You may have noticed in the last couple of days that some high resolution models which are normally super reliable with rain forecasts have missed developments of showery rain within frontal systems which have had dust or smoke particles within them.
There is a pretty good reason for this and it's down to how rain is formed. You see, if the air is totally pure with no dust or particulates then it's pretty much impossible for it to rain or snow (depending on temperature obviously). This is because raindrops are simply formed by water moisture which needs something to attach to. This usually comes in the form of microscopic particles or even bacteria which makes it to the upper atmosphere.
Once a water droplet latches onto a dust particle it then finds another and they merge and then keep bumping into others in a process called coalescence. One thing which the models aren't very good at is the forecasting of these particulates within actual numerical weather models. Granted, there are some great models which solely forecast the movement of dust and aerosols around the world, but they are rarely incorporated into the actual weather models which you see.
This means that when you get a plume of dust or smoke heading towards us with moist and unstable air, then the models don't take this into account and miss the fact that these can allow the moisture to merge with the dust and form rain further ahead than it would normally expect.
There is only one global model which does take this into account and that is the GFS which uses HYSPLIT data to calculate the chance of particulates above a certain size. So, if you are caught out in an unexpected shower in the next day or so, then don't blame the models, blame that stuff which you spend loads of money using Pledge and a yellow cloth trying to get rid of.
METEOROLOGIST : MARSH
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