Africa: Drought and floods hazards assessment: 21 Aug 2003
Update of the ITCZ Position
During the period from August 11-20, 2003, the African portion of the Intertropical Convergence Zone appeared to begin its transition southward, as the longitudinal averaged dekadal position was located around a full degree south from its position during August 1-10. If this does, indeed, mark the beginning of the ITCZ's southward movement, this year's peak position would have occurred around ten days earlier than climatologically normal. The 2003 peak northward location of around 20.0 degrees north would also be around 1.4 degrees north of the normal maximum northward position. The fact that the ITCZ has been located north of normal since around mid-June has led to a two-faced situation throughout much of the Sahel and western Africa. The northward bias of convective activity has led to enhanced rainfall across much of Africa between 10 degrees north and the Sahara Desert. Some regions to the south, however, have experienced locally severe dryness with much less rainfall than usual during the season.
As tropical waves continue to sweep across the continent, regions in western Africa were again exposed to torrential amounts of rain. Nearly continuous rain showers over regions in southern Senegal, Gambia, northern Guinea and western Mali produced areas of isolated flooding and some crop damage. Most of the area reported greater than 20 mm per day. Regions to the southeast in southern Cote d'Ivoire and Ghana continued with seasonably dry conditions, while just to the north in Burkina Faso, several waves brought heavy showers. One single event on August 23 produced greater than 50 mm in central areas of the country, but local regions reported extreme amounts. In eastern Africa, heavy rain in central and southern Sudan and Chad, again, brought severe flooding to isolated regions. RFE estimates showed daily amounts around 20 to 40 mm, with some isolated heavier amounts over all ready saturated grounds. Ethiopia had seasonal rains over much of the western and central growing areas and further eastward into pasturelands, while further to the south, western Kenya saw beneficial rains. A series of cold fronts with isolated heavy rains and gale force winds came across the region in the Western Cape of South Africa. Several reports indicated damage along the coastal regions, but much of the area received beneficial amounts of rain after a slow start to the winter crop-growing season. These cold fronts also produce below normal temperatures in the higher elevations of central South Africa and Lesotho.
WEEKLY AFRICAN WEATHER HAZARDS ASSESSMENT STATEMENT AUGUST 27, 2003 DISCUSSION:
1) Beneficial showers across northern Senegal and southern Mauritania favored local agriculture, while more substantial rains passed to the south across central and southern Senegal. Conditions this season are better than last season, when dryness and extreme heat resulted in severe drought. Rainfall totals so far this season, however, are still somewhat below normal across northwestern Senegal and southwestern Mauritania. This includes northern sections of Senegals Groundnut Basin. Additional rains are needed to ensure a good cropping season and to ease lingering impacts from last years drought, such as degraded pastures. Occasional showers are expected during the period, resulting in some further improvement.
2) Recent heavy rainfall and an overall wetter-than-normal season has resulted in saturated soils and swollen rivers across portions of the Sahel. Additional heavy rains are expected across northwestern Nigeria, extreme southwestern Niger, Burkina Faso and surrounding areas as easterly waves seem to be favoring this region. As a result, the potential for flooding exists in these areas throughout the period.
3) Locally heavy seasonal rains are expected to exacerbate flooding problems across the highlands of Eritrea and northwestern Ethiopia. Runoff from these rains may result in additional flooding problems in nearby Sudan along the lower stem of the Blue Nile, and some of its tributaries. Flooding may also continue to be a problem in Sudanese state of Kassala, which was recently impacted by serious flooding.
4) Much below normal rainfall three years in a row has resulted in long term drought across southern Sanaag and northern Sool regions in Somalia. Long term drought has degraded pastures across the area. Dry conditions are expected until mid and late September, when the second rainy season occurs. More substantial rains typically occur during the May-June wet season.
5) Lighter than normal rains across southern portions of the Ethiopian highlands and adjacent portions of southeastern Sudan has raised concerns about reductions in regional crop production. Rainfall amounts since June 1 are between 50 and 75% of normal. The wet season in the southern highlands usually runs into November. Over the next several days, showers are expected to continue, with either little change or a gradual decrease in rainfall deficits.
6) Rainfall totals for the 2002-03 rainy season were between 40 and 65% of normal across northeastern South Africa, southern Mozambique, and much of Swaziland. Seasonal rainfall deficits range from 150 to 400 mm across the region. This has resulted in a hydrologic drought across the region, reducing water supplies for wells, reservoirs and watersheds. Little improvement is expected until November, when the rainy season gets under way.
7) The mini dry season set in about a month early across southern Liberia, southern Cote DIvoire and southwestern Ghana. This resulted in a very dry July and may have stressed reproductive and maturing crops in the region. Seasonal rains across central Liberia and central Cote DIvoire have been lighter than normal, with some improvement courtesy of some recent rains. Further improvement is expected across northern parts of the area during the next several days. Southern areas will see scattered shower activity. However, rainfall across southern Cote DIvoire and southern Ghana will increase in subsequent weeks as the ITCZ approaches from the north and the second wet season sets in.
AUTHOR: Chester V. Schmitt