hehe thank you Kevin, and glad you arrived safely ^u^
so..I always recommend to check the official Japan Meteorological Agency website, which offers many useful info about the forecast:
another excellent website is Tenki.jp:
千葉市の今日明日の天気、気温、降水確率に加え、台風情報、警報注意報、観測ランキング、紫外線指数等を掲載。気象予報士が日々更新する「日直予報士」や季節を楽しむコラム「tenki.jpサプリ」などもチェックできます。
tenki.jp
(for both I've already selected Chiba city)
if you look at the typhoon forecast of the JMA website, you can see it basically starts on south-west Japan, and then it travels eastward: the first red and yellow circles are smaller but have a higher typhoon strenght, so it means strong winds and lots of rain during that day (today), so likely in that area flights and trains might be delayed/cancelled and some shops closed.
then, as you can see, the following days of the forecast show a bigger red and yellow circles, but they are much less strong,and it should reach Tokyo and Chiba area around September 2, but because the yellow circle is much bigger, it means at that time the typhoon area will have less energy, because it will cover a wider area. Typhoons in general are strong on their first day and second day (on this map, 08/28 and 08/29), then they lose energy each day ^^ red circle is center of typhoon, where it starts spinning ^^
so don't worry, looking at the forecasts, it will be fine, it seems there will be some raining on first day and second day of Magical Mirai in Tokyo, but nothing too strong. and Makuhari Messe is very safe location
usually any kind of big event in closed space is not affected by typhoons, but there can be some delays in public transports or some shops can be closed in advance, it really depends on the city and situation. do you remember the strong thunderstorm happened during Magical Mirai 2015 in Nippon Budokan? I think it only affected a little the official shops, which closed for a few hours because it was outside,but all the event went on smoothly (and there was also electricity black-out during one concert, I remember! nice experience!)