Weather at Howick, Auckland, New Zealand

Well despite today's instability, nothing of any significance occured across the Auckland region. Temperatures and dew points were fairly high for the time of year, so a bit unlucky this time around. 

Here was the MetService's updated outlook (the only difference from the morning's outlook was to increase chance of severe thunderstorms from low to moderate):

Issued at: 4:35 pm 17 Oct 2009 NZDT

"Amended at 4.10pm to extend the risk of both thunderstorms and severe thunderstorms further south and east.

A low approaching from the Tasman Sea is expected to deepen and lie over the south Taranaki Bight this evening and redevelop east of Wairarapa tonight.

Bands of squally showers and scattered thunderstorms are forecast to spread over northern and central parts of the North Island from afternoon and could last until night time.

The highest risk of thunderstorms is in the west from Taranaki to Northland but also parts of Bay of Plenty. Thunderstorms in these areas could produce locally heavy rain up to 25mm per hour, hail up to 15mm in diameter, and strong wind gusts of 90 to 110 km/h. There is even the possibility of a few severe thunderstorms during the afternoon and evening, with potentially damaging gusts exceeding 110 km/h, and possible small tornadoes, and a severe thunderstorm watch has been issued for some of these areas.

There is a moderate risk of thunderstorms spreading as far east as the ranges of Gisborne and Hawkes Bay and down to Wellington, and parts of Nelson and the Marlborough Sounds, as indicated on the chart.

The risk is lower over the remainder of the North Island and Marlborough, as well as northern Buller.

There is a low risk of isolated thunderstorms about the lakes of Otago and southern Canterbury this afternoon and evening, which could produce locally heavy rain up to 20mm per hour.

No thunderstorms or significant convection expected elsewhere."

Some thunderstorm activity occurred in other areas of the North Island, although nothing severe has been reported.

In the Auckland area, thunder is heard on about 15-20 days per year. 

Most thunderstorms are embedded in fronts which cross the area in a west-east orientation, although thunderstorms can occur in any wind direction given an unstable atmosphere. Frontal thunderstorms tend to bring heavy rain and sometimes squally winds with small hail, even the odd small tornado.  Thunderstorms which occur as frontal onslaughts from the west tend to be more dramatic over western Auckland much of the time.

Convective thunderstorms may occur in the warmer months, but this is more common further south in the Waikato. These types of storms can cause larger hailstones occasionally.

And then there are the rotating thunderstorms, called mesocyclones.  Very common in the Great Plains of the USA, but very rare in Auckland.  Such a thunderstorm may be severe, and the rotation is more conducive to tornado formation.  Supercell storms are virtually unheard of.

Since Auckland sits on an isthmus, opposing sea breezes sometimes create a convergence line - and when this phenomenon occurs simultaneously with an unstable amosphere, it is a classic set up for a narrow storm line, usually running in a north-south orientation.  Such storms are often very slow moving, and in the past such circumstances have created intense thunderstorms causing local flooding.

Here I will post info and reports of when thunderstorm activity occurs in the Auckland area, starting from 17th October 2009.