Read the period before the colour
On the three-hour accumulation layer, the amount belongs to the three hours ending at the selected time. A frame labelled 15:00 therefore covers the preceding interval, not automatically the three hours after 15:00. Check the provider's legend if you change the layer or accumulation setting.
Millimetres describe water depth: 1 mm spread evenly across one square metre is one litre. The total does not tell you whether it falls steadily or in one brief heavy shower.
Amount is not probability
A coloured area is a modelled precipitation amount, not a direct percentage chance of getting wet. The exact placement of showers can change between forecast runs. Compare neighbouring areas and several relevant time steps rather than treating the edge of a coloured patch as a precise boundary.
For snow, the precipitation amount may be expressed as liquid-water equivalent. It cannot be converted into an exact fresh-snow depth using one fixed ratio for every weather situation.
Use totals in context
For a garden task or longer walk, look at both the expected total and the timing. During prolonged autumn rain, consult official flood information where relevant; a precipitation map alone cannot establish how a particular river or drain will respond. In winter, compare the temperature and snow layers to understand possible changes in precipitation type.
Questions about this map
Is 6 mm in three hours the same as 6 mm per hour?
No. The first is a total for the whole interval. The second is a rate; if maintained for three hours it would produce 18 mm.
Why can a wet forecast move to another area?
Weather models update as new information arrives. Local showers and boundaries between air masses are especially sensitive to small changes.