I'm well into my instrument training and was working with a different instructor today on some knowledge prep. I know the 1-2-3 rule for requiring an alternate, but got tripped up on a question today that has me re-thinking my interpretation of how the rule is applied. The scenario presented was something like this: ETA: 1500Z TAFs: FM111400 20009KT P6SM BKN045 FM111800 27012KT P6SM OCV015 My interpretation was that I did not need an alternate because the wx 1 hour before (1400Z) and after (1600Z) my ETA was greater than 2000'/3SM (ie, it was going to be BKN045 until 1800Z). However, he said that I did need an alternate because the later TAF was below 2-3 and there was no way to know when conditions would fall below 2000 leading up to that OVC015 @ 1800Z. I understand the logic and don't want to overthink this, but I'm sure that this will come up in my oral and I want to make sure that I can answer it correctly and confidently. Technically what's the correct answer here?