The platform provides coverage of the UK up to six days in advance. These are called 'UK' followed by a number. They are also at differing resolutions - 1Km, 2Km, 4Km and 12Km. A resolution is just the smallest area the forecast uses, so a 2Km forecast is higher resolution (more data) than 4Km or 12Km (i.e. more forecasts are made).
What do the Model names mean?
When you traverse the data, you will see 'UK12' which means 'UK at 12Km resolution for today'. 'UK4' is today's data at a higher resolution of 4Km, 'UK4+1' means tomorrow (+1) at 4Km resolution. 'UK12+2' to 'UK12+6' are forecasts at 12Km resolution for two or more days out, with 'UK12+6' the furthest away. 'UKGM' is Scotland at 2Km and 'UKGW1' is Wales at 1Km resolution. You should get the idea of the naming convention.
How much should I trust?
Bear in mind that the atmosphere is in constant flux, so the forecast for almost a week in advance is likely to change. So most pilots only really use up to two days ahead. But you can at least get an idea of the weather further away - just take the results with a pinch of salt.
In most cases, pilots only worry about tomorrow based on the afternoon run or first thing in the morning by looking at the 4Km run (or 2Km/1Km run if available in that region).
In the morning UK time, why are the runs not always what they say?
Forecasting on the day starts around 04:00 UK BST/GMT. As each forecast is made and completed this placed on the staging server so the forecast data can be reviewed. This overwrites the previous model data in that Internet-facing location. So until that run is complete, the data in that location is the last available data.
What this means is that until about 08:00 UK time, not all the UK4 and UK12 data is in place. UK2 data for tomorrow (UK2+1) is usually ready by lunchtime. This means that the viewer pages may show a run that has yet to complete - this is normal.