Arsenal and Newcastle are both in good form and moreover, there are home and away games which I think will make this match difficult to predict. Because it is possible that the away team in the first leg will only play defensively and will be at their best in the second leg, or maybe the other way around, who knows. But, based on their recent head to head record, I think Newcastle still has a chance to beat Arsenal in this semifinal match. Because after all, the team that has really improved to be very good lately, of course it will be very possible for Newcastle to get a good result too. Considering the factors they have had lately, then I just hope that in this first leg they will have a lot of goals at least with a total of three goals.
The match will start in a few minutes, the bookies give @2.15 for Arsenal while Newcastle @5.00, the difference feels quite far if we are talking about the semifinal match, maybe Arsenal factor as the host is the bookies' reason, but as you said Newcastle is currently in their best form in the Premier League so Arsenal will not easily get a win, or Newcastle could give a surprise, what is clear is that this match will be tight and whoever loses still has a chance to take revenge in the second leg.