The rainfall graphics use observed data generated by NOAA CPC using both satellite and ground station rainfall estimates. The normal data are from the Australian National University and are based on a minimum of 40 years of historical data through 1995.

The area for each country or region is an area defined to be an important crop area by the FAO.

The normalized difference vegetation index (NDVI) plots use satellite data processed by NASA GIMMS. The averages are calculated using data from 1982-1998.

The NDVI data are "smoothed" by an algorithm using 60% from this ten day period, 30% from the previous ten day period and 10% from the period ten days before that. This was done to moderate the effect of clouds and haze which generally decrease the NDVI values.

The area for each country or region is an area defined to be an important crop area by the FAO.

The graphic is based on ITD location data generated by NOAA CPC. The ITD position is determined by drawing a line through the 15 degree C dew point temperature on surface plots of 12 GMT data each day. For data-sparse areas, the 10m-wind direction convergence line at 00 GMT is used. The latitude of the ITD is determined at each 5 degrees of longitude and the number is entered into a spreadsheet. The 10-day numbers are based on averaging the daily values. The ITD correlates well to the leading edge of the rains moving across the area.

The rainfall line indicates the center of the line of heaviest rainfall. It was modeled by relating the ITD line to historical observed rain event locations for 1979-1995. The dotted line is the calculated midpoint of the rainfall line. If the dotted line falls within the green shaded area, it is north of the historical average of calculated rain line midpoints but within the historical range; if it falls in the beige area, it is south of the historical average of calculated rain line midpoints but within the historical range. Beyond the shaded areas indicates a record north/south location.

The dotted line on the plot is of the calculated rain line location (see above) averaged across the area 10 degrees west to 10 degrees east longitude for each ten day period across the season April 1 through October 31. If the dotted line falls within the green shaded area, it is north of the historical average of the calculated rain line locations but within the historical range; if it falls in the beige area, it is south of the historical average of the calculated rain line locations but within the historical range. Beyond the shaded areas indicates a record north/south location.

The yellow line is the calculated midpoint of the rainfall line. The rainfall uses observed data generated by NOAA CPC using both satellite and ground station information (see above).

As the sun moves north and south from the equator during its annual cycle, the land and water beneath is warmed. The warm air and water vapor rise and form a slight vacuum-a low-pressure zone. This area of calm, ascending air falls between the trade wind zones along the equator and was originally called the doldrums. Now, the more commonly used terms include the Intertropical Convergence Zone (ITZ or ITCZ), the Intertropical Discontinuity (ITD), and the Intertropical Front (ITF) . This zone is important for African agriculture, because the rising air and water vapor caused by the warmth of the sun lead to clouds and rainfall. The line of rains tracking the sun's movement forms a wave-shaped pattern stretching east to west across the continent. Although the line is tracking the sun, atmospheric conditions can speed or delay the progress of the line, so its location is not exactly the same from year to year. This difference in location of the rains can make or break crops in their path, especially in marginal areas, such as the sub-Saharan Sahel zone. Doug LeComte of NOAA tracks the movement of the ITD (see above). Vikki French generates the rain line (see above).

The top graphic shows the most recent location of the rain line versus its normal range. The lower graphic shows the average location of the line between 10W and 10E longitude every ten days for this season.

A simple method for identifying areas subject to problems of flooding or excess moisture has been developed through the joint use of satellite rainfall estimates (RFE) and digital maps of basin boundaries and river networks. Maps are produced which highlight basins experiencing above-average rainfall in the previous ten-day period, and river reaches with potentially higher-than-average stream flow.

NOAA has produced RFE images (Herman et al., 1997) on a dekadal (10-day) time step for FEWS NET since 1995. The images are composed of 0.1-degree (about 10 km) pixels whose values are the estimated number of millimeters of accumulated rainfall for the dekad. A straightforward application of these images has been their use in conjunction with USGS digital maps of basins and river networks. These have been derived from 1-km resolution topographic data, and are part of a topologically coded (Verdin and Verdin, 1999) global data set known as HYDRO1K (http://edcdaac.usgs.gov/gtopo30/hydro/index.html).

Rainfall estimates are summed over river basin areas for each dekad, and cumulatively for the season. These sums are divided by the corresponding values for long-term average conditions (Hutchinson et al., 1995), and excess rainfall scores are assigned to basin areas and river reaches accordingly - the higher the ratios, the greater the scores. Maps are then produced with color codes indicating relative levels of excess precipitation. These products have been named Basin Excess Rainfall Maps (BERMs).

BERM products reveal situations of sustained heavy regional rains that adversely affect food security through flooding and consequent widespread disruption of agriculture, transportation, and market systems. The basin (or catchment) map highlights subbasins (out of approximately 3,000 across the continent) receiving above-average precipitation for the dekad, and cumulatively for the season, by color coding the relevant polygons.

The river segment (or stream) map highlights reaches of river receiving above-average amounts of dekadal and seasonal cumulative precipitation according to a similar scoring system. The difference is that a reach of river may receive rainfall from a much larger upstream area than that of the subbasin polygon in which it lies. Thus, a subbasin may not be highlighted because only light rain is occurring locally, while the reach of river passing through it is highlighted, due to heavy rains in upstream catchments.

The rainfall plots use observed data generated by NOAA CPC using both satellite and ground station rainfall estimates summed over the area within basin boundaries for each river network.

The sea surface temperature (SST) plots use data generated by NOAA CPC using satellite, ship and buoy data.

SST has been correlated with long-term normal rainfall patterns for each region using FEWS station rainfall data for 1922-1995.

The sea surface temperature (SST) image is generated by NOAA CPC using satellite, ship and buoy data. The anomaly is the difference between observed monthly tempertures and the long-term average (1950-present). Redder hues indicate warmer than normal, bluer hues cooler than normal and yellow is near normal.

The plots of the El Niño/La Niña (ENSO) indicators use data generated by NOAA CPC using satellite, ship and buoy data.

Many indicators are used to monitor an ENSO event. Several of the more useful indicators are shown in the plot. Each of the graphed indicators is a 5-month lagged moving average expressed as an anomaly-that is, how different it is from what is usual. Each indicator is calculated so that a positive number indicates an El Niño and a negative number indicates a La Niña event. The indicators in the plot are as follows:

• Air pressure-The Southern Oscillation Index (SOI) compares air pressure in Darwin, Australia (which is typically low), and in Tahiti (which is typically high). When Tahiti has lower-than-normal air pressure and Darwin has higher-than-normal air pressure for a sustained period, it indicates that an El Niño event is occurring. Note: this indicator is expressed as the "negative" value of the usual SOI indicator so that it is comparable with the other indicators. >
• Ocean temperature-The Pacific sea surface temperature indicator (SST) tracks ocean temperatures near the Equator in the Pacific. These are typically warm in the west, near Indonesia, and cool in the east, near Ecuador. When the waters near South America become warmer than usual, it indicates that an El Niño is occurring. This SST indicator is for the part of the Pacific Ocean from 0 to 10 degrees south and 90 to 80 degrees west.
• Winds-Usually the winds along the Equator at 5,000 feet above sea level blow from east to west. During an El Niño, their intensity weakens. This indicator is for the area from 5 degrees north to 5 degrees south and 175 to 140 degrees west (the middle zonal wind anomaly).
• Clouds-During an El Niño, convection and cloudiness increase in the mid-Pacific (an outgoing longwave radiation (OLR) anomaly). This indicator covers the area from 5 degrees north to 5 degrees south and 160 degrees east to 160 degrees west.