You are not seeing things. Currently NTA does not have that capability on what you are trying to accomplish with triggering alerts. Improved alerting is definitely on our road map just unfortunately, I can not give a specific which release this will be addressed. You should check here for product updates. This site will link you to the most current roadmap items.
mebway Thank you for the information. Hopefully it will not take too long to get this functionality working, as these DDoS attacks are only increasing in number, and we need a better way to mitigate them...
I will keep a lookout for the product updates.