However, Adi grabbed himself two quick-fire goals in the final quarter, with Rodney Wallace also netting late on, as the hosts bounced back from their 5-0 demolition by Los Angeles Galaxy last time out.
With 12 minutes on the clock, Portland - whose defeat to Galaxy ended a four-game winning streak - moved ahead in the clash. It was midfielder Nagbe, who struck his first goal of the season as his 20-yard rocket left Stefan Frei with no chance.
On 23 minutes it was nearly two but, after some skillful play, Gaston Fernandez saw his far-post curler beaten away by Frei to preserve the one-goal deficit.
It would prove a crucial save as Sigi Schmid's men found a route back into the game when Neagle got himself on the end of Tyrone Mears's free kick to prod home from close range with seven minutes left in the half.
The Timbers came agonisingly close to re-establishing their lead early in the second half when Jorge Villafana's header was swatted off the line by Frei.
It proved a case of third time lucky for the hosts, who finally beat Frei again on 75 minutes when Adi latched on to Nagbe's ball before bending it around the Swiss keeper.
Moments later it got even better for Portland when Adi completed a quick-fire double to stun the visitors with an outside-of-the-box strike that deceived the stopper.
There was still time for Porter's men to add gloss to the result late on as Wallace struck a fourth to seal an emphatic win in the end.