If the storms hit around 3pm, parents should wait an hour before picking their children up from school.
"The best thing to do is to stay indoors for a few hours. And let it go by, because they do only last an hour or so."
But the violent storms are not the only worry for a region well versed in the devastating effects of wet weather. The heavy rain, falling in the Bay of Plenty since Monday, is expected to last two more days.
Tauranga and Rotorua are expected to receive about 200mm of rain in the deluge - the worst of the year so far.
But the hills above the flat populated areas are likely to be battered by two to three times that amount of rain, Mr McDavitt said.
That rain will swell the streams and rivers that drain in to the plains.
When the thunderstorms hit, the result could be widespread havoc. With lightning, they will bring sudden heavy torrents of rain to an already soaked region.
"We've seen severe damage up around the north. And when those thunderstorms come to the Bay of Plenty, by then the accumulation of rain there becomes a factor. All that rainfall over several days starts to pond."
Landslides were "on the agenda" and rivers could break their banks, he said.
"Several days of rain is the sort of event that can cause these things. And the thunderstorms, well, you get one of those and it can undermine a whole house, just like that. It can turn the days of rain into a torrent. And there's not much you can do about that in advance."
Three days of rain in July 2004 left the Bay of Plenty ravaged by severe flooding, river breaches and landslides. The damage was estimated to have cost the country more than $50 million.
This week's downpour was about half the size of that seen when Cyclone Bola hit the eastern parts of the North Island in March 1988.
During Bola, 916mm of rainfall over three days was recorded inland from Tolaga Bay, on the East Cape.
