Notes on Marsden Point Weather Maps 

The maps have been prepared by retrieving the latest data from the NOAA high-resolution GFS model using GrADS DODS.
Unfortunately, acronyms abound in this field, and it's easy to lapse into them, as I did in the last sentence.
Here is a translation:
The National Oceanographic and Atmospheric Administration (NOAA), a US Government agency, runs a weather forecasting model called the Global Forecasting System (GFS). In its high-resolution mode, the model provides forecasts of various atmospheric parameters at 1 degree intervals in latitude and longitude around the globe every 3 hours for 7.5 days ahead. The model is run every 6 hours.
Data from the model output files are retrieved using a program called GrADS, which stands for Grid Analysis and Display System. Details about GrADS are available here. We access the model files using a facility called the Distributed Oceanographic Data System (DODS), details here. The combined system is called GrADS DODS.

The NOAA convention is to call the initial conditions the "analysis". The analysis is derived from various measurements that have been assimilated into previous model runs to obtain a best estimate of the present conditions. Actually, the analyses are a pretty good indicator of pressure, as indicated here.

There are 109 atmospheric parameters available from the model, but the only one we retrieve is atmospheric pressure at mean sea level (MSL), because that is the parameter we've found is most important for generating long waves.

Unfortunately, the DODS server can get overloaded, especially during extreme weather events in the US, so we sometimes cannot download the data for a day or two. Also, if the NOAA system goes down during the weekend, it is usually Monday before someone fixes it. This seems very unsatisfactory, but we must bear in mind that this service is entirely free, so we have no rights.

Once we have retrieved the data, we plot them using Matlab and ftp the resulting figures to the web. The plots are contour maps of pressure, with low pressure being blue and high pressure being red. Professional meteorologists are scornful of pressure maps being shown in this way using colours. They reckon pressure should be represented as line contours (like weather maps in the newspaper), and colours should be reserved for other parameters. However, I wanted to produce maps that could be assessed at a glance, and colours are the best way to achieve that.

The maps have been drawn so that we can see the vast ocean to the north and east of New Zealand. Experience has shown that whenever a low pressure system comes out of the tropics and propagates southwards past New Zealand, we get long waves at exposed east coast ports like Marsden Point and Timaru.

Here is an example of the sort of weather pattern that generates long waves. A shallow depression emerged from the tropics on 13 March and drifted southwards past New Zealand. The resulting long waves at Marsden Point were over 1 m in height.