Darwin experienced its coolest morning so far this year on Monday, and it could get even chillier later in the week. A strong high pressure system centred to the south of Australia has caused a pulse of cool and dry air to spread north into the nation's tropics. The mercury dipped to 19.7 degrees in Darwin shortly after 7am on Monday, making it the city's first morning below 20 degrees so far this year. If this wasn't cold enough to warrant a pair of socks under the thongs, the next few mornings could be. Darwin is forecast to reach lows of 18 to 19 degrees each morning until Friday, which is three to four degrees below average for this time of year. The lowest temperature on record in Darwin during May was 13.8 degrees in 1990 and it's been three years since the city cooled below 20 degrees during May.