GENEVA (Reuters) -The heatwave engulfing the northern hemisphere is set to intensify this week, causing overnight temperatures to surge and leading to an increased risk of heart attacks and deaths, the World Meteorological Organization said on Tuesday. The WMO warned that the heatwave was in its early phases, saying it expected temperatures in North America, Asia, North Africa and the Mediterranean to be above 40 Celsius (104 Fahrenheit) "for a prolonged number of days this week as the heatwave intensifies".