A rather old topic this but I have just experienced the same problem.
OWS with Hobby Boards ACM/control board has been working fine indoors during testing for months.
Installed it two days ago in stable weather and it has been ticking over and reporting about 5m/s winds which is about right.
Today with little to no increase in wind speed it started raining.
From as far as i can tell exactly at that point onwards it has been reporting 40-80m/s
I have looked at the raw counter values and they are indeed going up at the speed indicated.
Did David W ever resolve his problem?
Has anyone else experienced this and resolved the issue?
thanks