A warning issued when sustained winds of 39 to 73 mph (34 to 63 knots) are expected within 24 hours. In Canada, it is described as an elongated area of relatively low pressure extending from the centre of a region of low pressure. Trough A warning issued when sustained winds of 39 to 73 mph (34 to 63 knots) are expected within 24 hours. In Canada, it is described as an elongated area of relatively low pressure extending from the centre of a region of low pressure. Turbulence A warning issued when sustained winds of 39 to 73 miles per hour (34 to 63 knots) are expected within 24 hours.