If your aim is to store the Davis forecastIcon value in your database for use in reports etc then you need to do one of two things. Either add a forecastIcon field/column to your database schema and WeeWX will automatically save forecastIcon to database. Or, if you are using the wview_extended schema and the forecast field/column is not being used, you can re-map the forecastIcon value to forecast by adding a line to [StdCalibrate] [[Corrections]] as follows (untested):
[StdCalibrate]
[[Corrections]]
forecast = forecastIcon
WeeWX will then automatically save the forecastIcon value in the forecast field/column in the database.
You probably should also tell WeeWX how to extract the forecast icon value from the accumulators as well. The default action is to average the values, this could give an incorrect result if there are multiple different values in an archive period, in this case the best action would be to extract the last value. To do this you need to add the following to the end of your weewx.conf(untested):
[Accumulator]
[[forecast]]
extractor = last
Of course you will need to restart WeeWX.
Gary