Occasionally we hear in the forecast that there’s a risk of thunderstorms developing… but in the end they never arrive.

We spoke to MetService Weather Ambassador Bob McDavitt, to get a full understanding of how to interpret the thunderstorm risks – and to explain the difference between a thunderstorm outlook, watch and warning.

Bob also explains why Aucklanders seem to constantly miss out on predicted thunderstorms…

1. MetService Thunderstorm Outlook maps.

“Making thunderstorms is something like popping corn. All the ingredients have to join together in the right way before anything will happen, and then they pop in a random fashion, but not all the corn will pop. Thunderstorms act slightly differently because they can interact with each other but the analogy may be useful.

The regions outlined in the MetService Thunderstorm Outlook are drawn by experienced and skilled meteorologists taking into account computer guidance and watching where the ingredients are coming together. Once an area for potential thunderstorms is identified, the forecaster then has to assess the period over which the storms are likely to be active and what significant weather may be associated with the storms. We are watching for strong wind, heavy rain, hail, and the potential for tornadoes. A decision has to be made about the risk (likelihood) of the thunderstorms occurring. Due to a combination of all these factors, there maybe many regions drawn, although in complicated situations the outlook picture may have to be simplified to clearly show the likely outcome.

The regions we draw are thus useful in that the boundaries demark these differences, but within each region there will be some places more prone to thunderstorms than others. Within a region it can be that only one or two thunderstorms eventually pop – in low risk regions the ingredients may or may not come together. Even within a high risk area, the nature of thunderstorms means that it is not certain that everywhere in the region will experience a thunderstorm.

Sometimes the risk region may appear large because one of the ingredients covers a large area…overseas they often draw risk regions that would be as large as the whole of New Zealand.

2. Thunderstorm Watch

When we have sufficient information to have high confidence that some of the thunderstorms likely in the next 12-24 hours may be SEVERE, we start a Severe Thunderstorm Watch for the region. This service is provided on a 24 hour basis for all parts of New Zealand as part of a contract to the New Zealand Government.

 

The criteria for a severe thunderstorm are:

* Heavy rain (from thunderstorms):

* Rainfall of 25 millimetres per hour, or more.

* Large hail:

* Hailstones 20 millimetres in diameter, or larger.

* Strong wind gusts (from thunderstorms):

* Gusts of 110 kilometres per hour (60 knots) or stronger.

* Damaging tornadoes:

* Fujita F1 (wind speeds greater than 116 kilometres per
hour (63 knots)) or stronger.

Historically, severe thunderstorms occur in New Zealand around once a month. During summer, these events are more likely in eastern and inland areas during the afternoon and early evening, helped by the warm daytime temperatures. However, severe thunderstorms can occur at anytime associated with frontal features, with a slight prevalence to western areas of both Islands.

 

3. Thunderstorm warning

A Severe thunderstorm warning is issued when a severe thunderstorm has been detected and can be tracked by MetService radar. The warning gives details of that severe thunderstorm and where it is most likely heading.

These bulletins only have a lifetime of a few hours This service can only be offered to places within 150 kilometres of and in clear view of a MetService radar , but we may extend this to 180 km if we get a clear enough signal. Some damaging thunderstorms may go undetected even when
close to one of our radars, but the good news is that the MetService radar network continues to grow.

 

4. Auckland

Auckland City is on an isthmus. Showers that move onto the Auckland region from elsewhere are likely to dump their load on nearby hills or sea rather than the CBD, and these events may be stored as selective memory. Depending on the time of day and the season of the year, if the land is cooler than the nearby sea this tends to kill the buoyancy needed for a good thunderstorm. But we need to be aware that sometimes a convergence zone may form right over the isthmus, and in those cases the most intense rain may be near the city. For many years Whenuapai held New Zealand’s record for hourly rain intensity at 105 mm/hr.

Cheers from Bob McDavitt, MetService Weather Ambassador