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September rainfall will be particularly important in determining seasonal outcome for the Sahel

  • Seasonal Monitor
  • West Africa
  • September 18, 2014
September rainfall will be particularly important in determining seasonal outcome for the Sahel

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  • Key Messages
  • Update on Seasonal Progress
  • Forecasts
  • Partner
    USGS
    Key Messages
    • Over much of Senegal and Mauritania August brought rainfall accumulation with levels more near normal. Favorable Sea Surface Temperature trends off the coast are likely to extend wetter conditions for the next few weeks. This improvement, however, has come late in the season and may not be able to reverse the effects of rainfall deficits that occurred earlier in the season.

    • For the rest of the Sahel, the observed warming trend off the Gulf of Guinea may result in a bad distribution of rainfall across the Sahel for the end of the season, which could have a negative impact on crop development, particularly in areas that experienced long planting delays.

    • Average to above-average rainfall continues over much of the Guinean-Sudanian and Bi-modal Zones. Good crop development can continue to be expected, even in areas that are experiencing relative rainfall deficits as cumulative rainfall totals are enough to meet crop water requirements.


    Update on Seasonal Progress
    • The southward retreat of the Intertertropical Front (ITF) appears to have started seasonably during the last 10 days of August as it was positioned one to two degrees of latitude south of its average position for the previous 10-day period. Though it generally maintained a seasonally average position over the region during the last 10-day period of August, the ITF’s southward retreat had negative impacts on rainfall as it was generally met with warming Sea Surface Temperatures (SSTs) in the Gulf of Guinea, leading to relatively weak rainfall. Despite a drier than average end of August, total rainfall accumulation for the month was generally above average due to the good rains seen the first three weeks of the month.
    • Most of the Sudanian-Sahelian Zone received average to above-average rainfall (Figures 1 and 2) with only a few scattered areas experiencing light deficits.
    • Over the Guinean-Sudanian Zone, rainfall was average to above-average, at the exception of Nigeria and southern Cameroon with scattered areas of light to moderate deficits.
    • August rainfall in the Bi-modal Zone was above average except over southern Liberia/Cote d'Ivoire and southern Cote d'Ivoire/Ghana border areas.
    • The impact of August rainfall on crop and pastures by Zone:
      • In the Bi-modal Zone the first three weeks of August were seasonally dry but during the last 10 days rainfall was average to above average, which could mark the start of the second season in the zone.
      • In the Guinean-Sudanian Zone crop water requirements continue to be met everywhere including over areas with below-average rainfall where, despite relative deficits, total accumulation provides sufficient moisture for crop growth.
      • The generally average to above-average August rainfall in the Sudanian-Sahelian Zone provided adequate moisture for continued crop development over most of the region. It brought relief even in the western part of the Sudanian-Sahelian Zone, which had experienced significant planting delays due to prolonged dryness. There are two main cases to consider in terms of August rainfall impact on crop and pasture production:
        • In the western part of the Sudanian-Sahelian Zone (Senegal-Mauritania) the risk for reduced rainfed and flood recession crop yield remains high. However, if the warming SST trend in the eastern Atlantic observed recently along the Senegal-Mauritania coastal line continues, known to be favorable for rainfall over the area, there is a slight chance for rains to continue until mid-October, which could allow for closer to normal harvests in some areas.
        • Elsewhere conditions have been generally good for the second consecutive month and crops are at their reproductive phase with phenology varying from flowering where planting delays were observed at grain filling and complete maturation where the season start was near normal. At least an average harvest could be expected provided rains continue until the end of September. However, if the warming SST trend observed during the last couple of weeks over the Gulf of Guinea continues, the ITF retreat will be atypically fast, which will compromise crop maturation process at least in areas where planting delays were significantly long.
    • According to the short and medium term forecasts from NOAA/CPC, the next two weeks are expected to be wet with moderate to heavy rainfall forecasted over most of the region. The north of the Sudanian-Sahelian Zone is an exception, which includes the regions of Maradi, Zinder and Diffa in Niger, the northern parts of Yobe and Borno States in Nigeria, the regions of Lac, Kanem, Bahr el Ghazal the northern parts of Batha East and Batha West, the western part of Ouaddai and Wadi Firra Regions in Chad.

    Forecasts

    Most forecast models indicate a continued warming trend for the tropical eastern Pacific Ocean SSTs, but chances of El Niño during the rest of the summer have decreased to below 50 percent. Consequently, no significant influence in rainfall pattern over West Africa is expected.

    The latest NOAA-CPC Northern American Multi-Model Ensemble seasonal forecast updated in early September showed little skill over the region. The greatest SST influence on West African rainfall will be determined by the Gulf of Guinea and Eastern Tropical Atlantic SSTs that have started a warming trend for the last couple of weeks, which is not favorable for Sahel rainfall. If this warming continues there may be an end of season in the Sahel characterized by below-average and poorly distributed rainfall.

    Figures Figure 1. Total rainfall estimate (RFE) in mm, month of August 2014

    Figure 1

    Figure 1.

    Source: USGS/FEWS NET

    Figure 2. Rainfall estimate (RFE) anomaly compared to the 2001-2010 mean, month August 2014

    Figure 2

    Figure 2.

    Source: USGS/FEWS NET

    Seasonal Calendar in a Typical Year

    Figure 4

    Seasonal Calendar in a Typical Year

    Source: FEWS NET

    FEWS NET’s Seasonal Monitor reports are produced for Central America and the Caribbean, West Africa, East Africa, Central Asia, and Somalia every 10-to-30 days during the region’s respective rainy season(s). Seasonal Monitors report updates on weather events (e.g., rainfall patterns) and associated impacts on ground conditions (e.g., cropping conditions, pasture and water availability), as well as the short-term rainfall forecast. Find more remote sensing information here.

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