Checking In at Barcelona Airport

Since you are traveling in the Zone, 2hrs should be fine.

https://www.norwegian.com/en/travel-info/check-in-and-boarding/check-in-deadlines/

Checkin opens 2 hours prior
Checkin closes 45 mins prior
Gate closes 30 mins prior

If it was me, I'd aim to be there BEFORE 6am.
It is a very efficient, and beautiful terminal, with many facilities.
Good news for you is that it's a Sunday morning, and hence no traffic. Checkout at 5am, with a taxi at 5:15am, from a central Barcelona hotel.

Thank you. I was hoping we didn't need to be at airport 2 hours prior to flight. Where we live 1 hour before is usually ok on short domestic flights.

checkin online use the bag drop for a shorter line

Yours might be different - but we just flew Barcelona to Oakland last week... so it was an international flight. The line to check in at Norwegian was insanely long. We had to check our bags since their carry-on restrictions are pretty low.... we could not check in online, they don't do it (maybe for shorter flights they do?) but luckily we had everything already paid and had entered our passport numbers on time, so we could check-in at the kiosk and go to the bag drop line. The regular line was super long - 1-2 agents and was probably at least an hour... not sure why those people did not go to kiosk. Also, our vrbo host had told us there has been a lot of strikes at the airport so leave extra time. We got there about 3.5 hours early since we didn't want to miss our flight home. Using the bag drop line was fast, and there was not much line at security... there was also another passport check line that was pretty empty when we went through but I heard it was more crowded later. YMMV though since you are not flying internationally.

Another note: some airline.. Aena or something? OMG those lines were a complete disaster!

Kim

Aena is the company that owns/runs Spanish airports. They were probably generic multi airline counters.

... there's an industrial action (partial strike) going on these days at the security controls of the airport. The dispute is "apparently" happening because the workers of the security firm, EULEN, are demanding a pay raise and better working conditions -which they, in fact, deserve. The problem is much more complex though, as the airports are run by AENA -a sort of 'federal' company belonging to the Spanish state. AENA is notorious for being a cesspit of incompetence but it's a good tool for the Spanish government to keep a grip on all the airports in Spain -which are under the AENA umbrella- so as to ensure political pressure can be applied "when needed". In this case, since there are ongoing plans by the government of Catalonia (Barcelona, capital city of Catalonia) to hold an independence referendum this October, the Spanish government is "suspected" to be boycotting the airport to cause international bad rep for Barcelona in the highest month of the year regarding tourism. Furthermore, EULEN is run by a fat cat with political ties with Madrid's government and has been involved in numerous scandals in the past.... 'nuff said, right?

Sooo, if you're flying out of Barcelona during these days, stay tuned for the development of this conflict and be prepared to get to the airport way in advance to minimize the risk of losing your flight. Let's keep fingers crossed and hope it will sort itself out soon.

More info:
Editorial - Those responsible for the chaos at Barcelona's El Prat airport
http://www.elnacional.cat/en/

I have never taken an intra-European flight out of Barcelona, but I was in the United check-in line for nearly an hour for my August 2016 flight back to the US. I had allowed 3 hours at the airport before my scheduled departure; if I had not done so, I'd have had some nervous moments in that line.