INDIA METEOROLOGICAL DEPARTMENT

 

SOUTHWEST MONSOON 2006

END-OF-SEASON REPORT

 

 

HIGHLIGHTS

 

  • For the country as a whole, the seasonal rainfall from 1st June to 30th September was 99% of its long period average (LPA)

 

  • Seasonal rainfall over Central India was excess by 16% and it was 94% and 95% of LPA respectively over Northwest (NW) India and South Peninsula. It was however deficient over northeast (NE) India by 17%

 

  • Out of the 36 meteorological sub-divisions, the seasonal (June-September) rainfall was excess in 6 and normal in 20 sub-divisions. However, it was deficient in 10 sub-divisions.

 

  • Out of 533 meteorological districts, 60% of the meteorological districts received excess/normal rainfall and the remaining
    40% received deficient/scanty rainfall during the season. 130 districts (25%) experienced moderate drought and 30 districts (6%) experienced severe drought conditions at the end of the season.

 

  • Five sub-divisions (Andaman and Nicobar Islands, Arunachal Pradesh, Assam and Meghalaya, west Uttar Pradesh and Haryana experienced moderate drought conditions (rainfall deficiency of 26% to 50%) at the end of the season.

 

  • IMD’s long range forecasts for July rainfall over the country as a whole and the 2006 seasonal rainfall over NW India and south Peninsula were proved to be accurate. However, the 2006 monsoon seasonal rainfall over the country as a whole was slightly more than the predicted value.

 


1.       ONSET OF SOUTHWEST MONSOON

 

          Southwest monsoon advanced over the south Andaman Sea and parts of southeast Bay of Bengal on 17 May. It advanced over the remaining parts of southeast Bay and parts of southwest and east-central Bay by 22 May. Monsoon arrived over Kerala on 26 May, almost a week prior to the normal date.  It advanced further steadily and covered the western parts of Peninsular India and northeast India by 6 June. The advance along the west coast was rapid in association with an off shore trough. There was a prolonged hiatus from 7 June to 22 June, caused by the intrusion of mid-latitude westerlies. During the last week of June, it advanced further, as a weak current over the remaining parts of Peninsula, central India and also the northern parts of the country outside Haryana, Punjab and Rajasthan. By the end of June, monsoon covered most parts of north Arabian Sea, entire Gujarat state, some parts of east Rajasthan, entire west Madhya Pradesh, some parts of west Uttar Pradesh, entire Himachal Pradesh and parts of Punjab. The second hiatus lasted for 8 days (1 to 8 July). Further advance over west Rajasthan took place on 20 July. Monsoon covered the entire country on 24 July, with a delay of 9 days (Fig.1).

 

2.           SYNOPTIC FEATURES

 

The season as a whole had been quite active in terms of the number of low pressure systems. In all, 16 systems (1 severe cyclonic storm, 8 depressions/ deep depressions and 7 low pressure areas/ well marked low pressure areas) formed during the season. All the systems formed over the Bay of Bengal except one land depression and one severe cyclonic storm over Arabian Sea. The systems formed over the Bay of Bengal generally had a west-northwesterly track causing heavy rainfall over central India, especially over Orissa, West Madhya Pradesh, Maharashtra, Gujarat, and West Rajasthan.

 

          The only low pressure area in June which formed over the North Bay and adjoining Gangetic West Bengal (6–8 June) was short lived and dissipated over Jharkhand and neighbourhood. In July, one depression, 3 low pressure areas and one well marked low pressure area formed. All these systems moved west-northwestwards, except one which moved northwestwards. In August, one deep depression, 3 depressions and one low pressure area formed. All of them formed over the north Bay and crossed Orissa coast. They also had long tracks mostly in a westerly/west-northwesterly direction across central India and moved up to west Rajasthan as remnants. In September, one severe cyclonic storm formed over the Arabian Sea. It dissipated over the Sea itself due to large vertical wind shear and cold air advection. In addition, 3 depressions, including one land depression and one low pressure area formed. The last depression of the season formed over the Bay of Bengal in the afternoon of 28 September and crossed Orissa coast close to Gopalpur on 29 evening. It then moved westwards and weakened gradually.

 

 

3.           RAINFALL DISTRIBUTION DURING MONSOON SEASON

 

The southwest monsoon rainfall (June to September) for the period 1 June to 30 September 2006 for the country as a whole and the four broad homogeneous regions are as follows:

 

Region

Actual (mm)

Normal (mm)

Percentage Departure

All-India

886.6

892.2

-1%

Northwest (NW) India

573.7

611.6

-6%

Central India

1152.2

993.9

16%

South peninsula

684.6

722.6

-5%

North east (NE) India

1177.6

1427.3

-17%

 

In 2006, the southwest monsoon seasonal (June to September) rainfall over the country as a whole was 99% of its LPA.  Seasonal rainfall over NW India, NE India and South Peninsula was below its LPA with the largest deficiency over NE India (17%). The near normal performance of the monsoon rainfall over the country was contributed mainly due to the excess rainfall observed over Central India. Out of 533 districts, 209 districts (40%) received deficient rainfall (rainfall deficiency more than 19%) during the season, out of which 130 districts (25%) experienced moderate drought conditions (rainfall deficiency 26% to 50%) and 30 districts (6%) experienced severe drought conditions (rainfall deficiency 51% and more).

 

During the 2006 monsoon season, rainfall activity was not well distributed in space and time. The cumulative rainfall from 1 June to 30 September 2006 was excess in 6, normal in 20 and deficient in 10 meteorological sub-divisions. The sub-divisionwise cumulative rainfall distribution is shown in Fig.2. Five sub-divisions (Andaman and Nicobar Islands, Arunachal Pradesh, Assam and Meghalaya, west Uttar Pradesh and Haryana experienced moderate drought conditions (rainfall deficiency of 26% to 50%) at the end of the season.

 

Month-wise distribution of rainfall departure over the country as a whole is given below:

 

June:    13% below LPA,                  July:            2% below LPA,

August: 5% above LPA                              September: 1% below LPA. 

 

The spatial distribution of monthly rainfall is shown in Fig.3.

 

Figures 4 and 5 depict the monsoon rainfall as received week by week and the cumulative rainfall during the season. Large rainfall deficiency was observed from the second to fourth week of June and July, last week of August and during the middle of September. During the season, cumulative seasonal rainfall over the country as a whole remained always below its LPA, except for the first week of June. By 26 July, cumulative seasonal rainfall was deficient by 14%, which ultimately improved and the deficiency was just 1% at the end of September. The excess rainfall during the first three weeks of August, especially over the Central India was responsible for the revival of the rainfall scenario over the country. In the absence of break like conditions in August, monsoon trough was south of its normal position. These conditions, however, led to deficient rainfall over northern parts of India, especially, over Uttar Pradesh, Bihar and NE India.

 

4.           WITHDRAWAL OF SOUTHWEST MONSOON

 

Southwest monsoon withdrew from western parts of Punjab and most parts of west Rajsathan on 21 September. It further withdrew from some parts of Jammu and Kashmir, entire Punjab, most parts of Haryana, Chandigarh & Delhi, west Rajasthan and some parts of east Rajasthan on 25 September. After two days, on 27 September, it withdrew from remaining parts of Jammu and Kashmir, entire Himachal Pradesh, Uttaranchal, remaining parts of Haryana, some parts of west Uttar Pradesh and east Rajasthan, and some more parts of west Rajasthan.

 

5.       LONG RANGE FORECAST OF MONSOON RAINFALL

 

In May 2006, using an indigenously developed statistical model, IMD predicted that monsoon onset over Kerala would take place on 30 May with a model error of ±3 days. This year, the monsoon onset over Kerala was on 26 May, six days earlier than its normal date.

 

As per the long range forecast for the 2006 Southwest monsoon seasonal rainfall issued in April, the seasonal rainfall for the country as a whole was expected to be near normal and quantitatively 93% of LPA with a model error of ± 5%. In the updated forecast issued on 30 June, the forecast for the country as a whole was revised as 92% of LPA with a model error of ± 4%.  The season ended with the area-weighted rainfall for the country as a whole as 99% of the LPA, slightly more than error limit of the IMD’s long range forecast. Considering 4 broad homogenous regions of India, rainfall was expected to be 91% of its LPA over NW India, 90% of LPA over Central India, 94% of LPA over NE India and 97% of LPA over South Peninsula with a model error of ±8%. The actual rainfall over these 4 regions was 94%, 116%, 83 % and 95% of the LPA respectively. The long range forecasts for the seasonal rainfall over NW India and South Peninsula were proved to be accurate. However, IMD’s long range forecasts for Central India and NE India were not correct. IMD also issued the long range forecast for rainfall over the country as a whole in July 2006 as 97% of its LPA. The actual rainfall in July 2006 was 98% of LPA, very close to the predicted value. The Table below gives the summary of the verification of the long range forecasts issued for the 2006 Southwest monsoon.

 

 

                                    Table

 

    Details of long range forecasts and actual rainfall.

 

Region

Period

Issued on

Forecast

 

Actual

 

All India

June  to September

24th  April, 2006

93% of LPA ± 5%

 

99% of LPA

30th June, 2006

92% of LPA ± 4%

 

All India

July

30th June, 2006

97% of LPA ± 9%

98% of LPA

Northwest India

June  to September

30th June, 2006

91% of LPA ± 8%

94% of LPA

Northeast India

94% of LPA ± 8%

83% of LPA

Central India

90% of LPA ± 8%

116% of LPA

South Peninsula

97% of LPA ± 8%

95% of LPA

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


 

 

 

 

 

Fig.2.Sub-divisionwise rainfall distribution over India during southwest monsoon

         season-2006

 

 

 

Fig.3.   Spatial Distribution of Monthly Rainfall Departure (%) over India during southwest Monsoon season 2006

 

 

 

Fig. 4:  Progress of the Weekly Monsoon Rainfall – 2006

 

Fig. 5:  Progress of the Cumulative Monsoon Rainfall - 2006