Precipitation amount

Here, you can easily adjust the units.

The value given is a total predicted for the previous 3 hrs and includes the time of the forecast being looked at.

While the amounts are given as water equivalents, one needs to bear in mind that if the precipitation is falling as snow for example, then the depth/amount of snow is likely to be considerably greater, in some cases up to 10 times the precipitable water equivalent.

Say our forecast indicated a total of 10mm in a three hour period this could be the equivalent of up to 10CM of snow. You also need to think carefully about what the precipitation figure is saying to you.

Sometimes you may see very small amounts but looking at this in conjunction with the weather symbol can tell you a lot more about how that rainfall would be distributed.

For example with a small amount of precipitation, say only 0.2 mm in 3 hours one might think that this is insignificant, however, this should be interpreted in context of the weather symbol and cloud cover. 0.2 mm with overcast conditions is likely to be more indicative of prolonged persistent and widespread very light rain or drizzle. However, 0.2 mm with well broken cloud would be indicative of showers which tend to be more scattered with some places getting them and others not and because the 0.2 mm is an area wide average amount it is likely that if you are in one of the showers you could get considerably more than 0.2mm while many other nearby places have considerably less.

Our models simply do not allow us to model at very small scale in real time, that is at the scale of individual showers.

Hence the reason for these numbers, precipitation totals being an average across an area. Similarly, if you are on a wind facing slope then you might anticipate more precipitation under most situations and vice versa in the lee of a hill where there can be rain shadow areas.

It is always useful to bear this in mind when looking at our forecasts.