The weather over the Top End, specifically in the greater Darwin area, can be broadly divided into 2 distinct seasons. Hazards to aviation are possible in both seasons, albeit different in nature.
Wet season
This season typically runs from 1 October to 30 April and is broadly divided into the Build-Up, Monsoon and Monsoon Break Periods.
Monsoon
The first monsoon burst of the season usually occurs after mid-December, when the monsoon trough moves south of Darwin and the Top End. The monsoon is characterised by west to north-west winds from the surface to at least FL 200 and is accompanied by widespread rain, squally showers and some thunderstorm activity.
The intensity of the rain can vary, but generally results in periods of low visibility as well as extensive areas of low cloud that may persist through the day. When the monsoonal flow is strong, gusty winds and squally showers moving eastwards off the water can be experienced at any time of the day. Thunderstorms can occur at any time but are more likely around the Darwin area during the night and usually move eastwards off the water. Watch for organised lines of showers and storms on the radar, called squall lines, that move into the Top End off the Timor and Arafura Seas, mostly at night. Squall lines have the potential for very strong wind gusts,
potentially in excess of 41 knots.
Daytime thunderstorms are more likely inland around Jabiru and Tindal Airports and the Kakadu region.
Build-up and monsoon break
The Build-Up occurs in the wet season prior to the first monsoon burst, while the monsoon break occurs in periods between and following monsoon bursts. Both periods are characterised by hot, humid and unstable conditions, resulting in thunderstorms being the primary aviation hazard.
While thunderstorms can occur at any time, the most likely time for thunderstorms about the Darwin area is the afternoon and early evening, as well as the early hours of the morning.
While they can form directly over Darwin, afternoon thunderstorms usually form inland, then move back towards the coast with the upper-level steering winds, if they are strong enough. Look for well-formed thunderstorms on radar anywhere from Batchelor to Humpty Doo, or even as far east as Jabiru. If the cells are well-developed, they can overcome the sea breeze and affect the greater Darwin area. In some situations, squall lines can form well east of Darwin, even as far away as Arnhem Land, then move quickly to the west. Look for long lines of organized storms on radar moving to the west. These storms can have wind gusts greater than 41 knots.
Early morning storms over Darwin are often the result of Gulf lines that start on the west coast of Cape York in Queensland and move across the Gulf of Carpentaria 24 hours earlier. These lines continue to move though the eastern Top End during the afternoon and arrive at Darwin in the early morning. Look for obvious cloud lines on satellite during the previous night and morning, as well as showers and storms moving west on the Gove radar through the day.
Through the late morning and afternoon, a westerly sea breeze usually moves over the Darwin area, easing into the evening hours.
Dry season
While the threat of thunderstorms is rare during the dry season, other aviation hazards occur.
Turbulence
Strong easterly winds occur when a large high over southern Australia extends a strong ridge into the Northern Territory. Winds at 1,000 to 2,000 ft in excess of 35 knots can occur at times, mostly during overnight and morning periods. Wind shear warnings for Darwin Airport are issued when these winds are at their strongest, usually between 22 Z and 02 Z. An area to be particularly mindful of is to the west of the Arnhem Plateau about the Kakadu area, where even moderate strength easterly winds can create turbulence.
Smoke
The dry season is utilised extensively to burn off excess grasses and weeds, resulting in widespread areas of smoke. The fires usually intensify during the afternoon due to gustier winds and warm, dry conditions. Visibility is usually lowest close to the fires, but inversions forming in the early morning period before sunrise can trap smoke near the surface, causing widespread visibility reductions.
Fog
Fog forms in the hours before dawn and can persist into the late morning. Generally, the conditions required for fog to form are clear skies, light winds and plenty of moisture near the ground. These conditions mostly occur during the mid to late dry season and are often more prevalent closer to the coast due to the sea breeze transporting in moisture during the previous afternoon. Fog is rare at Darwin airport, occurring on average twice a year, mostly likely during August.
Additional ATC information
Pilots should be prepared to land at YDLV, YMKT, YBCR or YBTI and return to Darwin when weather has cleared. Sometimes weather will affect the aerodrome for greater than 60 minutes, after which ATC has to clear the backlog of aircraft. VFR aircraft will not get priority landing when severe weather is in the vicinity, so planning for an alternate is critical.