Southerly buster on the way

A classical southerly buster looks likely to reach Wellington near to the evening rush hour today (Tuesday 25th March).

Img1a

Image courtesy of ECMWF. Blue shading indicates “strong” winds (25kt)
Green shading indicates “gale” winds (35kt).

This image shows the forecast sea level pressure and winds at 4pm today. The green shadings show predicted areas of gale force winds – though they are likely to be even stronger than this for a time.

The current expectation is that the southerly should reach Wellington in the early evening. The following forecast is valid at 7pm:

Img2a

Image courtesy of ECMWF. Blue shading indicates “strong” winds (25kt)
Green shading indicates “gale” winds (35kt)

There are a few interesting dynamical features which should help this southerly. If we look at the temperature forecast, at around 700m above sea level, for 3pm:

Img3a

Temperature at 925hPa (about 700m above sea level). Image courtesy of ECMWF.

You can see quite a strong temperature difference across the boundary of the southerly change. This is shown by the tightly packed isotherms (the black contours marking areas of equal temperature). These strong thermal gradients can really help the following southerly buster – and it is likely to be further strengthened by afternoon heating over the Wellington, Marlborough and Wairarapa regions.

Next up is a fairly unusual model field which isn’t seen very often:

Img4a

Image courtesy of ECMWF

The blue shading is showing areas where the surface pressure is forecast to have risen in the period 1pm to 4pm, whereas the orange shaded areas show when it is forecast to fall over the same period. The dotted contours mark out the areas where the pressure rise or fall is particularly significant.

Two interesting areas are the rapidly rising pressures over the South Island behind the southerly change, and the (more modest) falling pressures over the lower North Island. Such conditions create a so-called “isallobaric push”, where the wind has an inclination to surge from an area of rising pressure towards an area of falling pressure.

While the model has given us a good heads-up, forecasters will be closely watching what happens during the afternoon and reconciling the observed data with the forecast. Keep up with the latest at metservice.com, via MetService New Zealand on Facebook and @MetService on Twitter.