If the US got nailed with an EMP attack we would loose ALOT! Most satalites in orbit would be lost (therefor most cellphones, internet, GPS, etc., wouldn't work). The electric grid would go down, cars would stop running...
It would be most likely this would be followed by a full scale nuclear strike...but in the off chance it was a rogue nation or terrorist with only 1 nuke...then i think the 1st thing i would do is fill everything that could hold water...it wouldn't be long before those water towers were empty.
Early warning? during any power failure, just look at a battery operated clock, wristwatch, or turn on a battery operated radio...if it stopped as well, it MIGHT be an EMP attack...if all of these devices stopped along with a power outage AND my car wouldn't start...yup...i'd start filling water containers!
That electric grid ain't gonna be fixed in a day or 2.