[NeilT (OP)]

Having gone for a cycle yesterday morning on the basis of a good weather forecast I ended up getting drenched for 45 minutes. Listening to the weather forecast at lunchtime this was explained (I think) as a 'sympathetic trough'. Are these things particularly rare and/or unpredictable as although I appreciate the vagaries of weather forecasts, this even seemed to take the Met Office unawares.   

[alancalverd]

I've not come across  the term before, and can only find one online reference to it

https://www.convectiveweather.co.uk/forecast.php?date=2019-01-29

which doesn't help at all!  An hour ago I flew through a trough that could not possibly be described as sympathetic, but it did give the plane a good wash.

[NeilT (OP)]

I had tried a quick search of the interweb although there was very little to be found. There was one pdf that mentioned it here:-
ftp://ftp.library.noaa.gov/docs.lib/htdocs/rescue/mwr/084/mwr-084-11-0391.pdf [nofollow]

although without any explanation of what it is or how it occurs.