Rainfall Scenarios of OND 2022 (Based on Astronomical Observations)

September, 18 2022

1.????Introduction

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 / 3rd week of December (cessassion). It has a peak in the 1st and 2nd week of November. Our forecasts and climate predictions are based on astronomical observations and predictions of key phases of the planets of the solar system and do not factor in the common climate drivers, e.g. MJO, ENSO and IOD. An astronomical-based forecast was issued for the season and was set on a week by week basis. This forecast is based on Astronomical cycles of Jupiter, Saturn, Mercury and Venus. Normally, the forecasting process involves analogue years associated with Jupiter, Saturn, Mercury, Venus, Mars and the Moon on experimental basis. The cycles are referenced on key phases of the planets, e.g. Conjunctions and oppositions of the planets as well as the Moon's Apogee-Perigee and Full Moon-New Moon phases. A summary of this planets and their associated phases are given in Table 1.

A pool of analogue years are selected by use of these cycles and it is from these years that a careful analysis of the rainfall events is done. The forecast, given in section 2, was developed and is based on this analysis.

?For the purposes of this forecast, the country is divided into 8 regions. The regions are areas of homogenous rainfall and are the traditional areas used in seasonal forecasting in the country (with minor adjustments). 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.

Figure 1: Regions in Kenya used in the forecast.

2.????Forecast Summary

The season is likely to be generally wet in almost the whole country. However, dry conditions will dominate the month of October (and probably 1st and 2nd week of November) where isolated cases of sporadic rainfall are likely to occur from time to time. A general seasonal onset will be observed in the 1st and 2nd week of November especially the eastern sector and 3rd week of November in the western sector. The heaviest rainfall is likely to be received in most areas mainly from the 4th week of November 2022 to the 2nd week of January 2023. Generally, the rainfall season will shift from the traditional October-December period to November-January period and will most likely be longer in the eastern sector and relatively shorter in the western sector.

3.????Forecast by Region

Below is a more detailed forecast which gives rainfall conditions week-by-week (see Table 2) in the 8 regions (Figure1);

?a)?????????Most areas of South East may get a generally dry in the first 3 weeks of October. Onset may be observed in the last week of October. Significant rains will be experienced in two phases; from the 4th week of October to the 4th week of November and 2nd week of December 2022 to the 1st week of January 2023. Relatively heavier rainfall may come in two events between 2nd and 4th week of December 2022.

?b)?????????The Highlands East of the Rift valley, may get a generally dry October, however, there may be some isolated storms sometime in the 2nd or 3rd week of October. Onset may be observed in the 1st week of November in some areas and 4th week in others. Significant rains will be experienced in two phases; the 1st / 2nd week of November and from 4th week of November 2022 to the 3rd week of January 2023. Relatively heavier rainfall may come in two events; between 1st and 2nd week of December 2022 and between 4th week of December 2022 and the 2nd week of January 2023.

?c)?????????In the Coast region, the season may begin with a relatively dry October but may have occasional rains from time to time. The region may have an onset in the 2nd week of November with significant rains falling from time-to-time all the way to the 2nd week of January. Extreme cases are expected in two phases; between 3rd week of November and 1st week of December 2022 and from the 4th week of December 2022 to the 2nd week of January 2023.

?d)?????????The North Western region may have a relatively short rainfall season dominated by dry conditions most of the time but may also get isolated cases of rainfall occasionally during the season. The season may be very dry in most of October up to the 2nd week of November and nearly similar conditions may be felt the rest of the period. However, extremely heavy and episodic rainfall may occur from the 2nd / 3rd week of October and the 4th week of November to 1st week of December.

?e)?????????The North Eastern region; the month of October will be very dry. Significant rains are likely to fall from the 1st week of November to the 2nd Week of January 2023, however, heavy rainfall may be episodic in the region in the 1st / 2nd week of November, 2nd week of December and from the 4th week of December 2022 to the 2nd week of January 2023.

?f)??????????The Eastern region; the month of October will be very dry, however, a 1 day or two day sporadic rainfall is possible in the 3rd week. Significant rains are likely to fall from the 1st week of November to the 2nd Week of January 2023, however, heavy rainfall may be episodic in the region from the 4th week of December 2022 to the 2nd week of January 2023.

?g)?????????Most of the Highlands to the West of the Rift Valley may have a relatively dry October with the dry conditions extending to the first two weeks of November. The region may get significant rainfall from the 3rd week of November to the 1st week of December then again from the 4th week of December 2022 to the 2nd week of January 2023. The heaviest rainfall may be observed in the last week of November to the 1st week of December 2022.

?h)????????Most of the Rift Valley may have a relatively dry October with the dry conditions extending to the first two weeks of November, however, October may experience some occasional rains from time-to-time especially in the 2nd and 3rd week. The region may get significant rainfall from the 3rd week of November to the 2nd week of December, then again, from the 4th week of December 2022 to the 3rd week of January 2023. The heaviest rainfall may be observed in the 1st and 2nd week of December 2022.

Table 2: Temporal distribution timeline of rainfall during the OND 2022 season per region

4.????Disclaimer

The forecasted conditions may deviate due to influences of other non-astronomical factors, e.g. ENSO, IOD an MJO. However, MJO has been given a special consideration for those interested in seeing how climate conditions associated with it, interacts with the main forecast.

The MJO and Associated Climate Conditions in Kenya

In the past the forecasts done have been found to deviate from the intended conditions due to influences of other climate drivers that are not incorporated in the forecasts. Three climate drivers come into the picture; ENSO, IOD and Madden-Julian Oscillation (MJO). While the first two are fairly stable, MJO is highly unstable and unreliable in seasonal forecasting. The MJO is associated with weekly to monthly periods of enhanced and suppressed rainfall in many parts of the tropical world that includes Kenya which lies across the Equator approximately between latitudes 5oN and 5oS.

Past experiences have shown that the MJO is a significant causative factor in variations of short-term climate conditions in Kenya. Its evolution is highly erratic and therefore its prediction skill is fairly low even within 14 days and the skill is almost negligible beyond two weeks.?In the last two seasons, MJO was found to have altered or shifted the periods of forecasted rainfall by as much as 1-2 weeks in some instances, however, most of the key conditions forecasted remained as forecasted.?This is a little setback to our astronomical based predictions.?

Figure 2: Common climate conditions in Kenya and associated 8 phases of the MJO based on the MJO phase diagram.?

For the last 10 years, it has been observed that the evolution of MJO goes with a certain persistent climate pattern in Kenya throughout the year. After observations between 2010 and 2021, a phase diagram with the climate conditions in Kenya which are associated with each of the 8 phases was developed and is provided here.?It is meant to assist the reader with a better picture of the conditions in Kenya and you can tell what kind of conditions that are likely to dominate with a 2 weeks lead time by referring to the phase diagram provided by;

BOM ( https://www.bom.gov.au/climate/mjo/ ) or

NOAA (https://www.cpc.ncep.noaa.gov/products/precip/CWlink/MJO/mjo.shtml )

By comparing the two scenarios (the forecast and the MJO climate condition given in the table) you will likely get a better picture of the general conditions as the season progresses.

___________________________________________________________________