights convective storms at different stages i.e. trigger, growing, maturity or decaying. This information is important to weather forecasters in determining when to issue warnings for early action, especially in aviation, which heavily depends on nowcasting. An example of the RDT product is as presented in Figure 1.

The rapidly developing storm was observed using the Severe Convection RGB and RDT products from the NWC SAF.

On 2 October, the public forecast released indicated high chance for several areas over the western parts of the country  receiving showers and thunderstorms. On 3 October the SYNOP observations at 09:00 UTC, for stations over western Kenya, indicated the development of convective clouds, with cumulus clouds being reported in Kisumu and Kitale (Figure 2).

Deep convective clouds, made up of ice particles and characterised by strong updraft (yellow in colour), developed over western Kenya, especially over the Kitale area. The corresponding RDT product shows convective clouds in various stages of development over the area, regions surrounded by red circles indicating areas with growing cells.

Cumulonimbus type 9 clouds were reported in the 12:00 UTC SYNOPS for Eldoret Airport, a station in western Kenya, not far from Kitale (Figure 6).

TThe convective activities over western Kenya grew so rapidly that within an hour, at 13:00 UTC, several new convective cells developed. The new, developing thick convective cells can be seen as yellow colours on the Convection RGB in Figure 6. In respective NWC SAF product this new development is indicated by a small red circle, Figure 7.

The animations below (Figures 8 and 9) demonstrate the rapid development, providing views of the thunderstorm from its development to its dissipation stage.