Large parts of Queensland and New South Wales face the risk of slow-moving thunderstorms over the next couple of days, with the potential for localised flash flooding in many areas.

When we think of thunderstorms, we tend to think of them rushing rapidly across the landscape, either as individual cells or as part of a squall line. Such storms usually dissipate within an hour or less, and wild wind gusts are part of the package.

Slow-moving thunderstorms pose a different kind of danger.

While damaging winds can still occur in slow-moving storms, the biggest danger tends to be from prolonged bursts of intense rainfall, as storms effectively park themselves over a location for an extended period.

That’s the risk for parts of NSW and Queensland this Friday and into the weekend.

Why are the current storms so slow-moving?

In order for thunderstorms to form, you need three key elements:

Low-level atmospheric moisture is one requirement. You also need instability (where a parcel of warm air rises through cooler air). Thirdly, you need a mechanism to lift air.

Sometimes the trigger mechanism that forces air to rise is the strong wind associated with frontal systems. Sometimes it’s topography (like a mountain range which forces air to rise).

And sometimes it’s a low pressure trough, where air that has flowed in from surrounding areas of higher-pressure areas is forced upward because it has nowhere else to go.

Areas of low pressure with numerous near-stationary troughs are the dominant broad-scale weather features over Australia this Friday. With such a pattern, storms tend to be slow-moving due to a lack of strong steering winds.

Heavy Qld rainfall totals under slow-moving storms

In the 24 hous to 9am Friday, numerous Queensland locations saw more than 100mm of rainfall. They included:

142mm at Upper Finch Hatton Creek, just west of Mackay

135mm at Alligator Creek, 25km southeast of the Townsville CBD

112mm at Coolagh, near Longreach, a huge total for a location closer to the NT border than the east coast

It’s also worth noting that accumulations of 40mm per hour were recorded in relatively slow-moving storms just west of Townsville, while Townsville itself saw more than 50mm in just two hours between 6:30am and 8:30am.

That’s the sort of downpour which can cause localised flooding.

Queensland is currently experiencing widespread riverine flooding in the state’s west, after recent heavy rainfall events. Numerous warnings are in place.

For the latest thunderstorm information for your area, keep checking the Weatherzone warnings page, especially during the afternoon and evening.