1st half or 2nd half of Sept for Portugal ,

I take the lazy way out and use the climate summary charts in cities' Wikipedia entries to check on precipitation. I'm not thrilled that the time period over which the averages are calculated often cuts off in 2010--as is the case for Porto and Lisbon--but it's such a simple way to view the data and see whether it appears there will be a big difference if one slips a trip by a month or so.

https://en.wikipedia.org/wiki/Porto

https://en.wikipedia.org/wiki/Lisbon

Looking at those two charts, one can see that as of 2010 there was much, much more rain in Portugal in the month of October than in September, and much, much more rain in September than in August. Therefore, I deduce that it's quite likely the last half of September will be significantly wetter than the first half. Now, we're just talking about averages, the average conditions in the second half of the month may not be bad enough to worry about, and this year may be drier than usual, so it could be perfectly lovely even at the very end of September. However, purely from the rain perspective, the second half of the month looks considerably riskier.

If you want to look at actual, historical, day-by-day weather statistics over the last ten years or so, you can find them at timeanddate.com. I use that website primarily to look at temperatures. I've linked you to Lisbon's data for September 2020. That's only one year; it's really important to check out more than one year. As you can see, Lisbon had some miserably hot days in early September last year, but that may be very atypical (for all I know).

If there's not a typo in your original post, you'd have an extra two days if you chose the end of the month for your trip, which for me would be a significant argument in favor of traveling then.

Portuguese schools return around the middle of September, so beach places will be busier in the first half of the month, as that’s when the weather is best plus there are more daylight hours. Early September is busy with Northern European pensioners who go on holiday as soon as their schools return, usually around 1 September. The weather is better at the start of the month.

No one knows as yet what museums etc will be open and whether numbers will be restricted. Covid cases are rising again in the Lisbon area and other locations, where the easing of lockdown has been postponed.

If you want to see Lisbon as well as Porto and the Algarve, 2 weeks is tight if your dates include travel dates. If you want great beaches and cliff formations, you don’t have to go as far as the Algarve, which is tourist central. Bookings in Portugal have soared recently, as it’s one of the few places where Brits can go for some sunshine without quarantining on return home, so the Algarve will be very busy.

I agree a climate summary is a good idea. In early September 2016 we started our tour in Lisbon. It was extremely hot, 104 degrees for a day. As we headed North it cooled down and was tolerable. We ended in Porto. Never encountered rain.

I did a 9-day bike tour in Portugal 1st half September 2019. Temps got up to high 80s (F) in eastern Portugal. Ditto Lisbon. Weather was dry and sunny, not terribly humid, either. Lisbon was quite busy, but not terribly crowded.

Wow ! Thank you all so much. I did not know of these weather/percipitaion resources.
This will be our first trip to Portugal. We are exploring a future relocation. That’s why rainfall is moreimportant than temps. It’s hard to get a feel for a community if folks are hunkered down to stay dry.
Each of y’all’s replies are so helpful. Thank you.

in 2018 we spent 3 weeks in Portugal beginning August 27. We found the timing perfect as far as crowds go. For weather, we didn't encounter any rain, barely and clouds in fact. During the entire 3 weeks I only wore a sweater 2 times - both times on the plane ride over and back. The evenings and early mornings were still in the high teens for temperature, and we considered the weather ideal at that time.

Thanks.
We are actually considering a relocation to Portugal and that’s why rain is an issue. It’s hard to get a feeling for a place if everyone is humkering inside.

We went from 8/23/19 to 9/9/19 ... weather was fantastic, cool up north with a few extra warm days in the Algarve ... not very crowded but the second part of the month should be even less crowded ... started in Lisbon for 2 days, then a day trip to Sintra before heading up to Porto for a few days ... then through the Douro valley and back down the other side of the country to the Algarve ... 3 days in the south then back to Lisbon for a day before heading home ... as with any trip, could have used a few more days to see a few more sights but overall we saw/did what interested us ... I'd go back in a heartbeat ...