NSW is in the middle of a classic summer storm outbreak as a warm and humid air mass provides the right mix of ingredients for a daily dose of lightning and thunder.
Severe thunderstorms are likely to continue across the state into at least the middle of next week, continuing a spell of dangerous storms that began on Monday.
Thunderstorms have already generated tens of thousands of lightning strikes over the state this week, along with torrential rain and hail.
Some the most extreme weather observations include:
- 133mm of rain from storms in Dorrigo
- 21mm in just 10 minutes at Coffs Harbour on Tuesday night
- 15mm in just 10 minutes at Terry Hills on Tuesday night
- Giant hail at Macksville on Wednesday afternoon
- 83kmh wind gust at Kempsey on Wednesday afternoon
Northern NSW storm risk
A handful of towns along the north coast were already struck by storms each day from Monday to Wednesday, including Kempsey and Lismore, and they formed again on Australia Day along the northern ranges.
While the north-east is the most storm prone part of NSW, the current outbreak has higher longevity compared to an average stormy spell.
Thunderstorms will continue for many towns and cities every day for a least the next week.
Armidale, typical of the prediction for northern NSW, has an identical forecast each day that includes "the chance of a thunderstorm".
Severe thunderstorms, defined as those which bring at least one of flash flooding, winds above 90kmh or hail greater than the size of a $2 coin, are likely each day over north-east districts.
Much of NSW will have the chance of thunderstorms during the coming week. Source: ABC News
Southern NSW and Sydney storm risk
For southern parts of the state the peak storm day is Sunday, but storms are possible on most days along the coast and ranges during the coming week.
Sydney's risk of a thunderstorm is only small on Friday and Saturday but will increase rapidly later on Sunday.
The storm forecast will become complicated from Monday for southern and central NSW due to excess cloud.
It may seem counter intuitive that a thunderstorm, which by definition is a cloud that produces lightning, is less likely if its cloudy.
For a towering cumulonimbus storm cloud to grow vertically it needs energy, and the process all begins with the sun heating the ground, which is why the most powerful storms usually follow a warm and sunny morning.
Cloudy mornings rarely lead to explosive afternoon storms, and from Monday overcast skies will potentially bring rain or showers to southern districts with perhaps just the odd rumble or two in the mix.
Flash flood threat this week
Heavy rain and flash flooding pose the greatest risk of severe weather during the current storm outbreak due to a humid air mass and slow-moving storms.
When winds above the surface are strong, thunderstorms move quickly, however, on most days during the coming week the winds aloft that carry storms will be relatively weak, allowing thunderstorms to remain sluggish and sit over a region for a sufficiently long time to produce flash flooding.
Along with torrential downpours, a few pockets of hail and damaging winds are also likely although with less regularity than heavy rain.
Forecasting thunderstorms
Forecasting the exact location and timing of a thunderstorm before it forms is near impossible.
Even powerful supercomputers sourcing data from satellites, weather balloons and ground observations crunching quadrillions of calculations per second will still struggle to predict the complex dynamics involved in thunderstorms.
What weather models can predict with high accuracy is regions where thunderstorms 'could' develop.
It's the reason a summer forecast will often include the "chance of a thunderstorm", meaning the atmosphere has all the ingredients for a storm, but its uncertain whether they will all come together at just the right levels, at just the right time exactly over your location.
A classic example of the complexity of storms was witnessed across Sydney on Australia Day when well-formed thunderstorm over the inland suddenly weakened before reaching the coast due to a cooler and therefore more stable coastal sea-breeze.
