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by G. B. Samarasinghe

The rainfall during the preceding Southwest monsoon (May - September) and Northeast monsoon (December-February) periods were even below the long term average value in the Southwest quarter and the inland areas of the island. Only the Northern and Eastern regions recorded above average rainfall.

These occurrences could be attributed to variations in the global climate and weather. Severe snow falls and floods in Europe, bush fires in Indonesia and Australia are very good examples to these unusual changes.

Intermonsoon weather

Intermonsoon season usually begins in March and could extend up to the first week of May. Moreover, by mid April, pre-monsoon showers occur along the southwestern coastal areas. But characteristics of an Intermonsoon atmosphere are not clearly evident due the existence of a deep subsidence in the atmosphere and a surface pressure gradient during this March Convective showers occurred during the periods of slackened pressure gradient.

When convection occurs only at isolated places (without being widespread) they could produce severe lightning hazards and heavy rains.

A separate leaflet is issued by the Department of Meteorology on lightning hazards and precautionary methods.

The global trend is for the increase occurrence of thunder storms and associated phenomena such as tornadoes, gales and lightning.

Up draft and down draft air currants in a thunderstorm cloud produce severe and cold down draft with wind speed exceeding 100 km/h just before a thundershower.

Intra-cloud, inter-cloud and cloud to ground lightning flashes are associated with these thunder clouds, and the cloud to ground stroke is the most destructive and hazardous of all. A tornado affects a limited area of about 100-1000M and lasts about 5-20 minutes with the rotating wind speed exceeding 500 km/h. The destruction caused is immense and occasionally associated with rain.

A mature tornado resembles a funnel hanging down from a thunderstorm cloud, or as an elephant's trunk swinging in the sky.

Forming under special meteorological circumstances, these systems are becoming somewhat common in Sri Lanka too now.

Outlook

According to global numerical weather prediction products and other climatological information, long term average rainfall or a little above that value could be expected during the rest of the Intermonsoon period.

Thus, in spite of occurrences of severe lightning and thunder in April, the associated rainfall may not be that heavy. Global seasonal forecasts indicate much significantly heavy rainfall only during Southwest monsoon in late May and June.

This warm conditions will continue, with the over head sun during 5-15 April. When the atmosphere is moist but not enough to produce large clouds to produce precipitation, sticky and uncomfortable conditions will prevail.

During calm winds, it could be much worse.

Temperatures over the island could be above normal but not as high as during the preceding two years.

But days with sticky discomfort will be more, with much warmer nights specially in April. Temperatures in the East would be a little above normal as usual.

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