An attempt is made to predict the occurrence of wet periods in the Middle East 4 to 6 days in advance. Such a forecast is of great value for cotton cultivation in Egypt during the cold season (December to March).
Generally, wet periods in the Middle East are known to be associated with the development of low pressure systems (the so-called Cyprus lows) over the eastern Mediterranean Sea. Such systems have been observed at upper levels, particularly at 500 mb, and have been found to originate as upper lows further west or west-southwest over northwest Africa, south of latitude 30°N, where they usually first appear on our charts.
These upper lows move east-northeastward, and after an average time of five days they give rise to active Cyprus depressions. At this stage their displacements are frequently checked either by the blocking effect of higher pressures in advance, or by the introduction of extremely cold air currents from northern and central Russia.
Rainfall has been observed to be heavy and to extend to southern Egypt during wet periods with the passage, at the 500-mb level, of troughs extending from northern or central Russia southwards to Egypt or the northern Sudan.