November brings chance of thunderstorms

Moisture key to November thunderstorms | The Outlook

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What is the difference between October and November to increase the risk of thunderstorms?

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The Bureau of Meteorology climate maps give a great overview of the recent days, weeks and months weather conditions.

The daily loop of daytime maximum temperature across the country shows the west to east progression of hot air as it is drawn south ahead of frontal systems.

Over a three to five day cycle, very hot air is drawn down to southern WA then taken east across the top of the Bight to SA, Vic and NSW and then back north to Qld with daytime maximum temperatures this month in the high 30s to 40.

When the hot, dry pre-frontal conditions are associated with strong winds, then extreme fire weather results.

However, as we move from October to November, there will also be an increasing risk of thunderstorms ahead of, and with, the fronts over south-eastern Australia.

What is the difference between October and November to increase the risk of thunderstorms?

Moisture is the key ingredient missing at present. The three key factors for thundery outbreaks are:

  1. A mechanism to get the air to move vertically; widespread lifting with a trough or front.
  2. Low level moist air - an increase in humidity; with air drawn into NSW from the warmer oceans to the north.
  3. An unstable atmosphere; cool air aloft.

Late spring is when all these factors become more significant - as fronts or troughs move across NSW every few days and very warm to hot and humid air is drawn in ahead.

As well as the lifting with front/trough systems, lifting is also enhanced by afternoon heating and/or when the weather system has to cross the mountains.

While the air is always cooler aloft, if it is colder than normal, then the resulting thunderstorms can be more intense with extreme rain, hail and strong outflow wind.

At the present time, both the surface and upper air is relatively warm and the moisture levels are not great but this should start to change next month.

Thunderstorms are generally most common on the NSW north coast and northern slopes and tablelands.

Regions further south and west have less frequent storms.

Late this week there will be good rain on the NSW north coast and eastern tablelands.

However, fronts with widespread showers and thunderstorms across more of the state are still unlikely.

Humidity levels need to climb over the north of the state and there are no signs for that to occur.

The positive Indian Ocean Dipole (IOD) and weak tropical wave (MJO) next week look like delaying the start of the wet season up north

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