1.1 WEATHER SUMMARY FOR MARCH 1999
During March, the highlands west of the Rift Valley, central Rift Valley, Lake Basin, highlands east of the Rift Valley and Nairobi areas received amounts of rainfall generally above their respective long term means (LTM). The coast, northeastern, northwestern and parts of southeastern districts received rainfall below their respective long-term means (LTM). The highest amounts of rainfall for the month were reported over Kericho, Narok and Kakamega stations, which received 297.6 mm, 289.0 mm and 264.8 mm respectively (see attached table). Temperatures were high particularly during the first half of the month. The rainfall was characterized by large spatial and temporal variations.
1.2 SYNOPTIC SITUATION DURING MARCH 1999
During this period, the high pressure systems around Spain (the Azores region) and North Africa remained fairly strong. The Arabian high pressure ridge weakened gradually, at times inducing moisture influx over the eastern sector of the country. Marked diffluent flow was however maintained over the northeastern parts of the country during the period. Weak tropical cyclone systems were reported over Southwest Indian Ocean giving rise to diffluence over the eastern sector of the country during the first ten days of the month. The high pressure system over Southwest Indian Ocean was relatively weak, but strengthened slightly towards the end of the month. The La Nina conditions continued to exist over the Pacific Ocean while pockets of positive SSTAs (Sea Surface Temperature Anomalies) were also observed over parts of Southern Atlantic and Southwestern Indian oceans. The North Atlantic Ocean reported negative SSTAs.
2. DAIRY FARMING WEATHER CONDITIONS DURING MARCH 1999.
During the month of March, the onset of rainfall was realised in most areas in the north rift valley with Nakuru registering over 100mm of rainfall by the end of the 2nd dekad. Temperatures were on the decline, ranging between 10 and 28 degrees Celsius. Regeneration of vegetation was realised in most places following the onset of the seasonal rainfall, thus improving pasture availability.
3. EXPECTED SYNOPTIC SITUATION DURING APRIL 1999.
The high pressure systems over the Mediterranean region and North Africa will gradually weaken during the period. The southern Atlantic Ocean high pressure system will be moderately strong and will achieve near normal intensity for the period. The Arabian high pressure ridge will collapse as the Month progresses while the establishment of the Southwestern Indian Ocean high pressure system will give rise to the East African high pressure ridge. The warm SSTs over the Mozambique Channel area and south of the Malagasy Island will slow the development of the East African high pressure ridge. The meridional (North-South) arm of the ITCZ will overlie central Africa, Uganda and western sector of our country. The zonal arm of the ITCZ will be located over our country though diffuse. The cooling over the Northern Atlantic and parts of the Southern Atlantic and warming over the Southwestern Indian Oceans will assist in marked eastward displacement of the north-south (meridional) arm of the ITCZ to affect the Western, Central Rift Valley, and parts of central districts and Nairobi.
The Global models indicate that the cold sea surface temperatures associated with the La Nina event would be sustained up to June 1999. The warming observed to the south of Mozambique channel will have negative effects on pressure build up over southwestern Indian Ocean leading to a generally weaker than normal East African ridge and less moisture influx into the eastern parts of the country.
From these developments, it is expected that the rainfall performance in April will be below normal in the eastern part of the country. The western parts of the country, parts of the southeastern lowlands and central parts of the country are expected to receive near normal to normal rainfall amounts. However, the rainfall is expected to peak during the month and episodes of flash floods are expected over various parts of the country.
4. EXPECTED WEATHER IMPACTS OF DAIRY PRODUCTION DURING THE MONTH OF APRIL 1999.
From the foregoing, it is expected that the Lake Victoria basin, highlands west of the Rift Valley, central Rift Valley including Narok and northwestern districts, The highlands east of the Rift Valley, Nairobi area and parts of Machakos Kitui and Makueni will receive moderate rainfall in April. However, the rainfall will be characterized by poor spatial and temporal distribution. The amounts will be near the climatological long term means. These conditions are expected to improve pasture regeneration thus providing adequate fodder for the animals and subsquently increase dairy production in these areas.
Coastal region, Northeastern districts and the remaining parts of southeastern districts will receive below normal rainfall with poor spatial and temporal distribution.
Despite the poor rainfall performance, few areas may experience occasional outbursts of heavy rainfall causing flash floods. The peak rainfall performance is expected towards the end of the month at the coast. These conditions will however benefit the dry areas as a result of increased soil moisture conditions.
KEY OF SCIENTIFIC WORDS USED
INTER-TROPICAL CONVERGENCE ZONE (ITCZ): An extensive area of seasonal low pressure caused by the seasonal migration of the sun and associated with heavy rainfall, lightning and thunder.
HIGH PRESSURE SYSTEM: An area associated with clear skies or fine weather clouds.
RIDGE: Asymptote of high pressure from which winds flow outward.
TROUGH: Asymptote of low pressure towards which winds flow and meet.
MOST PLACES: Between 66% and 100%.
SEVERAL PLACES: Between 33% and 66%
FEW PLACES: Between 0 and 33%