Choosing between そうだ, ようだ and はずだ

Question

From an article written on the 7th of the month:

東京から日本のほかの場所に向かう「下り」の飛行機は、１１日が特に混みそうです。東京に向かう「上り」の飛行機は、１５日〜１６日と１９日〜２０日が混みそうです。
For planes heading from Tokyo to other places in Japan, the 11th will seem particularly crowded. For planes heading towards Tokyo, the 15th to 16th and 19th to 20th will seem crowded.

I'm confused by the nuance that そう is adding to these sentences. I'm assuming these are guesses based on previous years' flight statistics, so I would have expected something like はず or よう to be used instead.

Could you please provide some explanation as to why そう is the best choice, and whether はず or よう would also be appropriate here?

My understanding is that 混みそうです means that when I look around I will have a feeling that it is crowded ("It will seem crowded"). Whereas 混むようです means that based on evidence I predict it will be crowded ("It seems that it will be crowded"). Could you please confirm or correct my error in understanding?

Answer 1

At this link you can find a very good explanation, but it's only in Japanese.

Let me take out of that site the picture below and give a quick explanation. First of all we can divide all the ways to express a judgement in three groups:

1. と思う、だろう、だろうと思う、かもしれない、にちがいない.
2. そうだ（様態｛ようたい｝)、ようだ、らしい、そうだ(伝聞｛でんぶん｝）.
3. はずだ、わけだ.

This image belongs to Japan foundation: 「Copyright Japan Foundation」

Basically the main difference is the degree of dependence that the speaker has on the information. As you go from group 1 to 3, the degree of dependence on the information increases. On the other hand, if you go from 3 to 1, there is an increase in subjectivity on the judgment. That is, the speaker's subjective judgement becomes stronger than the information. So not surprisingly this is basically saying that the judgment is either objective or subjective depending on whether the judgment itself is stronger than the dependence on the information or the other way around. I realize this is probably more confusing than helpful so let's look at some concrete examples as well:

Suppose you come back home and find the house messed up (open drawers, a flower bin dipped over on the table etc). What would you say?

• 泥棒に入られたかもしれない。
• 泥棒に入られたにちがいない。

These are from group 1: you think thieves came into your house but you don't have enough confidence to state that clearly.

• 泥棒に入られたようだ。
• 泥棒に入られたらしい。

These are from group 2: your confidence that actually thieves broke into your house is somewhat getting stronger. For example you noticed that jewels and money are gone, and with this added information your imagination gets close to conviction.

Suppose you also casually look at the window and notice that the glass is broken. Then you have no doubt someone broke into your house and you would use one from group 3 such as:

• 窓が割れている。そうか、あそこから泥棒に入られたわけだ。

The article also goes on with another interesting example which I suggest you read. If you do not understand it or feel this answer is still not clear enough, please let me know and I will expand it.

EDIT: I got carried away with the general answer and forgot to mention your specific case. Another common use of そう is when reporting not first-hand information but rather something that was heard from a friend or the news. So in the case of your plane そう is most likely used to indicate that such information is coming from some other source (as it seems to be if as you say they're likely to come from past statistics etc) rather than a direct observation. However... if this was the case the construction would be "verb (dictionary form) + そう" so it would be 混むそう. 混みそう indeed makes us think of some impression of something that "looks crowded". Where is this sentence coming from? Could it be a typo and it should actually be 混むそう instead of 混みそう? I realize now my answer might be off. Please leave a comment and if it doesn't help or it's too off I'll remove it.

EDIT 2: So, I would say it this way. Suppose who wrote the news is familiar with the information he/she is delivering (which makes sense). In this case 混みそう is not wrong and I think it is a good choice because it expresses, also according to this answer, a lower degree of certainty than ようだ. Besides, to the extent of my knowledge　ようだ expresses a speaker's subjective conjecture based on information obtained through his/her sensory organs (hear, smell, touch etc). Not sure how this is related in this specific case but also it seems that そう involves a big degree of concern on the speaker's side while in the case of よう such degree is lower. See for example see this page as well. Anyway, putting all these together makes me feel 混みそう is the right choice here. About はず, I think it just doesn't work here, maybe it would sound as "it should be crowded (according to my (the writer's) subjective feeling rather than objective data).

Answer 2

Please bear with me, I am unsure it meets your expectation.

Probably you know this

「そうです①」は動きや変化を表わす動詞につくと、そのような動き・変化が起きる兆候があることを表わします。

It means that if ”そうです” is accompanied with the verb which expresses movement or change, there is the sign of such movement or change is going to happen.

From my understanding of English, it indicates future tense in English.

"going to" sections of English grammar book(Practical English Usage by Michael Swan) explains that one of it is used as things that are on the way: She's going to have a baby.

For example,

Sandra's going to have another baby in June.: サンドラは六月にもう一人赤ちゃんを産みそうです。

Look at the sky. It's going to rain.: 空を見なよ。雨が降りそうだ。

Therefore I deduce, "１１日が特に混みそうです。" means It's going to be crowded on 11th.

Because the writer possibly has been checking this year's flight schedule well and that day's flight has already been booked a lot.

Edit1 I found you had hyperlinked to NHK news-site. So, I found the grammatical reason of using "そう" from the following sentence:

" ８月中旬は、夏休みでふるさとに帰ったり遊びに行ったりする人が多なります。このため、飛行機や新幹線や高速道路が混みます。今年は、飛行機と新幹線の予約が去年より１０％ぐらい多くなっています。"

pick up this

"今年は、飛行機と新幹線の予約が去年より１０％ぐらい多くなっています。"

It means

This year, the number of reservations of the flight and Shinkansen is greater than last year by approximately 10%.

Therefore NHK says they have been checking this year's reservations and that's why they are using "混みそう”.

Hope it helps.