An upper level cirrus outflow boundary brought thunderstorms to Lake Victoria basin in Western Kenya and Uganda overnight 24/25 March.
22 October 2020
24 March 2013
by Sarah W Kimani, IMTR /WMO-RTC
Western Kenya and the Lake Victoria basin are generally hot and humid with rainfall throughout the year round, mostly in the evenings. The region does not experience a dry season, but records low levels of rainfall in January and February.
During the long rains season (March/April/May) Western Kenya records the highest rainfall — annual rainfall of 1,200 mm and above, with thunderstorms occurring in the afternoon. The thunderstorms in Western Kenya are caused by the high temperatures and abundance of atmospheric moisture creating unstable conditions.
Southern Uganda shares the same tropical climate as Kenya — warmers days and cooler nights — but has two wet seasons: September/October/November and March/April/May.
Much of Uganda is wetter than Kenya due to the influence of Lake Victoria, half of which lies within Uganda’s borders, and is an important local source of atmospheric moisture and thunderstorms — making the shores of Lake Victoria the wettest areas in the country.
The Meteosat-10 WV 6.2 and IR 10.8 images from 24 March, 19:00 UTC to 25 March 09:00 UTC show an upper level cirrus outflow boundary over the Lake Victoria basin, both in Western Kenya and Uganda.
An outflow is air that flows outward from a storm system and radiates from thunderstorms in the form of a wedge of rain-cooled air, creating an arc shaped outflow boundary.
Outflow boundaries can exist at low or high levels, depending on the corresponding surface or upper level conditions. High level outflow boundaries exist when the cumulonimbus cloud bumps or crashes against the tropopause and the cirrus ice particles (downdraft) diverge rapidly in all directions. For strong high level winds and divergence the outflow boundary can be observed from its development stage to its dissipation stage, as in this case.
Figure 1: Meteosat-10 IR 10.8, 25 March 08:00 UTC
Upper level cirrus outflow boundary due to thunderstorm downdrafts during the dissipation phase of the convective system.
Thunderstorms form from the rapid upward movement of warm, moist air and can occur inside warm, moist air masses and at fronts. As the warm, moist air moves upward, it cools, condenses, and forms cumulonimbus clouds that can reach heights of more than 20 km.
Figure 2: Meteosat-10 WV 6.2, 25 March 00:00 UTC
High temperatures and an abundance of moisture are favourable conditions for creating thunderstorms in Western Kenya.
The WV image shows the overshooting tops, which lasted for much longer than the 10 minutes indicated.
Figure 3: KNMI cloud precipitation product, 25 March 05:15 UTC
Cloud precipitation products from The Royal Netherlands Meteorological Institute (KNMI) indicated high amounts of rainfall recorded in parts of Western Kenya and most parts of Southern Uganda, especially around the shores of Lake Victoria.
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