 They would usually cause heavy snowfall in the mountains, but would provide clear and relatively warmer conditions on the lee side of the mountain range in the Sydney region (albeit still cool due to wind chill factor).Katabatic winds are dry and usually warm drainage winds, akin to land breezes, that occur when air of higher density in the Blue Mountains descends under gravity force into the Sydney metropolitan area and the Hawkesbury Basin.They are westerly winds that would tend to arrive during nighttime, after the heat of the day ends, and into the early morning, but can occur any time of the day.