No woes: Weather for the Round the Bays 2012

A day later and it’s looking good for Sunday morning now. The H+120 picture was pretty grim with cold southerlies and showers, but now the models have decided on a different outcome. At H+96 it looks like we can expect a dry morning with northerly winds freshening (hopefully not freshening too quickly!).

The high over the Tasman Sea which has been a feature present in the forecasts right back to H+240 is still there, but a little further north. At midday on the 26th it extends a ridge over the North Island, producing northerly winds for Wellington. The ACCESS G is the only model trying to hold on to a southerly for Wellington. The latest MetService forecast for Sunday is “Fine weather. Fresh northerly.”

From showers and southerlies to fine with northerlies – It just goes to show that you can’t get too precious about extended range forecasts (forecasts more then 2 days ahead). Small changes in the initial conditions of the model run can lead to large changes in outcome after a few days. This is exactly what Lorenz discovered while running early atmospheric simulations in the 1960s, leading to the birth of a new field of mathematics called chaos theory. Lets hope the current picture for Sunday doesn’t change too much – Dry with light winds please!

ECMWF H+96 26 Feb 2012 00Z

Fig.1 ECMWF H+96 MSLP/850hPa wind, 26 Feb 2012 00Z. Ridge over the North Island, fresh northerlies for Wellington. Image courtesy ECMWF.

US GFS H+96 26 Feb 2012 00Z

Fig.2 US GFS H+96 MSLP, wind barbs, precip for 26 Feb 2012 00Z. Ridge over the North Island, fresh northerlies in Wellington and no broadscale precip for us either. Image courtesy Weatherzone.

Fig. 3 ACCESS G H+96 MSLP, Thickness, Precip for 26 Feb 2012 00Z. Looks like there is still a ridge over the upper South Island which would produce southerlies for Wellington. This seems to be the outlier of this set of models. Image courtesy Weatherzone.

Fig.4 ECMWF H+96 MSLP ensemble mean and normalised standard deviation, 26 Feb 2012 00Z. The ensemble mean is similar to the deterministic forecast for the North Island, but there is some variation about pressures over the South Island. The message is fresh northerlies. Image courtesy ECMWF.