It’s like a forecast, but for now.
In other words, it is a status report.
NEA 3 Hour Nowcast
A daily nowcast of recreational water conditions
FiveThirtyEight.blogs.nytimes.com also offers a Presidential now-cast (note the hyphen) in addition to its November 6 forecast.
I have not seen the term before but I can find instances of the verb in documents dating back to 2008, so it’s not brand spanking new.
EDIT: I was being a bit simplistic in saying it is a status report: it is an estimate of the current status based on data acquired previously.