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Poor rainfall over central West Africa observed at the start of the season

  • Seasonal Monitor
  • West Africa
  • June 10, 2015
Poor rainfall over central West Africa observed at the start of the season

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  • Key Messages
  • Update on Seasonal Progress
  • Forecasts
  • Partner
    USGS
    Key Messages
    • Over the past two months the Intertropical Front (ITF) northward migration has been generally slower than typical over the middle of the Sudanian-Guinean zone, resulting in below average rainfall over a large portion of the region.

    • The Bi-modal zone and the southern part of the Sudanian-Guinean zone have received generally well distributed and average to above average rainfall so far this season. This has contributed to favorable conditions for cropping in these regions.

    • Favorable rainfall over much of the region led to timely or early planting over the western (northern parts of Guinea and Cote d'Ivoire, southern Mali) and eastern parts (southern Chad) of the Sudanian-Guinean zone. However, delayed planting has been observed over the central part of the region (Central Nigeria, northern Cameroon, northern Benin, Togo and southwestern Burkina Faso).

    • Given the favorable medium term forecast, sowing/planting is expected to continue northward into the Sudanian-Sahelian zone in June.


    Update on Seasonal Progress
    • The Intertropical Front (ITF) is the leading edge of the climate system that brings rain to West Africa during the northern hemisphere spring/summer/fall. Over the past two months the ITF northward migration had been generally near average over western (Guinea-Senegal-Mali) and eastern (Chad-Cameroon-CAR) areas relative to its climatological position. Over central West Africa, however, its northward migration was much slower than normal, hovering 2 to 3 degrees south of its climatological position.
    • The delayed northward migration of the ITF in over central West Africa resulted in below-average rainfall in the north-central Sudanian-Guinean zone. This area included Burkina Faso, northern Ghana and Togo, Benin, Nigeria and Cameroon (Figures 1 and 2) where rainfall deficits ranged from moderate to severe. The slow northward progression of the rains over affected areas resulted in some delayed planting in part of the northern Guinean-Sudanian zone. The rainy season in the Sahelian zone does not typically begin until June.
    • In the Bi-modal zone and western and eastern parts of the Guinean-Sudan zone rainfall totals from the beginning of April to the end of May have been generally average to above average with good temporal distribution, providing for favorable cropping conditions for the long season.
    • Warmer Gulf of Guinea Sea Surface Temperatures (SSTs) are known to weaken rainfall accumulation over Sahelian West Africa, but are favorable for rainfall over southern coastal areas. Gulf of Guinea SSTs have been warmer than average in recent months, which explains both the delayed northward migration of the ITF over central West Africa and generally above-average rainfall accumulation in the Bi-modal and Sudanian-Guinean zones.

    Forecasts
    • Most forecast models indicate El Niño will continue through the northern hemisphere summer. El Niño generally correlates with suppressed rainfall over the Sudanian-Sahelian zone and favorable conditions in the Bi-modal and Sudanian-Guinean zone, but this correlation is known to be weak. The Gulf of Guinea SSTs, however, constitute the strongest driver of the spring and summer weather pattern in West Africa.  According to forecasts, the recently observed Gulf of Guinea cooling trend will continue through the summer, which is often correlated with a deep penetration of the monsoon into the continent.
    • According to the short and medium term forecasts from NOAA/CPC, rainfall is expected to continue the next two weeks without any atypical dryness, with rains extending northward through most of the Sudanian-Sahelian zone.
    Figures Figure 1.Total rainfall estimate (RFE) in mm, 1st dekad of April to 3rd dekad of May

    Figure 1

    Figure 1

    Source: USGS/FEWS NET

    Figure 2. Rainfall estimate (RFE) anomaly compared to the 2010-2014 mean, 1st dekad of April to 3rd dekad of May

    Figure 2

    Figure 2

    Source: USGS/FEWS NET

    Figure 3. Start of Season (SOS) anomaly for the third dekad of May

    Figure 3

    Figure 3

    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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