It looks like this would need a customized weather script, which can be done but will require writing some Python script.
Another possibility is to make use of the IFTTT support: requests like “if the forecasted low for the night will be <32ºF, water program xxx" can be done using IFTTT (using the Maker channel feature). Take a look at this blog post to see how to make OpenSprinkler work with IFTTT:
To clarify: OpenSprinkler firmware 2.1.7 supports IFTTT notifications (i.e. if some OpenSprinkler events happened, send notifications); whereas what you want is the other way around: if some events (like weather events) happened, send requests to OpenSprinkler to trigger certain programs to run. The former requires upgrading to firmware 2.1.7 while the latter works on any firmware version.