Google Now sent out the old NWS tone and tornado warning alert on my N4. This message beat the sirens by about 15 seconds, and the TV weathermen by about 5 seconds. The tornado really was in my vicinity too. No other alerts for all the other twisters more than 10 miles away.
Maybe I'm the only one, but I thought this was pretty slick. Color me impressed with this service of Google Now.
Its less of a Google Now service and more of a carrier / FCC mandate that said handsets being manufactured after a certain date had to have the capability to get Cell Broadcasts of emergency alerts.
They are SMS Text Messages sent out and are based off the location of the tower you are connected to.
Once a geographic location is selected and the type of emergency is selected, the towers in that location are supposed to push a SMS text message in the form of the Emergency Alert Notification to your device.
But i did notice that Google Now also had a card on my Nexus 4 when we had some storms in our area a week ago. Prior to that i had a Note 2 and i got the SMS text but no google now card.
My guess is that it might be a 4.2.x tweak OR google now works better. LOL Either way im glad to see it helped you and the service is working like its supposed to.