vf-tropicom Location of Rains Across West Africa

Current Season | Back to historical analysis page
The Calculated Rain Line Location Across the Season

Rainline location


The Intertropical Discontinuity/Intertropical Front

From the African Desk, Climate Prediction Center, NOAA

October 21-31 1998 - the ITD/ITF retreated to an average position of 13.1 degrees N latitude across West Africa (10 W long - 10 E long). This was 0.5 degrees north of its long-term mean position but 1.6 degrees south of last period's location. The ITD has been north of normal for 6 consecutive weeks. The east portion of the ITD/ITF (20-35 E long) was at 12.5 degrees N latitude. This was 1.6 degrees north of its estimated normal location for this time period.

October 11-20 1998 - the ITD/ITF retreated to 14.7 degrees N latitude over West Africa. This was 1.4 degrees south of the position for the preceding period and 0.5 degrees north of the long-term mean position. The western ITD (10 W to 10 E longitude) was north of normal for the fifth consecutive period. The ITD in East Africa (20-35 E long.) was well north of normal, especially over Sudan, averaging 14.3 degrees N. This was about 2 degrees north of its estimated normal location and the farthest north for the time period in 9 years of record-keeping. The ITD is typically retreating southward quite rapidly this time of year but, during this year, the eastern portion of the ITD showed little movement between late September and October 17. The associated circulation anomaly is related to the above-normal rainfall recorded across Ethiopia and Sudan this month.

October 1-10 1998 - the ITD/ITF retreated to 16.1 degrees N latitude over West Africa. This was 1.0 degrees south of the position for the preceding period and 0.7 degrees north of the long-term mean position. The ITD was north of normal for the fourth consecutive period. The ITD in East Africa showed little change from the preceding period, averaging 14.6 degrees N between 20 and 35 degrees east longitude. This was 1.3 degrees north of its estimated normal location. The ITD is typically retreating southward quite rapidly this time of year, so the lack of movement was unusual, and a likely factor in the above-normal rainfall recorded across much of Ethiopia and Sudan during October 1-10.

September 21-30 1998 - the ITD/ITF retreated southward to 17.1 degrees N latitude over West Africa. This was 1.5 degrees south of its position for the preceding period and 0.6 degrees north of the long-term mean position. This was the third consecutive period the ITD was north of normal, and suggests that the rainy season lingered longer than usual in the Sahel zone. In the east (Chad and Sudan), the ITD retreated sharply this period, averaging 14.5 N. This was near its estimated normal location. For the June-September season, the ITD has averaged 18.0 degrees north in West Africa. This is slightly north of its long-term mean position of 17.8 degrees N and near last year's position (18.0). The eastern ITD, averaging 15.9 N, was also north of its expected location. The implication is that the overall circulation pattern this summer favored plentiful rainfall in the Sahel countries.

September 11-20 1998 - the ITD/ITF retreated southward to 18.6 degrees N latitude over West Africa. This was 0.8 degrees south of the position for the preceding period and 1.2 degrees north of the long-term mean position. This was the second consecutive period the ITD was well north of normal, and indicates that the rainy season is lingering longer than usual in the Sahel zone. As a result, rainfall was generally above normal across West Africa, though not quite as far north as during the previous period. For the third consecutive period, the ITD was also much farther north than normal over East Africa, averaging 18.1 N. This position was 2.7 degrees north of the estimated normal location. Though the slow retreat of the ITD this year has resulted in abundant rains in Sudan, Ethiopia, and elsewhere, the downside is increased flooding, as recently seen across much of the Sahel and in Sudan. For the season-to-date (since June 1), the western ITD has averaged 18.1 degrees north, which is slightly north of the long-term mean position of 17.9 degrees N and last year's position (18.0).

September 1-10 1998 - the ITD/ITF's mean position was 19.4 degrees N latitude over West Africa. This was 0.6 degrees north of the position for the preceding period and 1.3 degrees north of the long-term mean position. Typically, the ITD is retreating southward by early September, along with the seasonal rains. Instead, the ITD advanced to a position close to its maximum for the season. The ITD was also much farther north than normal over East Africa, though it did retreat southward during the period. For September 1-10, 1998, the ITD averaged 17.8 N in the east (20-35 degrees longitude east). This is 0.2 degrees south of the revised position for August 21-31 and 2.1 degrees north of the estimated normal position. For the season-to-date (since June 1), the ITD has averaged 18.0 degrees north, which is near the long-term mean position of 17.9 degrees N and similar to last year's position (18.0).

August 21-31 1998 - The ITD/ITF's mean position was 18.8 degrees N latitude over West Africa. This was 0.4 degrees north of the revised position for the preceding period and 0.2 degrees south of the long-term mean position. For the season-to-date (since June1), the ITD has averaged 18.0 degrees north, which is near the long-term mean position of 17.9 degrees N and slightly south of last year's position (18.1). In East Africa, the ITD during August 21-31 was at 18.0 N. This is 1.1 N of the estimated normal position and 0.5 N of last period's revised position. Note that the West Africa and East Africa ITD locations have been revised for the August 11-20 period to 18.4 and 17.5, respectively. In summary, it appears that the ITD peaked somewhat early this year in the west and has retreated from its late July-early August peak. However, the ITD is little changed from last period. In the east, the ITD during August has been north of normal, resulting in a delay in the rainy season's retreat.

August 11-20 1998 - The ITD/ITF's mean position was 17.7 degrees N latitude over west Africa. This was 1.8 degrees less than the preceding period and 1.6 degrees south of the long-term position. These figures would indicate a dramatic and unprecedented shift south. Only once since 1979 has the ITD been at such a southerly position at this time: in 1984, and in 1984, the line had been far south of normal for most of the season, so there was no dramatic shift south at that time.

However, the rains during the period, as shown in the 5-panel maps, indicate that the rain line may not have regressed as much as the figures would indicate. The rains, which typically are estimated well by the rain line calculation, are significantly north of the calculated rain line for the August 11-20 period.

For the season-to-date (since June 1), the ITD has averaged 17.7 degrees north. This is near the long-term mean position of 17.8 degrees N and similar to last year's position. In East Africa, the ITD during August 11-20 was located near 17.5 degrees, 0.1 degrees north of the estimated normal position.

Sahel seasonal rains typically peak from late July to late August, corresponding to the northernmost location of the ITD which typically occurs during August 11-20. Because of the high variability of location of the ITD line this season, and the sharp southerly drop during this period, it is likely that the rainy season has peaked early.

August 1-10 1998 - The ITD/ITF's mean position was 19.5 degrees N latitude over West Africa. This was the same as the preceding period and 0.5 degrees north of the long-term mean position. For the season to date (since June 1), the ITD has averaged 17.7 degrees N. This is near the long-term mean position of 17.6 degrees N and similar to last year's position. In East Africa, the ITD during August 1-10 was located near 17.9 degrees N., or 1.5 degrees north of the previous position and 0.7 degrees north of the estimated normal position.

July 21-31 1998 - The ITD/ITF advanced northward across West Africa to 19.5 degrees N latitude. This is 1.7 degrees north of its July 11-20 position and 0.6 degrees north of the long-term mean position. Over eastern Africa, the ITD advanced northward to 16.7 degrees N, which is 0.2 degrees north of its estimated normal position. Reflecting the jump northward of the ITD, rainfall picked up across the Sahel countries, and flooding was reported in late July and early August in several countries from Senegal to Ethiopia.

July 11-20 1998 - The ITD/ITF advanced northward 0.5 degrees latitude to 17.8 degrees N across West Africa. This is slightly south of the long-term mean position of 18.3 degrees N and 0.4 degrees south of last year's position. The ITD's modest northward advance was consistent with the observed rainfall increases over the Sahel region during July 11-20. The ITD-East (20 degrees E-35 degrees E) advanced to 16.2 degrees N, which is 0.3 degrees north of its estimated normal position. The ITD was relatively depressed (south of normal) over Chad and western Sudan, generally fluctuating between the 15th and 16th parallels.

July 1-10 1998 - The ITD/ITF advanced 1.1 degrees latitude to 17.3 degrees N across West Africa. This is only slightly south of the long-term mean position of 17.5 degrees N and near last year's position of 17.2 degrees N. (In the report for June21-30, 1998, the corresponding position for 1997 was erroneously given as 16.5 degrees N. This is actually the 1996 position. The correct value for June 21-30, 1997, is 17.3 degrees N.) Consistent with the sharp northward advance of the ITD, rainfall increased across the Sahel from Mali eastward and decreased along the Gulf of Guinea coast. The ITD remained well south of normal and last year's location from eastern Niger through Chad and Sudan. As a result, the ITD-East position was south of normal for the 4th consecutive week, at 14.6 degrees N versus the estimated normal of 15.2 degrees N.

June 21-30 1998 - The ITD/ITF over West Africa retreated south to 16.2 degrees latitude, a 0.5-degree move southward from June 11-20. This is consistent with the reduced rainfall and high temperatures noted over the Sahel countries during this period. The ITD was 0.8 degrees south of normal and 0.3 degrees south of last year's position at this time. For June 21-30, this was the farthest south the ITD has been since 1984, when it stood at 16.1 degrees N. Also, this is the first time since record-keeping began in 1979 that the ITD was farther south in late June than in early June. The ITD typically advances northward until mid-August. During June and July, it is unusual for the ITD to backtrack, as Northern Hemisphere circulation features are advancing northward during this time of year. In the east (20 degrees E-35 degrees E), the ITD also retreated, moving south 0.6 degrees to 13.3 degrees N latitude, which is about 1.3 degrees south of normal. This is the farthest south of record for this time period, but records are only available for 1989-97. As in the west, surface weather reflects the lack of the seasonal advance northward of the weather systems, and the rainy season is off to a very poor start in Sudan's main crop areas.

June 11-20 1998 - The ITD/ITF over West Africa was located at 16.7 degrees N latitude. This is 0.3 degrees north of the long-term mean position and last year's position as well. Though the ITD slipped southward by 0.1 degree from the June 1-10, 1998, location, rainfall generally advanced northward this period. Since April 1, 1998, the ITD has averaged 0.7 degrees north of its long-term mean position. Over eastern Africa, the ITD was near normal during June 11-20 at 13.9 degrees N.

June 1-10 1998 - The ITD/ITF over West Africa was located near 16.8 degrees N, or 0.9 degrees north of the long-term mean and 0.5 degrees north of the location during May 21-31, 1998. Over East Africa, the ITD this period retreated to 12.8 degrees N vs. the long-term mean of 13.6 degrees N and last period's value of 13.8 degrees N.

May 21-31 1998 - The ITD/ITF over West Africa was located near 16.3 degrees N, or 0.9 degree north of the long-term mean and 1.1 degrees north of the location during May 11-20. The ITD/ITF has been north of its long-term mean every period this year since the start of April. Its mean position during May 11-20 was 15.2 degrees N; during May 1-10, 14.6 degrees N; during April 21-30, 13.9 degrees N; during April 11-20, 12.7 degrees N; and during April 1-10, 11.9 degrees N.

May 11-20 1998 - The mean position of the ITD/ITF over West Africa was 15.2 degrees N, up from 14.6 degrees N during May 1-10. The long-term mean is 14.6 degrees N. The ITD/ITF has been north of normal every 10-day period since the beginning of April. Over eastern Africa, the ITD/ITF was located near 13.0 degrees N, up from 12.7 degrees N during the preceding period and 0.6 degree north of the estimated long-term mean.

May 1-10 1998 - The ITD/ITF advanced northward to 14.6 degrees N across West Africa. This is up from 13.9 degrees N during the last period. The long-term mean is 13.9 degrees N. The ITD/ITF's progress is about 10 days ahead of schedule, implying that the seasonal rains are moving into the Sahel earlier than usual. In East Africa, the ITD was also north of normal, averaging 12.7 degrees N during May 1-10, compared with the long-term mean of about 11.8 degrees N.

April 21-30 1998 - The ITD/ITF advanced northward to 13.9 degrees N latitude across West Africa. This is up from 12.7 degrees N during the preceding period. The long-term mean is 13.1 degrees N. Across eastern Africa, the ITD/ITF advanced to 11.8 degrees N, up from 11.0 degrees N during the preceding period. The rapid northward movement of the ITD/ITF implies that the low-level circulation pattern driving the seasonal rains is progressing northward toward the Sahel ahead of schedule.

April 11-20 1998 - The western ITD/ITF was located at 12.7 degrees N. This is up from 11.9 degrees N last period and is north of the long-term mean of 12.2 degrees N. It is close to last year's value of 12.6 degrees N. In East Africa, from 20 to 35 degrees E longitude, the ITD averaged 11.0 degrees N. It was 7.1 degrees N last period, and the estimated normal is 10.5 degrees N. The recent northward advance of the ITD is a favorable indicator for an increase of seasonal rains across the region.

April 1-10 1998 - The ITD was located over 11.9 degrees N latitude on average across West Africa (10 degrees W-10 degrees E). This is 0.7 degree north of the long-term (1979-96) mean and 2.1 degrees north of the March 21-31 position. This year's April 1-10 position was near last year's position (12.2 degrees N). The ITD over eastern areas (20 degrees E-35 degrees E) was much farther south, averaging 7.1 degrees N during April 1-10. This was 2.7 degrees south of the estimated long-term mean position. The ITD-East was located near 4.8 degrees N during March 21-31, 1998. The eastern ITD anomaly is consistent with satellite imagery that shows a lack of seasonal convection over eastern Africa from the equator to about 10 degrees N.


Back to top | Current Season | Back to historical analysis page