Rainfall Scenarios of OND 2024
(Based on Astronomical Observations)
The OND (October-December) season in Kenya is normally referred to as the short rains season. It begins about the 1st or 2nd week of October (onset) and runs all the way to the 2nd to 3rd week of December (cessassion). It has a peak in the 1st and 2nd week of November. The conventional seasonal forecasts in the region are based on the behavior of ENSO and IOD. In the past, we have seen cases of these forecasts performing poorly due to unknown factors blamed mainly on Climate Change. The unique behavior of the SST configurations in both the Pacific and Indian oceans in the recent times is giving forecasters around the world a difficult time.
The seasonal intensities, onsets, cessations and peaks shift from year to year due to changes in climate systems that affect rainfall in East Arica. Most of the systems are based on the evolution of SSTs on the oceans, e.g. the Indian Ocean Dipole (IOD) and El Ni?o Southern Oscillation (ENSO).
On the other hand, the oceans are affected by oceanic and atmospheric tides which have their origin in gravitational tides and thermal tides that come from the spin-orbit motions of the Earth, the moon and the planets of the solar system. As the celestial bodies orbit the sun, a complex combination of gravitational effects act upon the oceans and the atmosphere through the tides which ultimately affect rainfall behavior. Key relative positions of the celestial bodies, in relation to the Earth, are connected to the maximum and minimum distances between the bodies and the earth, and are a function of the gravitational forces. The key astronomical phases of the bodies are associated with these distances and are important in the forecasting process that bears the OND 2024 rainfall scenarios.
The rainfall scenarios are based on extrapolations of key astronomical phases of the planets of the solar system. The OND 2024 season is set on a week by week basis and is based on the phases of planets Jupiter, Saturn, Mars, Mercury, Venus and the Moon and the forecasting process involves the analysis of the analogue years. A summary of these planets and their associated phases is given in Table 1.
A pool of analogue years are selected and a qualitative analysis of the rainfall events (length and intensity) is done. This forms the basis of the rainfall displayed displayed as spatial and temporal distributions of rainfall for the season OND 2024 by use of daily and monthly rainfall. The daily rainfall station data (1957-2023) is from 16 meteorological stations distributed around the country while the monthly data (1948-2023) is the reanalyzed monthly data from NCEP/NCAR Reanalysis .
For the purposes of this seasonal rainfall scenarios, the country is divided into 8 regions. The regions are areas of homogenous rainfall and in each of the regions, two representative rainfall stations are used where actual daily rainfall data is used in the analysis. Figure 1 shows a map of Kenya with the 8 regions used. The regions are Highlands East of the Rift Valley; the Rift Valley; Highlands West of the Rift Valley; South East low lands; North West lowlands; North East lowlands; ?Eastern lowlands and the coast.
2. Forecast Summary
The OND 2024 season will have a mainly below average rainfall in most areas (Figure 2) except the coastal strip and parts of NW. The season is characterized into 5 stages that describe it as a period with 2 opposite extremes in the same season. This behavior is unique and only comes out clearly 2 or 3 times in approximately 30 years. Below are the five stages that constitute the season;
Stage 1 (3rd week of September to 1st week of October 2024)
This period will be largely dry but may have a few days of moderate rainfall
Stage 2 (2nd week of October to 3rd week of October 2024)
Isolated cases of moderate rainfall with some having some scattered stormy rainfall events. Although the period may have predominantly dry conditions, some places may have relatively wet conditions from time to time especially in the 3rd week of October.
Stage 3 (4th week of October to 3rd week of November 2024)
This period will be the main season. In most places, rainfall will be heavy and some places may experience extreme flooding conditions especially in the 1st week of November.???????
Stage 4 (4th week of November to 3rd week of December 2024)
The period will have characteristic dry conditions with many days of dry spells in some places. The dry conditions will be interrupted by a day or some days of moderate rainfall especially in the 1st or 2nd week of December. The dry conditions are likely to resume immediately the rains are over.
Stage 5 (4th week of December 2024 to the 2nd week of January 2025)
The effects of the dry conditions are likely to be lessened or brought to an end during this period. The period will be dominated by light to moderate rains that will come at a time when a normal rainfall season has ended and a dry season has begun. This “late” seasonal rains will continue into the off-seasonal period in January 2025.
3. Forecast in Detail
The detailed forecast presented here below is done for each and every of the 8 regions and in weekly and monthly time steps. This involves the 3 months; October, November and December 2024 for the monthly rainfall scenarios and also 16 weeks, during the period 15th September 2024 to 15th January 2025 for the weekly rainfall scenarios.
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3.1?????? The Month-by-Month Rainfall Scenarios
The spatial distribution scenarios of rainfall are given as month-by month and they show that most parts of the country will be generally be dry (Figure 3(a) to (d)). Most places show a normal above normal rainfall in October while most of the northern Kenya is under normal to below normal rainfall in November. Some areas in the western sector and those in the eastern sector to the south of the equator still maintain above normal rainfall in November. In December most places will be under normal to below normal rainfall levels with the exception of the coast. In January, areas in the highlands will have above normal rainfall while the rest of the country will remain in normal dry conditions.
3.2?????? Week-by-Week Rainfall Scenarios per Region
Below is a more detailed rainfall conditions week-by-week (see also Table 2) in the 8 regions (Refer to Figure1);
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a)????????? Most areas of South East (SE) may have dry conditions but moderate rains will be experienced from time to time from the 4th week of September to the 3rd week of October. Some isolated cases of high intensity rains will start off in the 4th week of October in the upper areas giving way to Moderate rains for most of November (mainly in the 2nd or 3rd week of November) but by the 1st week of December the rains will briefly subside. However, in the 2nd week of December we are likely to have moderate to heavy rains in upper areas of SE and 2nd and 3rd week of December in the lower areas of SE. The rest of the period will have a fews days of scattered rainfall from the 4th week of December increasing in intensity and area by mid-January 2025.
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b)????????? The Highlands East of the Rift valley may have generally dry conditions from the 3rd week of September to the 2nd week of October. However, some isolated storms will kick in sometimes in the 1st or 2nd week of October in areas around Mt. Kenya East and 3rd week of October in areas around the Aberdares / Mt. Kenya West. Onset may be observed in the 1st week of November in most areas and may extend to the 3rd week of November. It is likely that the onset will come with torrential rains between the dates 30th October and 8th November leading to flooding. From the 3rd week of November the dry conditions will set in but isolated cases of moderate to heavy rains may be experienced for some few days in mid-December and again in 1st to 3rd week of January 2025.
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c)????????? In the Coast, the season may start with scattered rainfall events from the 3rd week of September to the 1st week of October which may increase in area by the 3rd week of October. The region may have an onset in the 4th week of October with moderate to very heavy rains falling from time-to-time all the way to the 3rd week of November. In the north coast, very heavy rains will be experienced and this may cause enormous flooding conditions especially in the 4th week of October to the 1st week of November. These similar conditions will be experienced in the south coast but will extend to the 3rd week of November. By the 1st week of December, rainfall incidences will have reduced ushering in a brief period of relatively dry conditions at the coast, however, moderate rainfall? is possible for a few days in the 2nd week of December (North coast) and 3rd week of December (South coast). While the 1st half of January 2025 may not have rainfall, the 2nd half may have a few days of moderate rainfall especially in the south coast.
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d)????????? The North Western may have a predominantly dry with a few isolated and scattered cases of rain sometimes from the 1st week to the 2nd week of October. Onset is likely to be in the 4th week of October with rains extending to the 3rd week of November. The peak of the season will be the 1st week of November. The period from the 4th week of November to the 3rd week of December will be mainly dry conditions, however, some moderate rains may interrupt the dry conditions in the 4th week of December. The period 1st / 2nd week of January 2025 will be generally dry (for areas to the north of Samburu County and western part of Marsabit County) but areas in Turkana County may have a few rainfall incursions from the west.
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e)????????? The North Eastern may have predominantly dry conditions with a few isolated cases of moderate rain sometimes from the 4th week of September to the 2nd week of October due to rainfall incursions from the north and east. High intensity rainfall may be experienced in the 2nd week of October. Onset is likely to be in the 4th week of October with rains extending to the 1st and 2nd week of November. From the 3rd week of November the dry conditions will set in but isolated cases of moderate rains may be experienced in the 1st and 3rd week of December in northern parts of Marsabit County and the 2nd week of December in some parts of Wajir and Mandera counties.
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f)?????????? The Eastern may have predominantly dry conditions with a few isolated cases of rain sometimes from the 4th week of September to the 2nd week of October due to incursions from the north and east. High intensity rainfall may be experienced in the 3rd week of October. Onset is likely to be in the 4th week of October with rains extending to the 2nd week of November. From the 3rd week of November, the dry conditions will set in but isolated cases of moderate rains may be experienced from time to time especially in Garissa and Tana River counties mainly in the 1st week of December and the mid-January 2025.
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g)????????? Most of the Highlands to the West of the Rift Valley may begin with isolated rainfall events starting from the last week of September running to the 2nd week of October. An onset is likely to occur in the 3rd week of October with moderate to heavy rainfall going all the way to the 3rd week of November. From the 4th week of November, rainfall will tremendously reduce in intensity and area and relatively drier conditions may go up to the 2nd week of January. However, moderate to heavier rainfall may fall a few days in the 2nd week and 4th week of December and again in the 2nd week of January 2025.
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h)???????? Most of the Rift Valley may begin with some cases of moderate rainfall in form of isolated events starting from the 1st week of October running to the 3rd week of October. An onset is likely to occur in the 1st week of November to the 2nd week (central Rift) and to the 3rd week in the south Rift. From the 4th week of November to the 3rd week of December dry conditions will be dominant, however, the 2nd week of December may have moderate rains in the central Rift. The 4th week of December may have moderate to heavy rainfall mainly in the south Rift. In the1st and 2nd week of January 2025 the region may have moderate rainfall from time to time.
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